Understanding the role of plasticity and genetic variation in parallel invasion fronts: Study of the range expansion of Erodium cicutarium in Chile and California

Background/Question/Methods

Invasive species experience a variety of novel environments and establishment in a novel range is likely to be facilitated by some combination of genetic variation within a population and plasticity. Our research focuses on understanding the relative importance of genetic variation and plasticity in determining the relative success of populations of an invasive annual plant, Erodium cicutarium, undergoing a recent range expansion in California and Chile. Microenvironmental variation experienced by dispersing offspring can generate heterogeneity in the strength and direction of selection across generations. Fine-grained heterogeneity within the spatial scale of seed dispersal should select for adaptive phenotypic plasticity. Addressing this question requires using a two-pronged experimental approach: first, characterizing fine-scaled patterns of heterogeneity and how they affect plant phenotype and fitness under field conditions; second, using greenhouse studies to measure levels of genetic variation for phenotypic plasticity.

In 2009 we located populations of E. cicutarium in a range of habitats throughout California. We quantified several environmental variables including soil water content, soil availability, plant community composition, and percent cover, in microsites surrounding twenty randomly selected plants. Additionally, leaf shape and SLA were measured on each field collected plant. The same field work is currently in progress throughout Chile. For the first part of this ongoing project we address the following questions: 1) What is the level of fine grained heterogeneity in key environmental variables within naturally occurring populations of E. cicutarium in Chile and California? 2) How does microenvironmental variation covary with variation in phenotypic expression in natural populations of E. cicutarium?

Results/Conclusions

Mean seedling survival rates and environmental variables varied significantly among populations throughout California. Additionally, variance within populations in environmental factors and seedling survival differed significantly among populations. We found that phenotypic traits measured on field collected plants covaried with soil water content and percent cover, but that the strength of this relationship varied among populations. Ongoing field work in Chile will provide a complimentary dataset from the invasion of E. cicutarium into another Mediterrenean region. Overall these preliminary results suggest that E. cicutarium experiences a wide range of micro-environmental heterogeneity among populations throughout its range in California. As a result the strength and direction of selection on phenotypic plasticity may vary substantially among populations in its invaded range. Planned greenhouse experiments will enable us to characterize patterns of quantitative genetic variation and plasticity expressed in response to major axes of environmental variation observed in field surveys.