--- title: "Demography: Life Tables, Age Structure and What They Predict" description: "A life table turns survival and reproduction at each age into a single growth rate, and the age structure of a population says whether that rate will hold. It is the tool behind fishery quotas, pensio" canonical: https://lightmysky.com/learn/science/demography-life-tables-age-structure-and-what-they-predict-mt_raWfdLLACU source: https://lightmysky.com/learn/science/demography-life-tables-age-structure-and-what-they-predict-mt_raWfdLLACU.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Demography: Life Tables, Age Structure and What They Predict A life table turns survival and reproduction at each age into a single growth rate, and the age structure of a population says whether that rate will hold. It is the tool behind fishery quotas, pension forecasts and every claim that a population is recovering. Subject: Science · Area: Ecology & Conservation · Ages 20 to 21 Page: https://lightmysky.com/learn/science/demography-life-tables-age-structure-and-what-they-predict-mt_raWfdLLACU ## Ready when they can - Build a life table from survival and fecundity data and compute the net reproductive rate - Read an age pyramid and predict how the population changes over the next generation - Explain why two populations with the same size can have very different futures ## Lesson: Life tables: reading the future of a population A life table follows one group born together, called a cohort. At each age it records how many survive. Divide survivors by the starting number to get the survival rate for that age. Mortality and life expectancy come from the same counts. **Example.** Plot the survivors and three shapes appear. Type I stays high and drops late, like humans and elephants with well protected young. Type II falls steadily, like birds facing the same risk at every age. Type III crashes early, like fish that flood the water with eggs almost all of which die. To get the net reproductive rate, multiply survival to each age by births at that age, then add the products. A rate above one means growth and below one means shrinking. Read the table beside the age pyramid: a wide base of young means many breeders are arriving, so the population grows. **Tip.** Two populations of equal size can head in opposite directions. Salmon spend their budget on many abandoned eggs while elephants pour years into one calf, and the table shows this as age at first breeding and births per event. Pension forecasts and fishery quotas use the same arithmetic. **Recap.** A life table turns age by age survival and births into a forecast: the rate and the age pyramid say what comes next. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Population Growth Curves and Carrying Capacity](https://lightmysky.com/learn/science/population-growth-curves-and-carrying-capacity-mt_KX6rhHF5kj) - [Biogeography: Why a Species Is Here and Not There](https://lightmysky.com/learn/science/biogeography-why-a-species-is-here-and-not-there-mt_sRi0FPcK8r) ## Opens up - [Life-History Trade-offs and How Reproduction Is Allocated](https://lightmysky.com/learn/science/life-history-trade-offs-and-how-reproduction-is-allocated-mt_eGwlydfDWO) - [Global Change Ecology: Ranges, Phenology and Regime Shifts](https://lightmysky.com/learn/science/global-change-ecology-ranges-phenology-and-regime-shifts-mt_tJ7XpAADMF)