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 predictions of total species richness and community intactness vary when multiple environmental changes both harm and benefit particular species in the community. Finally, we conduct a sensitivity analysis, examining how the average species response to environmental change, as well as the variance among species, can further alter predictions. Overall, we find that predictions of co‐tolerance theory can break down when communities contain species that benefit from some environmental changes. As a result, the presence of multiple environmental changes can either compound or mitigate species loss when species’ responses are positively correlated, preventing a one‐size‐fits‐all statement regarding the effects of correlated responses. This finding highlights the need to carefully consider the underlying mechanisms of community change when making policy assessments regarding the consequences of correlations of species responses to environmental impacts.