Global biogeography of traits and extinction risk in birds: an elevational perspective

Mountains are hotspots of terrestrial species richness and endemism, but the reasons why are poorly understood. Extensive reviews of the literature, across taxa, reveal that research on trait and extinction risk variation with respect to elevational distribution are outnumbered by studies on latitudinal gradients and geographical range size, and are taxonomically and geographically restricted.

The aim of this thesis is to analyse interspecific variation in morphology, life history, ecology, and extinction risk with respect to elevational distribution – at the global scale and across a broad taxonomic range. To achieve this, I use birds as a model system, a global avian trait database and a comparative approach – employing both bivariate and multivariate statistical techniques.

Elevational distribution is shown to be positively associated with reproduction and niche breadth, whilst being negatively associated with morphology, growth and survival – even when controlling for body weight, geographical range, and latitude. Birds with larger elevational ranges and higher maximum and midpoint elevations possess traits consistent with a fast life history, and vice versa. Fast life histories at high elevations may result from exposure to more variable/seasonal environments compared to lowland birds. Global avian extinction risk is found to be greatest in lowland species and those with small elevational ranges. Overall, these relationships remained robust at the family level, for species within biogeographic realms, endemic subsets, and across phylogenetically independent contrasts.

This research will add to current understanding of large-scale ecology, trait biogeography, and conservation biology – assisting the incorporation of an elevational perspective into biogeography and macroecology theory, and conservation practice. Future work should focus on further identifying the underlying processes for the patterns shown here, and investigating their generality across other vertebrate groups, e.g. mammals.