Distribution of common stickleback parasites on North Uist, Scotland, in relation to ecology and host traits.

Analysing spatial differences among macroparasite communities is an important tool in the study of host-parasite interactions. Identifying patterns can shed light on the underlying causes of heterogeneity of parasite distribution and help to better understand ecological constraints and the relative importance of host and parasite adaptations. In the present study, we aimed to find correlational evidence that the macroparasite distribution patterns on the Scottish island of North Uist, which had been described by de Roij and MacColl (2012), are indicative of local processes rather than an unspecific influence of habitat characteristics. We therefore reinvestigated parasite abundances and tested for associations with habitat characteristics and host traits. Distribution patterns of the most common parasites were largely consistent with the observations of de Roij and MacColl (2012). In accordance with the published results, we found that the most obvious abiotic habitat characteristic varying among the lakes on the island, pH, did not statistically explain parasite abundances (except for eye fluke species inside the lens). Instead, we found that genetic differentiation between host populations, measured as pairwise FST values based on available microsatellite data, was significantly correlated with dissimilarity in parasite community composition. Our results indicate that individual lake characteristics rather than physicochemical variables shape parasite distribution on this island, making it an ideal place to study host-parasite interactions. Furthermore, additionally to geographic distance measures taken from maps, we suggest taking into account connectivity among freshwater habitats, indirectly measured via fish population structure, to analyse spatial distribution patterns.