Composition and Ecology of Avian Communities Along Elevational Gradients in Borneo

Abstract

A primary goal of ecology is to explain the diversity and distribution of organisms. Species distributions can be a function of abiotic factors, species interactions, dispersal limitations, and history, but the relative importance of these factors is widely debated. Elevational gradients are useful systems for studying these effects because many of these factors vary predictably with elevation, and because elevational gradients are replicated many times across the earth. However, few quantitative surveys of Southeast Asian bird communities have been conducted along elevational gradients. In this study, I surveyed birds using point counts and measured habitat and temperature across primary forest elevational gradients on three mountains in Borneo. I asked what the relationship is between elevation and species richness, what mechanisms produce varying elevational range limits for species among these mountains, and how species abundances are distributed through their elevational ranges. I found that bird communities in Borneo display a low-mid-elevation peak in species richness (at ca. 600 m), and that the overlap of distinct lowland and montane groups of species contributes to this pattern. The elevational ranges of species differ among mountains, but lowland species are distributed higher on smaller mountains, against predictions of the Massenerhebung effect. The upper edge of lowland species’ distributions appears to be limited by competition with congeners and members of the same foraging guild, whereas the lower edge of montane species appears to be dictated by temperature. Most lowland species occupy truncated distributions at their lower end, but one third exhibit an abundant center pattern. There is no evidence from my species co-occurrence data that habitat or pairwise interactions are responsible for range limits; rather, competition in these diverse tropical bird communities is likely to be diffuse rather than primarily pairwise. I provide evidence that competition can limit the upward movement of lowland bird species, against the common belief that lowland species are more aggressive and stronger competitors than montane species. This fits past observations that lowland communities are often phylogenetically overdispersed, suggesting a role of competition in community assembly, whereas in montane regions species are phylogenetically clustered. I also show that a majority of lowland bird species are most abundant at sea level and decline gradually with elevation. This does not fit the ‘abundant center’ hypothesis, that species are most abundant in the center of their ranges, and implies that these species are not currently near their thermal maximum at sea level, although sharp