Population Ecology: Population Growth (AQA A-Level Geography): Revision Notes

Population Ecology: Population Growth

Understanding population growth dynamics

In ecology, how populations grow depends on two main factors: birth rates and death rates. The balance between these determines whether a population increases, decreases, or stays stable.

The number of births in any species is influenced by its natural ability to reproduce. Different species have different reproductive strategies. For example, fish like herring produce thousands of young because most won't survive to adulthood. In contrast, mammals (including humans) typically have fewer offspring, but these young have a much higher chance of survival.

Environmental resistance and limiting factors

While biotic potential determines how many offspring can be born, survival is another matter entirely. The number of deaths in a population is controlled by various environmental factors that act as obstacles to survival.

Types of limiting factors

Environmental limiting factors fall into two categories:

Density independent factors:

• These factors affect death rates regardless of how large or dense the population is
• They are not influenced by population size
• Examples include:
  • Natural disasters (droughts, floods, volcanic eruptions)
  • Extreme weather events
  • Natural hazards

These events will increase death rates whether the population is small or large.

Density dependent factors:

• These factors become more significant as population size and density increase
• They are directly related to how crowded a population becomes
• Examples include:
  • Food supply limitations
  • Disease spread
  • Competition for resources
  • Waste accumulation

As populations grow larger and more densely packed, these factors become more effective at limiting further growth.

Model growth curves

When biotic potential exceeds environmental resistance, populations will grow. Ecologists use model growth curves to show how populations change over time. These curves typically show three distinct phases:

Lag phase:

• The initial period of slow growth
• Population is establishing itself
• Birth rates are beginning to exceed death rates
• Population numbers remain relatively low

Logarithmic phase (exponential phase):

• Rapid population growth occurs
• Birth rates are significantly higher than death rates
• Environmental resistance is low
• Resources are plentiful
• Population increases dramatically

Stable/stationary phase:

• Population growth slows and levels off
• Population size fluctuates around a relatively constant level
• Birth rates and death rates are approximately equal
• Environmental resistance has caught up with biotic potential
• Resources become limited

Application to human populations

Historical population patterns

For most of human history, population size remained relatively stable. Although birth rates were high and families were large, most people only survived into adulthood through having many children. High death rates effectively balanced out high birth rates, preventing significant population growth.

The main reasons for population stability included:

• Widespread diseases that killed large numbers of people
• Low life expectancy
• Frequent food shortages and famines
• Lack of medical knowledge and healthcare

The transformation: technological and medical advances

Significant changes began occurring from around 1800 onwards. Developments in technology dramatically improved food production in many parts of the world. At the same time, advances in medical science, healthcare, and sanitation brought about massive improvements in survival rates.

Key developments included:

• Medicines and vaccines that combat infectious diseases (such as smallpox, measles, tuberculosis)
• Control of water-borne diseases (typhoid, cholera)
• Improved sanitation systems
• Better food security
• Increased life expectancy

These improvements meant that humans overcame much of the environmental resistance that had previously limited population growth.

Exponential population growth

The impact has been dramatic. In 1800, Earth's population was estimated at around 1 billion. By 2020, it had reached approximately 7.8 billion - largely due to the control of death rates through medical and technological advances.

For the past two hundred years, human population has been experiencing a 'log phase' - a period of exponential growth. The global population is still growing rapidly, though recent data suggests the rate of growth is beginning to slow down.

According to demographer Nicholas Eberstadt: "We didn't start breeding like rabbits, we stopped dying like flies." This quote perfectly captures how population growth has been driven more by declining death rates than by increasing birth rates.