- Professor, Ecology and Evolutionary Biology
- Postdoc of the year to Eugenio Larios (I received a certificate for my contributuion)
- Ecological Society of America, Summer 2015
- Visiting Professorship for Senior International Scientists
- Chinese Academy of Sciences, Summer 2012
No activities entered.
DissertationECOL 920 (Spring 2020)
Natural History of SWECOL 230 (Spring 2020)
Pollination EcologyECOL 467 (Spring 2020)
Pollination EcologyECOL 567 (Spring 2020)
Population BiologyECOL 596B (Spring 2020)
DissertationECOL 920 (Fall 2019)
Directed ResearchECOL 492 (Spring 2019)
DissertationECOL 920 (Spring 2019)
DissertationECOL 920 (Fall 2018)
Independent StudyECOL 499 (Fall 2018)
ThesisECOL 910 (Fall 2018)
ThesisECOL 910 (Summer I 2018)
Directed RsrchMCB 392 (Spring 2018)
DissertationECOL 920 (Spring 2018)
Independent StudyECOL 299 (Spring 2018)
Natural History of SWECOL 230 (Spring 2018)
ThesisECOL 910 (Spring 2018)
Directed ResearchBIOC 392 (Fall 2017)
DissertationECOL 920 (Fall 2017)
Independent StudyECOL 199 (Fall 2017)
Independent StudyECOL 399 (Fall 2017)
Independent StudyPSIO 399 (Fall 2017)
Plant Population EcologyECOL 496J (Fall 2017)
Plant Population EcologyECOL 596J (Fall 2017)
ThesisECOL 910 (Fall 2017)
Directed ResearchECOL 492 (Summer I 2017)
ThesisECOL 910 (Summer I 2017)
Directed ResearchBIOC 392 (Spring 2017)
Directed RsrchMCB 392 (Spring 2017)
DissertationECOL 920 (Spring 2017)
Honors Independent StudyECOL 299H (Spring 2017)
Independent StudyECOL 199 (Spring 2017)
Independent StudyMCB 199 (Spring 2017)
Independent StudyPSIO 399 (Spring 2017)
Natural History of SWECOL 230 (Spring 2017)
ThesisECOL 910 (Spring 2017)
Directed RsrchMCB 392 (Fall 2016)
DissertationECOL 920 (Fall 2016)
Independent StudyBIOC 299 (Fall 2016)
Independent StudyECOL 199 (Fall 2016)
Independent StudyECOL 299 (Fall 2016)
Independent StudyECOL 499 (Fall 2016)
Independent StudyMCB 199 (Fall 2016)
Independent StudyMCB 499 (Fall 2016)
Plant Population EcologyECOL 496J (Fall 2016)
Plant Population EcologyECOL 596J (Fall 2016)
Independent StudyECOL 499 (Spring 2016)
Natural History of SWECOL 230 (Spring 2016)
ResearchECOL 900 (Spring 2016)
Rsrch Ecology+EvolutionECOL 610A (Spring 2016)
ThesisECOL 910 (Spring 2016)
- Saatkamp, A., Poschlod, P., & Venable, D. L. (2014). The functional role of soil seed banks in natural communities. In Seeds: the ecology of regeneration in plant communities. Ed. 3.(pp 263- 295). Wallingford, UK: CABI.
- Venable, D. L., & Kimball, S. (2013). Population and community dynamics of Sonoran Desert winter annuals. In Temporal Dynamics and Ecological Process(pp 140-164). Cambridge, England: Cambridge University Press.
- Horst, J. L., Kimball, S., Becerra, J. X., Noge, K., & Venable, D. L. (2015). Documenting the early stages of invasion of Matthiola parviflora and predicting its spread in North America.. The Southwestern Naturalist, 59, 47-55.
- Huang, Z., Liu, S., Bradford, K. J., Huxman, T. E., & Venable, D. L. (2015). The contribution of germination functional traits to population dynamics of a desert plant community. ECOLOGY, 97(1), 250-261.
- Larios, E., & Venable, D. L. (2015). Maternal adjustment of offspring provisioning and the consequences for dispersal. ECOLOGY, 96(10), 2771-2780.
- Rubio de Casas, R., Donohue, K., Venable, D. L., & Cheptou, P. (2015). Gene-flow through space and time: dispersal, dormancy and adaptation to changing environments. EVOLUTIONARY ECOLOGY, 29(6), 813-831.
- Angert, A. L., Kimball, S., Peterson, M., Huxman, T. E., & Venable, D. L. (2014). Phenotypic constraints and community structure: Linking trade-offs within and among species. EVOLUTION, 68(11), 3149-3165.More infoTrade-offs are central to many topics in biology, from the evolution of life histories to ecological mechanisms of species coexistence. Trade-offs observed among species may reflect pervasive constraints on phenotypes that are achievable given biophysical and resource limitations. If so, then among-species trade-offs should be consistent with trade-offs within species. Alternatively, trait variation among co-occurring species may reflect historical contingencies during community assembly rather than within-species constraints. Here, we test whether a key trade-off between relative growth rate (RGR) and water-use efficiency (WUE) among Sonoran Desert winter annual plants is apparent within four species representing different strategies in the system. We grew progeny of maternal families from multiple populations in a greenhouse common garden. One species, Pectocarya recurvata, displayed the expected RGR-WUE trade-off among families within populations. For other species, although RGR and WUE often varied clinally among populations, among-family variation within populations was lacking, implicating a role for past selection on these traits. Our results suggest that a combination of limited genetic variation in single traits and negative trait correlations could pose constraints on the evolution of a high-RGR and high-WUE phenotype within species, providing a microevolutionary explanation for phenotypes that influence community-level patterns of abundance and coexistence.
- Gremer, J. R., & Venable, D. L. (2014). Bet hedging in desert winter annual plants: Optimal germination strategies in a variable environment. Ecology Letters, 17(3), 380-387.More infoAbstract: In bet hedging, organisms sacrifice short-term success to reduce the long-term variance in success. Delayed germination is the classic example of bet hedging, in which a fraction of seeds remain dormant as a hedge against the risk of complete reproductive failure. Here, we investigate the adaptive nature of delayed germination as a bet hedging strategy using long-term demographic data on Sonoran Desert winter annual plants. Using stochastic population models, we estimate fitness as a function of delayed germination and identify evolutionarily stable strategies for 12 abundant species in the community. Results indicate that delayed germination meets the criteria as a bet hedging strategy for all species. Density-dependent models, but not density-independent ones, predicted optimal germination strategies that correspond remarkably well with observed patterns. By incorporating naturally occurring variation in seed and seedling dynamics, our results present a rigorous test of bet hedging theory within the relevant environmental context.© 2014 John Wiley & Sons Ltd/CNRS.
- Kimball, S., Gremer, J., Barron-Gafford, G. A., Angert, A. L., Huxman, T. E., & Venable, D. L. (2014). High water-use efficiency and growth contribute to success of non-native Erodium cicutarium in a Sonoran Desert winter annual community. Conservation Physiology, 2(1), 1-13.More infoThe success of non-native, invasive species may be due to release from natural enemies, superior competitive abilities, or both. In the Sonoran Desert, Erodium cicutarium has increased in abundance over the last 30 years. While native species in this flora exhibit a strong among-species trade-off between relative growth rate and water-use efficiency, E. cicutarium seems to have a higher relative growth rate for its water-use efficiency value relative to the pattern across native species. This novel trait combination could provide the non-native species with a competitive advantage in this water-limited environment. To test the hypothesis that E. cicutarium is able to achieve high growth rates due to release from native herbivores, we compared the effects of herbivory on E. cicutarium and its native congener, Erodium texanum. We also compared these two species across a range of environmental conditions, both in a common garden and in two distinct seasons in the field, using growth analysis, isotopic compositions and leaf-level gas exchange. Additionally, we compared the competitive abilities of the two Erodium species in a greenhouse experiment. We found no evidence of herbivory to either species. Physiological measurements in a common environment revealed that E. cicutarium was able to achieve high growth rates while simultaneously controlling leaf-level water loss. Non-native E. cicutarium responded to favourable conditions in the field with greater specific leaf area and leaf area ratio than native E. texanum. The non-native Erodium was a stronger competitor than its native congener in a greenhouse competition experiment. The ability to maintain relatively higher values of water-use efficiency:relative growth rate in comparison to the native flora may be what enables E. cictarium to outcompete native species in both wet and dry years, resulting in an increase in abundance in the highly variable Sonoran Desert.
- Larios, E., Burquez, A., Becerra, J. X., & Venable, D. L. (2014). Natural selection on seed size through the life cycle of a desert annual plant. ECOLOGY, 95(11), 3213-3220.More infoUnder stressful circumstances, seed size has important consequences for germination, survival, and reproductive success; all of these are important components of plant fitness. This study investigates the relationship between seed size and fitness in the Sonoran Desert winter annual Dithyrea californica. This species represents a unique opportunity to study natural selection on seed size in the wild due to a serendipitous detail of its life history: the seed coat remains attached and unchanged to the root throughout its life. It is thus possible to excavate the root and measure the seed size that originated each plant. We measured the relationship between seed size and germination by comparing seed sizes of germinated and dormant seeds in the field over four consecutive years. We also measured the effect of seed size on survival and reproductive success using data from censuses of plant mortality and fecundity of survivors, relating survival and fecundity to the size of their initial seed size, and the number of conspecific neighbors. Larger seeds had a higher probability of germination than smaller seeds. Plants originating from larger seeds had higher survival rates and higher fecundity than plants originating from smaller seeds. The amount of precipitation had a beneficial effect on plant fecundity and influenced seed-size survival selection. Plant competition decreased plant fecundity but not survival, creating a detrimental environment for plants only to grow and reproduce. This is the first study to show empirical evidence of seedsize selection throughout the whole life cycle in a natural setting. Further, maternal provisioning has benefits that persist into adulthood, and environmental interactions are important in determining survival and fecundity.
