Table of Contents
Why Age Structure Matters
Populations are not only described by how many individuals they contain, but also by how these individuals are distributed across age classes (e.g., young, adult, old). This age structure strongly influences:
- future population growth or decline
- survival chances during environmental changes
- reproductive output
- competition within and between species
Understanding age structure allows ecologists to predict how a population will develop and to assess its stability and vulnerability.
Age Classes and Life Stages
Age structure usually groups individuals into a few broad age or stage classes, not by exact age in days or years. Typical examples:
- Pre‑reproductive individuals
- Not yet able to reproduce (e.g., seedlings, larvae, juveniles).
- Represent the "potential" for future population growth.
- Reproductive individuals
- Sexually mature or otherwise capable of producing offspring.
- Directly determine current birth rates.
- Post‑reproductive individuals
- No longer reproduce (common in humans and some mammals; often absent or rare in many wild animals).
- Can still affect the population via behavior (e.g., care, knowledge, social structure).
In many species, developmental stages (egg, larva, pupa, adult) may be more relevant than strict chronological age. Age-structured and stage-structured views are conceptually similar: both describe how individuals are distributed among life phases that differ in survival and reproduction.
Typical Age Pyramids and What They Indicate
Age structure is often visualized as an age pyramid (age on the vertical axis; number or percentage of individuals on the horizontal axis). Different shapes reflect different population dynamics.
Expanding Populations
Characteristics:
- Broad base: many pre‑reproductive individuals
- Narrow top: relatively few older individuals
Implications:
- High birth rates, often high juvenile mortality as well
- Population is likely to grow if conditions remain similar
- Typical of rapidly growing human populations in some developing regions, and many wild populations during colonization of new habitats
Stable Populations
Characteristics:
- Columns or gently tapering shape
- Similar number of individuals in pre‑reproductive and reproductive classes
Implications:
- Births roughly balance deaths
- Total population size remains about constant over time (fluctuating around a mean)
- Seen in human populations with moderate, relatively stable birth and death rates, and in long‑established animal populations under relatively stable conditions
Declining Populations
Characteristics:
- Narrow base: few young individuals
- Proportionally more older individuals
- Top may be relatively wide compared to the base
Implications:
- Low birth rates, possibly combined with high survival of adults
- Population size likely to shrink in future generations
- Typical of human populations with very low fertility rates and some endangered animal populations with poor reproduction
Survivorship and Life History Strategies
Age structure is closely linked to how mortality is distributed across ages and to the life history strategy of a species.
Survivorship Curves
A survivorship curve shows the number (or proportion) of individuals surviving at each age. Three idealized types help illustrate common patterns:
- Type I
- Low mortality in youth and middle age; mortality increases sharply in old age.
- Most individuals reach or approach maximum life span.
- Typical of many large mammals, including humans in developed countries.
- Age structure: relatively many adults and older individuals; fewer very young (if birth rates are low).
- Type II
- Constant probability of death at all ages.
- Straight line decline in survivorship on a linear scale.
- Typical of some birds, small mammals, and some reptiles.
- Age structure: more evenly distributed across ages (assuming stable conditions).
- Type III
- Very high mortality of young; those that survive juvenile stages may live relatively long.
- Typical of many fish, invertebrates, and plants producing large numbers of offspring.
- Age structure: huge numbers of early life stages, with steep drop into later ages.
Real populations often lie between these extremes, but the concept helps connect survival patterns to age distribution.
r‑ and K‑Selected Strategies and Age Structure
In broad terms:
- r‑selected species
- Many small offspring, little parental investment, short life span, high juvenile mortality.
- Age structure often dominated by young individuals; rapid turnover.
- Common in unstable or newly available habitats.
- K‑selected species
- Few, well‑cared‑for offspring, longer life, lower juvenile mortality.
- Age structure includes significant proportions in several adult age classes; slower turnover.
- Common in stable environments near carrying capacity.
Thus, age structure reflects not only current environmental conditions but also evolved life history strategies.
Age Structure and Population Growth
Reproductive Value of Age Classes
Not all age classes contribute equally to future population growth.
- Pre‑reproductive: no current reproduction but important for the future; their survival strongly affects future size.
- Reproductive: contribute directly to births; slight changes in their survival or fertility can have large effects.
- Post‑reproductive: little or no direct genetic contribution but may influence survival and reproduction of younger relatives (e.g., through care, protection, or resource sharing).
The reproductive value of an age class expresses how much that class is expected to contribute to future generations, given its present age and expected survival and fertility.
Momentum and “Hidden” Growth Potential
A population with:
- many young individuals
- relatively few older individuals
can continue to grow for some time even if birth rates per individual decline, simply because so many individuals are entering reproductive age. This phenomenon is called population momentum.
Conversely, a population with few young individuals may continue to decline even after birth rates increase, because the number of reproductive individuals is temporarily small.
Cohort vs. Static Age Structure
Ecologists distinguish between two main ways of assessing age structure:
- Cohort (longitudinal) approach
- Follow a group of individuals born at the same time (a cohort) throughout their lives.
