Biogeography and composition of dry forest bird communities in Bolivia

Abstract

In Bolivia dry forests are distributed extensively in the southeastern lowlands and extend into the country's northern and southern Andes in rain-shadowed intermontane valleys, where they often form habitat islands. We examined the biogeography and composition of bird communities at 14 Andean and seven lowland dry forest localities in Bolivia. To minimise biases, analyses considered only core speciessensu Remsen (1994) of zonal vegetation types. Of a total of 608 recorded species, 454 were zonal forest core species and 99 (22 %) typical dry forest species. Core species richness varied from 65 to 156 at individual sites. One species occurred at all sites and 154 (34 %) at a single locality each; this tendency was most evident in the northern Bolivian Andes where 46 % of core species were recorded only once. Comparisons of species composition showed that sites in the northern valleys were the most heterogeneous and distinct. Localities in the southern valleys were the most homogeneous with affinities to lowland dry forests; the latter separated into Velasco forest and Chaco woodland sites, respectively. A continuous ordination of sites revealed a steep gradient from large habitat areas in the dry southeastern lowlands to small, relatively humid Andean habitat islands in the northwest. Typical dry forest species were most prevalent in large habitat areas in the dry south, whereas typical humid forest species predominated in the relatively humid northern Andean habitat islands. Frugi-granivores and nectarivores such as pigeons and doves, hummingbirds, and emberizid finches were most prevalent at Andean sites, whereas insectivores made relatively greater contributions to lowland bird communities. The biogeographical affinities of Bolivian dry forest birds are varied. Overall, lowland birds gradually decreased and Andean species gradually increased with elevation, which was most pronounced in the southern Bolivian Andes; we found no elevational threshold of increased avifaunal turnover. An examination of range limits of typical dry forest species revealed a pronounced northwestward reduction of the dry forest avifauna from Chaco lowlands to small, isolated valleys in the northern Bolivian Andes. With seven species restricted to the Andean dry forests of Bolivia, this area has a considerably higher level of endemism than the Chaco. Andean dry forests were colonised by lowland species in two different ways. In the southern valleys and upper northern valleys, dry forest species ascended directly from the adjacent Chaco. The lower northern valleys were probably colonised by dry forest species during drier and cooler periods of the Pleistocene that resulted in expansions of seasonally dry tropical forest in Amazonia.