- Willis, C. G., Baskin, C. C., Baskin, J. M., Auld, J. R., Venable, D. L., Cavender-Bares, J., Donohue, K., & Rubio, R. (2014). The evolution of seed dormancy: Environmental cues, evolutionary hubs, and diversification of the seed plants. New Phytologist.More infoAbstract: Seed dormancy, by controlling the timing of germination, can strongly affect plant survival. The kind of seed dormancy, therefore, can influence both population and species-level processes such as colonization, adaptation, speciation, and extinction. We used a dataset comprising over 14 000 taxa in 318 families across the seed plants to test hypotheses on the evolution of different kinds of seed dormancy and their association with lineage diversification. We found morphophysiological dormancy to be the most likely ancestral state of seed plants, suggesting that physiologically regulated dormancy in response to environmental cues was present at the origin of seed plants. Additionally, we found that physiological dormancy (PD), once disassociated from morphological dormancy, acted as an 'evolutionary hub' from which other dormancy classes evolved, and that it was associated with higher rates of lineage diversification via higher speciation rates. The environmental sensitivity provided by dormancy in general, and by PD in particular, appears to be a key trait in the diversification of seed plants. © 2014 The Authors.
- Barron-Gafford, G., Angert, A. L., Venable, D. L., Tyler, A. P., Gerst, K. L., & Huxman, T. E. (2013). Photosynthetic temperature responses of co-occurring desert winter annuals with contrasting resource-use efficiencies and different temporal patterns of resource utilization may allow for species coexistence. Journal of Arid Environments, 91, 95-103.More infoAbstract: A mechanistic understanding of population dynamics requires close examination of species' differences in how physiological traits interact with environmental variation and translate into demographic variation. We focused on two co-occurring winter annual species (Pectocarya recurvata and Plantago insularis) that differ in photosynthetic resource-use efficiency and demographic responses to environmental variation and covariation between temperature and water availability. Previous work showed that Pectocarya has higher water-use efficiency and nitrogen allocation to light-driven dynamics of the Calvin cycle (Jmax:VCmax) than Plantago, which is often associated with enhanced electron transport capacity at low temperatures and better light harvesting capacity. These traits could enhance Pectocarya photosynthesis during reliably moist but cool, cloudy periods following precipitation. We acclimated plants to low and high temperatures and then measured gas exchange across a 30 °C temperature range. As predicted, optimal temperatures of photosynthesis were lower for Pectocarya than Plantago. Additionally, Pectocarya experienced greater respiratory carbon loss than Plantago at higher temperatures (every 1 °C increase beyond 24 °C increased the ratio of carbon loss to gain 9% and 27% in cold and warm-acclimated plants, respectively). These differential patterns of photosynthetic optimization and assimilation in response to differing rainfall distributions may have important implications for population dynamic differences and species coexistence. © 2013 Elsevier Ltd.
- Gremer, J. R., Kimball, S., Keck, K. R., Huxman, T. E., Angert, A. L., & Venable, D. L. (2013). Water-use efficiency and relative growth rate mediate competitive interactions in Sonoran Desert winter annual plants. American Journal of Botany, 100(10), 2009-2015.More infoPMID: 24095798;Abstract: Premise of the study: A functional approach to investigating competitive interactions can provide a mechanistic understanding of processes driving population dynamics, community assembly, and the maintenance of biodiversity. In Sonoran Desert annual plants, a trade-off between relative growth rate (RGR) and water-use efficiency (WUE) contributes to species differences in population dynamics that promote long-term coexistence. Traits underlying this trade-off explain variation in demographic responses to precipitation as well as life history and phenological patterns. Here, we ask how these traits mediate competitive interactions. Methods: We conducted competition trials for three species occupying different positions along the RGR-WUE trade-off axis and compared the effects of competition at high and low soil moisture. We compared competitive effect (ability to suppress neighbors) and competitive response (ability to withstand competition from neighbors) among species. Key results: The RGR-WUE trade-off predicted shifts in competitive responses at different soil moistures. The high-RGR species was more resistant to competition in high water conditions, while the opposite was true for the high-WUE species. The intermediate RGR species tended to have the strongest impact on all neighbors, so competitive effects did not scale directly with differences in RGR and WUE among competitors. Conclusions: Our results reveal mechanisms underlying long-term variation in fitness: high-RGR species perform better in years with large, frequent rain events and can better withstand competition under wetter conditions. The opposite is true for high-WUE species. Such resource-dependent responses strongly influence community dynamics and can promote coexistence in variable environments. © 2013 Botanical Society of America.
- Huxman, T. E., Kimball, S., Angert, A. L., Gremer, J. R., Barron-Gafford, G. A., & Venable, D. L. (2013). Understanding past, contemporary, and future dynamics of plants, populations, and communities using Sonoran desert winter annuals. American Journal of Botany, 100(7), 1369-1380.More infoPMID: 23838034;Abstract: Global change requires plant ecologists to predict future states of biological diversity to aid the management of natural communities, thus introducing a number of significant challenges. One major challenge is considering how the many interacting features of biological systems, including ecophysiological processes, plant life histories, and species interactions, relate to performance in the face of a changing environment. We have employed a functional trait approach to understand the individual, population, and community dynamics of a model system of Sonoran Desert winter annual plants. We have used a comprehensive approach that connects physiological ecology and comparative biology to population and community dynamics, while emphasizing both ecological and evolutionary processes. This approach has led to a fairly robust understanding of past and contemporary dynamics in response to changes in climate. In this community, there is striking variation in physiological and demographic responses to both precipitation and temperature that is described by a trade-off between water-use efficiency (WUE) and relative growth rate (RGR). This community-wide trade-off predicts both the demographic and life history variation that contribute to species coexistence. Our framework has provided a mechanistic explanation to the recent warming, drying, and climate variability that has driven a surprising shift in these communities: cold-adapted species with more buffered population dynamics have increased in relative abundance. These types of comprehensive approaches that acknowledge the hierarchical nature of biology may be especially useful in aiding prediction. The emerging, novel and nonstationary climate constrains our use of simplistic statistical representations of past plant behavior in predicting the future, without understanding the mechanistic basis of change. © 2013 Botanical Society of America.
- Kimball, S., Gremer, J. R., Huxman, T. E., Venable, D. L., & Angert, A. L. (2013). Phenotypic selection favors missing trait combinations in coexisting annual plants. American Naturalist, 182(2), 191-207.More infoPMID: 23852354;Abstract: Trade-offs among traits are important for maintaining biodiversity, but the role of natural selection in their construction is not often known. It is possible that trade-offs reflect fundamental constraints, negative correlational selection, or directional selection operating on costly, redundant traits. In a Sonoran Desert community of winter annual plants, we have identified a trade-off between relative growth rate and water-use efficiency among species, such that species with high relative growth rate have low water-use efficiency and vice versa. We measured selection on water-use efficiency, relative growth rate, and underlying traits within populations of four species at two study sites with different average climates. Phenotypic trait correlations within species did not match the among-species trade-off. In fact, for two species with high water-use efficiency, individuals with high relative growth rate also had high water-use efficiency. All populations experienced positive directional selection for water-use efficiency and relative growth rate. Selection tended to be stronger on water-use efficiency at the warmer and drier site, and selection on relative growth rate tended to be stronger at the cooler and wetter site. Our results indicate that directional natural selection favors a phenotype not observed among species in the community, suggesting that the amongspecies trade-off could be due to pervasive genetic constraints, perhaps acting in concert with processes of community assembly. © 2013 by The University of Chicago.
- Rodriguez-Buritica, S., Raichle, H., Webb, R. H., Turner, R. M., & Venable, D. L. (2013). One hundred and six years of population and community dynamics of Sonoran Desert Laboratory perennials. Ecological Archives, E094-083.
- Rodriguez-Buritica, S., Raichle, H., Webb, R. H., Turner, R. M., Pierson, E. A., & Venable, D. L. (2013). Population dynamics of Sonoran Desert saguaro cactus (Carnegiea gigantea) at the Desert Laboratory (Tucson, Arizona). Ecological Archives, E094-151.
- Becerra, J. X., Noge, K., Olivier, S., & Venable, D. L. (2012). The monophyly of Bursera and its impact for divergence times of Burseraceae. Taxon, 61(2), 333-343.More infoAbstract: Bursera is one of the most diverse and abundant groups of trees and shrubs of the Mexican tropical dry forests. Its interaction with its specialist herbivores in the chrysomelid genus Blepharida, is one of the best-studied coevolutionary systems. Prior studies based on molecular phylogenies concluded that Bursera is a monophyletic genus. Recently, however, other molecular analyses have suggested that the genus might be paraphyletic, with the closely related Commiphora, nested within Bursera. If this is correct, then interpretations of coevolution results would have to be revised. Whether Bursera is or is not monophyletic also has implications for the age of Burseraceae, since previous dates were based on calibrations using Bursera fossils assuming that Bursera was paraphyletic. We performed a phylogenetic analysis of 76 species and varieties of Bursera, 51 species of Commiphora, and 13 outgroups using nuclear DNA data. We also reconstructed a phylogeny of the Burseraceae using 59 members of the family, 9 outgroups and nuclear and chloroplast sequence data. These analyses strongly confirm previous conclusions that this genus is monophyletic. New calculations of the age of Burseraceae date the beginning of its diversification to at least 93 million years ago.
- Gremer, J. R., Kimball, S., Angert, A. L., Venable, D. L., & Huxman, T. E. (2012). Variation in photosynthetic response to temperature in a guild of winter annual plants. Ecology, 93(12), 2693-2704.More infoPMID: 23431599;Abstract: How species respond to environmental variation can have important consequences for population and community dynamics. Temperature, in particular, is one source of variation expected to strongly influence plant performance. Here, we compared photosynthetic responses to temperature across a guild of winter annual plants. Previous work in this system identified a trade-off between relative growth rate (RGR) and water-use efficiency (WUE) that predicts species differences in population dynamics over time, which then contribute to long-term species coexistence. Interestingly, species with high WUE invest in photosynthetic processes that appear to maximize carbon assimilation, while high-RGR species appear to maximize carbon gain by increasing leaf area for photosynthesis. In high- WUE species, higher rates of carbon acquisition were associated with increased investment into light-driven electron transport (Jmax). We tested whether such allocation allows these plants to have greater photosynthetic performance at lower temperatures by comparing the temperature sensitivity of photosynthesis across species in the community. Six species were grown in buried pots in the field, allowing them to experience natural changes in seasonal temperature. Plants were taken from the field and placed in growth chambers where photosynthetic performance was measured following short-term exposure to a wide range of temperatures. These measurements were repeated throughout the season. Our results suggest that high-WUE species are more efficient at processing incoming light, as measured by chlorophyll fluorescence, and exhibit higher net photosynthetic rates (A net) than high-RGR species, and these advantages are greatest at low temperatures. Sampling date differentially affected fluorescence across species, while species had similar seasonal changes in Anet. Our results suggest that species-specific responses to temperature contribute to the WUE-RGR trade-off that has been shown to promote coexistence in this community. These differential responses to environmental conditions can have important effects on fitness, population dynamics, and community structure. © 2012 by the Ecological Society of America.