- Provides detailed information on survival at each age but requires long-term observation.
- Static (cross‑sectional) approach
- Measure the age distribution of all individuals at one point in time.
- Faster and easier, but can be misleading if birth and death rates fluctuate strongly between years.
In stable conditions, the static age structure approximates the long-term pattern seen in cohorts. In fluctuating environments, large differences between years (e.g., a strong “good year” for reproduction) can cause irregular or “wavy” age pyramids.
Modeling Age-Structured Populations
In simple population models, all individuals are often treated as identical. Age-structured models add realism by distinguishing age (or stage) classes.
Life Tables
A life table summarizes:
- number of individuals surviving to each age (
l_x) - age‑specific mortality (
q_x) - age‑specific fertility (
m_x)
From a life table, ecologists can calculate:
- net reproductive rate $R_0$ (average number of daughters produced per female over her lifetime)
- generation time (average age of mothers when they give birth)
- intrinsic growth rate $r$ (linking age structure to overall growth)
Matrix Models (Conceptual)
One common framework is an age‑structured matrix model (e.g., Leslie matrix). Conceptually:
- Each age class is a category.
- Survival probabilities determine how individuals move from one class to the next.
- Fertilities determine how many new young are added.
Repeatedly applying the matrix to a population vector of age class abundances shows how age structure and total population size change over time. In constant conditions, the model often converges to a stable age distribution and a constant growth rate.
Age Structure, Environment, and Regulation
Environmental Influences on Age Structure
Different age classes may be differently sensitive to environmental factors:
- Drought or food shortage may primarily affect young individuals (e.g., seedlings or juveniles), narrowing the base of the pyramid.
- Disease may disproportionately hit certain ages (e.g., very young or very old).
- Predation may be directed at a particular stage (e.g., eggs, larvae, or slow-moving old individuals).
Thus, changes in age structure can reveal past environmental stresses or disturbances.
Density Dependence and Age
Population regulation (see separate chapter) often acts through age‑specific mechanisms:
- Competition may reduce juvenile survival more than adult survival.
- Territorial adults might limit the number of younger individuals that can establish themselves.
- In crowded conditions, reproduction may be delayed or reduced, altering the ratio of young to adults.
Monitoring age structure over time helps identify whether regulation is acting primarily on survival of young, survival of adults, or reproduction.
Applications of Age Structure Analysis
Wildlife Management and Conservation
Age structure is crucial for:
- Assessing population health:
- Few juveniles may indicate poor reproduction or high juvenile mortality.
- Absence of older individuals may suggest heavy adult mortality (e.g., hunting, disease).
- Designing sustainable harvests:
- Managers choose which age classes to harvest (e.g., only older fish or only males beyond prime breeding age) to minimize impact on reproduction.
- Evaluating recovery efforts:
- Conservation programs for endangered species track whether breeding programs or habitat restoration increase the proportion of young and reproductive individuals.
Human Populations and Planning
Human population pyramids are widely used in:
- forecasting school, housing, and health care needs
- evaluating dependency ratios (ratio of non-working to working-age population)
- predicting economic and social challenges in aging or very young populations
For example:
- A large proportion of children implies future demand for education and jobs.
- A large proportion of elderly individuals implies increased need for medical care and pensions, and a potential shortage of working-age people.
Exploited Populations (Fisheries, Forestry)
In harvested populations:
- Overharvesting of older, larger individuals can flatten or truncate the upper part of the age pyramid.
- A population that contains almost only young individuals, with few reaching large, older stages, may show reduced reproductive capacity and increased variability.
Sustainable management often aims to maintain a balanced age structure with enough older reproductive individuals to ensure stable recruitment of young.
Limitations and Special Cases
Difficulties in Determining Age
In many species, exact age is hard to measure:
- Some animals show growth rings (bone, teeth, shells, scales), but others do not.
- In many plants and invertebrates, age must be estimated indirectly (size classes, developmental stage).
In such cases, stage-structured or size-structured approaches are used instead of strict age classes, but the ecological reasoning is similar.
Species With Overlapping Generations vs. Non‑Overlapping Generations
- Overlapping generations (common in vertebrates and perennials): several age classes are present simultaneously; age structure tends to be smoother.
- Non‑overlapping generations (e.g., many annual plants, some insects): most individuals are about the same age; age structure may be very simple or show strong seasonal patterns.
Recognizing which pattern applies is essential for interpreting age structure data correctly.
Summary
- Age structure describes how individuals in a population are distributed across age (or life stage) classes.
- Different shapes of age pyramids (expanding, stable, declining) reflect different population dynamics and growth prospects.
- Age‑specific survival and reproduction (summarized in survivorship curves and life tables) determine age structure and the contribution of each age class to future population growth.
- Environmental factors and density dependence often act differently on different age classes, so changes in age structure can reveal underlying ecological processes.
- In applied contexts (conservation, wildlife management, fisheries, forestry, human demography), analyzing age structure is a key tool for predicting future trends and planning appropriate interventions.