Intraguild predation and trophic transfer drive parasite abundance patterns in larval dragonfly communities

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Sarah Goodnight and Michael McCoy, Biology, East Carolina University, Greenville, NC

Background/Question/Methods
Intraguild predation (IGP) occurs when predators that share prey also consume one another. Parasites are important members of consumer communities and often parasitize species that engage in IGP, thus IGP can be a critical mechanism driving parasite transmission and population structure. IGP has long been considered broadly detrimental to parasite transmission in simple systems due to the elimination of viable hosts, however, IGP may in fact be beneficial for certain macroparasites like digenean trematodes which have complex life cycles and often exploit trophic transfer as a transmission mechanism.
Anisopteran (dragonfly) nymphs exhibit high rates of IGP and are parasitized by a variety of macroparasites with diverse life history strategies; thus, they are an ideal model system for examining how IGP in mixed communities affects parasite transmission and prevalence. Freshwater mesocosms (n = 36) were constructed to test the effects of IGP in dragonfly host communities on two different parasites’ prevalence and abundance (Halipegus and Haematoloechus spp. trematodes), in one mixed- and two single-parasite scenarios. These three parasite treatments were crossed with both High and Low IGP treatments, created using size-structured dragonfly cohorts (High) and non-size-structured cohorts (Low). After the experiment, all dragonflies were dissected to quantify individual parasite load as well as overall parasite abundance.
Results/Conclusions
Initial results show that High IGP treatments resulted in higher per capita parasite load as well as overall abundance for Halipegus parasites in both single- and mixed-parasite scenarios in spite of host losses, while the opposite relationship was true for Haematoloechus parasites. Halipegus infects dragonfly nymphs through predator-prey interactions while Haematoloechus has free-swimming larvae (cercariae), showing that IGP may have inverse effects on parasites that infect via trophic transfer vs. parasites that have non-trophic infection mechanisms. Here we show that host interactions like IGP will differentially affect parasites with diverse life cycle structures and transmission mechanisms, and may actually increase parasite transmission in some contexts.