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The Dynamics of an Insect Population Throughout its Altitudinal Distribution: Coleophora alticolella (Lepidoptera) in Northern England

Martin G. M. Randall
Journal of Animal Ecology
Vol. 51, No. 3 (Oct., 1982), pp. 993-1016
DOI: 10.2307/4018
Stable URL: http://www.jstor.org/stable/4018
Page Count: 24
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Dynamics of an Insect Population Throughout its Altitudinal Distribution: Coleophora alticolella (Lepidoptera) in Northern England
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Abstract

(1) The population ecology of the moth Coleophora alticolella was studied from 1977 to 1979 at a series of sites along an altitudinal transect from 15 to 610 m above sea-level in northern England. (2) The larvae feed on the seeds of the rush Juncus squarrosus; variations in the food supply were recorded along the transect and at each site between years. (3) Between-year variations in the food supply at each site were mainly a result of variations in the density of J. squarrosus inflorescences produced. Inflorescence density can be predicted from rainfall data. (4) There was no significant relationship between the number of seeds per seed capsule of J. squarrosus and altitude. However, there was a reduction in the mean number of florets per inflorescence, and also a reduction in the percentage of florets developing into seed capsules, with increasing altitude. Thus, the larval food supply was reduced at the higher sites. (5) The highest densities of Coleophora alticolella eggs and larvae, with respect to the food supply, were recorded in the middle region of the transect. (6) Survival between the egg stage and larval establishment inside the seed capsules is directly related to the proportion of florets developing into seed capsules. Mortality at this stage is almost entirely due to starvation of larvae inside unsuccessful florets. (7) The percentage of the C. alticolella larvae parasitized was reduced with increasing altitude. (8) Analysis of life-table data for two generations of C. alticolella at six different sites, shows that the main mortality factors acting on the population change systematically along the altitudinal gradient. The incidence of starvation of newly-hatched larvae increases, whilst parasitization decreases. The changing intensity of these two factors leaves a middle region where their combined effect is low. (9) The most important factor determining the population fluctuations in the middle region of the transect is density-dependent overwinter mortality and reduction in natality, as a result of the increased competition for food at the high larval densities. (10) The population at low altitudes is not limited by the food supply and appears to be held at low density by natural enemies. (11) The population at the highest sites is also at low density and is mainly affected by the harsh climate which limits seed capsule development by the food plant. Extinction of this population occurs in years when the food plant fails to set seed.

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