Integrating community ecology and infectious disease research

Background/Question/Methods

One of the most fundamental challenges facing contemporary disease ecology involves understanding host-parasite interactions within complex communities composed of multiple host species, coinfecting parasites, and a suite of non-host species. Although interest in applying ecological approaches to disease research has grown strongly in recent years, the ‘community ecology of disease’ remains in its relative infancy, with few efforts to tackle the dynamics of multi-host, multi-pathogen assemblages. This has become especially apparent in ongoing questions regarding the relationship between biodiversity and disease risk. Building from community ecology as an organizing framework, we mechanistically explore how host and parasite assemblages interact to determine transmission and disease severity, using interactions between amphibian hosts and trematode parasites as a model system. To this end, we integrate (1) lab-based measurements of host life history and trait variation (including competence), (2) field surveys to assess patterns of host and parasite assembly, (3) in situ measurements of parasite transmission across a diversity gradient, and (4) lab- and mesocosm experiments to identify the influence of host and parasite richness on transmission and host fitness.

Results/Conclusions

Across surveys of 345 wetlands in California, host and parasite communities assembled non-randomly in a nested pattern. Host species assembly order correlated strongly with life history and epidemiological traits, including relative abundance, body size, and host competence. As a result, increases in host richness led to progressive decreases in community competence, or the capacity of a community to support infection by the most virulent infection. Correspondingly, host richness interacted significantly with infection pressure (i.e., the density of infected intermediate hosts) to determine amphibian disease risk. Experimental results bolstered these findings, indicating that more diverse assemblages with greater variation in host competence led to lower infection success and pathology in amphibian hosts. Variation in parasite diversity led to similar albeit more variable reductions in transmission success of the most virulent parasite. Importantly, however, field results also indicated that increases in host (or community) diversity positively predicted the richness of parasite species overall and their individual colonization probabilities. These findings help to resolve seemingly divergent perspectives on the diversity-disease relationship; while diversity promotes parasite colonization (and therefore richness), particularly at larger spatial scales, its effects at more local scales relevant to transmission are often negative.