Testing the enemy release hypothesis in a damselfly with an expanding range

Background/Question/Methods

The enemy release hypothesis (ERH) states that an invasive species will be less impacted by natural enemies in a new habitat. ERH is rarely tested on native species that are expanding or moving their ranges. As ranges change more rapidly because of climate change, understanding whether species that are moving into new regions leave their enemies behind could have implications on species interactions and ecosystem function.  The objective of this study was to test ERH using a damselfly (Enallagma clausum) that is expanding its distribution and its interactions with external parasites (Arrenurus water mites) at ‘historic’ and newly established sites. A secondary objective was to compare parasitism levels between E. clausum and a closely related species (Enallagma boreale) that has been well established in all sites. I collected damselflies at three ‘historic’ sites, where E. clausum has been present for at least 100 years and three ‘new’ sites, where E. clausum was first recorded only a decade ago. Each site was visited twice in two weeks. I assessed water mite parasitism to assess differences between levels of infection (prevalence and intensity) for each species between the site types using GLMMs where site type (historic or new), site and collecting week were response variables. 

Results/Conclusions

A total of 718 E. clausum and 922 E. boreale were collected. In E. clausum, prevalence varied from 2-18% in historic sites and from 3-71% in new sites. Median intensity varied between 1-45 mites in historic sites, and from 1-30 in new sites. In E. boreale, prevalence varied from 30-59% in historic sites and from 6-57% in new sites. Median intensity varied from 7-30 mites in historic sites and from 6-25 in new sites. Enallagma boreale seems to be less parasitized in new sites than E. clausum. There is high variation in measures of parasitism for E. clausum within site types, especially among new sites, where prevalence varies from 3-71%. Host species and collecting week were significant for prevalence. Site type, host species  and collecting week were significant when considering intensity of infection. E. clausum had higher prevalence of parasitism in ‘new’ sites suggesting that it is not freed from enemies; rather E. boreale seems to be benefitting from the presence of a closely related species. Thus, ERH is not supported in this system. This may have implications for species moving or expanding their ranges, who may actually be under higher parasitism pressure in new sites.