Scale-dependent, contrasting effects of habitat fragmentation on host-natural enemy trophic interactions

Abstract

Habitat fragmentation can have contrasting effects on species and their interactions within communities, changing community structure and function. Parasitoids and pathogens are key natural enemies in invertebrate communities, but their responses to fragmentation have not been explored within the same community. This study aimed to explore the scale-dependent effects of habitat fragmentation on the population density of a lepidopteran host and particularly its trophic interactions with a specialist parasitoid and virus. Host density and host larval-mortality from the parasitoid and the virus were measured in twelve isolated sites and thirteen connected sites. An index of habitat isolation was created based on the amount of suitable habitat surrounding sites at a range of spatial scales (0.1–5 km radii), and the direct and indirect effects of habitat isolation were analysed using generalised linear mixed effects models. Consistent with predictions, habitat isolation had direct negative effects on host density at the smallest and largest spatial scales, and indirect negative effects on host mortality from the virus at the largest scale, but in contrast to predictions it had direct positive effects on parasitism at small and medium scales. Higher trophic level species may still display responses to habitat fragmentation contrary to predictions based on well supported theory and empirical evidence. The mechanisms underlying these responses may be elucidated by studying responses, contrary to expectations, shared by related species. Developing general predictions about the responses of host-pathogen interactions to fragmentation will require greater understanding of the system-specific mechanisms by which fragmentation can influence pathogen transmission.