Reconciling seemingly divergent effects of diversity on disease: A metacommunity approach

Background/Question/Methods

The effect of biodiversity loss on parasite transmission has caused heated controversy lately, partly due to confusion over how transmission rates and parasite diversity relate to disease risk. For instance, while empirical evidence suggests that host biodiversity can limit the transmission of multi-host parasites, evidence also indicates that parasite diversity can increase with host diversity. Rather than contradicting each other, however, these divergent viewpoints are essentially an issue of scale and scope. For parasites, host biodiversity loss is analogous to habitat diversity loss for free-living species, for which there is a strong conceptual and theoretical framework. Here we capitalize on this framework to build an agent-based metacommunity model of generalized symbiont communities (that include parasites) embedded in a host community. This framework, in which individual hosts are treated as habitat patches for symbionts, allows us to explore the simultaneous effects of host diversity on overall symbiont diversity and individual symbiont transmission. Thus, we explore the relationships between symbiont transmission, host richness, and host functional diversity. This approach facilitates a more general view of the relationship between diversity and disease, while laying a better foundation for making sense of the rapidly increasing amount of empirical data on symbiont communities.

Results/Conclusions

Increases in habitat diversity can cause non-linear responses in species richness and abundance for free-living species. Correspondingly, we found a unimodal relationship between host and symbiont diversity. In homogeneous host communities, few symbionts can utilize a narrow range of host resources. In more heterogeneous host communities, more symbionts can colonize, increasing symbiont diversity. In extremely diverse host communities, the abundance of each host species decreases, reducing symbiont transmission and persistence within the host community. We found that relationships between host diversity and symbiont transmission are generally consistent with current theory, predicting that for fixed host community density, host diversity loss will generally increase transmission. For host communities with little standing functional diversity, host extirpations should decrease symbiont diversity. In contrast, host losses from communities with high functional diversity could cause an initial increase then a decrease in symbiont diversity. Increased symbiont transmission and reduced symbiont diversity appear to be consequences of host diversity loss that are related, consistent with metacommunity theory, and not inherently contradictory. This framework may help catalyze development of general ecological theory for parasite and symbiont communities, while making sense of the apparently divergent effects of host diversity on parasite transmission and richness.