Though dilution has been observed in multiple systems, there remains a need for greater understanding of the mechanisms that influence this interaction of diversity and parasite abundance. To gain this understanding, we used the mosquito host Aedes triseriatus and species-specific parasite Ascogregarina barretti as a native host-parasite pairing, with the invasive host competitor Aedes albopictus added as a potential dilution host. The addition of A. albopictus represents a doubling of diversity within the experimental host community, with the potential for dilution by encounter reduction and host-host competition.
The native host (A. triseriatus) was exposed to one of 8 combinations of: parasite exposure (+/- A. barretti), early competition (+/- A. albopictus in early larval development), and late competition (+/- A. albopictus in late larval development). Infection prevalence and infectious burden in the native host were assayed by both larval and adult dissection. Survivorship to adulthood, development time, and adult size were recorded as measures of native host fitness.
Results/Conclusions
Survival of parasitized A. triseriatus was increased by the presence of A. albopictus early in development. Native host development time was increased by competition from A. albopictus, indicating a negative effect of the invasive host. Assays of larval infection prevalence and infectious burden (assayed as measures of dilution by encounter reduction) indicated no significant difference due to either stage of competing A. albopictus. Adult infection prevalence and burden (assayed as measures of dilution by host-host competition) were significantly increased by the presence of A. albopictus, indicating that parasite success may be increased by interspecific competition impacting the host, with increased development time of the native host being a potential causal factor.
Overall, our results support the general hypothesis that the presence of the invading host alters the native host/parasite relationship. Though we found weak evidence for a dilution effect, it appears that host competition with the invader is driving changes in this interaction in a way that suggests a synergism between the invading host and parasite that results in increased parasite success.