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.

We measured 9 morphological traits on 37 ant species found in 20 sites along a 9° latitudinal gradient in Quebec, Canada. At each sampling site, we randomly established a 50×50 m plot. Within each corner and at the center of this plot, we placed a 10×10 m subplot. At each of these five subplots, we registered all soil and leaf-dwelling ant nests found by systematically searching in the leaf-litter, under rocks, dead trees and logs found within this area for 20 min (modified “Intensive sampling” technique described by Bestelmeyer et al., 2000). During this period, we put a flag where the colony was located and proceeded to continue our search. We only collected ant workers after this 20-min search period had ended. We collected a subset of at least 5-10 workers to sample.

We measured nine raw morphological traits that are regularly used in the literature: (1) Body size (Weber's length), (2) pronotum width, (3) head length and (4) head width, (5) eye length and (6) eye width, (7) mandible length, (8) antenna (scape) length and (9) Femur length, following a standardized protocol suggested by Parr et al. (2017). At each site, we found between 1 to 24 colonies per species, and we measured all nine traits on 1 to 3 individual workers from each colony. for monomorphic species, we measured 1 to 3 specimens per colony. Then, for polymorphic species (e.g., Pheidole spp., Camponotus spp.), we visually selected and measured one representative of each size class (i.e., majors, intermediate and minors). We took standard linear measurements using an ocular micrometer mounted on a dissecting microscope, accurate to 0.01 mm.

The macroenvironmental variables for each of the 20 sites: annual mean temperature and annual precipitation were extracted from WorldClim 2.0 (Fick & Hijmans, 2017).

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