Population growth depends not only on how many individuals exist but also on when individuals survive and reproduce.

A life table records survival and reproduction across age or stage classes. For each age $x$, ecologists often track:

• $l_x$: the fraction of the original cohort still alive at age $x$;
• $m_x$: the average number of offspring produced per individual at age $x$.

A cohort is a group of individuals born during the same interval and followed through time.

Example

Suppose 100 individuals are born. At age 1, 80 remain; at age 2, 50 remain; at age 3, 10 remain. Then survivorship is

$$l_0=1.00,\qquad l_1=0.80,\qquad l_2=0.50,\qquad l_3=0.10.$$

If reproduction occurs mostly at age 2, survival to age 2 has a strong effect on future population growth.

A survivorship curve summarizes how mortality is distributed across the life span. Some populations show low mortality until old age, others have roughly constant mortality, and others lose many young individuals while a small fraction survive much longer.

These curve shapes are descriptions, not rigid categories that every species fits perfectly.

Life tables matter because two populations with the same current size can have very different futures if one contains many individuals approaching reproductive ages while the other contains mostly individuals with low future reproduction. Demographic structure therefore links individual life histories to population dynamics.