Changes in population size Revision Notes

Population growth typically follows three distinct phases that describe how the number of individuals in a population changes over time.

The first phase is the lag phase, where population growth is slow. This occurs because the population is small and individuals are adapting to their environment. Resources are abundant, but reproduction rates are initially low.

Next is the exponential or log phase, characterised by rapid population increase. During this phase, birth rates exceed death rates significantly, and resources are plentiful, allowing the population to grow at an accelerating rate. For example, a newly introduced species in a favourable habitat may experience exponential growth as it reproduces quickly without many predators or competitors initially present.

• Lag phase: slow growth as population establishes.
• Exponential phase: rapid growth due to high birth rates and abundant resources.

The third phase is the carrying capacity phase, where population growth slows and eventually stabilises. This happens because resources such as food, water, and space become limited as the population size approaches the environment's maximum sustainable number, known as the carrying capacity.

At carrying capacity, birth rates tend to balance death rates, and the population size fluctuates around this limit. For example, a forest can only support a certain number of deer; if the population grows beyond this, food shortages and increased disease may increase death rates, reducing the population back to sustainable levels.

• Carrying capacity: maximum population size an environment can sustain.
• Population stabilises as birth and death rates balance.

Birth rate is the number of live births per 1,000 individuals in a population per year, while death rate is the number of deaths per 1,000 individuals per year. The difference between these rates determines whether a population grows, shrinks, or remains stable.

For example, if a country has a birth rate of 20 per 1,000 and a death rate of 10 per 1,000, the natural increase is 10 per 1,000, leading to population growth. Conversely, if the death rate exceeds the birth rate, the population will decline.

Changes in birth and death rates over time cause fluctuations in population size. Improvements in healthcare can reduce death rates, while social changes can influence birth rates.

• Birth rate: births per 1,000 people per year.
• Death rate: deaths per 1,000 people per year.
• Population change = birth rate - death rate (natural increase or decrease).

Several factors affect birth rates in human populations. Cultural beliefs and traditions can encourage larger families or promote family planning.

Economic conditions also play a role; in wealthier societies, families often have fewer children due to higher living costs and different social priorities.

Access to education and family planning services enables people to control the number and spacing of children. For example, countries with widespread contraception use tend to have lower birth rates.

• Cultural beliefs can encourage or discourage large families.
• Economic status influences family size decisions.
• Family planning access reduces birth rates.

Death rates are affected by healthcare quality, nutrition, sanitation, and disease prevalence.

Better healthcare reduces mortality from diseases and injuries. Improved nutrition strengthens immune systems, lowering death rates.

Sanitation reduces the spread of infectious diseases. For example, the introduction of clean water supplies in urban areas has historically decreased death rates significantly.

• Healthcare improvements reduce death rates.
• Good nutrition supports survival.
• Sanitation prevents disease spread.

Migration is the movement of people into (immigration) or out of (emigration) an area, affecting population size independently of birth and death rates.

Factors influencing migration include employment opportunities, safety, political stability, and environmental conditions.

For example, people may migrate from rural areas to cities seeking jobs, increasing urban populations. Conversely, conflict or natural disasters can cause emigration, reducing population size in affected regions.

• Migration changes population size beyond birth and death rates.
• Driven by jobs, safety, political and environmental factors.
• Urbanisation often results from rural-to-urban migration.