With rising interest in understanding the links between diversity and infections, however, a polarizing debate has emerged over whether biodiversity generally decreases pathogen transmission (via the ‘dilution effect’), generally increases transmission (‘amplification effect’), or whether responses are idiosyncratic and highly variable among systems – issues that are especially contentious when it comes to management policies. Because evidence exists to support all of the above positions, a more productive approach may be to delineate under what host, parasite, and environmental conditions changes in diversity amplify or dilute disease risk. For most systems, however, we lack the essential combination of (1) empirical data from replicated communities, (2) parameterized transmission models, and (3) ecologically relevant experiments necessary to address these questions, particularly across a range of host and parasite taxa.

To address these missing links, we developed a predictive framework built on mechanistic approach to identify the conditions under which biodiversity affects the capacity of pathogens to spread or cause illness. Our research thus far has already significantly impacted the field of disease ecology generally and the study of the biodiversity-disease relationship more specifically. In the main, our finding has helped to address sources of confusion surrounding the diversity-disease relationship and specifically, understanding how changes in host community composition influence parasite transmission. We showed, for instance, that community level changes in infection levels owed to shifts in host species composition, rather than richness per se. However, when composition patterns mirrored empirical observations along a natural assembly gradient, each added host species reduced infection success by 12–55%. These findings highlight the potential for combining information on host traits and assembly patterns to forecast diversity-mediated changes in multi-host disease systems. We also examined how parasites themselves – through their choices in infecting specific host species within a community – alter the overall pattern of transmission in complex assemblages, particularly with trematodes which support complex communities and have multiple hosts. Using detailed experiments that tested parasite choice for alternative host species, both individually and through varying ‘choice sets,’ we showed that trematode cercariae demonstrated consistent and predictable preference patterns for specific host species; however, preferred hosts were not the most susceptible to infection, which is important for understanding whether forecasted shifts in diversity will alter overall infection success in multi-host communities.

Lastly, our work also made progress on the development of empirical and theoretical approaches for understanding diversity-disease relationships. We have built statistical models that are helping to assess the relative influence of proposed mechanisms underlying the influence of biodiversity losses on pathogen transmission, including shifts in host abundance, species composition, and predation. These findings emphasize the core importance of understanding the empirical relationship between species richness and species composition: thus, whether diversity leads to reduced parasite transmission depends critically on the degree to which the most competent hosts predominate in low-richness communities, which is a strongly consistent pattern in this system. For the four most closely studied trematode parasites, we found that host diversity leads to reductions in transmission both at the individual host scale (infection risk per host) and at the community scale (total parasite infection success among all co-occurring hosts). Yet the magnitude of these effects differed broadly. Empirical data from field surveys and laboratory experiments have contributed to four manuscripts related to (1) deterministic patterns in ecological community assembly, (2) variation in competence across host-parasite-dosage combinations throughout communities, (3) a review of the epidemiological concept of competence and identification of core knowledge gaps, and (4) empirical analysis of field data to assess how richness, density, and predators affect transmission of different trematodes and at two biological scales (host level vs. community level).

Broader impacts:

This research capitalized on the public’s interest in amphibians generally – and in amphibian deformities specifically – to foster educational outreach and conservation awareness. PI Johnson and supported science personnel (Dana Calhoun) worked closely with students across a range of educational levels and backgrounds to foster research aptitude, encourage science career opportunities, and promote recruitment by under-represented groups. Partnering with programs such as the Colorado Initiative, the Science Discovery Center, Undergraduate Research Opportunities Program (UROP), and the Biological Science Initiative (BSI), this grant provided experiential research opportunities for 24 undergraduates and 20 technicians. In addition, we supported a RAHSS student, 4 REU students, three graduate students, and a postdoctoral researcher. To reach the public and disseminate research finding nearly 13 talks were given across multiple venues (e.g. Zoological Society of London, Wildlife Disease Association Meeting, California-Nevada Amphibian Population Task Force and, American Society of Parasitologist, Ecology and Evolution of Infectious Disease). In addition of the 27 peer-reviewed publications produced by this project, 12 included undergraduate or graduate student co-authors and 8 were lead-authored by a student.

Last Modified: 04/15/2025
Modified by: Pieter T Johnson