Per capita birth and death rates are environmentally influenced, e.g. birth rate by maternal nutrition and death rate by abundances of predators, and it could be instructive to identify the more variable demographic rate and then try to determine the causes of its variability. Ideally, censuses of many distinguishable cohorts would allow one to use variability in the relationship between females and eggs or offspring as a measure of variability in birth rate (natality). Similarly, variability in the relative abundances of consecutive age groups of juveniles would signal variability in mortality, and also indicate at what age(s) such variability occurs; it is not necessary that the ages in which mortality is greatest are also those in which it is most variable. Planktonic copepods, as arthropods, pass through a series of recognizable ontogenetic stages—egg, six naupliar, five juvenile copepodite and adult. Except where reproduction is strictly synchronized, resulting in distinct cohorts, categorization of juveniles by stage is easier than determination of age, but variability in the durations of successive stages being compared could be confounded with variability in mortality.
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