Isolated trees support lower bird taxonomic richness than trees within habitat patches but similar functional diversity

Abstract

Large isolated trees are keystone structures that can help maintain biodiversity in fragmented landscapes, with evidence that open areas with isolated trees may support similar levels of taxonomic diversity to nearby patches of habitat. However, it is not clear if isolated trees can support the same diversity of ecological functions as trees in habitat patches. We compared species richness, community composition and functional diversity of birds in trees at forest edges and isolated trees. Twenty isolated trees and ten edge trees of American muskwood (Guarea guidonia) were selected, and sampled on 11 occasions, each of 20 minutes, over four months. All individual birds that landed at trees were recorded using a standardized protocol. Species richness was, on average, almost twice as high at edge trees than at isolated trees. Taxonomic composition differed between edge and isolated trees, with many forest-dependent birds restricted to edge trees, and some open-area birds restricted to isolated trees. Overall functional diversity was similar at edge and isolated trees, but some ecological functions (e.g. frugivory) were less frequent, while others (e.g. granivory) were more frequent at isolated compared to edge trees. Isolated trees are important for supporting many ecological functions in modified areas. However, the maintenance of forest patches is essential to complement the provision of such functions in modified landscapes.