Mutualistic interactions along a fragmentation gradient

Abstract

Forest cover loss and fragmentation due to land use changes are one of the principal causes of global biodiversity loss. Indirectly, forest fragmentation can reduce biodiversity and ecosystem functioning by disrupting species interaction networks. Interaction networks, such as seed dispersal networks, are fundamental in maintaining ecosystem services. In tropical forests, frugivorous animals are the most important seed dispersers and thus the main agents of forest regeneration. In general, large tree species produce large seeds that are dispersed by large frugivores. Therefore, in absence of large frugivores, the average size in the tree community is hypothesized to eventually decrease which would lead to lowered carbon storage capacity. The objective of this study was to examine how fragmentation-related factors affect interactions between seed dispersers and trees in highly fragmented Brazilian Atlantic Forest biome. We utilized spatially and taxonomically comprehensive data on 407 frugivorous animal and 1426 zoochoric tree species' occurrences. We fitted spatially explicit joint species distribution models to the occurrence data to combine information on environmental covariates, life-history traits, and phylogenetic correlations. To examine the co-occurrence patterns of frugivores and trees, we first used the fitted models to generate predicted communities spanning over the entire biome. We then calculated the co-occurrence probability of each frugivore-tree pair and examined how the total number of interactions, as well as the proportion of interactions provided by keystone frugivores varied with fragmentation-related covariates (core-to-edge ratio, area of functionally connected forest and distance to nearest road). We found that the proportion of interactions provided by the keystone frugivores experienced an overall decline with increasing level of fragmentation. Furthermore, we observed a decline in the number of frugivore interactions per tree species with increasing level of fragmentation. However, the effect was solely contributed to the functionally connected forest area, not to the edge effect-related covariates. Even though the effects of fragmentation were not as strong as the climatic effects in shaping the seed dispersal interactions, they were consistent and showed that fragmentation changes the networks both quantitatively (via total number of interactions) and qualitatively (via proportion of interactions provided by keystone species). Based on the results, we conclude that identifying and conserving the keystone frugivores could be an efficient way to maintain the stability of the tropical forests.