Life Tables and Demography

Life tables record how survival and reproduction vary with age or stage, turning the schedule of an individual's life into predictions about whether a population will grow or decline.

Definition

Life tables and demography are the description and analysis of age- or stage-specific survival and reproduction within a population, used to summarise its life history and project its growth and structure.

Scope

This topic covers the demographic accounting of populations: cohort and static life tables, age-specific survivorship and fecundity, survivorship curve types, and the calculation of net reproductive rate, generation time, and the intrinsic rate of increase. It extends to age- and stage-structured projection using Leslie and Lefkovitch matrices and the stable age distributions they predict.

Core questions

• How are survival and reproduction distributed across ages or stages?
• What do survivorship curves reveal about a species' life history?
• How are net reproductive rate, generation time, and population growth rate calculated from a life table?
• How do matrix models project age- and stage-structured population dynamics?

Key theories

Age-specific schedules and survivorship curves

Survivorship and fecundity vary systematically with age, and the resulting survivorship curves fall into characteristic types reflecting whether mortality is concentrated early, late, or spread evenly across life.

Matrix population models

Leslie and stage-classified matrices project structured populations forward in time, yielding the asymptotic growth rate, the stable age or stage distribution, and sensitivities that show which vital rates most influence growth.

Mechanisms

A life table tabulates, for each age or stage, the probability of survival and the average number of offspring produced. Multiplying age-specific survivorship by fecundity and summing yields the net reproductive rate, while the Euler-Lotka equation links these schedules to the intrinsic rate of increase. Arranging the same vital rates in a projection matrix lets one iterate population structure forward and extract its dominant eigenvalue as the long-term growth rate.

Clinical relevance

Demographic analysis supports population viability assessment for endangered species, the design of harvest and culling strategies, and the identification of life stages whose protection most improves population growth. This is educational context, not management prescription.

History

Human mortality tables date to Graunt and Halley in the seventeenth century, and Lotka linked age schedules to population growth in the early twentieth century. Leslie introduced the age-classified projection matrix in 1945, Lefkovitch generalised it to stages in 1965, and Caswell synthesised matrix demography for ecology from the 1980s.

Key figures

• Alfred Lotka
• Raymond Pearl
• Leonard Lefkovitch
• Hal Caswell

Seminal works

• gotelli2008
• begon2006
• caswell2001

Frequently asked questions

What is a survivorship curve?

A survivorship curve plots the proportion of a cohort still alive against age; its shape shows whether mortality falls mainly on the young, the old, or is roughly constant through life.

What is the net reproductive rate?

The net reproductive rate is the average number of offspring an individual produces over its lifetime, accounting for age-specific survival; a value above one indicates a growing population.