What is Life table? — Biology

Definition 25.4 University Biology — Year 1 · Chapter 25 — Populations and Demography

A life table follows a cohort of individuals born together through their lives. For each age $x$ it records $l_x$ , the survivorship — the fraction of the cohort still alive at age $x$ — and $m_x$ , the fecundity — the mean number of daughters born to a female of age $x$ . From them: the mortality between ages, $q_x = 1 - l_{x+1}/l_x$ ; the net reproductive rate $R_0 = \sum_x l_x m_x$ , the number of daughters a newborn female will have on average in her lifetime; and the generation time $T = \sum_x x\,l_x m_x / R_0$ , the mean age of the mothers of those daughters. A population with $R_0 > 1$ grows, with $R_0 < 1$ declines; over one generation it is multiplied by $R_0$ .

Three types of survivorship curve, on a logarithmic scale. Type I dies late, type III dies early, type II dies at a constant rate; each is a life history written as a line.

Examples

Example 25.6 (A life table for a deer herd)

Of a thousand fawns, six hundred reach their first birthday, and the survivors then lose a tenth of their number each year until few pass nine; does bear from age two, about eight tenths of a daughter a year in their prime. Summing $l_x m_x$ gives $R_0 \approx 1.5$ : each doe leaves one and a half daughters, and the herd grows by half each generation of about four years — an eleven percent increase per year until something stops it.

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Examples

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