Genetic variation in plant traits and environmental conditions are recognized to work in conjunction to structure associated arthropod communities. However the temporal dynamics of such effects are not well understood, requiring long-term studies. Similarly, the consequences of plant local adaptation to the abiotic environment for associated arthropod communities remains largely unstudied. To this end, environmental gradients can serve as powerful tools to understand plant adaptation to abiotic clines, providing insight into how populations could respond to future conditions. We established a common garden of Artemisia californica sourced from five populations along a 5° latitudinal gradient, where precipitation is on average lower but more variable in the south. Previous work in this system demonstrates local adaptation to this cline and suggests that A. californica populations may differ in herbivore resistance and response to drought, with drought having larger effects on northern populations that have evolved to historically more stable and mesic environments. In this study we documented the structure of arthropod communities on A. californica for seven consecutive years. We then tested for genetic, temporal, and genetic-by-temporal variation on arthropod density (per gram plant biomass), diversity, and richness to discern patterns of genetic variation in arthropod communities and stability over time.
Results/Conclusions
We found strong clinal variation in resistance to herbivores within our common garden, with herbivore densities being 70% greater on plants sourced from northern than southern populations. However, richness and diversity did not differ among plant populations. Furthermore, herbivore richness, diversity, and density decreased over time, with herbivore density experiencing a 6.5-fold reduction, indicating a strong increase in resistance over time. While average annual rainfall decreased ~55% over the course of the experiment, plants were also aging. Thus, temporal variation in herbivore resistance could be due to either ontogenetic shifts in plant defense, plasticity in response to abiotic stress, a direct effect of abiotic stress on herbivore performance, or a combination of these effects. Lastly, we did not find genetic variation in plant resistance to increasing drought conditions (time-by-population interaction), suggesting that plants are responding to abiotic stress similarly, such that clinal variation in resistance is maintained despite an overall increase in resistance over time. While clinal variation in resistance persisted each year, the change in resistance over time was greater than that across populations. These results suggest that while landscape-scale genetic variation plays an important role in plant-arthropod interactions, broad year-to-year variation can also significantly influence these interactions.