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

Abstract

The gall-producing aphid Pemphigus betae is a plant parasite which colonizes the leaves of narrowleaf cottonwood, Populus angustifolia. Habitat quality and aphid fitness can be quantified easily, and one may census not only those colonizers which succeed in reproduction but also those which die attempting colonization. Using five measures of relative fitness and other data, selection pressures which drive the habitat selection process have been quantified. (1) Colonizing stem mothers have evolved the ability to discriminate and select only the best habitats for colonization. (2) 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. (3) 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. ...