Host density drives macroparasite abundance across populations of a critically endangered megaherbivore
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What determines the abundance of parasites is a central question within epidemiology. Epidemiological models predict that density-dependent transmission has a principal influence on parasite abundance. However, this mechanism is seldom tested in macroparasites, perhaps because multiple, comparable populations of the same host-parasite relationship are rare. We test the influence of a range of factors on parasite abundance across 18 populations of black rhinoceros (Diceros bicornis) in South Africa. Here we show that host density strongly predicts parasite abundance at the population level for both directly and indirectly transmitted parasites. All other models were not supported. The surprising influence of a single key factor, host density, within a complex ecological system demonstrates the validity of simple epidemiological models. Establishing this previously assumed relationship between host density and parasite abundance has major implications for disease control and parasite ecology. For instance, it is central to the idea of population density thresholds for parasitism, below which a parasite would become extinct. Density-dependent transmission is also essential for calculations of the basic reproductive number, and the hypothesis that parasites may regulate host population size.
KeywordsAnoplocephala gigantea Strongyle nematodes Disease transmission Epidemiology Diceros bicornis
Special thanks go to Eastern Cape Parks and Tourism Agency and associated staff, Dr. Dean Peinke, Ezemvelo KwaZulu-Natal (KZN) Wildlife and associated staff, Chris Kelly, Nelson Mandela Metropolitan University, Prof Graham Kerley, Dr. Shirley Parker-Nance, Victoria University of Wellington, Prof. Graham Le Gros, Dr. Roan Plotz, Dr. Rosalyn Anderson-Lederer, the World Wide Fund for Nature, Dr. Jacques Flamand, Dr. James Martin, Megan Towers, the South African Weather Service, and the owners, staff and black rhino monitors from numerous private reserves for their assistance. This project was made possible by funds from the US Fish and Wildlife Service (grant numbers 98210-6-G102, 98210-4-G920), and a Victoria University of Wellington Doctoral Assistantship.
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