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Demographic Strategy of Vertical Migration by a Marine Copepod

Ian A. McLaren
The American Naturalist
Vol. 108, No. 959 (Jan. - Feb., 1974), pp. 91-102
Stable URL: http://www.jstor.org/stable/2459738
Page Count: 12
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Demographic Strategy of Vertical Migration by a Marine Copepod
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Abstract

Recent explanations of the adaptive values of vertical migration by zoo-plankton are difficult to express in terms of individual fitnesses. An earlier explanation of the demographic consequences of vertical migration incorporated an unnecessary and probably erroneous metabolic model. From empirical data on the effects of temperature on fecundity and development rates of the copepod Pseudocalanus minutus, the demographic effects of vertical migration in thermally stratified waters are worked out. In seasonal or otherwise interrupted life cycles, increased fecundity from part-time residence in colder waters is advantageous in near-equilibrium populations, but this is not always true where life cycles are continuous and adequate food is present. However, residence in colder waters during later developmental stages is shown to be advantageous for near-equilibrium populations if mortality rate is sufficiently higher in earlier stages. The conclusions are sufficiently general to imply that vertical migration in thermally stratified waters confers a demographic advantage to individuals of this copepod and other zooplankters in which size and fecundity are negative functions of temperature.

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