Do correlated responses to multiple environmental changes exacerbate or mitigate species loss?

Biological communities face multiple global changes simultaneously, and predicting how they will respond remains a key challenge. Co-tolerance theory offers a framework for understanding how species-level responses to multiple stressors affect community properties. Co-tolerance theory predicts that positive correlations in species responses (i.e., species that are susceptible to one stressor are more likely to be highly susceptible to a second) lessen total species loss, essentially because species cannot be eliminated from a community twice. However, it is unclear whether several of the tenets of co-tolerance theory describe real-world communities, and what consequences result from such deviations. Here, we use an empirical dataset of bird community response to land-use change over a climate gradient to examine co-tolerance theory’s tenet that environmental changes only harm species (not benefit them). We show that this tenet is not met, and then use simulations to examine how predicti...