Biodiversity loss may increase the spread of infectious disease in a phenomenon known as the dilution effect. Circumstances that increase the likelihood of disease dilution are: i) when host species vary in their competence (or, “transmission potential”), and ii) when communities disassemble predictably, such that the least competent hosts are also the most likely to go locally extinct. Despite the central role of competence in diversity-disease theory, we lack a clear understanding of the factors underlying competence, as well as the drivers and extent of its variation. Here, we build a framework for systematically quantifying competence and identify the underlying drivers and extent of variation in competence across scales. We apply our framework to five amphibian hosts and four commonly occurring parasitic trematodes, representing a total of 20 unique host-parasite combinations. Using longitudinal data from experimental infection assays, we explore the host traits that comprise competence, ask whether competence is a general host attribute or a unique outcome of a specific host-parasite interaction, and we compare variation in competence at both the within- and among-species levels. Finally, we use long-term observational data to assess whether species’ competence values are linked with their demographic patterns across the natural landscape.
Results/Conclusions
We observed broad variation in competence among the full suite of amphibian host-trematode parasite interactions. Initial susceptibility to infection (mediated by host resistance) was a strong driver of among-species variation in competence. At the within-species scale, parasite dose and host developmental stage generated further intraspecific variation in competence. Although for some trematode infections the amphibian’s competence value was negatively correlated with its risk of local extinction (supporting dilution effect theory), this was not the case for all trematode species. Moreover, within-species variation in competence could rival that of among-species variation, suggesting that species identities alone may not be sufficient predictors for parasite transmission. Our results bridge within-host processes to the community ecology of infectious disease and provide a powerful empirical test of diversity-disease theory.