Abstract
BackgroundPathogens and parasites can have major impacts on host population dynamics, both through direct mortality and via indirect effects. Both types of effect may be stronger in species whose populations are already under pressure. We investigated the potential for blood parasites to impact upon their hosts at the immunological, physiological and population level during the non-breeding season using a declining population of yellowhammers Emberiza citrinella as a model.ResultsYellowhammers infected by Haemoproteus spp. showed both a reduced heterophil to lymphocyte (H:L) ratio, and an elevated standardised white blood cell (WBC) count compared to uninfected birds, indicating an immunological response to infection. Infected birds had shorter wings during the first winter of sampling but not during the second, colder, winter; survival analysis of 321 birds sampled across four winters indicated that increased wing length conferred a survival advantage.ConclusionsWe suggest that the potential impacts of blood parasite infections on over-wintering birds may have been underestimated. Further research should consider the potential impacts of sub-clinical parasite infections on the dynamics of vulnerable populations, and we suggest using declining populations as model systems within which to investigate these relationships as well as examining interactions between sub-clinical disease and other environmental stressors.JEL CodeQ5
Highlights
Pathogens and parasites can have major impacts on host population dynamics, both through direct mortality and via indirect effects
Of the 225 samples from 203 birds that were screened for the presence of haematozoa, 47% of samples were positive for Haemoproteus infection
We found two strains of Haemoproteus in our population [41], but as we only sequenced a subset of positive samples, we grouped these two strains together for further analysis
Summary
Pathogens and parasites can have major impacts on host population dynamics, both through direct mortality and via indirect effects. Pathogens and parasites can have major direct impacts on host population dynamics through increases in mortality and morbidity [1,2], sometimes leading to host extinction [3,4,5] They can have more subtle effects on host physiology [6,7], behaviour [8] and ecology [9], which can affect host survival and population dynamics [6]. The impacts of pathogens on their hosts are dependent upon the ecology, behaviour and life history of both host [10] and parasite [11] species Environmental factors such as variation in ambient temperature can increase or decrease host condition and susceptibility, as well as parasite behaviour and virulence, leading to a critical influence on the outcome of host-parasite interactions, even over relatively small temperature ranges [12]. Declining populations may be at greater risk than stable populations from the sub-lethal effects of pathogens and parasites
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