Symmetries in montane species abundance profiles elucidate the link between environment and abundance

We present a study of abundance profiles of bird species along a compact elevational gradient (200-2800 m) in the eastern Himalayas. We quantified asymmetry metrics using one or more width parameters on the two sides of the abundance peak (by fitting a narrow region around it: AP, and using root-mean-square deviation across the half ranges: AS, for 61 species. AS was observed to be negatively correlated with elevation, with low-elevation profiles skewed towards higher elevation, mid-elevation profiles symmetric and high-elevation profiles skewed towards lower elevation. However, AP was not correlated with elevation. Moreover, the averages of both AS and AP over all species were consistent with zero (i.e. symmetric). These observations are not compatible with the stress tolerance hypothesis or Stevens' rule. We suggest that the linear environmental gradient generates the community-averaged symmetry: the hard elevational limits at both ends of the mountain, mediated by interspecific competition, produce the elevation-asymmetry correlation. Investigations into the environment-abundance relationship have remained heuristic owing to challenges in deriving testable predictions from theoretical models for (complex) two-dimensional landscapes. Our findings suggest that the simplified geometry of steep elevational gradients can provide a tractable system that may bring together theory and real-world data.