Community-wide trait adaptation, but not plasticity, explain ant community structure in extreme environments

1. Quantifying trait-environment associations can help elucidate the
processes underpinning the structure of species assemblages. However, most
work has focused on trait variation across rather than within species,
meaning that processes operating at the intraspecific levels cannot be
detected. Incorporating intraspecific trait variation in community-wide
analyses can provide valuable insights about the role of morphological
adaptation and plasticity on species persistence and the composition of
ecological communities. 2. Here, we assessed geographic variation in the
direction (i.e., adaptation) and strength of selection, and the magnitude
of plasticity, by examining community-wide trait variation in ant
communities along an environmental gradient spanning 9° latitude in
Quebec, Canada. Specifically, we measured 9 morphological traits related
to foraging strategies, resource use and thermal regulation at 20
locations across temperate and boreal forests. We then examined how the
mean and variance of these traits varied along temperature and
precipitation gradients. Moreover, we examined how these trait-environment
relationships varied across levels of organization, from individual
workers (intraspecific) to colonies (intraspecific) and species
(interspecific). 3. We observed changes in mean trait values along
environmental gradients, but very little change in variance. Specifically,
we observed a decrease in the mean length of antennae and an increase in
the mean eye length from mild (warm and wet) to more extreme environments
(cold and dry). These shifts in trait means were mostly coordinated across
organizational levels (i.e., worker, colony, and species). We also
observed a general increase in trait variance from mild to extreme
environments, but only at the species level. 4. Our findings suggest that
stressful environmental conditions exert a strong selection pressure on
ant morphology causing shifts in optimal trait values. These adaptations
may enable persistence at the northern edge of the boreal forest and
therefore influence the composition of these ant communities.
Specifically, ants with large eyes and short antennae are overrepresented
at the transition zone between the boreal forest and the tundra, possibly
representing an adaptation to these more open habitats. Our study
demonstrates that combining spatial and community-wide intraspecific
functional trait data provides a promising way forward to gain new
insights on trait adaptations and processes underpinning community
structure along environmental gradients.