Shifts in optimal trait values rather than plasticity underpin ant community structuring in extremely cold and dry environments

Background/Question/Methods

Quantifying trait-environment relationships can improve our understanding of the processes underpinning species assemblages structuring. Examining intraspecific trait variation in community-wide analyses can provide valuable insights about the relative influence of local adaptation and plasticity on species composition and diversity. Adaptation to local conditions can influence the trait averages while the strength of selection and magnitude of plasticity can affect trait variance. Specifically, extreme environments could lower trait variation by filtering out species with suboptimal trait values whereas species that manage to persist could do so through plasticity and increasing intraspecific trait variation. In our study, we tested the hypothesis that ant communities in extreme environments exhibit a (1) shift in optimal morphology consistent with strong selection and (2) increase in morphological trait variance consistent with high plasticity. To test these hypotheses, we measured 9 morphological traits related to foraging strategies, resource use and thermal regulation on 1152 ant workers at 20 locations spanning 9° latitude across temperate and boreal forests. We examined how the mean and variance of these traits varied along temperature and precipitation gradients. In addition, we examined how these trait-environment relationships varied across levels of organization, from workers (intraspecific) to colonies (intraspecific) and species (interspecific).

Results/Conclusions

We observed changes in mean trait values along environmental gradients, but very little change in variance. Specifically, we observed an increase in the length of antennae and a decrease in eye length in more extreme environments (cold and dry). These shifts in trait means were mostly coordinated across organizational levels (i.e., worker, colony, and species). The observed differences in trait-environment relationships between organizational levels suggest that the strength of the selection pressure of extreme environments on specific traits differs among these levels. Furthermore, our findings suggest that extreme environments exert a strong selection pressure on certain traits across organizational levels, causing shifts in community-wide optimal trait values that allow species to persist under these climatic conditions. Our study demonstrates that examining community-wide patterns of trait mean and variance along environmental gradients can help disentangle the influence of selection and plasticity on species persistence and community structuring in extreme environments, which are particularly prone to climate change effects.