The Theory of Habitat Selection: Examined and Extended Using Pemphigus Aphids

Abstract

The gall-producing aphid P. betae is a plant parasite which colonizes the leaves of narrowleaf cottonwood, Populus angustifolia. Habitat quality and aphid fitness can be quantified easily; those colonizers which succeed in reproduction and those which die attempting colonization can be censused. Using 5 measures of relative fitness and other data, selection pressures which drive the habitat selection process were quantified. Colonizing stem mothers have evolved the ability to discriminate and select only the best habitats for colonization. As competitor density within a habitat increases, average fitness declines. Stem mothers settle in habitats of varying quality such that as habitat quality increases the density of competitors increases. An individual leaf is a highly heterogeneous habitat. Position within the habitat has a predictable and pronounced effect on fitness. Due to position, the best habitats produce individuals with the highest and lowest fitness. Because the number of optimal positions and best habitats can be low relative to the number of competitors, and since fitness declines as density increases, stem mothers have evolved territorial behavior which acts to limit density. Identical results from 5 measures of relative fitness indicate that stem mothers adjust their densities in habitats of varying quality such that the average fitness of stem mothers in habitats with the highest density of competitors is the same as those occupying habitats with fewer competitors. Theory of habitat selection was restricted to animals exhibiting territorial and nonterritorial behavior in homogeneous habitats. Assumptions of habitat homogeneity were discussed.