- Kimball, S., Gremer, J. R., Angert, A. L., Huxman, T. E., & Venable, D. L. (2012). Fitness and physiology in a variable environment. Oecologia, 169(2), 319-329.More infoPMID: 22116505;Abstract: The relationship between physiological traits and fitness often depends on environmental conditions. In variable environments, different species may be favored through time, which can influence both the nature of trait evolution and the ecological dynamics underlying community composition. To determine how fluctuating environmental conditions favor species with different physiological traits over time, we combined long-term data on survival and fecundity of species in a desert annual plant community with data on weather and physiological traits. For each year, we regressed the standardized annual fitness of each species on its position along a tradeoff between relative growth rate and water-use efficiency. Next, we determined how variations in the slopes and intercepts of these fitness-physiology functions related to year-to-year variations in temperature and precipitation. Years with a relatively high percentage of small rain events and a greater number of days between precipitation pulse events tended to be worse, on average, for all desert annual species. Species with high relative growth rates and low water-use efficiency had greater standardized annual fitness than other species in years with greater numbers of large rain events. Conversely, species with high water-use efficiency had greater standardized annual fitness in years with small rain events and warm temperatures late in the growing season. These results reveal how weather variables interact with physiological traits of co-occurring species to determine interannual variations in survival and fecundity, which has important implications for understanding population and community dynamics. © 2011 Springer-Verlag.
- Gerst, K. L., Angert, A. L., & Venable, D. L. (2011). The effect of geographic range position on demographic variability in annual plants. Journal of Ecology, 99(2), 591-599.More infoAbstract: The abundant centre model predicts that species abundance will decline from the centre towards the periphery of the geographic range. Thus, we expect to find decreases from the centre towards the edge in variables related to population dynamics such as population density and reproductive output. However, evidence for this pattern is contradictory, suggesting that geographically peripheral sites may not be ecologically peripheral. Populations may thrive in pockets of suitable habitat at the edge of the range or may be locally adapted to peripheral conditions. This study examines how the position of a site within geographic and climatic ranges of 13 species is related to the population dynamics at one common location, The Desert Laboratory at Tumamoc Hill, Tucson, AZ, USA. We used data on survival, fecundity, germination fraction and population density from a 25-year long-term data set on winter annual plants to determine whether there was a relationship between distance to the centre of the range and population dynamics. Geographic distance was calculated by determining the distance from the Desert Laboratory to the centre of the observed range determined from locality records. Climatic distance was calculated using the niche modelling software, maxent, and subtracting the mean climatic profile for the species range from that of the Desert Laboratory. There was no relationship between mean population metrics and distance metrics. We found significant relationships between some geographic distance metrics and variance in fecundity, survival and per-germinant fecundity, but not germination fraction or population density. We did not find a relationship with any metric of population dynamic variation and climatic distance. Synthesis.Our results indicate that geographic distance from the centre of the range of 13 annual plant species more strongly predicts their population dynamics than climatic distance. This study reinforces the importance of examining vital rates and their variation in order to properly capture the effect of position within a range on population dynamics. © 2011 The Authors. Journal of Ecology © 2011 British Ecological Society.
- Kimball, S., Angert, A. L., Huxman, T. E., & Venable, D. L. (2011). Differences in the timing of germination and reproduction relate to growth physiology and population dynamics of sonoran desert winter annuals. American Journal of Botany, 98(11), 1773-1781.More infoPMID: 22003177;Abstract: Premise of the Study: Trait differences can promote distinct survival and fecundity responses to environmental fluctuations. In a Sonoran Desert winter annual plant community, we have identified a tradeoff between relative growth rate (RGR) and water-use efficiency (WUE) that predicts interannual variation in reproductive success. Here we test the hypothesis that traits underlying RGR and WUE differences are linked to seasonal phenology. Methods: We use long-term demographic data and finer-scale, short-term data to investigate timing of germination, reproduction, and death of several winter annual species in multiple years in open and under-shrub habitats. We hypothesized that species with high WUE and less interannual demographic variability would have life cycle transitions early in the winter to spring growing season. This would be due to an ability to use small amounts of rain and photosynthesize at low temperatures. By contrast, we hypothesized that species with low WUE whose survival and reproductive rates vary greatly from year to year would have life cycle transitions later in the season. Key Results: In any given year, species with high WUE germinated and reproduced earlier in the season than species with low WUE, whereas low-WUE species germinated later and had shorter reproductive phases. Conclusions: Our results demonstrate a direct relationship between phenology and physiological trait differences. This link between phenology and physiology is of interest because it clarifies the mechanism by which trait differences determine species ' relative abundances. © 2011 Botanical Society of America.
- Noge, K., Venable, D. L., & Becerra, J. X. (2011). 2-Phenylethanol in the leaves of Bursera velutina Bullock (Burseraceae). Acta Botanica Mexicana, 97(1), 9-16.More infoAbstract: The volatile composition of the leaves of Bursera velutina Bullock (Burseraceae) was determined by a gas chromatograph-mass spectrometer (GC-MS). The major component found was 2-phenylethanol (29.5%). This is the first report of 2-phenylethanol in the leaves of a species of the genus Bursera. In addition, B. velutina also produces monoterpenes, sesquiterpenes, diterpenes and alkanes, making it a species with one of the most complex chemical compositions in the genus. This diverse and unique blend of compounds may play an important role in plant defense against its herbivores.
- Angert, A. L., Horst, J. L., Huxman, T. E., & Venable, D. L. (2010). Phenotypic plasticity and precipitation response in sonoran desert winter annuals. American Journal of Botany, 97(3), 405-411.More infoPMID: 21622404;Abstract: Temporal environmental variation has profound influences on population dynamics and community structure. Examination of functional traits that influence resource uptake and allocation can illuminate how co-occurring species translate environmental variation into different demographic outcomes, yet few studies have considered interspecific differences in trait plasticity. We experimentally manipulated soil moisture to test the hypothesis that differences in morphological plasticity contribute to species differences in demographic response to unpredictable precipitation in Sonoran Desert winter annual plants. We compared plasticity of leaf traits and biomass allocation between Pectocarya recurvata (Boraginaceae) and Stylocline micropoides (Asteraceae), co-occurring species that differ in long-term demographic patterns. The species with highly variable population dynamics, Stylo-cline, had striking increases in leaf area and root biomass in response to an experimental increase in soil moisture. In contrast, the species with buffered long-term population dynamics, Pectocarya, did not differ in leaf morphology or biomass allocation between soil moisture treatments. Regardless of water treatment, Pectocarya had earlier reproductive phenology and greater fecundity than Stylocline, suggesting that differences in the timing of the phenological transitions from vegetative to reproductive growth may affect species' responses to precipitation pulses. Combining long-term observations with experimental manipulations provides a window into the functional underpinnings and demographic consequences of trait plasticity. © 2010 Botanical Society of America.
- Kimball, S., Angert, A. L., Huxman, T. E., & Venable, D. (2010). Contemporary climate change in the Sonoran Desert favors cold-adapted species. Global Change Biology, 16(5), 1555-1565.More infoAbstract: Impacts of long-term climate shifts on the dynamics of intact communities within species ranges are not well understood. Here, we show that warming and drying of the Southwestern United States over the last 25 years has corresponded to a shift in the species composition of Sonoran Desert winter annuals, paradoxically favoring species that germinate and grow best in cold temperatures. Winter rains have been arriving later in the season, during December rather than October, leading to the unexpected result that plants are germinating under colder temperatures, shifting community composition to favor slow growing, water-use efficient, cold-adapted species. Our results demonstrate how detailed ecophysiological knowledge of individual species, combined with long-term demographic data, can reveal complex and sometimes unexpected shifts in community composition in response to climate change. Further, these results highlight the potentially overwhelming impact of changes in phenology on the response of biota to a changing climate. © 2009 Blackwell Publishing Ltd.
- Angert, A. L., Huxman, T. E., Chesson, P., & Venable, D. L. (2009). Functional tradeoffs determine species coexistence via the storage effect. Proceedings of the National Academy of Sciences of the United States of America, 106(28), 11641-11645.More infoPMID: 19571002;PMCID: PMC2710622;Abstract: How biological diversity is generated and maintained is a fundamental question in ecology. Ecologists have delineated many mechanisms that can, in principle, favor species coexistence and hence maintain biodiversity. Most such coexistence mechanisms require or imply tradeoffs between different aspects of species performance. However, it remains unknown whether simple functional tradeoffs underlie coexistence mechanisms in diverse natural systems. We show that functional tradeoffs explain species differences in long-term population dynamics that are associated with recovery from low density (and hence coexistence) for a community of winter annual plants in the Sonoran Desert. We develop a new general framework for quantifying the magnitude of coexistence via the storage effect and use this framework to assess the strength of the storage effect in the winter annual community. We then combine a 25-year record of vital rates with morphological and physiological measurements to identify functional differences between species in the growth and reproductive phase of the life cycle that promote storage-effect coexistence. Separation of species along a tradeoff between growth capacity and low-resource tolerance corresponds to differences in demographic responses to environmental variation across years. Growing season precipitation is one critical environmental variable underlying the demographic decoupling of species. These results demonstrate how partially decoupled population dynamics that promote local biodiversity are associated with physiological differences in resource uptake and allocation between species. These results for a relatively simple system demonstrate how long-term community dynamics relate to functional biology, a linkage scientists have long sought for more complex systems.
