Threat intensification reshapes trait-response relationships in birds and mammals

Sarah Bull, Simone Blomberg, Rikki Gumbs, Katrina Davis, Rob Salguero-Gomez

Predicting which species are most at risk of extinction, and by which threats, is central to effective biodiversity management (1). Trait-based frameworks, linking species traits to extinction risk, are increasingly used to predict global biodiversity trends, with applications from species-specific prioritisations (2) to estimations of global diversity loss (3). However, despite the strong spatial heterogeneity (4) and rapid acceleration of pressures globally (5), current trait-based approaches implicitly assume that trait-response relationships are stable across gradients of threat intensity. Here, we test this assumption, using a dataset of 9,262 bird and mammal responses to three key threats: habitat modification, hunting, and invasive species/disease. We show that, for all threats examined, intensification fundamentally alters how species traits mediate population responses, frequently reversing whether traits buffer or amplify declines. The shape of the interaction between threat intensity and trait effects is strongly dependent on threat type, taxa, and the degree of intensification. Our findings provide strong evidence that trait-mediated sensitivity is not intrinsic to a species, as currently assumed, but dynamic. Consequently, spatial and temporal changes in threat intensity have the potential to restructure risk across species, strongly challenging current interpretations of trait-based frameworks in predicting present and future biodiversity change. Equally, the relationship between traits and population response to intensification presents an opportunity to integrate complex, non-linear responses to intensification into biodiversity forecasts at a generalisable scale.