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.
Highlights
All population dynamics influenced by biotic effects result from a combination of direct and indirect interspecific interactions (Lotka 1922, Elton 1924, Volterra 1928, Gause 1935)
In light of increasing rates of cowbird parasitism, likely resulting from inversely related host (Wood Thrush) and parasite population trends in the mid-A tlantic region of the United States, we show how within our study area, Wood Thrushes incurred greater reproductive costs related to breeding effort, had reduced clutch sizes, and had fewer fledglings per brood
In addition to these net reproductive costs to Wood Thrushes, we found negative correlations between cowbird parasitism rates and all aspects of Wood Thrush demographics, indicating that when cowbird parasitism increases, they can have negative effects on both reproductive and demographic parameters that are biologically meaningful
Summary
All population dynamics influenced by biotic effects result from a combination of direct and indirect interspecific interactions (Lotka 1922, Elton 1924, Volterra 1928, Gause 1935). Species’ population dynamics can have important influences and exhibit density-d ependent effects on the nature and patterns of interspecific interactions (Elton and Nicholson 1942, Hairston et al 1960, Burdon et al 2013). Host–parasite coevolution is a widely studied example of how complex interdependencies may arise and shape interspecific interactions (Anderson and May 1978, Price et al 1980, Morand and Poulin 1998). At large spatial scales, drivers of species-level population trends can give rise to unique regional interspecific interactions, which are in part dependent on relative population densities of locally interacting species (Hanski and Henttonen 1996, Laine and Hanski 2006)
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