Divergent response to forest structure of two mobile vertebrate groups

Taxonomic and functional diversity of animals respond to habitat heterogeneity. However, how structural heterogeneity at the stand level of differently managed forests drives species richness across taxa is not clear. Here we analyze how birds and bats, sharing the ability to fly, respond to the structural composition of the vegetation layers in differently managed forests. We combined datasets of birds and bats and compared their response as taxonomic vs. functional richness towards forest structure. Overall our results revealed that bird and bat species richness were positively correlated and species richness of both taxa peaked within the same forest stands. In contrast to our prediction that richness of mobile vertebrates with similar ecological traits (the ability to fly) should respond alike to forest stand characteristics, our results showed almost no congruence. Using Generalized Linear Mixed Effects Modeling (GLMM) with model selection based on AICc and model averaging, we found that taxonomic richness of birds and bats as well as functional richness (based on main diet and foraging mode) responded to forest structure in very different ways. While the taxonomic richness of birds was mainly associated with the structural parameters contrast and forest height, representing tall trees and a relatively closed canopy, richness of bat species increased with vertical and horizontal heterogeneity within forest stands. Our findings indicate that functional richness is more useful to understand responses of bird and bats species richness to land use, forest structure and forest management, rather than focusing solely on taxonomic groups.