Parasites warning us on ecosystem changes. The case study of fish parasites in anthropogenically impacted Lake Victoria

How parasites are affected by global change is largely unknown, despite their ubiquity and crucial role for ecosystem health in maintaining complexity and contributing to ecosystem robustness. Human-induced environmental changes are expected to alter parasite abundance and host-parasite interactions (e.g. spillovers), but the direction of such changes is unclear, as its consequences for ecosystem health. Lake Victoria, the youngest of the African Great Lakes, is a biodiversity hotspot that experienced simultaneous drastic anthropogenic changes, the main being: Nile perch invasions and eutrophication. We compared gill macroparasite communities of 13 cichlid fish species 20 years before and after the onset of these anthropogenic perturbations, using historical and recent fish collections. We observed a decline in parasite abundance, biodiversity indices, and co-infections. The host-parasite network rearranged, some parasites disappeared from some host species and colonized few new ones, in a way that reduces ecosystem stability. This highlights the need to preserve parasites and their ecosystem services in face of global change. We also disentangled the cause of such changes, by comparing parasite communities between lake ecosystems similar and close to Lake Victoria but differing in perturbation types. This space-for-time approach revealed that the observed changes in parasite communities in Lake Victoria are not due to natural fluctuations, but rather result from the effect of both studied perturbations. Since changes that have occurred in Lake Victoria are also occurring in other ecosystems, we can use parasites as sentinel for ecosystem health, which might contribute to better strategies for linking conservation and ecosystem health.