What Happens When Mortality and Natality Rates Are Constant in Each Age Group?

Population dynamics are shaped by two fundamental processes: natality (births) and mortality (deaths). When these rates vary by age, the structure and growth of populations can change dramatically. But what happens if both the mortality and natality rates become constant within each age group? This scenario leads to predictable and important outcomes for population biology and demographics. This article explores the consequences of constant age-specific natality and mortality, and which option best describes the resulting population dynamics.

Understanding Natality and Mortality

Natality refers to the birth rate within a population, while mortality refers to the death rate. Both are typically expressed as rates per age group, since the risk of death and the likelihood of reproduction change as individuals age. For example, infant mortality is usually higher than adult mortality, and reproductive output is concentrated in certain age classes.

What Does It Mean for Rates to Be Constant in Each Age Group?

If mortality and natality rates become constant in each age group, this means that the probability of giving birth or dying does not change over time for any specific age. For instance:

• Infants: Always have the same chance of dying.

• Adults: Always have the same chance of reproducing or dying.

• Elderly: Always have the same chance of dying.

This scenario is a theoretical ideal but is used in models to understand how populations might behave under stable conditions.

The Result: Stable Age Distribution

When mortality and natality rates are constant within each age group, the population eventually reaches a stable age distribution. This means that the proportion of individuals in each age group (children, reproductive adults, elderly) remains constant over time, even as the total population may grow or decline.

In a stable age distribution:

• Each age group grows or shrinks at the same rate.

• The population structure does not change over time.

• The number of individuals in each age class is predictable based on the constant rates.

What Does This Mean for Different Age Groups?

Let’s examine the options provided in the context of constant age-specific natality and mortality:

(1) Number of children increase

If natality and mortality rates are constant, the number of children will not increase relative to other age groups. The proportion of children in the population remains the same as the population grows or declines.

(2) Number of older one will increase

Similarly, the number of older individuals will not increase relative to other age groups. The proportion of elderly remains constant under stable age distribution.

(3) Number of individuals in reproductive stage will increase

Again, the number of individuals in the reproductive stage does not increase relative to other age groups. The proportion stays the same.

(4) The increase or decrease within any cohort will become independent of other

This is the correct answer. When mortality and natality are constant in each age group, each cohort (group of individuals born at the same time) grows or declines independently of other cohorts. The fate of one cohort does not affect another because the rates are fixed for each age. This independence is a key feature of stable age distribution models.

Why Is Option (4) Correct?

Under constant age-specific rates:

• Each cohort moves through the age structure at its own pace.

• The number of individuals in each cohort at any age is determined only by the fixed survival and birth rates for that age.

• There is no interaction or dependence between cohorts.

• The population as a whole may grow or decline, but the relative proportions of each age group remain stable.

This is the essence of stable population theory in demography and ecology.

Real-World Implications

In reality, natality and mortality rates are rarely perfectly constant over time. However, when they are, the population quickly stabilizes in terms of age structure. This concept is used in:

• Population projections: To predict future population sizes and structures.

• Wildlife management: To model the growth of animal populations under controlled conditions.

• Public health planning: To understand how changes in mortality or fertility will affect population age structure.

The Demographic Transition and Stable Age Distribution

The idea of stable age distribution is related to the demographic transition, where populations move from high mortality and fertility to low mortality and fertility. During the transition, age structure changes, but if rates stabilize, the population eventually reaches a new stable age distribution.

Summary Table

Conclusion

When mortality and natality rates become constant in each age group, the population reaches a stable age distribution. In this scenario, the increase or decrease within any cohort will become independent of other cohorts. This means that each age group’s size is determined only by its own fixed birth and death rates, and not by the fate of other age groups.

Correct answer:
(4) The increase or decrease within any cohort will become independent of other