Abstract
Parasites comprise a substantial portion of total biodiversity. Ultimately, this means that host extinction could result in many secondary extinctions of obligate parasites and potentially alter host-parasite network structure. Here, we examined a highly resolved fish-parasite network to determine key hosts responsible for maintaining parasite diversity and network structure (quantified here as nestedness and modularity). We evaluated four possible host extinction orders and compared the resulting co-extinction dynamics to random extinction simulations; including host removal based on estimated extinction risk, parasite species richness and host level contributions to nestedness and modularity. We found that all extinction orders, except the one based on realistic extinction risk, resulted in faster declines in parasite diversity and network structure relative to random biodiversity loss. Further, we determined species-level contributions to network structure were best predicted by parasite species richness and host family. Taken together, we demonstrate that a small proportion of hosts contribute substantially to network structure and that removal of these hosts results in rapid declines in parasite diversity and network structure. As network stability can potentially be inferred through measures of network structure, our findings may provide insight into species traits that confer stability.
Highlights
Networks by losses in biodiversity, without assessing changes to network structure
Host-specific contributions to nestedness and modularity were most associated with parasite species richness based on the relative contribution (RC) values obtained from the boosted regression tree analysis (Fig. 1)
We find that the differential impact a host has on network structure and parasite diversity can be partially explained by host traits
Summary
Networks by losses in biodiversity, without assessing changes to network structure. Here, we examine both losses to biodiversity (secondary extinctions) and changes to network structure with the removal of host species from a network. Network structure is commonly quantified using topological network measures to describe the tendency for interactions in a network to be organized in a particular manner. These metrics describing network structure have often been related to measures of network stability, such as structural stability[14]. The natural synthesis of these two approaches can provide predictions on what happens to network structure when species are systematically lost from the network We combine both approaches, using data from a Neotropical fish-parasite network[24], to distinguish hosts contributing differentially to network structure, traits associated with these key hosts, and the resulting effect of removing these hosts relative to random extinction simulations. We provide evidence that 1) a small proportion of key hosts are responsible for maintaining network structure (i.e. nestedness and modularity), 2) parasite diversity and host family are important predictors of species level contributions to network structure, and 3) simulated extinctions of hosts with high parasite richness, or large contributions to network structure (nestedness and modularity), tend to result in more precipitous declines in parasite diversity and network structure relative to random simulations
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