Biological diversification in a complex region: a spatial analysis of faunistic diversity and biogeography of the Andes of Colombia
Understanding large-scale patterns of beta diversity and endemism is essential for ecoregional conservation planning. We present a study of spatial patterns of faunal diversification and biogeographical relationships in the Andean region of Colombia. This region has a great geomorphological complexity, as it is formed by several mountain ranges with different geologic origins. We hypothesize that this complexity results in a high turnover in species composition among subregions. Location
The Andean region of Colombia, including the Santa Marta and Macarena mountain ranges. Methods
The region was divided into subregions, represented by the eastern and western slopes of each of the three Andean cordilleras, the Cauca and Magdalena valley bottoms, and the peripheral mountain ranges of Perijá, Macarena and Sierra Nevada de Santa Marta. Species lists for five animal taxa (rodents, bats, birds, frogs and butterflies) were compiled for each subregion and similarities in species composition were determined by cluster analysis. To explore biogeographical relationships, species were classified into one of four distributional categories: endemic, tropical Andean, Andean-Central American and wide continental distribution. Results
The highest species richness in the region was found in the Pacific and eastern versants of the Andes, and the lowest in the Cauca and Magdalena valley bottoms. Inter-Andean slopes were intermediate in species richness. However, when species richness was calculated per unit area, the most diverse regions were the Santa Marta and Macarena ranges, the Cauca Valley watershed and the Pacific slope. Although each taxonomic group had a different branching pattern, dendrograms indicated five common subregional clusterings: (1) Perijá-Sierra Nevada, (2) the Pacific slope, (3) the eastern Andean slope, (4) the Cauca and Magdalena valley bottoms, and (5) the inter-Andean slopes. Clustering patterns of inter-Andean slopes varied among taxa. In birds, bats and rodents, grouping was by opposite slopes of the same valley, whereas frogs were grouped by mountain ranges and butterflies by valleys and their respective slopes. Seventy-five per cent of species in all taxa were found in less than five subregions. The fauna of the Magdalena and Cauca valley bottoms was composed mostly of lowland species with wide geographical distributions, whereas the cordilleran fauna was mostly restricted to the tropical Andes. Main conclusions
The western and eastern versants of the Andes have the highest species richness, but are also the largest subregions. On a per unit area basis, the peripheral ranges (Santa Marta and Macarena) are the richest, followed by the western portion of the Andes (the Cauca Valley watershed and the Pacific versant). Clustering patterns in dendrograms suggest two major patterns of differentiation of the Andean fauna: one elevational (lowlands vs. highlands) and one horizontal (among ranges and/or slopes). Biogeographical affinities of the inter-Andean valley bottoms are with the lowland faunas of tropical America. In contrast, Andean faunas diversified locally, resulting in the evolution of a large number of endemic species, particularly among the less vagile taxa. Three different main branches of Andean fauna can be recognized, one confined to the Pacific, another to the eastern (Amazonian-Llanos) versant of the Andes, and the third one composed by the inter-Andean slopes of the Cauca and Magdalena valleys. The identification of five main biogeographical units in the Andean region of Colombia has important implications for the conservation of the regional biota. Conservation initiatives that seek to preserve representative samples of the regional biodiversity should take into account the patterns of diversification described here, and the evolutionary processes that gave rise to these patterns.