Abstract
BackgroundThe organization of networks of interacting species, such as plants and animals engaged in mutualisms, strongly influences the ecology and evolution of partner communities. Habitat fragmentation is a globally pervasive form of spatial heterogeneity that could profoundly impact the structure of mutualist networks. This is particularly true for biodiversity-rich tropical ecosystems, where the majority of plant species depend on mutualisms with animals and it is thought that changes in the structure of mutualist networks could lead to cascades of extinctions.Methodology/Principal FindingsWe evaluated effects of fragmentation on mutualistic networks by calculating metrics of network structure for ant-plant networks in continuous Amazonian forests with those in forest fragments. We hypothesized that networks in fragments would have fewer species and higher connectance, but equal nestedness and resilience compared to forest networks. Only one of the nine metrics we compared differed between continuous forest and forest fragments, indicating that networks were resistant to the biotic and abiotic changes that accompany fragmentation. This is partially the result of the loss of only specialist species with one connection that were lost in forest fragments.Conclusions/SignificanceWe found that the networks of ant-plant mutualists in twenty-five year old fragments are similar to those in continuous forest, suggesting these interactions are resistant to the detrimental changes associated with habitat fragmentation, at least in landscapes that are a mosaic of fragments, regenerating forests, and pastures. However, ant-plant mutualistic networks may have several properties that may promote their persistence in fragmented landscapes. Proactive identification of key mutualist partners may be necessary to focus conservation efforts on the interactions that insure the integrity of network structure and the ecosystems services networks provide.
Highlights
While there is much to be learned about the dynamics of mutualisms from the study of pair-wise interactions [1], there has been an upsurge of interest in how the analysis of multi-species networks can enhance our understanding of these pivotal interactions [2,3]
We found that networks in continuous forest had significantly more plant species than networks in forest fragments (N = 7.25 vs. N = 4.25, respectively, P = 0.015, Figure 1, Table 1), and there was a trend towards more ant species in continuous forests than forest fragments (N = 10 vs. N = 7, respectively, P = 0.092)
Because most plant and animal species are involved in mutualisms, it has been suggested that the extinctions of individual taxa could result in a ‘‘cascade of extinctions’’ reverberating throughout communities of interacting taxa [44]
Summary
While there is much to be learned about the dynamics of mutualisms from the study of pair-wise interactions [1], there has been an upsurge of interest in how the analysis of multi-species networks can enhance our understanding of these pivotal interactions [2,3]. They are likely to be built on weak and asymmetric links, meaning a plant species that is very dependent on a particular animal species is only weakly depended on by that animal species [4,7] Understanding these and other properties of network structure can provide insights into the assembly and evolution of species interactions, but can provide unique insights into community responses to anthropogenic disturbances such as habitat loss and species extinctions [8,9,10]. Habitat fragmentation is a globally pervasive form of spatial heterogeneity that could profoundly impact the structure of mutualist networks This is true for biodiversity-rich tropical ecosystems, where the majority of plant species depend on mutualisms with animals and it is thought that changes in the structure of mutualist networks could lead to cascades of extinctions
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