Winners, losers, and biotic homogenization in current global changes

Natural selection faces species with an evolutionary trade-off between specializing to perform a few activities well or generalizing to perform many fairly. This trade-off shapes the specie’s niche, and leads to more or less specialized strategies among species. Here, we tested the prediction that specialists, which are expected to be favored in stable environments, should be more negatively affected by current global changes. We used a continuous index of habitat specialization for the 105 commonest bird species monitored by the French Breeding Bird Survey and more than 1000 plots randomly distributed across the whole country. First, we tested if each species’ response to landscape fragmentation and disturbance, measured independently by two landscape surveys, was related to its specialization level. We found a very strong negative link between species specialization and its response to both sources of habitat degradation. Second, we extended this study at the community level to assess the functional homogenization of bird communities. We found that functional homogenization rapidly increased with increasing habitat fragmentation and disturbance. Finally, we showed that our results were robust across habitats and scales. This study is the first to demonstrate a strong empirical link between a quantitative measurement of niche width and human-induced landscape degradations. We also emphasize the need to consider both positive and negative species responses when assessing consequences of habitat change in communities. In this respect, our measure of species specialization and of biotic homogenization are straightforward to predict which species is likely to thrive in fragmented and disturbed landscapes.