Not so stable: variation in mixed-species flocks networks in a lowland amazonian forest

Terra firme forests in the Amazon host extraordinary species diversity and richness, mainly due to great availability of resources. Although it may give an impression of a homogeneous and stable environment, topography is sufficiently variable to affect landscape and species distribution. In this study, we aimed to examine the effect of habitat in overall richness and network properties of mixed-species flocks of birds (MSF) in a continuous terra firme Amazonian forest. We collected data on 33 MSF along an environmental gradient in the FLONA Saracá-Taquera, located in eastern Brazilian Amazon. The type of habitat influenced species richness and network properties. Vegetation (NDVI) was a good predictor of species richness in MSF (R2-adj = 0.217, p = 0.019), and elevation had a marginal effect (R2-adj = 0.044, p = 0.062). We found higher frequency of interspecific associations (weighted degree) in valley areas (“baixio”), which may present favorable niche features for the formation of more complex MSF. Previous studies observed the influence of the altitudinal gradient in bird assemblages, and our results emphasize the misconception of stability in terra firme Amazonian forests. Although altitudinal gradients are not heavily pronounced as in other regions, the resulting environmental gradient in these forests are sufficient to affect further interspecific associations.