- Becerra, J. X., Noge, K., & Venable, D. L. (2009). Macroevolutionary chemical escalation in an ancient plant-herbivore arms race. Proceedings of the National Academy of Sciences of the United States of America, 106(43), 18062-18066.More infoPMID: 19706441;PMCID: PMC2775328;Abstract: A central paradigm in the field of plant-herbivore interactions is that the diversity and complexity of secondary compounds in plants have intensified over evolutionary time, resulting in the great variety of secondary products that currently exists. Unfortunately, testing of this proposal has been very limited. We analyzed the volatile chemistry of 70 species of the tropical plant genus Bursera and used a molecular phylogeny to test whether the species' chemical diversity or complexity have escalated. The results confirm that as new species diverged over time they tended to be armed not only with more compounds/species, but also with compounds that could potentially be more difficult for herbivores to adapt to because they belong to an increasing variety of chemical pathways. Overall chemical diversity in the genus also increased, but not as fast as species diversity, possibly because of allopatric species gaining improved defense with compounds that are new locally, but already in existence elsewhere.
- Venable, D. L., & Rees, M. (2009). The scaling of seed size. Journal of Ecology, 97(1), 27-31.More infoAbstract: 1. Rees & Venable (2007; Journal of Ecology, 95, 926-936) critically evaluated ideas of Moles et al. to explain the cross-species positive correlation between offspring size and adult size, arguing that they had misinterpreted the theoretical literature, and used cross-species patterns to constrain the evolution of life-histories. 2. In a reply to Rees & Venable, Westoby et al. (2009; Journal of Ecology, 97, 23-26) claimed we had misrepresented their views. Here we try to clarify the arguments indicating points of agreement and disagreement. 3. Using simple models we then extend the current theory to allow (i) the time during which seedling survival is influenced by seed mass to scale with the duration of the juvenile period and (ii) the intensity of stress experienced by seedlings to scale with adult size. These new models predict that species with long juvenile periods or large adult size will have larger seeds. 4. We synthesize the new theoretical ideas with our current understanding of the evolution of seed mass, and suggest that much of the observed increase in seed mass with adult body size may be due to constraints correlated with adult body size. © 2008 The Authors.
- Becerra, J. X., & Venable, D. L. (2008). Sources and sinks of diversification and conservation priorities for the Mexican tropical dry forest. PLoS ONE, 3(10).More infoPMID: 18927613;PMCID: PMC2562985;Abstract: Elucidating the geographical history of diversification is critical for inferring where future diversification may occur and thus could be a valuable aid in determining conservation priorities. However, it has been difficult to recognize areas with a higher likelihood of promoting diversification. We reconstructed centres of origin of lineages and identified areas in the Mexican tropical dry forest that have been important centres of diversification (sources) and areas where species are maintained but where diversification is less likely to occur (diversity sinks). We used a molecular phylogeny of the genus Bursera, a dominant member of the forest, along with information on current species distributions. Results indicate that vast areas of the forest have historically functioned as diversity sinks, generating few or no extant Bursera lineages. Only a few areas have functioned as major engines of diversification. Long-term preservation of biodiversity may be promoted by incorporation of such knowledge in decision-making. © 2008 Becerra et al.
- Huxman, T. E., Barron-Gafford, G., Gerst, K. L., Angert, A. L., Tyler, A. P., & Venable, D. L. (2008). Photosynthetic resource-use efficiency and demographic variability in desert winter annual plants. Ecology, 89(6), 1554-1563.More infoPMID: 18589520;Abstract: We studied a guild of desert winter annual plants that differ in long-term variation in per capita reproductive success (lb, the product of per capita survival from germination to reproduction, l, times per capita reproduction of survivors, b) to relate individual function to population and community dynamics. We hypothesized that variation in lb should be related to species' positions along a trade-off between relative growth rate (RGR) and photosynthetic water-use efficiency (WUE) because lb is a species-specific function of growing-season precipitation. We found that demographically variable species have greater RGR and greater leaf carbon isotope discrimination (A, a proxy inversely related to WUE). We examined leaf nitrogen and photosynthetic characteristics and found that, in this system, variation in A is a function of photosynthetic demand rather than stomatal regulation of water loss. The physiological characteristics that result in low A in some species may confer greater photosynthetic performance during the reliably moist but low temperature periods that immediately follow winter rainfall in the Sonoran Desert or alternatively during cool periods of the day or early growing season. Conversely, while species with high A and high RGR exhibit low leaf N, they have high biomass allocation to canopy leaf area display. Such trait associations may allow for greater performance during the infrequent conditions where high soil moisture persists into warmer conditions, resulting in high demographic variance. Alternatively, high variance could arise from specialization to warm periods of the day or season. Population dynamic buffering via stress tolerance (low RGR and A) correlates negatively with buffering via seed banks, as predicted by bet-hedging theory. By merging analyses of population dynamics with functional trait relationships, we develop a deeper understanding of the physiological, ecological, and evolutionary mechanisms involved in population and community dynamics. © 2008 by the Ecological Society of America.
- Venable, D. L., Flores-Martinez, A., Muller-Landau, H. C., Barron-Gafford, G., & Becerra, J. X. (2008). Seed dispersal of desert annuals. Ecology, 89(8), 2218-2227.More infoPMID: 18724732;Abstract: We quantified seed dispersal in a guild of Sonoran Desert winter desert annuals at a protected natural field site in Tucson, Arizona, USA. Seed production was suppressed under shrub canopies, in the open areas between shrubs, or both by applying an herbicide prior to seed set in large, randomly assigned removal plots (10-30 m diameter). Seedlings were censused along transects crossing the reproductive suppression borders shortly after germination. Dispersal kernels were estimated for Pectocarya recurvata and Schismus barbatus from the change in seedling densities with distance from these borders via inverse modeling. Estimated dispersal distances were short, with most seeds traveling less than a meter. The adhesive seeds of P. recurvata went farther than the small S. barbatus seeds, which have no obvious dispersal adaptation. Seeds dispersed farther downslope than upslope and farther when dispersing into open areas than when dispersing into shrubs. Dispersal distances were short relative to the pattern of spatial heterogeneity created by the shrub and open space mosaic. This suggests that dispersal could contribute to local population buildup, possibly facilitating species coexistence. Overall, these results support the hypothesis that escape in time via delayed germination is likely to be more important for desert annuals than escape in space. © 2008 by the Ecological Society of America.
- Angert, A. L., Huxman, T. E., Barron-Gafford, G., Gerst, K. L., & Venable, D. L. (2007). Linking growth strategies to long-term population dynamics in a guild of desert annuals. Journal of Ecology, 95(2), 321-331.More infoAbstract: Combining long-term observational studies with comparative physiological ecology can yield a deeper understanding of the contribution of individual function to population and community dynamics. Sonoran Desert winter annuals exhibit striking year-to-year variation in population dynamics that is driven by variable precipitation, but species differ in the strength of demographic response to precipitation and hence in the degree of temporal variance in population dynamics. To understand the physiological mechanisms of differing population dynamic responses to environmental variation, we investigated interspecific differences in functional traits that mediate responsiveness to precipitation. We conducted sequential harvests throughout the growing season to examine relative growth rate and biomass allocation patterns. We then related growth parameters to leaflevel carbon isotope discrimination (a time-integrated measure of water-use efficiency) and long-term demographic variation. We hypothesized that water-use efficiency should trade-off with rapid growth rates. Furthermore, we hypothesized that species having efficient water use should have buffered population dynamics in dry years but sacrifice high growth and fecundity in wet years, resulting in low long-term variance in demographic success. Conversely, species with high growth capacity should be very responsive to infrequent periods of high precipitation and thus exhibit high temporal variance. Species differed in seasonal relative growth rate and allocation patterns. Species with the highest relative growth rates rapidly deployed large leaf area displays following mid-season rainfall. Species with intermediate relative growth rates exhibited high biomass assimilation rates per unit leaf area. Species with low relative growth rates exhibited low leaf area ratios and low assimilation rates per unit leaf area. Relative growth rate was positively related to leaf carbon isotope discrimination, consistent with a trade-off between growth rate and water-use efficiency. Seasonal relative growth rate did not predict long-term demographic variance. However, leaf area plasticity in response to precipitation was positively related to longterm demographic variance. Our results illustrate how morphological and physiological traits influence demographic tracking of environmental variability and demonstrate how species differences in functional strategies determine population and community dynamics. © 2007 The Authors.
- E., M., Ferrière, R., Kane, M. J., & Venable, D. L. (2007). Bet hedging via seed banking in desert evening primroses (Oenothera, Onagraceae): Demographic evidence from natural populations. American Naturalist, 169(2), 184-194.More infoPMID: 17211803;Abstract: Bet hedging is one solution to the problem of an unpredictably variable environment: fitness in the average environment is sacrificed in favor of lower variation in fitness if this leads to higher long-run stochastic mean fitness. While bet hedging is an important concept in evolutionary ecology, empirical evidence that it occurs is scant. Here we evaluate whether bet hedging occurs via seed banking in natural populations of two species of desert evening primroses (Oenothera, Onagraceae), one annual and one perennial. Four years of data on plants and 3 years of data on seeds yielded two transitions for the entire life cycle. One year was exceptionally dry, leading to reproductive failure in the sample areas, and the other was above average in precipitation, leading to reproductive success in four of five populations. Stochastic simulations of population growth revealed patterns indicative of bet hedging via seed banking, particularly in the annual populations: variance in fitness and fitness in the average environment were lower with seed banking than without, whereas long-run stochastic mean fitness was higher with seed banking than without across a wide range of probabilities of the wet year. This represents a novel, unusually rigorous demonstration of bet hedging from field data. © 2007 by The University of Chicago.
- Rees, M., & Venable, D. L. (2007). Why do big plants make big seeds?. Journal of Ecology, 95(5), 926-936.More infoAbstract: 1. The conventional explanations for large plant species producing larger seeds on average than small plant species have recently been challenged, and it has been suggested that the pattern is better explained by the theory developed by Charnov (1993). Here we use simple life-history theory to explore the logic underlying Charnov's models and show that under most reasonable conditions they predict no relationship between seed mass and size at maturity. 2. Using a simple general model incorporating size-specific growth and survival, we explore the joint evolution of seed mass and size at maturity, and argue that seed mass will be correlated with adult traits, such as the timing of reproduction and size at reproduction, only if seedling and adult growth and mortality rates are correlated. Evidence for such correlations is briefly explored. 3. It has also been suggested that the standard model for seed mass evolution (Smith & Fretwell 1974) has been misinterpreted, and that application of the model requires measurement of survivorship to reproductive maturity. Using a simple model incorporating size-specific growth and survival we show that this criticism is unfounded. 4. Our results differ from those of Moles and colleagues because they look at the effect of long juvenile period on survival to maturity, but do not recognize that this may be compensated by covarying life-history traits, such as plant size and reproductive lifespan. Also, they seem to argue that life-history evolution is constrained by cross-angiosperm correlations, such as that between seed mass and longevity, while the life-history models presented here seek selective causes of such correlations, rather than regarding them as constraints. 5. Models similar to those of Charnov (1993) only predict a positive relationship between seed size and plant size if unrealistic assumptions are made about the effects of seed mass on survival, such as the effect of seed mass on instantaneous survival persisting to adulthood. © 2007 The Authors.
