Group-size effects on virus prevalence depend on the presence of an invasive species.
The extent to which host group size affects the hosts' exposure to parasites and pathogens has been explored by behavioral ecologists for almost 50 years, and we know that host and parasite taxa, mobility of host and parasite, and the extent of spatial structure within groups all affect the group-size relationship. Here we examine how the prevalence of an arthropod-borne viral pathogen changes with host group size in a host-parasite system recently invaded by an introduced species that also serves as a host for the virus. Infection by the alphavirus, Buggy Creek virus (BCRV), in swallow bugs (Cimex vicarius) increased with colony size of the cliff swallow (Petrochelidon pyrrhonota; the bugs' principal host) in the absence of invasive house sparrows (Passer domesticus) but decreased with swallow colony size in the presence of sparrows. The sparrow-adapted lineage A of BCRV declined to near extinction in the largest cliff swallow colonies, regardless of sparrow presence. The results may reflect BCRV's divergence into a lineage (B) that amplifies mostly in bugs and thus is transmitted more effectively in large cliff swallow colonies that maintain high numbers of the blood-feeding bugs, whereas the other lineage (A) is adapted to house sparrows and does not require large numbers of bugs and cliff swallows for effective transmission and persistence. The results show that an alternative host can modify the group-size consequences for the original host and illustrate another complexity in analyzing the costs and benefits of coloniality whenever invasive species are present.