Clinal adaptation in Artemisia californica to a latitudinal gradient influences arthropod community composition

Background/Question/Methods

The structure and composition of arthropod communities are largely influenced by inter- and intraspecific variation in plant functional traits. Patterns of geographic variation in plant functional traits may drive corresponding patterns of variation in arthropod communities across broad environmental gradients. As such, patterns of local adaptation in plants to the abiotic environment will likely cascade up to influence arthropod community structure. Here we examine geographic patterns of intraspecific variation in the foundation species, Artemisia californica, collected from 20 populations along a 700 km (5.3^o) latitudinal gradient. We monitored plant growth and arthropod communities for one year on plants grown in a common garden at the southern portion of the species range. We in turn interpreted variation in plant growth and associated arthropod communities with respect to the abiotic conditions characterizing each population's source site.

Results/Conclusions

We found clinal population (genetic) variation in plant growth rate, with plants from the south being 66% larger than those from the north. This variation in plant growth rate was best explained by the intra- and inter-annual variation in precipitation at population source sites, suggesting this is a key selective factor driving clinal adaptation in this plant. Sampling arthropods from 204 plants once in 2012, a total of 265 morphological species from 16 orders and 84 families were found, with a mean arthropod density of 180 individuals per kg plant dry biomass ± 17 SE (and 198 individuals per plant volume m³ ± 19). We found clinal population (genetic) variation in arthropod density but not species richness, with plant from the north having 79% higher arthropod densities than those from the south. These findings for total arthropod density also held when herbivorous and predatory arthropod where analyzed individually. Overall, we show that patterns of intraspecific variation in arthropod community attributes correspond to patterns of variation in plant growth for this species. This study therefore contributes to our overall understanding of the indirect effects of plant adaptation to abiotic clines for arthropod community structure.