Population dynamics of a vertically transmitted, parasitic sex ratio distorter and its amphipod host

We investigate the population dynamics of a vertically transmitted, parasitic sex ratio distorter ( Nosema granulosis ) and its amphipod host ( Gammarus duebeni ), using field measurements to quantify and test alternative theoretical models of the interaction. We measure parasite, host population and transmission parameters at four locations on the Isle of Cumbrae, Scotland at monthly intervals for two years. We develop a simple infinite population model and test its predictions for parasite prevalence using field measurements of the parameters. Parasite prevalence is maintained at relatively low levels at all four sites (20–42%), consistently below that predicted. The parasite imposes a slight fitness cost on its host by reducing female fecundity, but this cost cannot account for the relatively low prevalences observed. We also investigate the importance of population structure, comparing parasite prevalence across sites and sampling intervals to look for evidence of spatial and temporal asynchrony as predicted by metapopulation models. We find significant temporal and spatial heterogeneity in parasite prevalence although there was also evidence that parasite dynamics were synchronous across sites. These data suggest that the parasite is unlikely to drive local population dynamics through cycles of extinction and recolonization at the scale measured. As the host (adult) population sex ratio was male‐biased, local population crashes are unlikely to be induced by the parasite, contrary to theoretical predictions.