Niche-width expansion by Anopheles gambiae (Diptera: Culicidae) in western Kenya: Larvae found in novel habitats generated by human disturbance

When released from interspecific competition, intraspecific competition drives niche-width expansion. Outbreaks of Anopheles gambiae mosquitoes in Africa suggest that populations of this species have escaped regulation by predators, setting the stage for intraspecific competition in the nutrient-poor aquatic habitats of their dense larval populations. To determine the sources of malaria outbreaks in western Kenya, several surveys of aquatic habitats were undertaken. Analysis of habitat use by Anopheles mosquitoes in western Kenya revealed high densities of An. gambiae s.s. in novel habitats: brick-making pits excavated from highland bogs and natural treehole cavities in exotic ornamental trees. Follow-up studies demonstrated previously undescribed adaptations in An. gambiae: preferential colonization of disturbed ground pools shown to be associated with low predator diversity, and desiccation-resistant eggs deposited into treeholes lacking standing water. These adaptations highlight the need to maintain vigilance in monitoring disease vectors in human-disturbed areas, where intraspecific competition may facilitate the evolution of traits which permit the use of novel habitats.