Biodiversity and the Ecology of Emerging Infectious Diseases
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The question of how biodiversity influences the emergence of infectious diseases is the subject of ongoing research. A set of nonlinear differential equations is been used to explore the interactions between ecology and epidemiology. The model allows for frequency-dependent transmission of infection within host species, and density-dependent transmission between species, via the environment or a vector. Three examples are discussed. It is shown that removing a pathogen may increase a consumer population, decreasing its resource. It is then shown that the presence of a pathogen could enable a predator and a prey species to coexist. Finally the dilution effect, by which increasing biodiversity reduces the transmission of an infectious disease, is investigated.
KeywordsBiodiversity Ecology Epidemiology Infectious diseases
Emerging infection diseases present a major threat to world health. On average, two new species of human virus are reported each year , most having an animal origin [2, 3, 4, 5]. Recent examples are SARS , swine flu  and avian influenza [8, 9]. In 2014, Ebola virus re-emerged from a bat reservoir [10, 11], causing a major epidemic [12, 13, 14, 15]. Climate change could lead to Aedes mosquitoes establishing in New Zealand [16, 17], and with them dengue and Zika viruses .
Large complex ecosystems interacting at random are almost certain to be unstable . Adding structure to the community matrix produces a different picture [20, 21, 22, 23], competitive interactions are stabilising, whereas mutualism is destabilising . A major component of an ecosystem is the food web: the network of feeding interactions among species. Adding pathogens increases the web’s complexity [24, 25, 26, 27, 28], and parasites have been described as the dominant or missing links [29, 30]. An infection may make prey easier to catch, or unpalatable to a predator, or reduce a predator’s hunting ability, but the overall influence of pathogens on an ecosystem may be unexpected . The influence of ecosystem dynamics on epidemiology can also be unexpected  and may lead to a pathogen jumping host species causing a pandemic.
We present a model that describes how an infectious disease can modify the dynamics of host and non-host species, and how changes in ecosystem dynamics can modify the epidemiology of a pathogen. We illustrate our model with three examples. In the first, eliminating a pathogen led to an increase in biodiversity, whereas in the second the presence of a pathogen is necessary to maintain a prey–predator relationship. The third example directly addresses the dilution effect—how a change in biodiversity may result in a change in the dynamics of an infectious disease.
2 The Model
A model described by a set of nonlinear differential equations has been used to explore the interactions between ecology and epidemiology. Three examples have been discussed. In the first, it was shown that removing a pathogen increased a consumer population and decreased the resource. The second example showed that the presence of a pathogen could enable a predator and prey species to coexist. Finally, the complex issue of the dilution effect was addressed. The question of how biodiversity influences the emergence of infectious diseases is the subject of ongoing research.
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