Populations, Communities & Ecosystems (Cambridge (CIE) O Level Biology): Revision Note

Populations, Communities & Ecosystems

• There are several key terms that we use when referring to the different components of an ecosystem and their levels of organisation:

  • A population is defined as a group of organisms of the same species living in the same place at the same time

  • A community includes all of the populations living in the same area at the same time

    • Within a community, each species depends on other species for food, shelter, pollination, seed dispersal etc

    • If one species is removed it can affect the whole community

    • This is called interdependence

  • A habitat is the place where an organism lives

    • E.g. badgers, deer, oak trees and ants are all species that would live in a woodland habitat

  • An ecosystem is defined as a unit containing the community of organisms and their environment, interacting together

  • This means,all the biotic factors and all the abiotic factors that interact within an area at one time

    • The term 'biotic factors' includes all the living components such as plants and animals

    • The term 'abiotic factors' includes all the non-living components such as light intensity, mineral ions, water availability

    • Ecosystems can vary greatly in size and scale

      • A small ecosystem might be a garden pond

      • A large ecosystem might be the whole of Antarctica

Ecology Definitions Diagram

Levels of organisation in an ecosystem

Population Growth

• All living organisms compete with each other for food, water and living space

• Those who are the best adapted to their environments generally increase their populations at the expense of those less well-adapted

• Population growth in most organisms is controlled by the following four factors:

  • Food supply

  • Competition

  • Predation

  • Disease