Enigmatic freshwater mussel declines could be explained by the biodiversity‐disease relationship.

The biodiversity‐disease relationship states that increased species richness leads to lower pathogen pressure (i.e. the dilution effect), an effect that erodes with biodiversity loss. However, whether losses of the dilution effect can trigger extinction cascades remains largely unexplored.To explore this idea, we consider declines in freshwater mussels, on average the most threatened non‐marine group of organisms in the world. We argue that anthropogenically stimulated declines in mussel richness amplify pathogens in remaining species. Pathogen amplification triggers further local losses in both mussel abundance and richness, explaining the observed so‐called "enigmatic" declines in freshwater mussels.Vulnerable communities could become trapped in cycles of pathogen amplification and host decline. We highlight knowledge gaps and provide key steps to assess the likelihood of this occurring; these key steps are applicable to any host group.Policy implications. Our argument constitutes a testable hypothesis that may explain richness or abundance declines in previously intact communities. We provide further impetus for the consideration and preservation of diversity at a local scale and show that effective conservation requires integration of both host and parasite ecology. [ABSTRACT FROM AUTHOR]