Abstract
Parasites are a major component of all animal populations. Males and females often differ in their levels of parasite prevalence, potentially leading to sex differences in the impact of parasitism on fitness, with important implications for the evolution of parasite and host traits including resistance, tolerance, and virulence. However, quantitative measures of the impact of parasitism under free‐living conditions are extremely rare, as they require detailed host demographic data with measures of parasite burden over time. Here, we use endoscopy for direct quantification of natural‐parasite burdens and relate these to reproductive success over 7 yr in a wild population of seabirds. Contrary to predictions, only female burdens were associated with negative impacts of parasitism on breeding success, despite males having significantly higher burdens. Female reproductive success declined by 30% across the range of natural parasite burdens. These effects persisted when accounting for interannual population differences in breeding success. Our results provide quantitative estimates of profound sub‐lethal effects of parasitism on the population. Importantly, they highlight how parasites act unpredictably to shape ecological and evolutionary processes in different components of the same population, with implications for demography and selection on host and parasite traits.
Highlights
Understanding the key drivers of demographic processes is crucial in predicting population growth and persistence (Agnew et al 2000)
We found parasite burden had a significant impact on the reproductive success of female free-living shags
We found no evidence of the environment mediating the effects of parasitism in either sex
Summary
Understanding the key drivers of demographic processes is crucial in predicting population growth and persistence (Agnew et al 2000). Levels of individual parasite burdens may interact synergistically with environmental conditions, with impacts of parasitism often more apparent when animals are operating under greater pressure in more marginal conditions (Laaksonen et al 2002, Tompkins et al 2011, Granroth-Wilding et al 2014) Such interactions may be important during reproduction, because parasites can affect the condition, behavior, and energy use of the host (Sheldon and Verhulst 1996, Binning et al 2012, Hicks et al 2018a), thereby limiting the resources available for breeding. We used mean population productivity as an annual proxy for environmental conditions (as in Reed et al 2008b, Granroth-Wilding et al 2014, Bogdanova et al 2014) This was calculated as the average number of fledged young per incubated nest in a series of unmanipulated, long-term monitoring plots completely independent of the birds included in the parasite study (see Newell et al 2015 for monitoring methods). The span of data collection beyond the dates of the present study allowed us to include lag mean population productivity in some analyses
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