Extreme weather shrinks estimated range boundaries and alters biodiversity predictions

Anthropogenic climate change has been linked to rapid changes in ecological patterns and processes, including species re-distributions and phenological shifts, and is threatening one out of six species with extinction. While most work to date has explored the ecological consequences of gradually rising mean temperatures, the influence of increasingly frequent and intense extreme weather for wildlife species and biodiversity patterns has been largely underexplored but is increasingly becoming appreciated. Many organisms are physiologically constrained by their upper and lower thermal limits and thus may be more likely to be pushed past their physiological limits by an extreme weather event than gradually shifting mean conditions. Species might be especially sensitive to extreme weather at the edges of their geographic ranges, where they are often already living near their physiological limits.  Thus, understanding how the incidence of extreme weather events limits the boundaries of species distributions is critical for accurate ecological forecasts and better conservation outcomes under rapidly accelerating climate change. However, the influence of climatic variability and extreme weather is often ignored in favor of climatic means when estimating distributional and richness patterns. Here we use hundreds of millions of citizen science bird observations from 2004-2024 and high-resolution extreme weather risk maps to explore how climatic variability and extreme weather risk alters summer and winter distributions and biodiversity patterns for 540 North American species. We find that species distribution models accounting for historical extreme weather risk performed better at predicting richness and species’ presence or absence across ~250 sites. These models predicted much narrower geographic distributions than traditional models relying on only climatic means, with range truncation observed primarily at the range edges. Additionally, we observed these effects in both seasons, though they were particularly strong in winter. Richness estimates were substantially lower when extreme weather was accounted for, especially in the US southwest and central plains, regions highly prone to extreme heat, cold and drought. Our results suggest that more mechanistically informed biodiversity predictions that account for extreme weather are critical for appreciating and reliably predicting shifting biodiversity distributions.