Data from: 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 toward the
center. 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.