- Venable, D. L. (2007). Bet hedging in a guild of desert annuals. Ecology, 88(5), 1086-1090.More infoPMID: 17536393;Abstract: Evolutionary bet hedging encapsulates the counterintuitive idea that organisms evolve traits that reduce short-term reproductive success in favor of longer-term risk reduction. It has been widely investigated theoretically, and many putative examples have been cited including practical ones such as the dormancy involved in microbe and weed persistence. However, long-term data on demographic variation from the actual evolutionarily relevant environments have been unavailable to test for its mechanistic relationship to alleged bet hedging traits. I report an association between delayed germination (a bet hedging trait) and risk using a 22-year data set on demographic variation for 10 species of desert annual plants. Species with greater variation in reproductive success (per capita survival from germination to reproduction X per capita fecundity of survivors) were found to have lower average germination fractions. This provides a definitive test using realistic data on demographic variance that confirms the life history prediction for bet hedging. I also showed that the species with greater long-term demographic variation tended to be the ones with greater sensitivity of reproductive success to variation among years in growing-season precipitation. © 2007 by the Ecological Society of America.
- E., M., Hearn, D. J., Hahn, W. J., Spangle, J. M., & Venable, D. L. (2005). Climate and life-history evolution in evening primroses (Oenothera, Onagraceae): A phylogenetic comparative analysis. Evolution, 59(9), 1914-1927.More infoPMID: 16261729;Abstract: Evolutionary ecologists have long sought to understand the conditions under which perennial (iteroparous) versus annual (semelparous) plant life histories are favored. We evaluated the idea that aridity and variation in the length of droughts should favor the evolution of an annual life history, both by decreasing adult survival and by increasing the potential for high seedling survival via reduced plant cover. We calculated phylogenetically independent contrasts of climate with respect to life history in a clade of winter-establishing evening primroses (sections Anogra and Kleinia; Oenothera; Onagraceae), which includes seven annuals, 12 perennials, and two variable taxa. Climate variables were quantified from long-term records at weather stations near collection localities. To explicitly account for phylogenetic uncertainty, contrasts were calculated on a random sample of phylogenetic trees from the posterior distribution of a Bayesian analysis of DNA sequence data. Statements of association are based on comparing the pertree mean contrast, which has a null expectation of zero, to a set of per-tree mean contrasts calculated on the same trees, after randomizing the climate data. As predicted, increased annual aridity, increased annual potential evapotranspiration, and decreased annual precipitation were associated with transitions to the annual habit, but these trends were not significantly different from the null pattern. Transitions to the annual habit were not significantly associated with increases in one measure of aridity in summer nor with increased summer drought, but they were associated with significantly increased maximum summer temperatures. In winter, increased aridity and decreased precipitation were significantly associated with transitions to the annual habit. Changes in life history were not significantly associated with changes in the coefficient of variation of precipitation, either on an annual or seasonal (summer vs. winter) basis. Though we cannot attribute causality on the basis of a correlational, historical study, our results are consistent with the idea that increased heat and drought at certain times of the year favor the evolution of the annual habit. Increased heat in summer may cause adult survival to decline, while increased aridity and decreased precipitation in the season of seedling recruitment (winter) may favor a drought-avoiding, short-lived annual strategy. Not all of the predicted patterns were observed: the capability for drought-induced dormancy may preclude change in habit in response to summer drought in our study group. © 2005 The Society for the Study of Evolution. All rights reserved.
- Adondakis, S., & Venable, D. L. (2004). Dormancy and germination in a guild of sonoran desert annuals. Ecology, 85(9), 2582-2590.More infoAbstract: To investigate bet hedging and species coexistence in a guild of Sonoran Desert winter annuals, we subjected seeds of eight species to factorial combinations of summer treatments (varying temperature and precipitation), germination conditions (representative of early, middle, and late germination season temperatures and day lengths), and experiment trial dates (spanning the germination season). In keeping with bet hedging theory, we found that many viable seeds would not germinate in response to any combination of treatments (germination usually
- Venable, D. L. (2004). Introduction: Allocation under multiple resource constraints. Evolutionary Ecology Research, 6(8), 1107-.
- Venable, D. L., & Lloyd, D. G. (2004). Allocation under multiple resource constraints. Evolutionary Ecology Research, 6(8), 1109-1121.More infoAbstract: Models of reproductive ecology and life history usually resort to arguments of optimal or ESS allocation of a single limiting resource. Yet real organisms may be limited by multiple resources. We present a verbal-graphical method for analysing allocation problems under multiple resource constraints, focusing on an example of allocation to two structures or functions, subject to two resource constraints. The logic behind this intuitive model is justified by an analytical model, which is applied to a sex allocation example. The evolutionarily stable strategy (ESS) or optimal allocation will usually depend on which constraints are limiting at the ESS. For example, if the carbon:nitrogen ratio required to produce seeds differs from that required to produce pollen, the ESS numbers of seeds and pollen grains will differ depending on which resource is limiting (and take on yet another value if both resources are limiting at the ESS). We show how to determine whether one, the other or both constraints are limiting at the ESS. The degree of difference in the ESS allocation under different constraints is determined by the degree of difference in the 'resource recipes' of the structures or functions in question. ESS allocations to structures or functions are predicted to be more sensitive to changes in resource availability when both constraints are limiting than when only one is limiting. In contrast, ESSs are predicted to be more sensitive to changes in fitness equation parameters when one resource is limiting than when both are limiting. Nutrient manipulation experiments are suggested that can test the importance of these ideas. Plastic developmental responses in resource acquisition may result in multiple resource constraints frequently being binding at ESSs. To the degree that resources are substitutable, rather than being required in specific ratios, multiple resource allocation problems can be rephrased as single resource problems.
- Miller, J. S., & Venable, D. L. (2003). Floral morphometrics and the evolution of sexual dimorphism in Lycium (Solanaceae). Evolution, 57(1), 74-86.More infoPMID: 12643569;Abstract: Plants of Lycium californicum, L. exsertum, and L. fremontii produce flowers that are either male-sterile (female) or hermaphroditic, and populations are morphologically gynodioecious. As is commonly found in gynodioecious species, flowers on female plants are smaller than those on hermaphrodites for a number of floral traits. Floral size dimorphism has often been hypothesized to be the result of either a reduction in female flower size that allows reallocation to greater fruit and seed production, or an increase in hermaphroditic flower size due to the increased importance of pollinator attraction and pollen export for hermaphroditic flowers. We provide a test of these two alternatives by measuring 11 floral characters in eight species of Lycium and using a phylogeny to reconstruct the floral size shifts associated with the evolution of gender dimorphism. Our analyses suggest that female flowers are reduced in size relative to the ancestral condition, whereas flowers on hermaphrodites have changed only slightly in size. Female and hermaphroditic flowers have also diverged both from one another and from ancestral cosexual species in several shape characteristics. We expected sexual dimorphism to be similar among the three dimorphic taxa, as gender dimorphism evolved only a single time in the ancestor of the American dimorphic lineage. While the floral sexual dimorphism is broadly similar among the three dimorphic species, there are some species-specific differences. For example, L. exsertum has the greatest floral size dimorphism, whereas L. fremontii had the greatest size-independent dimorphism in pistil characters. To determine the degree to which phylogenetic uncertainty affected reconstruction of ancestral character states, we performed a sensitivity analysis by reconstructing ancestral character states on alternative topologies. We argue that investigations such as this one, that examine floral evolution from an explicitly phylogenetic perspective, provide new insights into the study of the evolution of floral sexual dimorphism.
- Miller, J. S., & Venable, D. L. (2002). The transition to gender dimorphism on an evolutionary background of self-incompatibility: An example from Lycium (Solanaceae). American Journal of Botany, 89(12), 1907-1915.More infoPMID: 21665619;Abstract: Populations of three North American species of Lycium (Solanaceae) are morphologically gynodioecious and consist of male-sterile (i.e., female) and hermaphroditic plants. Marked individuals were consistent in sexual expression across years and male sterility was present throughout much of the species' ranges. Controlled pollinations reveal that L. californicum, L. exsertum, and L. fremontii are functionally dioecious. Fruit production in females ranged from 36 to 63%, whereas hermaphrodites functioned essentially as males. Though hermaphrodites were mostly male, investigation of pollen tube growth reveals that hermaphrodites of all dimorphic species were self-compatible. Self-fertilization and consequent inbreeding depression are commonly invoked as important selective forces promoting the invasion of male-sterile mutants into cosexual populations. A corollary prediction of these models is that gender dimorphism evolves from self-compatible ancestors. However, fruit production, seed production, and pollen tube number following outcross pollination were significantly higher than following self-pollination for three diploid, cosexual species that are closely related to the dimorphic species. The data presented here on incompatibility systems are consistent with the hypothesis that polyploidy disrupted the self-incompatibility system in the gynodioecious species leading to the evolution of gender dimorphism.
