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

Abstract

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.

Biotic interactions help explain variation in elevational range limits of birds among Bornean mountains

“…We found that bird species distributions can differ significantly and consistently among mountains, and that both abiotic factors and species interactions contribute to these differences. This knowledge is important as we construct models to predict elevational range shifts under future climate change scenarios (Burner, Styring, Rahman, & Sheldon, ). Most future projections have been based solely on temperature (Chen et al, ; Forero‐Medina et al, ; Grytnes, Beaman, Romdal, & Rahbek, ), or temperature plus precipitation (Tingley, Monahan, Beissinger, & Moritz, ), but evidence now suggests that reactions of natural communities to climate change have more complex underpinnings (Devictor et al, ; Dulle et al, ; Tingley et al, ).…”

Varying support for abundance‐centre and congeneric‐competition hypotheses along elevational transects of mammals

“…Freeman and Beehler (2018) found that only one‐third of avian species showed a unimodal symmetric abundance distribution along a New Guinean elevational gradient. A similar proportion (30%) of species displayed such spatial patterns on a Bornean mountain (Burner et al., 2019). These previous findings together with our research demonstrate that species may show heterogeneous abundance patterns along an elevational gradient.…”

“…Our study of species' elevational abundance patterns, including evidence from five commonly distributed and abundant small mammal species along 12 elevational gradients, provides a rigorous assessment of the ACH along the elevational centre‐periphery axis, which is largely underrepresented in the ACH literature (Burner et al., 2019). We found very limited evidence supporting the consistent relationship between species abundance distributions and the ACH.…”

“…Elevational gradients have been frequently used for testing and developing ecological hypotheses regarding abundance and distribution and their relationship (Brown, 1984;Elsen et al, 2017;. However, several recent studies (only two to our knowledge) have explored the validity of the ACH along elevational gradients, with the elevational abundance patterns of the studied taxa (birds in both studies) showing relatively low support for the hypothesis (Burner et al, 2019;Freeman & Beehler, 2018). Studies along elevational gradients present a number of merits.…”

“…temperature differences) are mostly one‐dimensional along elevational gradients, which provides more straightforward insights into the mechanism underlying the abundance pattern (Sagarin & Gaines, 2002b). Third, habitat is relatively continuous along elevational gradients, which minimizes the impact of dispersal limitations (Burner et al., 2019). However, our understanding of the ACH along elevational gradients could be skewed by the fact that much of the evidence so far is drawn from a single animal group.…”