Abstract
AbstractAimThe 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.LocationMt. Mulu, Borneo.TaxonRain forest birds.MethodsWe 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.ResultsFor 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 conclusionsHigh 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.
Highlights
Ecologists have long assumed that species are most abundant at the centre of their ranges and become rarer near the edges (Brown, 1984)
To determine potential causes limiting the upper distributional ranges of these lowland species, we examined co‐occurrence patterns of species pairs and tested two hypothesized mechanisms: (a) patchy habitat, that is that multiple lowland species will cluster near their upper range edges, presumably in pockets of suitable habitat or microclimate; and (b) interspecific competition, that is that a lowland species’ upper limit is determined by the lower limit of likely montane com‐ petitors, and that these pairs of species will be segregated from each other where ranges overlap
While some lowland species had the highest occupancy in their ele‐ vational range centres, most were at peak occupancy at sea level and declined gradually with elevation
Summary
Ecologists have long assumed that species are most abundant at the centre of their ranges and become rarer near the edges (Brown, 1984). Studies have tried with mixed results to min‐ imize these challenges by testing multiple distance metrics (Dallas et al, 2017; Soberón, Peterson, & Osorio‐Olvera, 2018), employing multiple definitions of range centres and margins (Santini, Pironon, Maiorano, & Thuiller, 2018) and measuring environmental rather than spatial distances (Martínez‐Meyer, Díaz‐Porras, Peterson, & Yáñez‐Arenas, 2013; Pironon et al, 2015; Van Couwenberghe et al, 2013) In addition to these inherent complications, comparisons of abundance estimates across geographical distances are often diffi‐ cult because sampling usually depends on different observers oper‐ ating at different times using different methods (Santini et al, 2018). Appear to be more abundant at sites pre‐ dicted to be more suitable by species distribution models (Lunghi et al, 2018; Weber, Stevens, Diniz‐Filho, & Grelle, 2017)
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