- Till-Bottraud, I., Gouyon, P., Venable, D. L., & Godelle, B. (2001). The number of competitors providing pollen on a stigma strongly influences intraspecific variation in number of pollen apertures. Evolutionary Ecology Research, 3(2), 231-253.More infoAbstract: Variation in the number of pollen apertures has been widely described among and within angiosperm species. Apertures are weak points of the pollen wall where the pollen tube germinates. Pollen aperture heteromorphism (pollen grains with different numbers of apertures in a single individual) is common in flowering plants, whereas polymorphism (among-individual variation) is rare. Previous work on Viola has shown that pollen with few apertures has a better survival rate, whereas pollen with more apertures germinates faster. Here we develop game-theoretic models of competition between several pollen donors. These show that heteromorphism can be a stable strategy for all finite numbers of competitors per stigma for some parameter values where one pollen type germinates faster but has lower longevity. In contrast, polymorphism is not stable in pairwise contests (two pollen donors). When more than two pollen donors interact on stigmas, polymorphism can be stable for certain parameter values. In both heteromorphism and polymorphism, selection operating on the number of pollen apertures is an example of soft selection if each flower in a population produces a fixed number of seeds, regardless of the average fitness of the particular pollen composition present on its stigma. This results in stigma-level and population-level frequency dependence, which makes stable heteromorphism and polymorphism possible. Selective scenarios vary among stigmas due to variation in the pollen present. Thus, a particular pollen type may be more fit than average on some stigmas but less fit on others. As a pollen strategy increases in frequency in a population, the frequency of different kinds of pollen contests shift. This may result in the pollen strategy's fitness advantage being lost at an intermediate frequency, resulting in heteromorphism or polymorphism. Low numbers of pollen donors per stigma result in greater variance in pollen composition among stigmas, resulting in a broader parameter range for stable heteromorphism or polymorphism. For any number of pollen donors per stigma, the conditions for polymorphism are a more restrictive subset of those for heteromorphism. We show that heteromorphism can invade a polymorphic population, whereas heteromorphism is stable against polymorphism, thus explaining why polymorphic species are rare.
- Clauss, M. J., & Venable, D. L. (2000). Seed germination in desert annuals: An empirical test of adaptive bet hedging. American Naturalist, 155(2), 168-186.More infoAbstract: Temporal variability in survivorship and reproduction is predicted to affect the evolution of life-history characters. Desert annual plants experience temporal variation in reproductive success that is largely caused by precipitation variability. We studied several populations of the desert annual Plantago insularis along a precipitation gradient. Whereas models of bet hedging in unpredictable environments generally predict one optimal germination fraction for a population, empirical studies have shown that environmental conditions during germination can cause a range of germination fractions to be expressed. In a 4-yr field study, we found that populations in historically more xeric environments had lower mean germination fractions, as is predicted by bet-hedging models. However, populations exhibited significant variation in germination among years. Two experimental studies measuring germination under several environment conditions were conducted to elucidate the source of this in situ variation. Germination fractions exhibited phenotypic plasticity in response to water availability and date within the season. Populations differed in their norms of reaction such that seeds from more xeric populations germinated under less restrictive conditions. A pattern of delayed germination consistent with among-year bet-hedging predictions arose in the field through the interaction of seed germinability and the distribution of environmental conditions during germination.
- Evans, P. H., Becerra, J. X., Venable, D. L., & Bowers, W. S. (2000). Chemical analysis of squirt-gun defense in Bursera and counterdefense by chrysomelid beetles. Journal of Chemical Ecology, 26(3), 745-754.More infoAbstract: The genus Bursera produces resin stored in canals in the leaf. When leaves are damaged, some, but not all, species release abundant resin. Species of Blepharida are specialized herbivores of Bursera, and they exhibit variation in their counterdefensive behavior. Species feeding on resin- releasing plants cut the leaf veins before feeding, which often makes them more prone to predation. They also adorn their backs with their feces and may regurgitate and release an anal secretion when attacked or disturbed by predators. Species that feed on Bursera species that release no fluids do not sever the leaf veins prior to feeding, and they do not carry their feces on their backs. Instead, they face their predators, raise their heads in a 'boxing-like' display, and rapidly swing their abdomens from side to side. We performed a comparative chemical analysis of the compounds found in Bursera schlechtendalii, a species that releases abundant resins, and B. biflora, a species that does not. We also analyzed the frass, enteric discharges, and larvae of the two species of Blepharida that feed on each of these plants. The compounds found in the body, feces, and discharges of the Blepharida species that adorns itself with feces match the chemical mixture of its host plant, suggesting that this beetle species can compensate its higher risk of predation by using the compounds present in the plant for defense. The chemical mixture of B. biflora is more complex and does not match the compounds found in the body or frass of its beetle herbivore, suggesting that the defensive strategy of this insect is behavioral and does not rely on its host's constituents.
- Miller, J. S., & Venable, D. L. (2000). Polyploidy and the evolution of gender dimorphism in plants. Science, 289(5488), 2335-2338.More infoPMID: 11009416;Abstract: Gender dimorphism and polyploidy are important evolutionary transitions that have evolved repeatedly in many plant families. We show that gender dimorphism in North American Lycium (Solanaceae) has evolved in polyploid, self-compatible taxa whose closest relatives are cosexual, self-incompatible diploids. This has occurred independently in South African Lycium. We present additional evidence for this pathway to gender dimorphism from 12 genera involving at least 20 independent evolutionary events. We propose that polyploidy is a trigger of unrecognized importance for the evolution of gender dimorphism, which operates by disrupting setf-incompatibitity and reading to inbreeding depression. Subsequently, mate sterile mutants invade and increase because they are unable to inbreed.
- Moriuchi, K. S., Venable, D. L., Pake, C. E., & Lange, T. (2000). Direct measurement of the seed bank age structure of a Sonoran Desert annual plant. Ecology, 81(4), 1133-1138.More infoAbstract: We describe a new approach to determining the age structure of seed banks of natural plant populations and apply it to a natural population of the Sonoran Desert winter annual, Pectocarya recurvata (Boraginaceac). Unlike other 14C techniques, tandem accelerator mass spectrometry (TAMS) counts the number of carbon isotope atoms, permitting high precision with small samples. Aboveground nuclear bomb tests caused atmospheric 14C levels to peak in 1963. Their subsequent gradual decline provides a signal for aging seed banks with TAMS. We constructed a calibration curve using seeds with known dates of production during 1980-1995, then used it to age 53 seeds sampled from a natural seed bank in 1993, at the Desert Laboratory in Tucson, Arizona. Seed number declined with age at an approximately exponential rate, with the oldest recovered seed having an estimated age of 5 yr (95% CI = ±2.3 yr). The seed bank age structure was judged more than adequate to buffer this population from typical fluctuations, based on an examination of 15 yr of population dynamic data. The TAMS technique has strong potential for answering a broad range of ecological and evolutionary questions requiring post-1963 age determinations and for which a several-year confidence interval is acceptable.
- Becerra, J. X., & Venable, D. L. (1999). Macroevolution of insect-plant associations: The relevance of host biogeography to host affiliation. Proceedings of the National Academy of Sciences of the United States of America, 96(22), 12626-12631.More infoPMID: 10535973;PMCID: PMC23020;Abstract: Identifying the factors that have promoted host shifts by phytophagous insects at a macroevolutionary scale is critical to understanding the associations between plants and insects. We used molecular phylogenies of the beetle genus Blepharida and its host genus Bursera to test whether these insects have been using hosts with widely overlapping ranges over evolutionary time. We also quantified the importance of host range coincidence relative to host chemistry and host phylogenetic relatedness. Overall, the evolution of host use of these insects has not been among hosts that are geographically similar. Host chemistry is the factor that best explains their macroevolutionary patterns of host use. Interestingly, one exceptional polyphagous species has shifted among geographically close chemically dissimilar plants.
- Becerra, J. X., & Venable, D. L. (1999). Nuclear ribosomal DNA phylogeny and its implications for evolutionary trends in Mexican Bursera (Burseraceae). American Journal of Botany, 86(7), 1047-1057.More infoPMID: 10406728;Abstract: The genus Bursera (Burseraceae) is one of the most diversified and abundant groups of plants of the tropical dry forests of Mexico. In order to provide a basis for better understanding of its evolutionary biology, we reconstructed a phylogeny of 57 species and varieties using the nucleotide sequences of the internal transcribed spacer regions (ITS1 and ITS2) of 18S-26S and the 5.8S coding region of nuclear ribosomal DNA. We used four species of the allied genera Commiphora and Boswellia and one species of Spondias (Anacardiaceae) as outgroups. Our results support the views that Bursera is monophyletic and more closely related to Commiphora than to Boswellia. The division of Bursera into sections Bullockia and Bursera is also strongly supported by our phylogeny. Several other subclades also had high bootstrap values, especially within section Bursera. We use the phylogeny as a basis for discussing evolutionary tendencies in bark, leaves, breeding systems, and fruits.
- Venable, D. L., Dyreson, E., Piñero, D., & Becerra, J. X. (1998). Seed morphometrics and adaptive geographic differentiation. Evolution, 52(2), 344-354.More infoAbstract: Adaptive geographic differentiation is documented for seed morphology of 36 populations of Heterosperma pinnatum Car. (Asteraceae), a seed heteromorphic annual plant in the central highlands of Mexico. Achenes (single-seeded fruits) vary continuously within heads but are classified by shape and position as central, intermediate, or peripheral morphs and as having adhesive awns or not. Here we quantify shape as a principal component score contrasting log length and width of achenes. Heads and their variation among populations are described in terms of maximum, minimum, and range of shape scores; the number of achenes per head; quantitative indices of the abruptness of shape shift; where in the head the most abrupt change in shape occurs; and what achene shapes have awns. First and second principal components of these descriptors summarize 86% of among-population variation in achene and head morphology and correlate strongly with percent central achenes per head (%C) and percent of achenes with awns (%A), respectively. Awns are associated with greater dispersibility and achene shape is correlated with speed of dormancy loss. We hypothesized that dispersal morphology would be associated with vegetation attributes indicative of population ephemerality and that dormancy morphology would be associated with precipitation patterns during the early germination season. Morphological distance matrices were calculated using Euclidean distances among populations in %A and %C. Geographic distances among populations were calculated, as were genetic distances based on isozyme frequencies from 29 bands of six enzymes. Vegetation was classified as open or closed and early spring (germination season) and summer precipitation means were determined for each site. Closed vegetation was assumed to provide only ephemeral habitats for H. pinnatum. Partial matrix correlations between morphology and environment controlled for geographic but not genetic distance among sites, since the latter was not significantly correlated with either morphology or geography. A significant relationship was found between %A and closed vegetation, lower spring, and higher summer precipitation. %C was only correlated with lower spring precipitation. Independence of isozyme and morphological traits is interpreted in terms of selection on the latter but not the former.
