Abundance Patterns of Species along Elevational Gradients

Abstract

A species’ distribution or range is the expanse of geographical space where it is commonly encountered. The environment-abundance paradigm (EAP) links the abundance of species to the environment through traits and fitness and forms the core of evolution theory for species distributions. However, EAP studies so far are purely heuristic or empirical due to (i) a lack of testable theoretical frameworks, (ii) the complex, multivariate nature of most natural landscapes, and (iii) the functional link between trait and fitness that can neither be measured observationally, nor calculated from theory. Consequently, progress towards understanding the processes that shape species distributions has been slow.

Using a database of ∼35000 bird records collected by our research group in both summer and winter along a 2600 m gradient in the eastern Himalayas, we demonstrate that montane elevational gradients – which are effectively univariate, one dimensional and host high species diversity – are suitable for testing theoretical frameworks of the EAP.

We find that the species community average abundance profile shape is close to symmetric – peaked with extended tails – as predicted by the model for a linear environmental (elevational) gradient. We also detect a small but significant residual asymmetry arising mainly from outlier range-edge populations indicating that ranges are likely getting compressed by the limits of the mountain slope. Analysis of abundance-range width patterns suggests that summer distributions have soft boundaries and may be at carrying capacity, but this may not be true in winter. We find that species were more likely to move to lower elevations in winter, but range expansions are as common as range contractions. We find some evidence that migratory propensity is likely to be driven by body size (thermoregulation) rather than flight ability, and is influenced by certain habitat/dietary preferences as well as phylogenetic relatedness. Our study suggests a path towards a more mechanistic understanding of the determinants of species distributions while bringing to light a highly diverse, yet understudied, tropical montane ecosystem which has been recognised as a globally important biodiversity hotspot.