Application of Microbiome Feedback Theory to Animals: Can Parasites Drive Coexistence in Ungulate Communities?

Synopsis Parasites can have large impacts on host populations, but the extent to which parasite dynamics impact or respond to multi-species community structure remains uncertain. Empirical and theoretical studies within the host–microbiome feedback framework (often called plant–soil feedback) has provided strong evidence of the importance of soil pathogens to plant community structure and function. We adapt this framework to herd animals by extending the mathematics of host–microbiome feedback theory to accommodate increased likelihood of exposure to microbiomes from conspecific hosts rather than heterospecific hosts. We then integrate this framework with a model of interguild frequency dependence. Coupling this model with empirical observations, we estimate the host-specific fitness of gastro-intestinal nematodes living on ungulate species of Western USA. We find evidence that host-specific differences in nematode fitness could generate negative feedback on host fitness and contribute to coexistence of ungulates. Moreover, we find that this is more likely to be the case for pairs of ungulate species with high habitat overlap. If nematodes can indeed drive such negative feedbacks, then negative impacts of nematodes on their ungulate hosts should decline, i.e., be diluted, with increasing host diversity. While more work is necessary to confirm the underlying assumptions driving these conclusions, our work highlights the possibility that parasites play under appreciated roles in structuring animal communities.