Award: OCE-1420273

Intellectual Merit Changes in host density can alter disease dynamics. Epidemiological theory predicts that contact rate between hosts and parasites should increase with host density, thereby increasing parasite transmission and per capita infection risk. At the same time, for a given number of infective stages in the environment, high host density can decrease per capita infection risk because infective stages are divided (diluted) among available hosts. Risk dilution, or safety in numbers, occurs when infective stages move far enough to decouple local production from local transmission, which is common among environmentally-transmitted parasites. As an NSF postdoctoral fellow at UC Santa Barbara, I tested these predictions using field experiments and observational studies on the marine snail, Cerithidea californica, and its trematode parasites in salt marshes. I found that infective stage input and host density increased parasite recruitment (evidence for recruitment limitation and bottom-up control respectively), whereas intraguild predation reduced parasite recruitment (evidence for top-down control). Furthermore, although host density increased parasite recruitment, it simultaneously decreased per capita infection risk in both the field experiment and the field survey. In collaboration with Dr. Kevin Lafferty, I constructed a mathematical model that shows that this counterintuitive result is due to scale-dependence. Adding a second host species to the model allows us to examine how scale alters the relationship between biodiversity and disease risk (dilution/amplification effects), a controversial topic in disease ecology. Our model reconciles empirical studies, which usually occur at local scales, with theoretical work, which typically occurs at global scales. Broader Impacts The supported research activities contributed to our understanding of the role of infectious agents in ecosystems and the forces controlling their populations – topics with important implications for conservation and human health. Additionally, the project supported several activities that broadened the participation of underrepresented groups and the public in ocean sciences. Specifically, (1) numerous UCSB undergraduate and graduate students gained experience in study design and execution, data management and analysis, (2) I developed and taught a course for high school students about the role of parasites in hosts, populations, communities, and ecosystems through the UCSB School for Scientific Thought, (3) I co-led two Earthwatch Ignite expeditions, a program that allows high school students to participate in scientific research at no cost to themselves, and (4) I initiated a blog to help disseminate research from the UCSB Ecological Parasitology lab to diverse audiences. Finally, the project contributed to my career development. Specifically, the fellowship allowed me to expand my work into marine systems, thereby broadening my knowledge base and further developing my scientific and technical skills. In addition to initiating my own research projects, I contributed to collaborative projects of the UCSB Ecological Parasitology research group and gained experience in a supervisory role by mentoring graduate and undergraduate students. Last Modified: 09/26/2017 Submitted by: Julia C Buck