Fragile plant-frugivore interaction networks in tropical forest edges

Abstract

Current incessant threats of environmental changes, such as habitat fragmentation, may disrupt vital mutualistic plant-frugivore networks, threatening the whole ecosystem. To advance our knowledge of the magnitude of the impacts of such disruption, we need a better understanding of its structure and robustness. We addressed this by comparing several aspects of plant-frugivore networks in the forest edge and interior habitats of a fragmented forest, using field data on fruit removal by a community of frugivorous birds and lemurs from 40 tree species over 30 months. While the plant-frugivore networks in both habitats were significantly less nested than random expectations, the network in the forest edge habitat had higher value of nestedness compared to the network in the forest interior. Also, the networks in both habitats were highly modular compared to random expectations, with a dominance of species that are highly interactive within its modules. We also found that fruit size, weight, and color as well as tree height predicted the structural role of plants in the networks and their degree of specialization. Finally, the removal/loss of best-connected plants or animals resulted in a weak robustness of the networks in both habitats; this vulnerable pattern is more pronounced in forest edges than interior habitats. Interestingly, the loss of large-bodied seed dispersers from either habitat did not affect much the robustness of the networks; in contrast, when small-sized species were eliminated first, the network became less robust, especially for the edge community. These findings highlight the need to preserve species interactions in forest edges to avoid the collapse of the network, mitigating the impacts of species extinctions on ecosystem equilibrium.