Long‐term dynamics in local host–parasite interactions linked to regional population trends

Abstract

AbstractTemporal changes in the relative abundances of host–parasite populations can influence the magnitude of the effects of corresponding interspecific interactions. When parasite populations are at relatively low abundance, the negative effects on host populations may be insignificant, but when parasite abundance increases beyond critical thresholds, they can have population limiting effects on the host. Here, we used data from a 40‐yr demographic study on breeding Wood Thrushes (Hylocichla mustelina) and avian brood parasitic Brown‐headed Cowbirds (Molothrus ater) in the mid‐Atlantic United States to disentangle host–parasite interactions. The relative abundance for these two species has changed both locally and regionally over this time period with a reduction in host abundance coincident with an increase in the parasite population. We detected a fivefold increase in Brown‐headed Cowbird parasitism rates of Wood Thrushes over the 40‐yr time period leading to a reduction in Wood Thrush fitness (i.e., adult survival, fecundity, and recruitment). After accounting for the effects of Wood Thrush age, individual, and annual and within‐season variation in reproduction, we found that Wood Thrushes exhibited increased reproductive effort (produced more nests per year) as nest parasitism rates increased. Additionally, we found that as parasitism rates increased, both Wood Thrush clutch size and fecundity declined. In conjunction with widespread habitat loss and land use change on both wintering and breeding ranges, increasing rates of Brown‐headed Cowbird parasitism are reducing Wood Thrush fitness, and are likely contributing to observed regional Wood Thrush population declines. Coordinated local and regional efforts to reduce Brown‐headed Cowbird populations, particularly in fragmented landscapes, may help reduce the decline for Wood Thrushes, and likely other parasitized Neotropical migratory species.