Intraspecific tests of growth-defense trade-offs within Streptanthus tortuosus

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Danielle De La Pascua, Population Biology, UC Davis, Davis, CA, Elena Suglia, Evolution and Ecology, University of California, Davis, Davis, CA, Sharon Y. Strauss, Department of Evolution and Ecology, University of California, Davis, Davis, CA and Jennifer R. Gremer, Center for Population Biology, University of California, Davis, Davis, CA

Background/Question/Methods
Plants have evolved numerous strategies to defend themselves against herbivores.. Due to the potentially costly nature of investing in defenses over other functions (e.g., growth), evolutionary trade-offs between plant defenses and growth have been predicted in plant defense theory. Plant defense trade-offs between growth and defense as well as tradeoffs among types of defenses have been a central topic in defense theory, but much of this work has been done across species rather than within species. Further, it is unclear why supported trade-offs among species are often not observed within species, such as in the growth-defense trade-off.
One possible explanation for the discordance between trade-offs among and within species is the limitation of resources, and how this may vary across environmental gradients within species distributions. One reason for this is because of how environmental factors vary within species across different environmental gradients. In this study, we measured rates of herbivory across 9 populations, and compared population herbivory rates with a physical constitutive defense along an elevational gradient. Streptanthus tortuosus exhibits wide phenotypic variation and spans a climatic, elevational, and latitudinal gradient across California, southern Oregon, and eastern Nevada - making an ideal candidate to test intraspecific growth-defense trade-offs.
Results/Conclusions
Indeed, at high elevations, where resources are expected to be higher (less drought, more rain/snow), herbivory rates were lower. Correspondingly, constitutive defense traits measured in a common garden were decreased for low elevation, low resource populations. For example, leaf thickness was lower in populations from low elevation, low resource environments. Our data suggests that plants from populations that receive high rates of herbivory tended to have thin leaves and occur in low elevations, while individuals from high elevations have thicker leaves and receive significantly less herbivory - a pattern also observed within other species. In future studies, I will investigate whether growth-defense and inducible-constitutive defense trade-offs occur in S. tortuosus, and whether defense trait covariances and trade-offs are more strongly associated across a particular environmental gradient across the range (elevational, climatic, latitudinal.) In conclusion, herbivore pressure and resource levels influence intraspecific defense variation, and selection pressures associated within elevational gradients may drive intraspecific defense trait evolution.