Abstract
Understanding the importance of parasites in affecting the biodiversity of host species in ecosystems is a central aim of conservation biology. Recent advances in ecology have suggested that differential parasite susceptibilities between taxonomically related host species may be a determinant of animal community structure. Although conceptually appealing, such an hypothesis suffers from a lack of field evidence. Here, we report that the populations of two congeneric and sympatric host species (Gammarus insensibilis and G. aequicauda), infected by the same parasite (Microphallus papillorobustus), exhibit a strongly contrasted pattern of parasite-induced mortality.