Roles of competition and predation in the North American invasion of the Asian bush mosquito aedes japonicus

Background/Question/Methods

The invasion of a nonindigenous species can be facilitated or impeded by biological interactions such as competition and predation.  Keystone predation occurs when the preferential predation suppresses competitive interactions between two populations, allowing the coexistence of a competitively inferior organism.  The invasive mosquito Aedes japonicus has established and spread into the range of Aedes albopictus, the dominant resident mosquito.  Aedes albopictus is thought to be competitively superior to A. japonicus because of its active foraging behavior, but this behavior may also put it at greater risk of predation.  We hypothesized that keystone predation in the larval phase by indigenous mosquito Toxorhynchites rutilus is allowing the successful invasion of Aedes japonicus.  To test this, we established a microcosm experiment in which populations of A. japonicus and A. albopictus competed in the presence or absence of T. rutilus.  Microcosms consisted of five density combinations of A. japonicus : A. albopictus (150:0, 300:0, 150;150, 0:300, and 0:150) to create conditions of low and high per capita resource availability. There were three replicates for each density combination without T. rutilus and three replicates for each density combination with T. rutilus.  Development time and survival to adulthood were measured for each population. 

Results/Conclusions

In non-predator treatments for both species, survival to adulthood decreased in high density microcosms when compared to the low-density microcosms, which indicates high levels of competitive stress at low per-capita resources.  Aedes japonicus had a lower survival than A. albopictus in all cases.  Aedes albopictus negatively impacted development time and survival for A. japonicus in interspecific microcosms, indicating competitive superiority.   For both A. japonicus and A. albopictus, the addition of a predator significantly decreased survival to adulthood.  Predation in interspecific conditions negatively impacted the survivorship of A. japonicus more than interspecific conditions without a predator, which did not support the hypothesis of keystone predation as a mechanism for coexistence.  However, both species suffered less proportionate predation when in interspecific microcosms than intraspecific, indicating that the presence of more than one species decreases the efficiency of predation.  Our hypothesis of preferential predation of A. albopictus is not supported, but our results show that more than one species of prey may negatively affect the predation success of T. rutilus.