Hosts and parasites are embedded in communities where species richness and composition can influence disease outcomes (diversity–disease relationships). The direction and magnitude of diversity–disease relationships are influenced by variation in competence (ability to support and transmit infections) of hosts in a community. However, host susceptibility to parasites, which mediates host competence, is not static and is influenced by environmental factors, including pollutants. Despite the role that pollutants can play in augmenting host susceptibility, how pollutants influence diversity–disease dynamics is not well understood. Using an amphibian–trematode model, we tested how NaCl influences diversity–disease dynamics. We predicted that NaCl exposure can alter relative susceptibility of host species to trematodes, leading to cascading effects on the diversity–disease relationship. To test these predictions, we exposed hosts to benign or NaCl environments and generated communities that differed in number and composition of host species. We exposed these communities to trematodes and measured disease outcomes at the community (total infections across all hosts within a community) and species levels (average number of infections per host species within a community). Host species differed in their relative susceptibility to trematodes when exposed to NaCl. Consequently, at the community level (total infections across all hosts within a community), we only detected diversity–disease relationships (dilution effects) in communities where hosts were exposed to NaCl. At the species level, disease outcomes (average number of infections/species) and whether multi‐species communities supported lower number of infections relative to single‐species communities depended on community composition. Notably, however, as with overall community infection, diversity–disease relationships only emerged when hosts were exposed to NaCl.
Understanding parasite transmission in communities requires knowledge of each species' capacity to support transmission. This property, ‘competence’, is a critical currency for modelling transmission under community change and for testing diversity–disease theory. Despite the central role of competence in disease ecology, we lack a clear understanding of the factors that generate competence and drive its variation. We developed novel conceptual and quantitative approaches to systematically quantify competence for a multi‐host, multi‐parasite community. We applied our framework to an extensive dataset: five amphibian host species exposed to four parasitic trematode species across five ecologically realistic exposure doses. Together, this experimental design captured 20 host–parasite interactions while integrating important information on variation in parasite exposure. Using experimental infection assays, we measured multiple components of the infection process and combined them to produce competence estimates for each interaction. With directly estimated competence values, we asked which components of the infection process best explained variation in competence: barrier resistance (the initial fraction of administered parasites blocked from infecting a host), internal clearance (the fraction of established parasites lost over time) or pre‐transmission mortality (the probability of host death prior to transmission). We found that variation in competence among the 20 interactions was best explained by differences in barrier resistance and pre‐transmission mortality, underscoring the importance of host resistance and parasite pathogenicity in shaping competence. We also produced dose‐integrated estimates of competence that incorporated natural variation in exposure to address questions on the basis and extent of variation in competence. We found strong signals that host species identity shaped competence variation (as opposed to parasite species identity). While variation in infection outcomes across hosts, parasites, individuals and doses was considerable, individual heterogeneity was limited compared to among‐species differences. This finding highlights the robustness of our competence estimates and suggests that species‐level values may be strong predictors for community‐level transmission in natural systems. Competence emerges from distinct underlying processes and can have strong species‐level characteristics; thus, this property has great potential for linking mechanisms of infection to epidemiological patterns.
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- Award ID(s):
- 1754171
- NSF-PAR ID:
- 10380728
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Functional Ecology
- Volume:
- 36
- Issue:
- 8
- ISSN:
- 0269-8463
- Page Range / eLocation ID:
- p. 1845-1857
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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