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Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition

Thomas G. Whitham
Ecology
Vol. 59, No. 6 (Autumn, 1978), pp. 1164-1176
DOI: 10.2307/1938230
Stable URL: http://www.jstor.org/stable/1938230
Page Count: 13
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Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition
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Abstract

The leaf galling aphid, Pemphigus betae can be highly resource and habitat limited and has evolved to reduce these limitations. Over a 3-day period in spring as much as 83% of the overwintering population migrates to immature leaves of Populus angustifolia where individuals are rapidly entombed by expanding leaf tissue. The size of the mature leaf is critical to ultimate fitness of the individual colonizing stem mother. Probability of stem mother failure, body weight of stem mother and progeny, number of progeny, development rate of progeny to maturity, and number of embryos in mature progeny are all correlated with leaf size. Due to these selection pressures, stem mothers search out and colonize large leaves. Thirty-two percent of the tree's leaves were so small as to result in an 80% probability of total aphid failure. Nearly all of these leaves were avoided. In comparison, only 1.6% of the tree's leaves were so large as to result in a 0% probability of failure, and 100% of these leaves were colonized. Even though most stem mothers were forced to colonize. Even though most stem mothers were forced to colonize suboptimal leaves because competitor density (35/100 leaves) was much > the availability of optimal leaves, the average stem mother colonized a leaf 60% > the mean leaf size of the tree. Resulting from this degree of selectivity, the minimum increase in fitness was 2 times > expected if leaves had been selected at random. At low competitor densities reproductive output would still be limited by energy intake such that if there were only 1 gall on a tree, the colonizing stem mother and her progeny would still be resource limited. Apparently, no leaf produced by the tree is so large that a further increase in leaf size would not result in a further increase in fitness. Thus, even at low competitor densities, resources would still be important in the habitat selection process and in determining the parasite distribution.

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