Seed-dispersal networks lose complexity in tropical forest fragments

Abstract

Seed dispersal interactions are a key ecological process in tropical forests that help to maintain ecosystem functioning. Yet this functionality may be threatened by increasing habitat loss, defaunation and fragmentation. However, generalist species, and their interactions, can benefit from the habitat change caused by human disturbance while more specialized interactions mostly disappear. Therefore changes in the structure of the local, within fragment networks can be expected. Here we investigated how the structure of seed-dispersal networks changes in a gradient of increasing habitat fragmentation. We analysed 16 bird seed-dispersal assemblages located in forest fragments of a biodiversity-rich ecosystem. We found significant species-, interaction- and network-area relationships. The number of frugivore bird and plant species, their interactions, and the number of links per species decreases as area is lost in the fragmented landscape. Contrarily, network nestedness has a negative relationship with fragment area, suggesting an increasing generalization of network structure in the gradient of fragmentation. Network specialization was not significantly affected by area though. The local extinction of partner species, paralleled by a loss of interactions and specialist-specialist bird-plant seed dispersal associations suggests the lost of community complexity linked to a functional homogenization of the system as area is lost. Our study provides empirical evidence for network-area relationships, yet the lack of significant changes in specialization point towards some resilience of the network structure in fragmented landscapes.