They show that if there is frequency-dependent transmission, a host can be rescued from pathogen-mediated extinction by the presence of a second host with which it shares a pathogen. Sharing a pathogen among one or more alternative species may actually be limiting the spread of the pathogen in any one particular host population. Thus, an epidemic on one species may result because of loss or reduction in the abundance of species in the community, without any numerical or genetic change in the host or pathogen. Studying only the target species or the abiotic environment would not reveal the cause of the epidemic. Their results have not only important consequences for understanding the role of pathogens in species interactions but also provide a potential mechanism linking host biodiversity and ecosystem functioning.
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Volker H. W. Rudolf and Janis Antonovics, "Species coexistence and pathogens with frequency-depend ent transmission" 165:7 July 2005.