Background/Question/Methods: Our understanding of the ecology and biogeography of microbes, including those that cause human disease, lags behind that for larger species. Despite intense recent focus on the geographic distribution of bacteria, in places as varied as oceans and the human gut, the distribution of human pathogens on Earth remains incompletely understood. Islands have long inspired basic ecological insight, but we do not know if the micro-organisms that cause human disease in modern times follow patterns common to insular plants and animals. We sought to answer the question: Does the distribution of human parasites and pathogens occurring on different islands conform to the general predictions of the equilibrium theory of island biogeography (MacArthur and Wilson 1967), specifically that pathogen richness is a positive function of island size and a negative function of distance to the nearest mainland. Our methods included testing these predictions against the spatial distribution of nearly 300 human parasites and pathogens across 66 island nations and territories extracted from the Global Infectious Disease and Epidemiology Online Network Database (GIDEON). We also tested two regional subsets, one for Caribbean islands (n = 25) and one for Pacific islands (n=21). We used univariate linear regressions (ANCOVA generalized to allow unequal slopes) to investigate relationships between species richness and both island surface area and distance to the nearest mainland. The analysis was conducted on the whole set of island pathogen species and then on subsets by (i) host-requirement (human-only, zoonotic with human being a dead-end, zoonotic with multi-hosts), (ii) transmission mode (vector-borne, directly transmitted) and (iii) taxonomy (bacteria, virus, fungus, protozoans, helminths).
Results/Conclusions: Results support the theory’s basic predictions: pathogen species richness increases with island surface area and decreases with distance to the nearest mainland. However, differences in the effects of area, distance, and pathogen biogeography suggest that globalization, notably through travel and the animal trade, has softened these relationships. The effect of area (y = 0.017x + 2.022, R²adj=0.423, p<0.0001) is greater than that of distance (y = - 6.39.10-6 x + 2.087, R²adj=0.0766, p=0.014), and the strongest relationships are for pathogens that are zoonotic, vector-borne, protozoan, or helminth. These findings support the spirit of theoretical insight as much as the substance. In effect, parasites and pathogens that primarily target non-human species, whose distributions are more constrained by island life than are those of human hosts, drive the island biogeography of human disease.
