Bird community composition across an Andean tree-line ecotone

Abstract

Few studies have found strong evidence to suggest that ecotones promote species richness and diversity. In this study we examine the responses of a high-Andean bird community to changes in vegetation and topographical characteristics across an Andean tree-line ecotone and adjacent cloud forest and puna grassland vegetation in southern Peru. Over a 6-month period, birds and vegetation were surveyed using a 100m fixed-width Distance Sampling point count method. Vegetation analyses revealed that the tree-line ecotone represented a distinctive high-Andean vegetation community that was easily differentiated from the adjacent cloud forest and puna grassland based on changes in tree-size characteristics and vegetation cover. Bird community composition was strongly seasonal and influenced by a pool of bird species from a wider elevational gradient. There were also clear differences in bird community measures between tree-line vegetation, cloud forest and puna grassland with species turnover (β-diversity) most pronounced at the tree-line. Canonical Correspondence Analysis revealed that the majority of the 81 bird species were associated with tree-line vegetation. Categorizing patterns of relative abundance of the 42 most common species revealed that the tree-line ecotone was composed primarily of cloud forest specialists and habitat generalists, with very few species from the puna grassland. Only two species, Thlypopsis ruficeps and Anairetes parulus, both widespread Andean species more typical of montane woodland vegetation edges, were categorized as ecotone specialists. However, our findings were influenced by significant differences in species detectability between all three vegetation communities. Our study highlights the importance of examining ecotones at an appropriate spatial and temporal scale. Selecting a suitable distance between sampling points based on the detection probabilities of the target bird species is essential to obtain an unbiased picture of how ecotones influence avian richness and diversity.