Hidden Losses: Assessing the Risk of Coextinction Among Ectoparasitic Flies and Their Bat Host Species in Brazil

ABSTRACT Thousands of species are at risk of extinction and frequently, one lost species can lead to the loss of other co‐dependent ones. Known as coextinction, this is a common but neglected form of biodiversity loss, affecting highly species‐specific organisms, like parasites. One example of such specificity is the Streblidae and Nycteribiidae ectoparasitic flies and their obligate bat hosts. Here, we investigated the risk of coextinction among 106 fly and 182 bat species in Brazil, using network analysis to visualize the modularity, nestedness, and connectivity between them. Only 1/3 of the bat species had documented interactions, but 33% of the bats and 64% of the flies had species‐specific links. Hosts had a higher degree of centrality (7.5) than parasites (5.0). The interaction network formed 33 distinct modules, with two to 24 species each. The average modularity (0.89) indicated a strong connection between the module nodes, but the network's structure is primarily driven by modularity and degree constraints rather than a distinct nested hierarchy. Within the modules, bats presented higher centrality values (0.25–0.75) than flies (0.25–0.5), indicating their importance in the network. Simulations of up to 100 extinction events indicated that the loss of more connected bat species would lead to a faster collapse of the network compared to null models. Four fly species are at risk of coextinction because their host species are close to or are currently threatened. In some cases, the potential extinction of a single bat species would cause five fly coextinctions. We highlight that coextinctions must be correctly considered and ignoring this would result in incorrect estimates of extinction rates and biodiversity losses.