Mixed species flocks along an elevational gradient in the bolivian andes: a network perspective

Mixed-species flocks represent a prevalent social system of most tropical forests on which species of several taxa move and forage together. Despite the importance of understanding flocking dynamics, the underlying processes driving the structure and assembly of mixed-species flocks are still poorly understood, particularly in mountain ecosystems. Here, I assess how mixed-species flocks are structured in the Andes of Bolivia and how the characteristics of these ecological networks change across elevations. For each flock along a continuous elevational gradient (3550 – 1300 m asl), I (1) constructed weighted networks and calculated species-level and flock-level metrics and (2) calculated a pair-wise association strength for each species pair, and related this matrix with pair-wise phenotypic similarity matrix and a phylogenetic similarity matrix. Overall I found that positive interactions were more important in the assembly of mixed-species flocks than negative associations. However, positive interactions were frequent in high elevation flocks, whereas negative interactions (i.e. competition) were more frequent at lower elevations. My results showed a decay in species richness with elevation along with patterns of in less modular and simpler networks at high elevations. Thus, networks with fewer and more even pair-wise interactions are found at high elevations, whereas more modular and complex networks are found at lower elevations. I discuss how this information can be used to better understand how mixed-species flocks respond to changes in environmental conditions, such as those expected by climate change.