Winter conditions structure extratropical patterns of species richness of amphibians, birds and mammals globally

Aim: The aim was to derive global indices of winter conditions and examine their relationships with species richness patterns outside of the tropics. Location: All extratropical areas (>25° N and 25° S latitudes), excluding islands. Time period: 2000–2018. Major taxa studied: Amphibians, birds and mammals. Methods: We mapped three global indices of winter conditions [number of days of frozen ground (length of frozen ground winter); snow cover variability; and lack of subnivium (below-snow refuge)] from satellite data, then used generalized additive models to examine their relationships with species richness patterns derived from range data. Results: Length of frozen ground winter was the strongest predictor of species richness, with a consistent cross-taxonomic decline in species richness occurring beyond 3 months of winter. It also often outperformed other environmental predictors of species richness patterns commonly used in biodiversity studies, including climate variables, primary productivity and elevation. In areas with ≥3 months of winter conditions, all three winter indices explained much of the deviance in amphibian, mammal and resident bird species richness. Mammals exhibited a stronger relationship with snow cover variability and lack of subnivium than the other taxa. Species richness of fully migratory species of birds peaked at c. 5.5 months of winter, coinciding with low species richness of residents. Main conclusions: Our study demonstrates that winter structures latitudinal and elevational gradients of extratropical terrestrial species richness. In a rapidly warming world, tracking the seasonal dynamics of frozen ground and snow cover will be essential for predicting the consequences of climate change on species, communities and ecosystems. The indices of winter conditions we developed from satellite imagery provide an effective means of monitoring these dynamics into the future.