The Productivity Gradient Explains Global Bird Specialisation Better Than Climate Stability

Aim

Historical and contemporary environmental factors are hypothesised to influence the degree of ecological specialisation of species. Long-term climate stability might facilitate specialisation by promoting stable environments and diversification (climate stability hypothesis). In contrast, current stress–productivity gradients could also moderate specialisation through: (i) environmental filtering in stressful (e.g., arid) environments or (ii) accumulation of specialised species in highly productive regions.

Location

Global.

Time Period

Pliocene-present.

Major Taxa Studied

Birds.

Methods

We tested whether different specialisation facets (climate, diet and habitat) in bird assemblages are better explained by long-term climate stability or current stress-productivity gradients while accounting for latitude, longitude, biogeographic realm, taxonomic species richness and the evolutionary age of the assemblages at a global scale.

Results

Long-term climatic stability was a weak predictor of bird specialisation after accounting for latitude. In contrast, aridity showed a consistent negative association with climate, diet, and habitat specialisation, even after controlling for latitude and species richness. Species richness was strongly positively associated with diet specialisation, suggesting the influence of niche filling processes. In addition, specialisation was more pronounced in high-productivity environments, indicating that greater niche availability fosters specialisation. Notably, the effects of aridity and assemblage mean evolutionary age on specialisation differed between hemispheres. While negative associations dominated in Southern realms, the Palearctic and Nearctic realms in the Northern Hemisphere showed more positive trends. This hemispheric contrast underscores the context-dependency of environmental effects on specialisation and points to biogeographic history as a potential modulator of these patterns.

Conclusions

Globally, stress-productivity gradients better explain patterns of avian specialisation than long-term climate stability. Overall, our results did not support the climate stability hypothesis and challenged the idea that abiotically stressful conditions promote specialisation. The present study suggests that ecological processes, especially niche filling in productive regions with accumulated species, play a key role in driving and maintaining specialisation in birds.