Abstract
The growth or decline of a population over a period of time can be expressed in terms of the changes in biomass (ΔB) or of numbers (ΔN). The changes that occur are the result of the sum of the individuals born (R) minus those dying (M) during the interval, the growth (G) of the individuals in the population, and of the net difference between emigration (E) and immigration (I). Thus we can write for biomass
all expressed in units of biomass. The change in the number of individuals (ΔN) is expressed by
In this case the growth of each individual is disregarded. The two equations are interchangeable by simply accounting for the biomass of the individuals in the population. We will return to biomass expressions in Chapter 7, where we deal with secondary production. In this chapter we concentrate on numbers of individuals, since most of the work on growth of populations is done on this basis.
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A collection of all the individuals present at any one time, indicated by the vertical dotted line, would include all the live individuals present. This is L x , the survivorship table age structure, and is shown in the histogram at top right of Fig. 3–1.
This is the same problem presented by life insurance rates calculated on the basis of outdated human survivorship data. Since current human survival rates are nearly always higher than earlier rates, the fairer (and cheaper) rates will be those calculated for the very latest survival data. Incidentally, most of the development of life tables for ecological work was done based on actuarial techniques.
In human populations there has been too much attention given to total family size and too little to age at first reproduction. Cole (1965) shows that human populations that started reproducing at 12 years of age could attain over twice the growth rate of a population whose age at first birth was 30 years of age. The effect of total number of children was small compared to the effect of age at first reproduction. The countries where growth of population has been most successfully curtailed are those where economic and social restrictions have led to delayed reproduction until the parents were in their 30s, as well as to reductions in the number of children per family.
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© 1984 Springer Science+Business Media New York
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Valiela, I. (1984). Dynamics of Populations of Consumers. In: Marine Ecological Processes. Springer Advanced Texts in Life Sciences. Springer, New York, NY. https://doi.org/10.1007/978-1-4757-1833-1_3
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DOI: https://doi.org/10.1007/978-1-4757-1833-1_3
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