Isotopic Niche Partitioning in Avian Scavenger Communities of South America

Abstract

Interspecific competition plays a central role in structuring ecological communities and is a particularly strong force among avian scavengers. Niche partitioning among scavengers has been thoroughly investigated for Old World vultures (family Accipitridae), yet we know much less about its effect on the structure and functioning of the complex scavenger guilds of the New World (family Cathartidae). We studied how niche partitioning differed between ecosystems featuring species-poor and species-rich guilds of avian scavengers in South America. We compared dietary overlap between avian scavenger assemblages at two sites in Argentina where Andean Condors (Vultur gryphus) reside: Patagonia, featuring a rich guild of five main avian scavengers, and the High Andes, with only two main species of avian scavengers. We compared the δ13C and δ15N isotopic signatures, proxies for animal diets, of molted feathers to quantify niche breadth for individual species, and niche overlap between species. We found greater isotopic overlap between species and a smaller niche breadth for Andean Condors in Patagonia, where the richest assemblage of scavengers exists. These site-level differences in niche size and overlap are likely a result of greater isotopic diversity of ungulate prey around our sampling area in the High Andes, and future research should test this potential mechanism. The smaller niche breadth and greater isotopic overlap for the Andean Condor in Patagonia could signify competitive interactions with other scavengers and negative population-level effects for this species of conservation concern.