Population density drives increased parasitism via greater exposure and reduced resource availability in wild hosts

Exposure to environmental parasites should increase with host population density due to the accumulation of infective parasites in space. However, competition for resources also increases with density, lowering condition and increasing susceptibility, which offers an alternative pathway for density-dependent infection to act. To test how these two processes act independently or together to drive greater parasite counts, we used a long-term study of red deer to examine associations between host density, resource availability, and counts of three common helminth parasites. We found that greater density correlated with reduced resource availability, and while density was positively associated with both strongyle and tissue worm burdens, resource availability was independently and negatively associated with the same burdens, supporting separate roles of density-dependent exposure and susceptibility in driving infection. This study provides evidence that competition for resources is an important driver of infection in higher-density areas, exacerbating the effects of density-dependent increases in exposure.