vol. 164, supplement the american naturalist november 2004 Pathogen Spillover in Disease Epidemics

abstract: In field experiments manipulating generalist pathogens and host community composition, the presence of a highly suscep-tible reservoir species drove disease dynamics in multiple nonres-ervoir species, sometimes decreasing their abundance through ap-parent competition. The dynamics of generalist pathogens in multispecies host communities remain a major frontier for disease ecology. Of particular interest are how host community structure controls pathogen transmission and how disease spread feeds back to influence the host community. Pathogen spillover occurs when epidemics in a host population are driven not by transmission within that population but by transmission from a reservoir population. Here we review examples of spillover in pathogens infecting humans, domesticated animals, and crops, noting that most empirical evidence for spillover results from nonmanipulative, observational studies. We then present results from two field experiments utilizing an exper-imentally tractable model system of annual wild grasses and a gen-eralist virus, the barley yellow dwarf virus. In these experiments, the presence of a highly susceptible reservoir species, Avena fatua (wild oats), greatly increased pathogen prevalence in several other species. This result demonstrates pathogen spillover and illustrates the crucial role of host community structure in controlling the dynamics of generalist pathogens. Further, pathogen spillover from A. fatua de-creased the abundance of two other host species through pathogen-mediated apparent competition. Thus, our results provide experi-mental support for theoretical predictions of strong feedbacks between host community structure and generalist disease dynamics