Likely range expansion of ectoparasites in an Australian damselfly under future climates

Climate change driven range expansion of parasites may increase parasite prevalence and intensity in host population and thereby contributing to the decline of host populations. Parasites impose strong evolutionary forces on their hosts and this interaction can be impacted by changes in local climate. Here we are interested in predicting parasitism in insect populations in future climates by mapping host-parasite interactions across a temperature and latitudinal gradient. We test the assumption that parasitism is more abundant in the tropics by analysing the spatial variation and effects of climatic factors on the prevalence and intensity of ectoparasitic water mites (Arrenurus sp.) and endoparasitic protozoans (Apicomplexa: gregarine) on Australian common bluetail damselflies (Ischnura heterosticta (Burmeister, 1839)). We collected damselflies across an extensive latitudinal gradient along the east coast of Australia from 16°52'00.8"S to 34°24'42.0"S, covering ~18° of latitude. We sampled 43 different populations covering three distinct climatic zones: tropics, subtropics and temperate. In support of our predictions, we found that water mite prevalence and intensity were higher in the tropics than in subtropical and temperate biomes. Gregarine parasitism in male damselflies but not in females was greater in the tropics. We found that water mite ectoparasitism (prevalence and intensity) but not gregarine endoparasitism correlated with latitude. Bioclimatic analysis suggests that annual temperature is the main driver for water mite prevalence, and precipitation is the main driver for water mite intensity whereas gregarine parasitism was unaffected by these climate variables. Based on these data, our future climate analysis suggests that ectoparasitism is likely to extend their range in this species under a warming climate. Considering the fitness cost imposed by parasites, the expansion of parasitism is likely to contribute to the decline of already vulnerable populations.