03. Population Ecology and Human Demographics

Population Characteristics and Change

Population ecology examines how and why populations change through time. A population is a group of individuals of the same species living in a particular area and interacting with one another. Population change affects resource use, pollution, land conversion, and ecosystem stability.

Describing a population

Ecologists use several characteristics to describe populations:

• Population size is the total number of individuals.

• Population density is the number of individuals per unit area or volume.

• Population distribution describes how individuals are arranged in space:

  • Clumped: individuals occur in groups, often around resources or social structures.

  • Uniform: individuals are evenly spaced, commonly because of territorial behavior or competition.

  • Random: individuals show no predictable spatial pattern.

• Age structure gives the relative proportions of prereproductive, reproductive, and postreproductive individuals.

• Sex ratio gives the proportion of males to females or other reproductive sexes.

• Birth rate and death rate measure births and deaths per unit population per unit time.

• Immigration is movement into a population, while emigration is movement out.

Population change can be represented as:

Here, $B$ represents births, $I$ immigration, $D$ deaths, and $E$ emigration. In a closed population, migration is absent, so births and deaths determine the change. In human populations, migration can substantially alter local population size.

The intrinsic rate of increase summarizes a population’s potential growth under ideal conditions:

If $r > 0$, the population increases; if $r < 0$, it decreases; and if $r = 0$, it remains approximately stable when migration is negligible.

Takeaway: Population size changes through four demographic processes: births, deaths, immigration, and emigration.

Population Growth Models

When resources are abundant and environmental resistance is minimal, a population may undergo exponential growth. Each generation adds more reproducing individuals, so the growth rate accelerates and the graph becomes J-shaped. Bacteria in a nutrient-rich culture may initially show this pattern.

Exponential growth is temporary in a finite environment. Food, water, space, nutrients, and other resources eventually become limiting.

Logistic growth

The logistic growth model incorporates environmental limits:

In this equation, $r_{\max}$ is the maximum per-capita rate of increase, $N$ is the current population size, and $K$ is carrying capacity. When $N$ is much smaller than $K$, growth approaches exponential growth. As $N$ approaches $K$, competition and resource shortages slow growth. The resulting graph is S-shaped.

The logistic model is useful but simplified. Carrying capacity can change with seasons, disasters, climate, technology, and human management.

Estimating doubling time

The rule of 70 estimates how long a population takes to double:

For example, at an annual growth rate of $2\%$, the estimated doubling time is $70/2 = 35$ years. This estimate assumes that the growth rate remains constant.

Takeaway: Exponential growth assumes ideal conditions, whereas logistic growth shows how environmental limits slow a population near carrying capacity.

Survivorship and Life-History Strategies

A life table records survival and mortality at different ages. Ecologists use these data to construct a survivorship curve, which shows the proportion of individuals surviving to each age.

• Type I survivorship: Most individuals survive through early and middle life, followed by a steep decline in old age. Humans and many large mammals commonly show this pattern. These species generally produce relatively few offspring and provide substantial parental care.

• Type II survivorship: Individuals face a relatively constant probability of death at each age. Many birds and some small mammals show this pattern.

• Type III survivorship: Mortality is very high among young individuals, but those that reach adulthood have relatively high survivorship. Many fish, marine invertebrates, plants, and insects show this pattern. These organisms often produce many offspring with little parental care.

Survivorship is connected to life-history strategy. Species differ in age at first reproduction, number of offspring, frequency of reproduction, parental investment, and life span. These traits reflect trade-offs in allocating limited energy among growth, maintenance, and reproduction.

Takeaway: Survivorship patterns reveal when mortality is concentrated during a species’ life span and how reproduction and parental investment are related to survival.

Carrying Capacity and Population Regulation

Carrying capacity is the largest population size that an environment can sustain over a particular period with available resources and technology. It is not a fixed universal number. Rainfall, temperature, food and water availability, habitat quality, soil fertility, disease, predation, pollution, natural disasters, and human activities can all change it.

The collection of conditions that restrict population growth is called environmental resistance. If a population temporarily exceeds carrying capacity, resource depletion may lead to increased disease, reduced fertility, starvation, or a population crash. Populations may also fluctuate around carrying capacity rather than remaining exactly at $K$.

Human carrying capacity

Technology can increase the number of people supported by a particular area, but it may shift environmental costs elsewhere. For example:

• Irrigation can increase food production while depleting aquifers.

• Synthetic fertilizers can raise crop yields while contributing to eutrophication.

• Fossil-fuel energy can support dense cities while increasing greenhouse-gas emissions.

Therefore, human carrying capacity depends not only on population size but also on consumption patterns, technology, trade, and ecological impacts.

Limiting factors

Density-dependent factors become stronger as population density increases. Examples include competition for food, water, space, nutrients, or mates; predation; parasitism; infectious disease; waste accumulation; territorial conflict; and overcrowding. A contagious disease generally spreads more rapidly in a dense population because infected individuals have more frequent contact with susceptible individuals.

