Biology IGCSE (0610): Study Notes – Populations (Section 19.4)

Welcome to the world of Ecology! In this chapter, we step back from individual organisms and look at life on a grander scale: how groups of living things interact with each other and their environment. Understanding Populations is crucial because it helps us predict how species survive, how ecosystems function, and how human activities impact the world around us.

Don't worry if the vocabulary seems new; we’ll break down these concepts one step at a time!


1. Defining Key Ecological Groups

To study how living things interact, we use specific terms to define different levels of organization. These terms build upon one another:

1.1 Population

A population is the simplest ecological group.

  • Definition: A group of organisms of one species, living in the same area, at the same time.
  • Example: All the wild deer living in a specific national park in July 2024.

1.2 Community

When you put different populations together, you get a community.

  • Definition: All of the populations of different species living and interacting in an ecosystem.
  • Example: The deer, the wolves, the mice, the trees, and the fungi all living and interacting in the national park.

1.3 Ecosystem

The ecosystem is the largest term we need to define here.

  • Definition: A unit containing the community of organisms (the biotic, or living, parts) and their environment (the abiotic, or non-living, parts), interacting together.
  • Example: The deer, wolves, mice, trees (the community), plus the soil, the air, the water in the river, and the temperature (the environment).

Quick Review Aid: Think of a classroom.
A Population is just the students.
A Community is the students, the teacher, and the pet hamster.
An Ecosystem is the students, teacher, hamster, plus the desks, the air conditioning, and the light.


2. Factors Affecting Population Growth Rate

The size of a population is not fixed; it is constantly changing due to four major factors that either increase or limit its growth. These are called limiting factors when they restrict growth.

2.1 Food Supply

Food supply (or nutrient availability for plants) is usually the most important factor in determining the maximum size a population can reach.

  • If food is abundant, organisms can obtain energy easily, leading to high birth rates and fast growth.
  • If food is scarce, organisms starve, leading to low birth rates and high death rates.

2.2 Competition

Competition occurs when organisms need the same limited resource.

  • Intraspecific competition: Competition *between* members of the same species (e.g., two male lions fighting over a mate or two pine trees fighting for sunlight).
  • Interspecific competition: Competition *between* members of different species (e.g., foxes and eagles hunting the same type of rabbit).
  • High competition slows down growth as less energy is available for reproduction.

2.3 Predation

Predation is when one organism (the predator) eats another (the prey).

  • If the number of predators increases, the population of prey decreases.
  • If the population of prey decreases, the predators eventually run out of food and their population decreases too (a classic cycle).

2.4 Disease

Infectious diseases can spread rapidly through dense populations.

  • The higher the population density (the more crowded the organisms), the easier it is for pathogens to move from one host to the next.
  • This leads to an increased death rate and slows population growth.

Key Takeaway: Population growth is a balancing act between the factors that encourage reproduction (like plentiful food) and the limiting factors that increase death or reduce birth (like disease and competition).


3. Interpreting the Sigmoid Population Growth Curve

When a population of organisms (like bacteria in a flask or rabbits introduced to an island) is growing in an environment where resources are limited, their growth usually follows a characteristic S-shape, known as the sigmoid curve.

This curve demonstrates how limiting factors affect the growth over time. You must be able to identify and explain the four phases.

Step-by-Step: The Phases of the Sigmoid Curve

Phase 1: Lag Phase

The Slow Start

  • What happens: The population grows very slowly.
  • Why: Organisms are adapting to the new environment, maturing, establishing territories, or finding mates. The initial numbers are small, so reproduction takes time to become noticeable.
  • Limiting Factors: Minimal impact from limiting factors yet, but time is required for reproduction to begin seriously.
Phase 2: Exponential (Log) Phase

Rapid Growth!

  • What happens: The population increases extremely quickly. This part of the graph is steep.
  • Why: Resources (food, space, etc.) are non-limiting (or unlimited). The birth rate is much higher than the death rate. Every organism that reproduces significantly boosts the overall population size.
  • Did you know? This phase is also called the "Logarithmic Phase" because population size plotted against time results in an exponential curve.
Phase 3: Stationary Phase

Hitting the Ceiling

  • What happens: The population size stops increasing and fluctuates around a maximum level. The curve flattens out.
  • Why: The population has reached the **carrying capacity** of the environment.
  • Carrying Capacity: This is the maximum population size that a specific environment can sustain indefinitely, given the available resources.
  • Role of Limiting Factors (Supplement 7): Limiting factors are now having a major effect.
    • Food supply becomes scarce.
    • Competition for space and resources is high.
    • Waste products may build up (e.g., carbon dioxide or bacteria waste).
    • Disease transmission increases due to high density.
  • In this phase, the Birth Rate is approximately equal to the Death Rate, resulting in zero net growth.
Phase 4: Death Phase (or Decline Phase)

The Collapse (Sometimes)

  • What happens: The population size begins to decrease significantly.
  • Why: This occurs if the environment degrades drastically, often due to the population size itself (e.g., permanent depletion of the food source, or lethal buildup of toxins). The death rate now exceeds the birth rate.

Common Mistake Alert: Students often confuse the Stationary Phase with the death phase. Remember, in the Stationary Phase, the population is stable (at carrying capacity), whereas in the Death Phase, the population is actively declining!


Key Takeaway: The sigmoid curve is a model showing how a population’s growth is initially rapid (exponential) until environmental constraints (limiting factors) slow the growth and stabilize the population at the environment's carrying capacity.