Nestedness in a fish-parasite ecological network in the Gulf of California: a potential buffer against extinction

• Nestedness in host-parasite networks buffers coextinctions. • The nestedness is provided by the phylogeny and ecological traits of the hosts. • Peripheral species assume the functional roles of the extinct core species. • The loss of the most connected fish causes higher rates of parasite coextinction. Marine biodiversity is declining due to overfishing, pushing many species to the brink of extinction and profoundly affecting fish–parasite networks. However, the impact of coextinctions of core species within these networks remains unexplored. In this study, we analyzed the effects of coextinctions on a network comprising 10 carangid fish species and 39 metazoan parasite species in the southeastern Gulf of California. Four scenarios were simulated: the removal of a fish species vulnerable to extinction, the removal of all core fish species, the removal of core parasite species, and the removal of both core fish and parasite species. Core species were identified based on network centrality, and network structure was assessed using metrics of nestedness, modularity, robustness, and connectance. Nestedness and modularity were calculated using NODF and MODULAR software, respectively, while robustness and connectance were computed with the “bipartite” package in R. The network’s core consisted of two fish species and five parasite species; their removal triggered a reorganization that shifted peripheral species into core roles. In all scenarios, the network was nested and robust, with low modularity and connectance. Most post-extinction metrics were significantly lower than those from null models, and the highest coextinction rate occurred when species were removed from most to least connected. Recognizing parasites as essential components of ecosystems may contribute to more inclusive and effective conservation strategies, integrating their protection into fisheries management and marine conservation policies.