The role of climate, habitat, competition, and land-use in the distribution and conservation of montane species

Abstract

Understanding the factors that limit species distributions and determining how anthropogenic climate and land-use change will affect these distributions are fundamental yet unresolved goals of ecology and conservation biology. For my dissertation, I answered questions concerning these issues in the western Himalayas, a highly biodiverse region undergoing agricultural intensification and climate change.

In the first chapter, I determined the relative importance of biotic and abiotic factors in setting the altitudinal distribution of Himalayan birds. My results differ from earlier, foundational research in the tropics, suggesting contrasting mechanisms limiting bird elevational ranges in temperate and tropical systems, with particularly strong abiotic influence in temperate regions.

Climate change is expected to lead to distributional shifts as species track their climatic niches over time. In the second chapter, I assess whether Himalayan altitudinal migratory birds track their thermal niches seasonally to better understand their potential sensitivity to climate-induced range shifts. I find greater niche tracking by warm-adapted species, suggesting cold-adapted species may lag behind rates of warming.

Himalayan landscapes are rapidly losing primary forest due to agriculture and grazing, yet the consequences to biodiversity are largely unknown. In the third chapter, I surveyed bird communities in primary forest, agriculture, and pastures during winter, a period of resource limitation. Low-intensity agricultural lands had significantly higher bird richness and abundance than pastures and primary forest, suggesting they are important for many Himalayan birds during winter.

In my final chapter, I challenge the long-held assumption that surface area declines with elevation in mountains. Using a global dataset, I show that a diversity of mountain range topographies exists, with many ranges having more surface area at higher elevations. These results suggest that population-level responses from climate-induced range shifts likely depend on topography. Moreover, accounting for topography can reveal priority areas for montane species conservation under climate change.

Altogether, my dissertation shows that temperature plays a critical role in structuring Himalayan bird distributions across seasons, and points to likely future climate-induced range shifts. Such range shifts may force low-elevation species into a topographic bottleneck, given Himalayan topography. Conservation priorities should target such bottlenecks to conserve vulnerable species.