Fitness and Its Correlates Assessed by Intra- And Interspecific Interactions among Tree-Hole Mosquitoes

Abstract

(1) Larvae of Aedes geniculatus from southern England (U.K.) and A. sierrensis from western Oregon (U.S.A.) were reared alone or together at varying densities and on varying amounts of natural tree-hole substrate from each locality. (2) The major determinants of fitness (replacement rate) in both species were the linear effects of substrate level, larval density and source of substrate; each of the linear effects was greater than that of higher order interactions combined. (3) Neither species achieved higher fitness than the other at low density but Aedes geniculatus was competitively dominant to A. sierrensis at high density. (4) The interaction between the two species did not depend upon the source locality (U.S.A., U.K.) or upon holes within localities; consequently, tree holes provide replicate habitats for these two species over intercontinental distances. (5) Factors affecting fitness in single-species populations of either species persisted in interspecific encounter but some factors that have no effect on single-species populations affected fitness during interspecific encounter. (6) Individual correlates of fitness accurately reflected the major, proximal factors affecting fitness in both species but no one of them was consistently, linearly correlated with fitness. Composite, rather than component, indices of fitness therefore still provide the best comparative assessment of the effect of environmental variables on fitness both within and between species.