Spatial covariation of local abundance among different parasite species: the effect of shared hosts

Within any parasite species, abundance varies spatially, reaching higher values in certain localities than in others, presumably reflecting the local availability of host resources or the local suitability of habitat characteristics for free-living stages. In the absence of strong interactions between two species of helminths with complex life cycles, we might predict that the degree to which their abundances covary spatially is determined by their common resource requirements, i.e. how many host species they share throughout their life cycles. We test this prediction using five trematode species, all with a typical three-host cycle, from multiple lake sampling sites in New Zealand's South Island: Stegodexamene anguillae, Telogaster opisthorchis, Coitocaecum parvum, Maritrema poulini, and an Apatemon sp. Pairs of species from this set of five share the same host species at either one, two, or all three life cycle stages. Our results show that when two trematode species share the same host species at all three life stages, they show positive spatial covariation in abundance (of metacercarial and adult stages) across localities. When they share hosts at two life stages, they show positive spatial covariation in abundance in some cases but not others. Finally, if two trematode species share only one host species, at a single life stage, their abundances do not covary spatially. These findings indicate that the extent of resource sharing between parasite species can drive the spatial match-mismatch between their abundances, and thus influence their coevolutionary dynamics and the degree to which host populations suffer from additive or synergistic effects of multiple infections.