Demography: life tables, age structure and what they predict
Life tables summarize age-specific survival and reproduction. The net reproductive rate estimates lifetime replacement, while age structure helps interpret population change under assumptions about vital rates and migration. Survival estimates inform pension forecasts; demographic data also inform fisheries management.
What a learner can do afterwards
- Build a life table from survival and fecundity data and compute the net reproductive rate
- Read an age pyramid and make conditional predictions using survival, reproduction and migration assumptions
- Explain why two populations with the same size can have very different futures
1 · Read
A cohort life table follows individuals born together; a period life table uses age-specific data from one period. l_x is survivors at age x divided by the starting cohort: survival to that age, not within one interval. m_x is daughters per surviving female in interval x. The net reproductive rate is R0 = sum(l_x times m_x), expected lifetime daughters per newborn female. Example: the only breeding intervals have (l_x, m_x) = (0.8, 3) and (0.4, 4). R0 = 0.8 times 3 + 0.4 times 4 = 4.0.
Survival data support mortality and life expectancy estimates. Survivorship is the proportion alive at each age. Type I has low early mortality and high late mortality, often illustrated by humans and elephants. Type II has constant death probability per equal age interval, as in some birds: survivors fall by a constant proportion, not a constant number. Type III has high early mortality, as in many fish and plants. The chart illustrates half surviving each interval.
Vital rates here are survival and reproduction by age. With fixed vital rates and no migration, R0 above 1 predicts long-term growth; below 1 predicts long-term decline. R0, the net reproductive rate, measures lifetime replacement, not annual growth. An age pyramid shows numbers by age. A wide base means many young, who may breed if they survive. Age alone cannot predict growth: births, deaths and migration matter. Different age structures can cause short-term change even with the same R0.
Growth, maintenance, reproduction and care compete for limited energy. Many marine invertebrates produce many small offspring with little care; elephants usually have one calf at a time and care for it for years. These tradeoffs help explain survival and births by age. Equal-sized populations can have different futures because their age structures and vital rates differ. Pension forecasts use survival to estimate payments. Fishery forecasts also need recruitment, harvest and other mortality.
Use survival and reproduction to calculate lifetime replacement. Predict population change from age structure only with stated assumptions about vital rates and migration.
Lesson and practice partly adapted from Population Demography and Life Histories and Natural Selection textbook records supplied through the hoofer-content collection, with original demographic explanations and worked exercises.
Reading summaries adapted from 45.1 Population Demography, 45.2 Life Histories and Natural Selection, and 36.1 Population Demography, supplied through the hoofer-content textbook collection.
2 · Watch
به انگلیسیTake it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.