Occupancy patterns and upper range limits of lowland Bornean birds along an elevational gradient

Aim The traditional view of species' distributions is that they are less abundant near the edges of their ranges and more abundant towards the centre. Testing this pattern is difficult because of the complexity of distributions across wide geographical areas. An alternative strategy, however, is to measure species' distributional patterns along elevational gradients. We applied this strategy to examine whether lowland forest birds are indeed less common near their upper range limits on a Bornean mountain, and tested co-occurrence patterns among species for potential causes of attenuation, including signatures of habitat selection and competition at the periphery of their ranges. Location Mt. Mulu, Borneo. Taxon Rain forest birds. Methods We surveyed lowland forest birds on Mt. Mulu (2,376 m), classified their elevation-occupancy distributions using Huisman-Olff-Fresco (HOF) models, and examined co-occurrence patterns of species pairs for signatures of shared habitat patches and interspecific competition. Results For 39 of 50 common species, occupancy was highest at sea level then gradually declined near their upper range edges, in keeping with a "rare periphery" hypothesis. With respect to habitat selection, lowland species do not appear to cluster together at sites of patchy similar habitat near their upper range limits; neither are most lowland species segregated from potential montane competitors where ranges overlap. Main conclusions High relative abundance at sea level implies that species inhabit "truncated niches" and are not currently near the limits of their fundamental niche, unless unknown critical response thresholds exist. However, indirect effects of increasing temperature predicted under climate change scenarios could still influence lower range limits of lowland species indirectly by altering habitat, precipitation regimes and competitive interactions. The lack of non-random co-occurrence patterns implies that patchy habitat and simple pairwise species interactions are unlikely to be responsible for upper range limits in most species; diffuse competition across diverse rain forest bird communities could still play a role.