Abstract
BackgroundA generalized decline in populations of Old World avian scavengers is occurring on a global scale. The main cause of the observed crisis in continental populations of these birds should be looked for in the interaction between two factors - changes in livestock management, including the increased use of pharmaceutical products, and disease. Insular vertebrates seem to be especially susceptible to diseases induced by the arrival of exotic pathogens, a process often favored by human activities, and sedentary and highly dense insular scavengers populations may be thus especially exposed to infection by such pathogens. Here, we compare pathogen prevalence and immune response in insular and continental populations of the globally endangered Egyptian vulture under similar livestock management scenarios, but with different ecological and evolutionary perspectives.Methods/Principal FindingsAdult, immature, and fledgling vultures from the Canary Islands and the Iberian Peninsula were sampled to determine a) the prevalence of seven pathogen taxa and b) their immunocompetence, as measured by monitoring techniques (white blood cells counts and immunoglobulins). In the Canarian population, pathogen prevalence was higher and, in addition, an association among pathogens was apparent, contrary to the situation detected in continental populations. Despite that, insular fledglings showed lower leukocyte profiles than continental birds and Canarian fledglings infected by Chlamydophila psittaci showed poorer cellular immune response.Conclusions/SignificanceA combination of environmental and ecological factors may contribute to explain the high susceptibility to infection found in insular vultures. The scenario described here may be similar in other insular systems where populations of carrion-eaters are in strong decline and are seriously threatened. Higher susceptibility to infection may be a further factor contributing decisively to the extinction of island scavengers in the present context of global change and increasing numbers of emerging infectious diseases.
Highlights
Recent research has revealed the worrying conservation status of Palearctic avian scavenger populations [1,2,3]
Carcass consumption is lower in insular Egyptian vultures (8% of prey items correspond to intensively raised livestock vs. 25.8% in the Ebro Valley [5])
On Fuerteventura Egyptian vultures come into contact with each other continuously throughout the year, at the ‘‘vulture restaurant’’, and at other feeding points and communal roosts [9]
Summary
Recent research has revealed the worrying conservation status of Palearctic avian scavenger populations [1,2,3]. It has been suggested that insular populations have naturally impoverished pathogen communities [17,18] and diminished immunocompetence, probably as a result of low exposition and reduced selection for parasite resistance during their evolutionary history [19,20,21] Population constraints such as isolation, sedentary habits, high density and a reduction in genetic diversity make insular organisms especially susceptible to infection [22,23,24,25]. Taking advantage of parallel long-term studies in two areas of continental Spain and the Canary archipelago [32,33], we compare here the vulture’s vulnerability to pathogen infection and the impact pathogens have on individual health in two populations under two scenarios (insular vs mainland) with socio-economic similarities (increasing intensification of livestock managing), but differing in ecological and biogeographical constraints (isolation, higher density, sedentarism and lower genetic variability of the insular population)
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