Diversity Loss and Disease Emergence
How Do Changes in Biodiversity Influence Disease?
Disease emergence and biodiversity loss are two of the most pressing environmental challenges of the modern era. Each is driven predominantly by human-mediated alterations in the environment. Although changes in biodiversity and infectious diseases have often been studied separately, the importance of interactions between them has received comparatively little attention. Under what circumstances will disease epidemics drive species losses? And, reciprocally, can higher levels of biodiversity reduce parasite transmission (i.e., the dilution effect) or lead to increases in infection (i.e., the amplification effect)? Through what ecological mechanisms and at what magnitude relative to other pathways known to affect parasite spread, such as changes in host density?
We use a combination of field surveys, laboratory manipulations, and large-scale experiments to understand the links among biodiversity, community composition, and infectious disease risk. While a common approach to studying the relationship between biodiversity and disease is test for bivariate correlations between species richness and some metric of disease or infection, this simplified strategy fails to recognize the dynamic nature of most diseases. Many infections vary strongly both spatially and temporally, and the large number of response variables related to ‘disease risk’ can lead to conflicting or confusing results. We have been using trematode parasites to study these questions in more detail, including Schistosoma mansoni (the causative agent of human schistosomiasis) and Ribeiroia ondatrae (the causative agent of amphibian malformations). Results from both laboratory- and more complex mesocosm experiments indicate that higher levels of host diversity can reduce transmission of both parasites (i.e., the dilution effect), but that such patterns depend critically on the order and pattern of host community assembly. Mechanisms that mediate the relationship between diversity and infection include variation in host competence, the presence of non-competent ‘decoy’ hosts, predators that consume parasites or alter host exposure to infection, and interactions between coinfecting parasites.
Using field data from simple pond ecosystems, our findings highlight the importance of identifying potential links between the order of species assembly and a given host’s competence to support an infection. In cases in which the most widespread and abundant hosts are also the most competent, with subsequent colonizers functioning as lower competence hosts, the transmission potential of the community will decrease predictably with increased diversity (Johnson et al. 2013). This can happen, for instance, when tradeoffs between immune defenses and colonization ability occur among ‘fast’ vs. ‘slow’ species (e.g., Johnson et al. 2012). However, whether this pattern is common remains unclear, particularly given how little we know about host competence, host assembly, and transmission for many multi-host infections.
Johnson P. T. J., Wood, C. L., Joseph, M. B., Preston, D. L., Haas, S. E., and Y. P. Springer (2016). Habitat heterogeneity drives the host diversity-begets-parasite diversity relationship: evidence from experimental and field studies. Ecology Letters 19: 752-761.
Orlofske, S. A., Jadin, R. C., Hoverman, J. T. and P. T. J. Johnson (2014). Predation and disease: understanding the effects of predators at several trophic levels on pathogen transmission. Freshwater Biology 59: 1064-1075.
Orlofske, S. A., Jadin, R., Preston, D. L., and P. T. J. Johnson (2012). Parasite transmission in complex communities: predators and alternative hosts alter pathogenic infections in amphibians. Ecology 93: 1247-1253.
Johnson, P. T. J., Rohr, J. R., Hoverman, J. T., Kellermanns, E., Bowerman, J. and K. B. Lunde (2012). Living fast and dying of infection: host life history drives interspecific variation in infection and disease risk. Ecology Letters 15: 235-242.
Johnson, P. T. J., Preston, D. L., Hoverman, J. T., Henderson, J. S., Paull, S. H., Richgels, K. L. D. and Redmond, M. D. (2012). Species diversity reduces parasite infection through cross-generational effects on host abundance. Ecology 93: 56-64.
Johnson, P. T. J., Dobson, A., Lafferty, K. D., Marcogliese, D., Memmott, J., Orlofske, S., Poulin, R., and D. W. Thieltges (2010). When parasites become prey: ecological and epidemiological significance of eating parasites. Trends in Ecology & Evolution 25: 362-371.
Johnson, P. T. J., Lund, P. J., Hartson, R. B., and T. P. Yoshino (2009). Community diversity reduces Schistosoma mansoni transmission and human infection risk. Proceedings of the Royal Society of London, Series B 276: 1657-1663.
Johnson, P. T. J., Hartson, R. B., Larson, D. J. and D. R. Sutherland (2008). Linking biodiversity loss and disease emergence: amphibian community structure determines parasite transmission and pathology. Ecology Letters 11: 1017-1026.
In the Press
“Many species, one health”: a February 2015 News Feature story in PNAS focuses on the research done by the Johnson Lab on the links between changes in biodiversity and disease risk in wildlife. In particular, the feature synthesizes recent research testing the diversity-disease relationship in amphibians as a model for understanding emerging human diseases, which are often more difficult to study with the same level of replication and experimentation. Click here to see the news article.
“Ecology advancing the fight against infectious diseases”: A new paper in the journal Science by Pieter Johnson, Jaap de Roode and Andy Fenton, is advancing a multidisciplinary framework that could provide a better mechanistic understanding of emerging outbreaks. View the press release here or read the publication here.
“Biodiversity protects against disease”: in a 2013 publication in Nature, Dr. Johnson and colleagues integrated lab experiments, extensive field surveys, and large-scale field manipulations to comprehensively test the influence of biodiversity losses on pathogen transmission. Their results offer clear insights into the mechanisms through biodiversity can protect against disease and provide one of the most rigorous tests to date of the dilution effect hypothesis. Read the press release here or a story about the research by National Geographic.