Survivorship Curve

Plotting $l_x$ from a Life Expectancy table on a logarithmic scale versus $x$ gives a curve known as a survivorship curve. There are three general classes of survivorship curves, illustrated above.

1. Type I curves are typical of populations in which most mortality occurs among the elderly (e.g., humans in developed countries).

2. Type II curves occur when mortality is not dependent on age (e.g., many species of large birds and fish). For an infinite type II population, $e_0=e_1=\ldots$, but this cannot hold for a finite population.

3. Type III curves occur when juvenile mortality is extremely high (e.g., plant and animal species producing many offspring of which few survive). In type III populations, it is often true that $e_{i+1}>e_i$ for small $i$. In other words, life expectancy increases for individuals who survive their risky juvenile period.

See also Life Expectancy