Density-independent factors affect populations regardless of density. Examples include hurricanes, floods, wildfires, droughts, extreme heat or cold, volcanic eruptions, and some forms of pollution. A drought can affect all individuals while also creating increased competition for the food that remains, so a single event may have both density-dependent and density-independent effects.

Takeaway: Environmental limits are dynamic, and population regulation often results from several interacting factors rather than one fixed constraint.

Human Demographics and Environmental Impact

Human populations are governed by the same basic demographic processes as other populations, but agriculture, sanitation, medicine, transportation, energy use, and technology have altered environmental limits.

Important demographic measures

• Crude birth rate: births per $1{,}000$ people per year.

• Crude death rate: deaths per $1{,}000$ people per year.

• Total fertility rate: the average number of children a woman is expected to have during her lifetime.

• Infant mortality rate: infant deaths per $1{,}000$ live births.

• Life expectancy: the average number of years an individual is expected to live.

• Replacement-level fertility: the fertility rate at which a population replaces itself over the long term, assuming no migration. It is often near $2.1$ births per woman in populations with low mortality, but it varies among populations.

• Net migration rate: immigration minus emigration relative to population size.

A population can continue growing even after fertility declines if many people are young and approaching reproductive age. This population momentum can sustain growth for years or decades. In contrast, a population with a large older population may remain stable or decline even if fertility rises slightly.

Global population growth has slowed compared with the rapid growth of the twentieth century, but regional patterns differ substantially. The United Nations 2024 revision estimated a global population of approximately $8.2$ billion in $2024$ and projected about $10.3$ billion in the mid-2080s before a gradual decline. Some countries have already reached population peaks, while many countries in sub-Saharan Africa are projected to continue growing rapidly.

Population and environmental impact

Population size alone does not determine environmental impact. A useful conceptual relationship is:

Here, $I$ is environmental impact, $P$ is affluence or consumption per person, and $T$ is technology, including environmental impact per unit of consumption. A smaller population with very high per-person consumption can exert greater environmental pressure than a larger population with low per-person consumption.

Takeaway: Human demographic change must be considered together with age structure, migration, consumption, technology, and the social conditions that shape fertility and mortality.

Demographic Transition and Population Structure

The demographic transition model describes a general relationship among economic development, birth rates, death rates, and population growth. It is a model rather than a law, so countries may not follow every stage in the same way or at the same speed.

Stages of the model

• Stage 1: Preindustrial. Birth and death rates are high, producing slow growth or stability. Sanitation and medical care are limited, infant mortality is high, and subsistence agriculture is common.

• Stage 2: Transitional. The death rate declines rapidly while the birth rate remains high, producing rapid growth. Improved sanitation, nutrition, medicine, and food supply reduce deaths before birth rates fall.

• Stage 3: Industrial. Birth rates decline while death rates remain low, so growth slows. Education, urbanization, access to contraception, lower infant mortality, and delayed marriage contribute to lower fertility.

• Stage 4: Postindustrial. Birth and death rates are low, producing stability or slow growth. Education and income are high, family planning is widespread, and childbearing occurs later.

• Stage 5: Possible decline. Fertility remains very low, and deaths may exceed births as populations age, resulting in population decline.

In Stage 2, the gap between a falling death rate and a still-high birth rate produces rapid growth. In Stage 3, education, economic opportunity, health care, family planning, urbanization, and postponement of marriage and childbearing contribute to declining birth rates.

Reading population pyramids

A population pyramid displays the age and sex structure of a population:

• An expansive pyramid has a broad base, indicating many young people and likely future growth.

• A stable pyramid has relatively similar proportions across age groups, suggesting slow growth or stability.

• A constrictive pyramid has a narrow base and larger older cohorts, indicating low fertility and possible population decline.

Population pyramids help predict future demand for schools, jobs, housing, health care, food, water, and energy. They also help identify whether population momentum is likely.

Takeaway: The demographic transition model links changes in mortality and fertility to development, while population pyramids show how age structure influences future trends.

Applying Population Ecology to Environmental Problems

Population ecology provides tools for understanding environmental problems at several scales.

• Local scale: A deer population may exceed the carrying capacity of a forest, causing overbrowsing and vegetation loss.

• Regional scale: Rapid urban growth may increase demand for water, housing, transportation, and waste disposal.

• Global scale: Population growth combined with high per-capita consumption can increase greenhouse-gas emissions, land conversion, freshwater withdrawals, and material extraction.

• Conservation scale: A small population may experience inbreeding, reduced genetic diversity, and vulnerability to random environmental events.

Effective responses can address both demographic conditions and the social and economic factors shaping resource use. Possible approaches include improving education and health care, expanding voluntary access to family planning, reducing infant mortality, increasing resource efficiency, protecting ecosystems, planning sustainable cities, and reducing high-impact consumption.

A strong analysis asks several connected questions: How is the population changing? Which demographic processes are responsible? What is the current carrying capacity? Which limiting factors are operating? How do consumption and technology affect environmental impact? What interventions reduce both ecological pressure and social vulnerability?

Final takeaway: Population size matters, but environmental outcomes also depend on age structure, fertility, mortality, migration, consumption, technology, and the resilience of ecosystems.