An empirical test of the aggregation model of coexistence and consequences for coexistence of container-dwelling Aedes mosquitoes

Background/Question/Methods

Coexistence of similar competing species in a single community presents a challenge to ecological theory and understanding the mechanisms permitting coexistence of these competitors is an important goal of ecologists. Multiple species of developing mosquito larvae often coexist in container communities and frequently experience intense intra- and interspecific competition resulting from limited resources. Aggregation of competitors across habitat units, such as containers, may contribute to coexistence by allowing some fraction of the population of the poorer competitor to escape the competitive effects of the superior competitor.

I investigated the potential impact of aggregation on coexistence between the competing mosquitoes Aedes albopictus and A. aegypti. The invasive A. albopictus has displaced A. aegypti in many areas in southern Florida although in certain locations the species coexist. I sampled larval distributions in 6 cemeteries in Florida to determine if these species are aggregated in nature. I then conducted a laboratory study investigating the consequences of aggregation of the superior competitor A. albopictus on performance of A. aegypti. Four levels of A. albopictus aggregation, based on field observations and ranging from zero to high aggregation, were created across replicate cohorts of 80 A. aegypti larvae uniformly distributed across 8 containers.  

Results/Conclusions

Aedes mosquitoes showed greater intraspecific aggregation than interspecific aggregation in nature, consistent with the hypothesis that intraspecific aggregation of the superior competitor A. albopictus may facilitate coexistence with A. aegypti. In the laboratory, performance of A. aegypti cohorts uniformly distributed across identical containers was influenced by the degree of A. albopictus aggregation. Mass of emergent A. aegypti females, which is positively correlated with fitness, was lowest when A. albopictus was uniformly distributed and increased steadily as A. albopictus became more strongly aggregated. The difference in mass between uniform and high A. albopictus aggregation was significant (p=0.0028).

These results suggest population performance of A. aegypti increases as intraspecific aggregation of competing A. albopictus increases. Values of A. albopictus aggregation used in the laboratory experiment spanned the range of values observed in the field, suggesting that the aggregation hypothesis may account for coexistence of these competing mosquitoes in nature. This laboratory experiment addresses an important aspect of aggregation theory that has not been well tested. Theory predicts intraspecific aggregation of a superior competitor facilitates coexistence with an inferior competitor through increased availability of spatial refuges. My results support this general conclusion for container-dwelling mosquito competitors.