Can more parasites be good for you?
Most contemporary emerging disease threats involve multiple host species, and almost all hosts are infected with more than one parasite species. Nonetheless, much of disease ecology has historically focused on interactions between a single host and parasite species, despite growing evidence that changes in host and pathogen community structure can sharply influence host pathology, parasite evolution, and the efficacy of proposed management strategies.
We are interested in applying methods from community ecology to study the process of disease in complex assemblages of hosts and parasites. Unfortunately, the types of data necessary to address such questions – which require information on multiple host and parasite species from a range of spatial and temporal scales – are often lacking. Our efforts in this area have thus involved using comparatively simple host-parasite networks and extending generated insights to more challenging systems of relevance to public health or conservation. Over the last several years, we have become particularly intrigued by three, inter-related dimensions at the interface between community ecology and disease. First, we are combining cross-sectional surveys, experiments, and immunology to understand the full range of interactions among parasites in a community, which may include dozens or more different species. Although parasites frequently co-occur with individual hosts or host populations, how and when their interactions are important enough to affect patterns of transmission or pathology remains difficult to determine. Our work explicitly explores how the outcome of such interactions depends on the type of parasites involved (e.g., viruses, fungi, and helminths) or host condition, as well as identifying the consequences of parasite diversity for disease risk.
Second, the composition and diversity of the free-living community is also likely to have important ramifications for infection and disease. While many parasites infect multiple host species, those species often vary in their capacity to support and transmit infection, emphasizing the potential influence of host diversity on disease dynamics. Moreover, other members of the community, such as predators, can alter parasite spread by consuming infected hosts, consuming parasites before they infect hosts, or altering the behavior of hosts in ways that make them more or less likely to become infected. Although there is evidence that such effects can be significant for particular host-parasite systems, it is much less apparent whether these mechanisms are widespread or predictable. How can we use tools from community ecology to better forecast the frequency and influence of such community-based effects across a range of disease systems?
Finally, one of the most foundational concepts in community ecology is the importance of scale. With respect to parasites and disease, research is often focused at a specific scale, such as within hosts, between hosts, or among species, with relatively few studies traversing across scale-specific boundaries. Nonetheless, effects at one scale have enormous potential to alter patterns of infection both up and down the disease hierarchy, underscoring the crucial importance of understanding cross-scale effects for applied issues such as disease management. Our efforts here aim to integrate immunological, ecological, and macroecological tools to explore how patterns of infection within hosts ‘scale-up’ to alter transmission, population dynamics, and metacommunity structure.
Johnson, P. T. J., De Roode, J. C. and A. Fenton (2015). Why infectious disease research needs community ecology. Science 349: 1259504. PDF
Johnson, P. T. J., Ostfeld, R. S. and F. Keesing (2015). Frontiers in research on biodiversity and disease. Ecology Letters 18: 1119-1133. PDF
Johnson, P. T. J. and J. T. Hoverman (2014). Heterogeneous hosts: how variation in host size, behaviour and immunity affects parasite aggregation. Journal of Animal Ecology 83: 1103-1112. PDF
Hoverman, J. T., Hoye, B. and P. T. J. Johnson (2013). Does timing matter? How priority effects influence the outcome of parasite interactions within hosts. Oecologia 173: 1471-1480. PDF
Johnson, P. T. J., Preston, D. L., Hoverman, J. T., and B. E. LaFonte (2013). Host and parasite diversity jointly control disease risk in complex communities. Proceedings of the National Academy of Sciences 110: 16916-16921. PDF
Joseph, M. B., Mihaljevic, J. R., Arellano, A. L., Kueneman, J. G., Preston, D. L., Cross, P. C. and P. T. J. Johnson (2013). Taming wildlife disease: bridging the gap between science and management. Journal of Applied Ecology 50: 702-712. PDF
Hoverman, J. T., Mihaljevic, J. R., Richgels, K. L. D., Kerby, J. L. and P. T. J. Johnson (2012). Widespread co-occurrence of virulent pathogens within California amphibian communities. EcoHealth 9: 288-292. PDF
Johnson, P. T. J. and J. T. Hoverman (2012). Parasite diversity and coinfection determine pathogen infection success and host fitness. Proceedings of the National Academy of Sciences 109: 9006-9011. PDF
Romansic, J. M., Johnson, P. T. J., Searle, C. L., Johnson, J. E., Tunstall, T., Han, B. A., Rohr, J. R. and A. R. Blaustein (2011). Individual and combined effects of multiple pathogens on Pacific treefrogs. Oecologia 166: 1029-1041. PDF
Johnson, P. T. J. and I. D. Buller (2011). Parasite competition hidden by correlated coinfections: using surveys and experiments to understand parasite interactions. Ecology 92: 535-541. PDF