Abstract
Pathogens entering the marine environment as pollutants exhibit a spatial signature driven by their transport mechanisms. The sea otter (Enhydra lutris), a marine animal which lives much of its life within sight of land, presents a unique opportunity to understand land–sea pathogen transmission. Using a dataset on Toxoplasma gondii prevalence across sea otter range from Alaska to California, we found that the dominant drivers of infection risk vary depending upon the spatial scale of analysis. At the population level, regions with high T. gondii prevalence had higher human population density and a greater proportion of human-dominated land uses, suggesting a strong role for population density of the felid definitive host of this parasite. This relationship persisted when a subset of data were analysed at the individual level: large-scale patterns in sea otter T. gondii infection prevalence were largely explained by individual exposure to areas of high human housing unit density, and other landscape features associated with anthropogenic land use, such as impervious surfaces and cropping land. These results contrast with the small-scale, within-region analysis, in which age, sex and prey choice accounted for most of the variation in infection risk, and terrestrial environmental features provided little variation to help in explaining observed patterns. These results underscore the importance of spatial scale in study design when quantifying both individual-level risk factors and landscape-scale variation in infection risk.
Highlights
Marine pathogen pollution involves the transport of potentially pathogenic terrestrial-based microorganisms to the ocean, either directly by flows of water or air, or indirectly by mobile intermediate or transport hosts [1]
We examined the prevalence of T. gondii infection, based on serological testing, in sea otters along the North American Pacific coast from Alaska to California, and evaluated the influence of key landscape-scale drivers in coastal watersheds within the context of three independent analyses with very different spatial scales: (i) a conventional local-scale analysis of individual, behavioural and landscape-derived risk factors over a small spatial extent within California; (ii) a coarse-grained analysis among regions, comparing regional average values of terrestrial risk factors and mean regional T. gondii prevalence; and (iii) a fine-grained analysis covering a broad study area
Anticipating large differences in sea otter T. gondii infection prevalence among regions, we hypothesized that prevalence would be associated with land use/land cover and human population density
Summary
Marine pathogen pollution involves the transport of potentially pathogenic terrestrial-based microorganisms to the ocean, either directly by flows of water or air, or indirectly by mobile intermediate or transport hosts [1]. The sea otter as a host species, and this study system more generally, have several features that lend themselves to understanding marine pathogen pollution. Especially females, exhibit marked site fidelity [6], and so the nature of their habitat and exposures can be predicted with some accuracy based on capture locations. These animals live their whole lives near shore and bring all prey items to the surface to process, which facilitates accurate observation of diet and habitat use [6]. Since the sea otter lives permanently outside of its thermoneutral zone [8] it consumes large volumes of generally sessile invertebrate prey, effectively contacting any pathogens contained in that prey in the process
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