- Fishbein, M., & Venable, D. L. (1996). Diversity and temporal change in the effective pollinators of asclepias tuberosa. Ecology, 77(4), 1061-1073.More infoAbstract: Although pollination effectiveness is a central process underlying the evolution of plant and pollinator traits, it is difficult to measure and has rarely been reported for a diverse spectrum of visitors under natural conditions. We measured the effectiveness of all common flower visitors to Asclepias tuberosa (butterfly weed) at a site in southeastern Arizona, in terms of visitation rate, per-visit rate of pollinia removal and insertion, and pollinia load. Bombus and Apis (Hymenoptera) were the most effective pollinators, counter to predictions that A. tuberosa is butterfly-pollinated. We also documented large differences between 2 yr in the pollination effectiveness of visitors, primarily due to changes in visitation rate. Bombus were the most frequent and effective pollinators in 1992. In 1993, Apis were equivalent to Bombus. Battus (Lepidoptera) were the second most effective pollinators in 1992, but were scarce in 1993. Thus, conclusions about the identity of effective pollinators based on floral traits, casual observations of visitation, or even precise measurement of effectiveness in a single season are all potentially suspect. We compare our results to those of previous studies of Asclepias pollination.
- Fishbein, M., & Venable, D. L. (1996). Evolution of inflorescence design: Theory and data. Evolution, 50(6), 2165-2177.More infoAbstract: Very low fruit set ill milkweeds and other flowering plants often has been attributed to greater sexual selection on inflorescence size via male, rather than female, reproductive success. Although this explanation has been generally accepted, alternate explanations have been presented, and recently the 'male function' or 'pollen donation' hypothesis has been sharply criticized. In this paper, we make the distinction between selection on total flower number and on the size of inflorescence units, both of which have been termed 'inflorescence size.' We present an ESS model for the evolution of inflorescence design that considers reproductive success through male and female function. The model predicts that selection will balance the proportional changes in female and male reproductive success resulting from changes in inflorescence-unit size. We conducted a field study of selection on the size of inflorescence units (umbels) by manipulating umbel size and number in a natural population of Asclepias tuberosa, in southeastern Arizona, during two reproductive seasons. We found that the male fitness function reached a maximum at an intermediate umbel size in both years (although not significantly different from the smallest umbel size in either year), whereas the female illness function was highest for the smallest umbel size in one year, but was constant across umbel sizes in the other year. We also found that pollinator visitation rate corresponded well with male, but not female, function, and that between year variation in the male reproductive success of different umbel sizes corresponded with variation in the composition of the pollinator pool. Our empirical results, when inserted in the model, predict ESS umbel sizes similar to those observed in the study population and the species throughout its range.
- Pake, C. E., & Venable, D. L. (1996). Seed banks in desert annuals: Implications for persistence and coexistence in variable environments. Ecology, 77(5), 1427-1435.More infoAbstract: It is widely believed that desert annual plants maintain between-year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable environments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long-term data collected from permanent plots over a 10-yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet-hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between-year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year-to-year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss how a correlation between germination and reproductive success enhances the role of temporal variance in success hierarchies in promoting species coexistence.
- Venable, D. L. (1996). Packaging and provisioning in plant reproduction. Philosophical Transactions of the Royal Society B: Biological Sciences, 351(1345), 1319-1329.More infoAbstract: Plant reproductive ecologists investigate many aspects of reproductive design not covered by simple offspring size/number models or simple sex allocation models, such as inflorescence design, pollen packaging or fruit design. General models for hierarchical packaging of reproductive allocation which cover these and other cases are developed here. These demonstrate that selection will tend to equalize fitness elasticities of reproductive components when these are properly scaled to take account of reproductive costs. Elasticities are defined as the proportional change in a fitness component with a proportional change in the trait contributing to that component (e.g. the proportional change in the fitness per seed with a proportional change in seed size). For the simplest reproductive design models, selection will favour the equalization of the elasticities of all female hierarchical provisioning and packaging fitness components or all male packaging components, both in single sex models or cosexual models. For simple cosexual models, selection favours allocation to each sex in proportion to the gender-specific fitness elasticities. More generally, selection tends to equalize all component elasticities when these are properly scaled to account for the total resource costs of changes in each component. The models are extended to cover more complex biology, including links between female and male packaging components, packaging components that contribute to the fitness of both genders, accessory costs that may or may not contribute to both genders, and allometric costs and trade-offs. As assumptions about fitness interactions and life history trade-offs become less restrictive, the models more closely approach a general equal-marginal- advantage model. The models provide tools for understanding how and when different components of the reproductive design constrain and selectively impact each other. The utility of the model for aiding in the design and analysis of specific research problems is discussed with reference to some empirical examples.
- Pake, C. E., & Venable, D. L. (1995). Is coexistence of Sonoran Desert annuals mediated by temporal variability in reproductive success?. Ecology, 76(1), 246-261.
- Till-Bottraud, I., Venable, D. L., Dajoz, I., & Gouyon, P. H. (1994). Selection on pollen morphology: A game theory model. American Naturalist, 144(3), 395-411.
- Fleming, T. H., Venable, D. L., & Herrera, L. G. (1993). Opportunism vs. specialization: the evolution of dispersal strategies in fleshy-fruited plants. Vegetatio, 107-108(1), 107-120.More infoAbstract: In this paper we address two questions concerning the interaction between fleshy-fruited plants and their seed dispersers: (1) What determines optimal disperser coterie size (designated as Ĉ) and (2) Why does disperser specialization occur along taxonomic lines? We review factors that affect the evolution of Ĉ and conclude that seed size and disperser quality (designated as Q) are especially important. We present a simple graphical model for determining Ĉ based on Q and conclude that Ĉ will be small (i.e. a specialized disperser strategy) when Q declines rapidly with increasing coterie size; Ĉ will be large when Q declines slowly with increasing coterie size. We construct a model based on fitness set theory to predict that specialization on particular vertebrate taxa (e.g. birds or mammals) will be favored when different disperser taxa are perfectly substitutable or antagonistic (sensu Tilman 1982); mixed-taxa coteries are favored when different disperser taxa are complementary. Finally, we predict that when conditions favor taxonomic specialization, plants will evolve bird fruits more often than bat or primate fruits because of the greater species richness of birds compared with bats and primates. © 1993 Kluwer Academic Publishers.
- Pantastico-Caldas, M., & Venable, D. L. (1993). Competition in two species of desert annuals along a topographic gradient. Ecology, 74(8), 2192-2203.
- Venable, D. L., & Brown, J. S. (1993). The population-dynamic functions of seed dispersal. Vegetatio, 107-108(1), 31-55.More infoAbstract: We summarize some of the population-dynamic consequences of the mosaic structure of plant populations for the evolution of seed dispersal. A fairly elaborated set of theoretical ideas exist regarding the evolution of dispersal and we have synthesized some of them in an attempt to make them more accessible to field ecologists. We consider the relationship of these general theoretical ideas to our understanding of fruit and seed dispersal. We develop three related models to describe the similarities and differences in how dispersal functions for risk reduction (bet hedging), escaping the negative consequences of crowding, and escaping high concentrations of relatives. We also briefly discuss directed dispersal as a fourth population-dynamic aspect of dispersal. Dispersal can have a risk-reducing function only when there is global (metapopulation) temporal variance in success. Dispersal to escape the negative consequences of crowding requires only spatial and local temporal environmental variation. Dispersal for escaping high concentrations of relatives requires no environmental variation, but does require genetic population structure. Directed dispersal, defined as non-random into particular patch types contingent on the expectation of local success, is always valuable when possible and represents an advantage independent the others which can occur with random dispersal. In an effort to accommodate for the differences between simple mathematical models and the behavior of complex natural fruit and seed dispersal systems we have discussed the following issues: actual patterns of patch structure and dispersal distance; the implications of plant cosexuality, perenniality, and allocation costs of dispersal structures; and the impact of the detailed nature of density dependence, breeding systems, and genetic structure. We briefly compare the population-dynamic functions of dispersal presented here with the widely cited functions of colonization, escape, and directed dispersal. Finally, we suggest how the theoretical models can be used with field data to estimate the fitness consequences of dispersal. © 1993 Kluwer Academic Publishers.
- Lloyd, D. G., & Venable, D. (1992). Some properties of natural selection with single and multiple constraints. Theoretical Population Biology, 41(1), 90-110.More infoAbstract: The properties of selection restricted by single and multiple constraints are examined by using the Lagrange and Kuhn-Tucker conditions of calculus. We show for a general set of fitness equations containing any number of strategy components and subject to any single differentiable equality constraint that the marginal fitnesses of any two strategy components are equal at the evolutionarily stable strategy (ESS) when expenditures are measured in the same units, those of a binding constraint. Equal marginal advantages are a necessary, though not usually a sufficient, condition for an interior ESS. When selection is operating under more than one constraint, the marginal fitnesses of any two strategy components are equal at the ESS whenever both components are affected by only one, and the same, binding constraint. The equalization of marginal fitnesses allows the positions of constrained fitness maxima to be explored in theoretical models or empirical tests and is a convenient heuristic for understanding selection. © 1992.
- Venable, D. L. (1992). Size-number trade-offs and the variation of seed size with plant resource status. American Naturalist, 140(2), 287-304.More infoAbstract: Develops a general treatment of the effects of parental resource status on optimal offspring size. Even when there is a resource trade-off between size and number of offspring within individuals, positive correlations between size and number may occur among individuals due to individual variation in resources. Such positive correlations imply that parental resource status affects the fitness-maximizing offspring size, in contrast to the predictions of the standard Smith-Fretwell model. Parental resource status affects the fitness-maximizing offspring size whenever the size-number fitness function is nonhomogeneous in offspring number. This condition implies some sort of density-dependent interactions among offspring, although it is possible to have either positive or negative sib interactions in fitness functions that are homogeneous in offspring number. In the latter case offspring size should be insensitive to parental resource status. -from Author
- Brown, J. S., & Venable, D. L. (1991). Life history evolution of seed-bank annuals in response to seed predation. Evolutionary Ecology, 5(1), 12-29.More infoAbstract: We present a model of life history evolution for seed-bank annuals in temporally varying environments in which both the seed bank and the distribution of fecundity across year types evolve in response to seed predation. The fecundity distribution refers to the expected reproductive success of germinating seeds across a range of different year types. We assume that it is a function of traits pertaining to growth and survival under different environmental circumstances. Such traits are assumed to result in a trade-off between reproduction in favourable and less favourable years. The model is used to explore how seed predation selects for changes in the seed bank and fecundity distribution and how changes in each of these further select for changes in the other. The direction of selection is contingent upon: whether or not a seed bank exists; whether predation has a greater effect on fresh or buried seed; whether the predation rate differs in different year types, and if so, if it is positively or negatively density-dependent; whether or not predation rate is sensitive to individual variation in seed yield, and if so, whether and how such dependency varies in different kinds of year. Under a variety of predation regimes, seed predators select for a temporal clumping of reproduction; i.e. a specialisation on a favourable subset of year types. This effect usually requires negatively density-dependent seed predation of the sort created by predator satiation. In fact, the classic scenario favouring masting in perennials creates the strongest such effect in our model. Yet unlike the masting of perennial plants, this effect is favoured in a seed-bank annual. It can even occur in a strict annual without a seed bank, and it can occur in a seed-bank annual even if seed predation is density-independent. © 1991 Chapman and Hall Ltd.
