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Population Fluctuations in Complex Life Cycles: An Example From Pemphigus Aphids

Nancy A. Moran and Thomas G. Whitham
Ecology
Vol. 69, No. 4 (Aug., 1988), pp. 1214-1218
DOI: 10.2307/1941276
Stable URL: http://www.jstor.org/stable/1941276
Page Count: 5
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Population Fluctuations in Complex Life Cycles: An Example From Pemphigus Aphids
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Abstract

The population dynamics of organisms with complex life cycles are rarely examined due to the number of potentially interacting variables. Experimental manipulations of conditions and long-term data on abundances were used to investigate the bases of year-to-year fluctuations in population size in the aphid Pemphigus betae, which has a complex life cycle. Spring generations may live in leaf galls on cottonwood trees, whereas summer generations obligately inhabit roots of herbaceous Rumex and Chenopodium species. Experimental manipulation of irrigation levels of root colonies in both field and garden experiments showed that soil moisture affects aphid survival and reproduction on roots. The importance of this single factor in governing population size is further supported by 6 yr of gall-density and weather data showing a strong relationship between gall abundance and the amount of rainfall the previous season. Censuses of autumn migrants arriving at cottonwoods during three of these years corroborate this. Thus, weather affects success on summer hosts and consequently determines numbers of migrants returning to cottonwoods and gall abundances during the subsequent spring. This study demonstrates that conditions during one phase of a complex life cycle may largely determine the population size of future generations using alternate niches.

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