Professor Princeton University Princeton, New Jersey, United States
Abstract: The emergence of a novel pathogen can cause rapid population declines in immunologically naive host species. But the ecological consequences of an outbreak may not be limited to the primary host. Cascading effects on the rest of the community are mediated by networks of species interactions, and for most systems we lack empirical evidence about how pathogens could alter the structure of ecological communities. In this study, we explore how the communities of birds that use feeders as a major food source are changed following the emergence of a novel pathogen, Mycoplasma gallisepticum(MG). We use 52 thousand observations from the community science program, Project FeederWatch, to examine these changes. The house finch (Haemorhous mexicanus), rose to dominate species abundance distributions in these communities in the Eastern United States in the 1980s. Finch abundance declined severely over a 5-year period in the 1990s as a result of the emergence of Mycoplasma infections, and it has not recovered to peak abundance nearly three decades after the initial outbreak, with MG now endemic in the population. We quantify the impacts of the decline of house finches on the rest of the bird community at multiple locations across the United States. We provide evidence that the species with the strongest competitive interactions with house finches experienced the largest increases in abundance when house finches declined, indicating a pathogen-driven competitive release. We also explore other changes to the structure of these bird communities, including West Nile virus and the decline of introduced species, and we demonstrate that the communities are highly dynamic. Further, our analysis provides new insights into why MG caused less damage in the native Western range of house finch than in its introduced Eastern range. Our study explicitly demonstrates that an emerging pathogen can suppress the abundance of a dominant primary host, leading to increases in the abundance of competitors and changing the structure of an entire bird community, and how such changes to these massively observed avian communities diffuse in space and time.
