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

Thomas G. Whitham
The American Naturalist
Vol. 115, No. 4 (Apr., 1980), pp. 449-466
Stable URL: http://www.jstor.org/stable/2460478
Page Count: 18
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The Theory of Habitat Selection: Examined and Extended Using Pemphigus Aphids
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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. (4) 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. (5) Identical results from five 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. (6) Theory of habitat selection has been restricted to animals exhibiting territorial and nonterritorial behavior in homogeneous habitats. This study empirically extends theory to consider the more general condition of heterogeneous habitats. (7) Assumptions of habitat homogeneity should not be made without verification because predictions of expected fitness in heterogeneous and homogeneous habitats can be identical for very different animal behaviors.

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