Why are some epidemics devastating to host populations, while others have only small effects? This question is particularly timely given that epidemics of infectious disease in wildlife populations have increased worldwide. In this project, Duffy and Hall will study links between the severity of fungal parasite epidemics in zooplankton host populations and interactions between evolutionary and ecological processes. One key factor that may shape disease severity is rapid evolution of disease resistance, fueled by genetic variation within host populations. If true, genetic variation in resistance could potentially dictate the severity of epidemics. If genetically diverse host populations can better withstand disease, results from this project might then suggest worrisome, causal links between worldwide increase in disease and the recent, widespread loss of biodiversity. A second key factor that may shape disease severity is predation. In particular, Duffy and Hall will consider the role of predators which selectively prey on infected hosts (a common occurrence in nature). They are particularly interested in whether predation and genetic variation in resistance interact to determine epidemic severity. This project will use a three-pronged approach to tackle these questions. It will combine intensive studies of natural epidemics in lake plankton, laboratory experiments manipulating host diversity and predation, and development of epidemiological models that incorporate both rapid evolution and selective predation. The processes examined and the modeling work naturally extend to many other disease systems. Therefore, this work will ultimately produce theoretical guidance for efforts seeking to understand and control the severity of wildlife disease.
Why are some epidemics large, but others small? Why do some have dramatic effects on the host population, while others have small effects? In this project, we demonstrated that the ecological context in which host-parasite interactions are embedded determine the severity of the epidemic, and how hosts evolve in response to parasites. We showed that predators, primary producers, nutrient levels, and habitat structure can all exert strong influences on the size of epidemics. We also showed that the size of epidemics determines how host populations evolve in response to outbreaks of virulent parasites. We also discovered that the parasite on which this grant focused is extremely sensitive to UV radiation, and that variation between habitats in UV penetration is strongly linked with disease; this suggests that changes in UV penetration associated with global change is likely to alter levels of disease in these ecosystems. Other scientific findings on this project include: 1) investing in defenses against predators can increase host susceptibility to infectious disease, 2) predators can fuel epidemics through trophic cascades, 3) infection reduces grazing ability of Daphnia, which has the potential to influence algal blooms, 4) host density can suppress epidemic size, and 5) the genotype in which a host spore develops can influence its future infection success; there are numerous other research findings on this project, which have either already appeared in publication or that are currently in preparation for submission. Many different people were trained on this project, including 6 graduate students, 27 undergraduates, 2 high school students (both from underrepresented minorities), 2 postdoctoral researchers, and 3 laboratory technicians. Of the undergraduate students, 7 were from groups traditionally underrepresented in the sciences and 5 were from primarily undergraduate institutions; at least 5 of these students will be coauthors on publications resulting from their work on this grant. As part of this project, Duffy worked with the Piedmont Park Conservancy in Atlanta to develop activities for campers at Piedmont Park. Duffy and her lab led hands-on activities at the park during summers. These activities were conducted in the park on the dock of Lake Clara Meer in Midtown Atlanta, allowing urban children to get hands-on experience with ecological research. Groups of children between the ages of 4 and 11 learned how to collect samples from aquatic environments and learned about the diversity of organisms that can be found in local environments. Duffy and her lab also developed activities that allowed the campers to develop and test their own hypotheses (e.g., more microbes will grow from samples collected over a longer period of time). Hands-on, experiential learning activities for middle school students have been shown to retain their interest in science at a time when many students lose interests in STEM fields. This project has also generated a rich data set on the densities of hosts, parasites, and predators in a set of 18 lakes in Indiana, as well as on a number of important physical parameters (such as nutrient levels). This data set has served or will serve as the foundation for several PhD dissertations and undergraduate honors thesis. Our water quality and plankton data are also shared with the two state agencies responsible for managing our field sites.
