Climate, niche conservatism, and the global bird diversity gradient

Area of Science:

• Evolutionary Biology
• Ecology
• Biogeography

Background:

• The latitudinal diversity gradient, with more species in the tropics, is a key pattern in ecology.
• Previous hypotheses suggest factors like climate, area, and evolutionary rates contribute to this gradient.

Purpose of the Study:

• To test if the higher species richness in the tropics is due to the loss of ancient lineages from cooler regions.
• To investigate the roles of paleoclimate adaptation, niche conservatism, and speciation time in shaping global bird diversity patterns.

Main Methods:

• Utilized molecular phylogenies to classify bird species into "basal" (ancient) and "derived" (more recent) clades.
• Contrasted richness patterns of basal and derived clades across different climatic zones (lowland tropics, extratropics, high-altitude tropics).
• Employed path models to assess environmental predictors of species richness, differentiating between clade types.

Main Results:

• Basal clades showed higher diversity in the lowland tropics, while derived clades were more diverse in extratropical and high-altitude regions.
• Environmental variables explained 74% of bird species richness variation, with distinct models for basal and derived groups.
• The global richness gradient is primarily driven by the distribution of basal clades, with regional differences between the Old and New Worlds.

Conclusions:

• Global bird species richness gradients are shaped by both ecological responses to current climate and long-term evolutionary processes.
• Extirpation of ancient lineages from cooler regions over evolutionary time is a primary driver of the latitudinal diversity gradient.
• Niche conservatism and the "time-for-speciation" hypothesis effectively explain the observed diversity patterns, with dispersal and subsequent radiations as secondary factors.