8.2 Changes in Population Size

2026 📋 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe and explain the growth curve of populations
  2. Describe and explain the changes in human populations
  3. Understand and apply the following key ideas: lag phase, exponential (log) phase, carrying capacity; birth rates and death rates; factors affecting birth and death rates; factors affecting migration

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1. The Population Growth Curve

When scientists study how a population (the total number of individuals of one species in an area) grows over time, they draw a population growth curve — a graph with time on the x-axis and population size on the y-axis.

This curve has a characteristic S-shape (called a sigmoid curve). It is made up of three distinct phases:

Phase 1 — The Lag Phase

  • This is the slow start at the beginning of the curve.
  • The population is small, and individuals are settling into their new environment.
  • Organisms are reproducing, but because there are so few of them, the overall number is not rising quickly.
  • Think of it like a car just starting to move — the engine is running, but the speed is still very low.

Phase 2 — The Exponential (Log) Phase

  • This is the rapid growth phase — the curve shoots steeply upward.
  • Exponential growth means the population keeps multiplying by the same factor over and over. For example, if it doubles every month: 2 → 4 → 8 → 16 → 32…
  • This happens because:
    • There is plenty of food and space available.
    • There are few predators and little disease.
    • The birth rate (number of births per 1,000 people per year) is much higher than the death rate (number of deaths per 1,000 people per year).
  • This phase is also called the "log phase" because the numbers grow so fast that scientists often use a logarithmic scale to display them.

Phase 3 — The Stationary Phase (at the Carrying Capacity)

  • Growth slows down and eventually levels off at a maximum population size.
  • This maximum is called the carrying capacity — the largest population that an environment can support sustainably, given its available food, water, space, and other resources.
  • At the carrying capacity:
    • Resources become limited.
    • Competition, disease, and predation increase.
    • The birth rate and death rate become roughly equal, so the population stays stable (it no longer grows).
  • Think of it like a bucket of water — you can keep filling it, but once it reaches the top, no more water can fit.

Key Idea: In the natural world, populations rarely grow forever. The environment sets a limit — the carrying capacity. When a population exceeds this, death rates rise and/or birth rates fall until balance is restored.

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