- Becerra, J. X., & Venable, D. L. (1989). Extrafloral nectaries: a defense against ant-Homoptera mutualisms?. Oikos, 55(2), 276-280.More infoAbstract: Extrafloral nectaries may sometimes function to defend plants from ant-Homoptera mutualisms by weaning ants onto a plant-controlled diet of nectar. Extrafloral nectaries can be favored even in the absence of ant defensive behavior. Many studies have shown no net, defensive benefit to plants of nectar feeding ants. Many nectar-feeding ants tend Homoptera which are major plant disease agents and which may profoundly alter plant architecture and physiology. Ants can be distracted from tending Homoptera by feeding them sugar and they may even destroy the Homoptera. Nectar has very similar chemical composition to honeydew and its collection does not require the extensive husbandry that tending Homoptera does. Nectary production increases during infestations of Homoptera. It is more difficult to produce chemical defenses against sap feeders than against chewing insects. -from Authors
- Kelly, C. K., Venable, D. L., & Zimmerer, K. (1988). Host specialization in Cuscuta costaricensis: an assessment of host use relative to host availability. Oikos, 53(3), 315-320.More infoAbstract: The parasitic plant Cuscuta is considered a host generalist because of the many host species on which it can grow, but C. costaricensis in Costa Rica does not use all hosts equally. Proportion similarity (PS) between resource use and availability is 0.66 on a scale of 0-1, reflecting that 88% of parasite cover is on only 2 of the 10 potential host types, those 2 comprising 54% of the total cover. Host availability does not predict parasite use: similarity distributed hosts were not equally infested. However, C. costaricensis grows significantly more vigorously on the 2 most commonly used host types in 3 different measures of vigor. Thus, relative host use by the parasite is determined by differential parasite growth and/or mortality. However, C. subinclusa in southern California, has similar patterns of host use and possesses active mechanisms for choosing among host species as well as the above passive mechanisms. -from Authors
- Venable, D. L., & Brown, J. S. (1988). The selective interactions of dispersal, dormancy, and seed size as adaptations for reducing risk in variable environments. American Naturalist, 131(3), 360-384.More infoAbstract: Seed size, dormancy, and dispersal share 3 population-dynamic functions in temporally and spatially varying environments: risk reduction of bet hedging, escape from crowding, and escape from sib competition. A model was developed to explore ways they may interact to reduce risk. The risk-reducing properties of these seed traits evolve only in response to global temporal variance. Thus, to understand how selection impinges on the seed traits, creating fitness interactions, one must understand the factors contributing to global temporal variance and how they are mitigated by the various seed traits. Since the traits interact to reduce variance, arbitarily fixing any 1 trait at different values alters the fitness-maximizing values of the others, resulting in trade-offs among traits. The authors explore how changes in the number of independent environmental patches, probability of favorable conditions, radius of dispersal, and spatial and temporal autocorrelation of environmental conditions alter selection on the interacting syndrome of seed traits. -from Authors
- Venable, D. L., Burquez, A., Corral, G., Morales, E., & Espinosa, F. (1987). The ecology of seed heteromorphism in Heterosperma pinnatum in central Mexico.. Ecology, 68(1), 65-76.More infoAbstract: Achene behavior ranges from 'low-risk' (peripheral achenes) to 'high-risk' (central achenes) within the progeny of a single individual, while individuals and populations vary in the proportion of offspring exhibiting each type of behavior.-from Authors
- Brown, J. S., & Venable, D. L. (1986). Evolutionary ecology of seed-bank annuals in temporally varying environments.. American Naturalist, 127(1), 31-47.More infoAbstract: The production of long-lived seeds by annual plants introduces a unique form of age structure. In a temporally varying environment the dormant seed may experience many years with different weather, whereas the germinating individual experiences only the weather conditions of a single growing season. Natural selection operates on both the between-year dormancy and on non-seed-bank traits that affect the degree of specialization to conditions pertaining in different year types. An integrated model permits these 2 aspects of the life history to evolve simultaneously, and leads to predictions that are not attainable by considering the evolution of each in isolation. Changes in the survival probability of the between-year seed bank select for reinforcing changes in between-year dormancy and specialization. Changes in the probability of occurrence of different year types select for damping changes in between-year dormancy and specialization. Across a gradient in environmental quality, most change should occur in between-year dormancy, with little change in specialization. If between-year dormancy is fixed, however, a greater change in specialization should occur. The predictions of the model are discussed in terms of environmental gradients, seed bank versus non-seed-bank annuals, and a variety of plant traits modeled elsewhere that may be involved in specialization to different types of years. -Authors
- Venable, D. L. (1985). Ecology of achene dimorphism in Heterotheca latifolia III. Consequences of varied water availability.. Journal of Ecology, 73(3), 757-763.More infoAbstract: Disc achenes were more successful than ray achenes when a plentiful water supply resulted in a high overall biomass and density at the end of the experiment. The inferior performance of ray achenes is attributed to the combined effects of lower embryo provisioning, lower and germination, and suppression by the early-germinating disc achenes. Ray achenes were more successful than disc achenes when water was scarce and overall biomass and density were low at the end of the experiment. Their superior performance resulted from escaping the water shortage by dormancy.-from Author
- Venable, D. L. (1985). The evolutionary ecology of seed heteromorphism.. American Naturalist, 126(5), 577-595.More infoAbstract: Heteromorphism is promoted over monomorphism by low correlation of the demographic success of the different morphs and high temporal variation in seed success. Available experimental data from 2 heteromorphic-seeded species satisfy the model conditions for heteromorphism and produce morph proportions similar to those predicted. Seed heteromorphism can arise when the evolution of morph behavior is constrained by a single concave fitness set or when different seeds on the same individual plant are constrained by different convex fitness sets. The fitness sets represent constraints on demographic success in different year types (trade-offs resulting from constraints in design, development, physiology, or genetics). Two kinds of heteromorphism may arise when the constraints differ for different seeds on the same plant. Whether high-risk-low-risk heteromorphism or high-risk-high-risk heteromorphism evolves depends on the similarity of the evolutionary constraints. -from Author
- Venable, D. L., & Levin, D. A. (1985). Ecology of achene dimorphism in Heterotheca latifolia. I. Achene structure, germination and dispersal.. Journal of Ecology, 73(1), 133-145.More infoAbstract: Describes structural differences between achene types of Heterotheca latifolia (Compositae), when they are produced, where on the plant they are produced, laboratory germination, seedling growth and dispersal biology. -from Authors
- Venable, D. L., & Levin, D. A. (1985). Ecology of achene dimorphism in Heterotheca latifolia. II. Demographic variation within populations.. Journal of Ecology, 73(3), 743-755.More infoAbstract: The capacity for population increase of H. latifolia is high with the contribution from disc achenes usually considerably more important than that of ray achenes. Plants derived from ray achenes will contribute more, relative to those derived from disc achenes, in declining populations or populations in which disturbance reduces the survival of the growth stage more than the survival of the dormant seed stage.-from Authors
- Venable, D., & Levin, D. A. (1983). Morphological dispersal structures in relation to growth habit in the Compositae. Plant Systematics and Evolution, 143(1-2), 1-16.More infoAbstract: The relationship between gross morphological dispersal structures, growth habit, and weediness is reported for a worldwide sample of 5893 Compositae species from 18 regional floras. A significantly smaller percentage of annual species than perennial species has dispersal structures. Among species with dispersal structures, plumed types are more frequent in perennials while adhesive types are more frequent in annuals. Differences in the occurrence and nature of dispersal structures between perennial herbs, shrubs and trees were minor. Weeds are less likely to have well developed morphological dispersal structures than non-weeds. The data support the idea that dispersal in space may be more important for perennial or non-weedy plants than for annual and weedy plants. © 1983 Springer-Verlag.
- Venable, D., & Lawlor, L. (1980). Delayed germination and dispersal in desert annuals: Escape in space and time. Oecologia, 46(2), 272-282.More infoAbstract: A model is developed to consider the interplay between dispersibility and delayed germination in desert annuals. The model explores the effect of low levels of dispersal, considered realistic for annual plants, on optimal germination fraction. The model also demonstrates the effect of the amount and accuracy of "predictive" (responsive to the environment) dormancy on the optimal innate germination fraction (not responsive to environmental conditions). Optimal germination fraction is found to be very sensitive to changes in despersibility especially at the limited dispersibilities that are realistic for annual plants. As dispersibility increases, optimal germination fraction increases. If plants make two kinds of seeds with differing despersibility, reproduction is maximized if the low dispersal seeds have delayed germination and the high dispersal seeds have quick germination. If dormancy mechanisms permit seeds to germinate when environmental conditions allow successful maturation, and remain dormant when environmental conditions do not permit successful maturation, what fraction of seeds should remain dormant under predicted good conditions as a hedge against inaccurate prediction of the environment? If environmental cues that break dormancy are uncorrelated with environmental conditions that permit successful maturation, predictive dormancy has little or no effect on the optimal innate germination fraction. When predictive dormancy lowers the probability of germinating when environmental conditions preclude successful maturation, the optimal innate germination fraction increases with increasing germination control by predictive dormancy. With a moderate degree of germination control by predictive dormancy, the optimal innate dormancy is still sensitive to changes in dispersal in the low dispersal ranges characteristic of annual plants. Evidence is presented from plant species that have both dispersal and germination dimorphisms to support the predicted correlation of high germination fractions with high dispersal. © 1980 Springer-Verlag.