Aims The fauna of mountains and their surrounding regions are likely to be influenced principally by two biological processes: horizontal colonization along similar altitudinal levels by elements originating from lineages inhabiting higher latitudes; and vertical colonization by lineages from the same latitude, but at lower altitudes. We examine whether the expected patterns derived from the latter process can be observed in mountain dung beetle assemblages. Specifically, we study the variation in species composition and richness with altitude in five regions spanning elevation gradients, analysing whether the altitudinal rates of change in the number of species and genera differ, and whether beta-diversity scores for adjacent sites in each altitudinal gradient are different for species and genera.
Location Eastern Cordillera of the Colombian Andes.
Methods Field work was carried out in 1997–99 at 27 sites in five regions with elevation gradients, with 10–32 pitfall traps placed in each site. For each altitudinal level the numbers of species and genera were analysed with respect to altitude, and the slope of the linear regression between these variables was calculated. The slope of the curve of the altitude against the cumulative number of species and genera was also calculated for each altitudinal gradient to describe the compositional change between adjacent sites (beta diversity). Species and generic slopes were compared using analysis of covariance. The turnover of species along each altitudinal gradient was measured using presence/absence data and Cody's beta-diversity index between adjacent pairs of sites. A cluster analysis was used to detect faunistically homogeneous groups of localities.
Results Species richness always decreased with altitude, although the slopes did not differ significantly from zero. The number of genera also decreased with increasing altitude, but generally at a significantly slower rate than for species. Variation in the species beta-diversity scores between altitudinal levels did not follow a homogeneous pattern in the different regions. Two main altitudinal groups of sites with a boundary c. 1500–1750 m a.s.l. can be detected with respect to faunistic similarity. Low- and mid-altitude sites are inhabited by all of the genera (19) and 80% of all species collected. Eight genera and 61 species (c. 60% of the total) are unable to inhabit high-altitude sites, and only 20 species appear to be exclusive to these high-altitude environments (> 2000 m a.s.l.).
Main conclusions The dominant processes explaining dung beetle composition in the high north-eastern Andean mountains are probably those of vertical colonization. The limited role of horizontal colonization processes, or colonization from northern or southern lineages, could be a consequence of the isolation and recent geological origin of these mountains.