Climate and productivity shape a mid-elevation peak in bird diversity along the western slope of the Central Andes

Abstract Avian species richness and endemism peak at mid-elevations (3,500–4,000 meters above sea level [m.a.s.l.]) on the western slope of the Central Andes, driven primarily by high productivity and moderate climatic conditions. These patterns are expected to shift with climate change, leading to upslope range shifts, lowland species loss, and mountaintop extinctions. Using data from the literature and field survey of 212 bird species and environmental data from climate (WorldClim2), primary productivity (NDVI), topographic area, human footprint (HFI), and the mid-domain effect (MDE), we examined how avian diversity is structured along an elevational gradient in Peru. Species richness and endemism were tightly linked and positively associated with primary productivity and intermediate levels of temperature and precipitation. In contrast, land area, HFI, and MDE explained little of the observed diversity patterns. Forecasting under four climate scenarios for 2070 indicated that patterns of bird diversity are likely to shift, with total species richness likely to decline in response to (1) warming above 2,000 m and (2) increasing human pressure at both low (<500 m) and high (>3,000 m) elevations. Our results support the hypothesis that avian species richness on the western slope of the Central Andes is shaped primarily by climatic factors—specifically temperature and precipitation (i.e., climate optima hypothesis)—which influence biodiversity indirectly through their effects on ecosystem productivity. To further explore this mechanism, we outline 6 ways describing how temperature and precipitation may interact to shape productivity and, in turn, richness along elevational gradients globally. These pathways build on existing theory and emphasize the importance of evaluating climate–productivity–diversity relationships across different mountain systems worldwide. Our findings highlight the vulnerability of xeric tropical montane ecosystems to climate change and underscore the urgent need to conserve mid-elevation productivity hotspots as refugia for biodiversity.