Habitat geometry, a step toward general bird community assembly rules in mature forests

Community ecology has often been criticized because it seldom defines general rules that can be exported outside of the studied system. Although much progress has been achieved by studying functional traits instead of species identities per se, environmental gradients defined in terms of local resources can be measured only on a subset of ecosystems and, thus, are not deemed general. Here, we used state-of-the-art statistical approaches for modeling the interaction between species functional traits (body size, feeding substrate) and habitat geometry (object size, shape and color; packing, layering and texturing), such as to derive general assembly rules for bird communities in an even-aged mature forest (La Mauricie National Park, Québec, Canada). Our results show that habitat geometry filters the species pool through its interaction with bird body size and feeding substrate, both at the species, and even more so at the community level. We found comparable assembly rules using only photographic (image-based) descriptors of habitat geometry, thus providing forest managers with a fast, reliable, standardized and cost-effective protocol for characterizing forest stands. Because bird functional groups respond differently to environmental gradients, we conclude that mature forest should be managed to preserve a spatial mosaic of successional states.