The effects of odonate species abundance and diversity on parasitism by water mites (Arrenurus spp.): Testing the dilution effect

Abstract

Water mites (Arrenurus spp.) parasitize adult dragonflies.We collected dragonflies weekly at 11 waterbodies in Greenville Co. and Pickens Co., SC, USA, to: (1) compare parasitism prevalence across species, sites, and sampling periods; (2) test the hypothesis that prevalence correlates with host abundance; (3) test the hypothesis that prevalence is inversely related to host diversity (the “dilution effect”); and (4) test the hypothesis that prevalence and intensity vary with ecological conditions. Parasitism prevalence varied among well-sampled (N > 30) hosts; Perithemis tenera, Plathemis lydia, and Celithemis ornata had no mites, whereas prevalence exceeded 20% for Argia fumipennis, Celithemis elisa, and C. fasciata. Differences among species, however, varied across sites and through time, suggesting patchy or speciesspecific relationships not captured by our diffuse analysis at the generic level. Prevalence was positively correlated with species abundance and host site occupancy, as expected for generalist parasites. There was no evidence of a dilution effect: there were no significant negative relationships between prevalence and three measures of species richness (observed richness, extrapolated Sest, or CHAO2 estimated richness), considering all odonate species, parasitized species, or only species in the parasitized families Libellulidae or Coenagrionidae. Odonate communities in more pristine sites had higher mean prevalence (18.4 ± 6.0) and median intensity (4.5) than those in disturbed sites (13.1 ± 7.0; 3.0), but only intensities were marginally significantly different. Parasitism by Arrenurus spp. met the criteria for a dilution effect, but did not exhibit this effect as a diffuse community-level response.