19.3 Ecosystems and Biodiversity

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe a population as a group of organisms of one species, living in the same area, at the same time
  2. Describe a community as all of the populations of different species in an ecosystem
  3. Describe an ecosystem as a unit containing the community of organisms and their environment, interacting together
  4. Describe biodiversity as the number of different species that live in an area
  5. Identify and state the factors affecting the rate of population growth, limited to: food supply, competition, predation, and disease
  6. Understand that the growth of the human population is increasing the demand for global resources

📖 Main Content

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1. What is a Population?

A population is a group of organisms that:

  • All belong to the same species (the same type of living thing)
  • Live in the same area
  • Are present at the same time

Think of it this way: all the rabbits living in one particular field form a population. They are all the same species (rabbit), they live in the same place (that field), and they are alive at the same time.

🔑 Key point: A population is always just one species. You cannot have a population made up of two different types of animals.

More examples of populations:

  • All the oak trees in a forest
  • All the frogs in a pond
  • All the bacteria inside your gut

2. What is a Community?

A community is made up of all the populations of different species living in an ecosystem at the same time.

So if you take that same field, the community includes:

  • The population of rabbits
  • The population of grass
  • The population of foxes
  • The population of insects
  • The population of worms
  • ...and every other species living there

🔑 Key point: A community includes many different species. Every single type of organism in the area is part of the community.

Simple way to remember it:

  • One species living together = Population
  • Many different species living together = Community

3. What is an Ecosystem?

An ecosystem is a unit (a self-contained system) that includes:

  • The community of organisms (all the living things — plants, animals, bacteria, fungi, etc.)
  • Their environment (all the non-living things around them — water, air, temperature, soil, light, etc.)
  • All of these interacting together — meaning the living things affect each other and affect their environment, and the environment affects the living things

🔑 Key point: An ecosystem = living things + their non-living environment + interactions between them

Examples of ecosystems:

  • A pond — includes all the fish, plants, water insects, algae, the water itself, the temperature of the water, the rocks at the bottom, etc.
  • A forest — includes all the trees, birds, deer, fungi, soil, rainfall, sunlight filtering through the leaves, etc.
  • A coral reef — includes all the corals, fish, seaweed, and the ocean water around them

What do we mean by "interacting together"?

  • Plants use sunlight and minerals from the soil to grow (living things using non-living environment)
  • Animals eat plants (living things interacting with other living things)
  • Dead leaves fall and decompose, returning nutrients back to the soil (living things affecting the non-living environment)

All of these interactions happening together is what makes something an ecosystem.

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