Birds in Anthropogenic Landscapes: The Responses of Ecological Groups to Forest Loss in the Brazilian Atlantic Forest

Abstract

Habitat loss is the dominant threat to biodiversity and ecosystem functioning in terrestrial environments. In this study, we used an a priori classification of bird species based on their dependence on native forest habitats (forest-specialist and habitat generalists) and specific food resources (frugivores and insectivores) to evaluate their responses to forest cover reduction in landscapes in the Brazilian Atlantic Forest. From the patch-landscapes approach, we delimited 40 forest sites, and quantified the percentage of native forest within a 2 km radius around the center of each site (from 6 - 85%). At each site, we sampled birds using the point-count method. We used a null model, a generalized linear model and a four-parameter logistic model to evaluate the relationship between richness and abundance of the bird groups and the native forest amount. A piecewise model was then used to determine the threshold value for bird groups that showed nonlinear responses. The richness and abundance of the bird community as a whole were not affected by changes in forest cover in this region. However, a decrease in forest cover had a negative effect on diversity of forest-specialist, frugivorous and insectivorous birds, and a positive effect on generalist birds. The species richness and abundance of all ecological groups were nonlinearly related to forest reduction and showed similar threshold values, i.e., there were abrupt changes in individuals and species numbers when forest amount was less than approximately 50%. Forest sites within landscapes with forest cover that was less than 50% contained a different bird species composition than more extensively forested sites and had fewer forest-specialist species and higher beta-diversity. Our study demonstrated the pervasive effect of forest reduction on bird communities in one of the most important hotspots for bird conservation and shows that many vulnerable species require extensive forest cover to persist.

Fig 1. Map of the study area in southern Bahia, northeastern Brazil.

A: Atlantic Forest remnants (gray areas) and the 40 sampling sites (black circles). Dashed lines show the areas that were mapped for this study. Images of areas that are outside of the dashed lines were obtained from forest cover map “Atlas dos Remanescentes Florestais da Mata Atlântica” of open access [87]. B: Detail of some sampled landscapes (2 km radius), highlighting the percentage of forest cover (gray areas).

Fig 2. Total richness and abundance of forest-specialist and generalist species in the 40 sampling sites.

Landscapes vary in the amount of remaining forest cover from 6% to 85%. Lines correspond to the best fitting models.

Fig 3. Richness and abundance of frugivorous and insectivorous birds in the 40 sampling sites.

Landscapes vary in the amount of remaining forest cover from 6% to 85%. Lines correspond to the best fitting models.

Fig 4. NMDS ordination of the 40 sites that were sampled in the Brazilian Atlantic Forest.

Black points represent the scores of landscapes and the numbers indicate the percentages of forest cover (from 6% to 85%). A: Forest-specialist birds, B: Generalist birds, C: Frugivorous birds, D: Insectivorous birds. Pairwise ANOSIM tests showed significant differences (p < 0.05) between the bird compositions of landscapes with percentages of forest cover that were less than (left polygons) and greater than (right polygons) the threshold values.