Abstract
The usually positive inter-specific relationship between geographical range size and the abundance of local bird populations comes with exceptions. On continents, the majority of these exceptions have been described from tropical montane areas in Africa, where geographically-restricted bird species are unusually abundant. We asked how the local abundances of passerine and non-passerine bird species along an elevational gradient on Mt. Wilhelm, Papua New Guinea relate to their geographical range size. We collected data on bird assemblages at eight elevations (200–3,700 m, at 500 m elevational increments). We used a standardized point-counts at 16 points at each elevational study site. We partitioned the birds into feeding guilds, and we obtained data on geographical range sizes from the Bird-Life International data zone. We observed a positive relationship between abundance and geographical range size in the lowlands. This trend changed to a negative one towards higher elevations. The total abundances of the assemblage showed a hump-shaped pattern along the elevational gradient, with passerine birds, namely passerine insectivores, driving the observed pattern. In contrast to abundances, the mean biomass of the bird assemblages decreased with increasing elevation. Our results show that montane bird species maintain dense populations which compensate for the decreased available area near the top of the mountain.
Highlights
Many previous studies have found a positive inter-specific relationship between geographical range size and the abundance of local populations (Brown 1984; Gaston & Blackburn 2000; Gaston et al 2000)
Total bird species richness seemed to show a plateau at lower elevations and decreased with increasing elevation afterward (Figure 2a)
In this study we focused on the patterns and relationships in species richness, abundance and range size in assemblages of passerine and non-passerine birds along a tropical, elevational gradient
Summary
Many previous studies have found a positive inter-specific relationship between geographical range size and the abundance of local populations (Brown 1984; Gaston & Blackburn 2000; Gaston et al 2000). The authors hypothesized that (1) species utilizing a wider range or more abundant resources become more abundant and widely distributed (Brown 1984), (2) that high population growth leads to higher abundances and to more occupied sites or that (3). Intensive dispersal produces a positive inter-specific abundance-range size relationship (Borregaard & Rahbek 2010; Gaston et al 2000). Even fewer studies have combined these attributes of bird communities with an estimate of available resources (Ding et al 2005; Ghosh-Harihar 2013; Price et al 2014) and/or available area along mountain ranges (e.g., Ferenc et al 2016; Price et al 2014).
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