Parasitism is an interaction defined by negative exploitation of one organism by another. However, the effects of parasites extend far beyond harm to a single individual. In this review, we explore three distinct pathways by which parasite effects on individual hosts translate into profound consequences at higher scales of biological organization. First, parasite-associated changes in individual host traits, including morphology, physiology, behavior, and life history, affect survival and reproduction, with consequences for host population densities and genotypic and phenotypic distributions. Second, changes in host densities affect ecological and evolutionary processes at the community and ecosystem levels. Third, changes in host traits independently affect communities and ecosystems. We illustrate each pathway using examples spanning the effects of parasites on host populations, communities, and ecosystems. This holistic approach showcases the discordant effects parasites can have across scales. We conclude with a discussion of open research questions, highlighting opportunities for studying the consequences of parasitism as a multiscale phenomenon.
Ezenwa et al. (Tue,) studied this question.