Scale-dependent, contrasting effects of habitat fragmentation on host-natural enemy trophic interactions
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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.
KeywordsBiotic interactions Habitat connectivity Habitat loss Host-parasite Host-parasitoid Host-pathogen Landscape epidemiology Natural enemies Species interactions
We would like to thank Robert Possee and Helen Hesketh for their help and support with lab work, and Elizabeth Elliott and Steven White for their help with field work, and two anonymous referees for their helpful comments. We would also like to thank Mark Shaw for his expertise in identifying the parasitoid, and for providing information on its biology. Finally, we would like to thank the RSPB and other land owners for access to their land. This study was supported by a NERC DTG studentship awarded to JPH (NE/J50001X/1).
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