Ecosystems and interdependence

An ecosystem is all the living things in an area together with their non-living surroundings: a pond, a wood, a coral reef. The organisms depend on each other for food, shelter and pollination, and they compete for what they need. Living and non-living factors decide which organisms can survive there and how many.

Key facts

  • An organism lives in a habitat. A population is all the organisms of one species in a habitat. A community is all the populations living there. An ecosystem is the community and the non-living (abiotic) parts of its environment.
  • Abiotic factors are non-living: light, temperature, water, soil pH and minerals, wind, carbon dioxide and oxygen levels.
  • Biotic factors are living: the food available, predators, new diseases (pathogens), and competition from other species.
  • Plants compete for light, water, space and mineral ions from the soil. Animals compete for food, mates, water and territory.
  • Interdependence: in a community, each species depends on others for food, shelter, pollination and seed dispersal. A change in one population affects the others.
  • Predator and prey numbers rise and fall in cycles. When prey increase, predators have more food and increase too, a little later; then prey decrease, and later so do predators.
  • A stable community keeps roughly the same species and numbers over time, because its biotic and abiotic factors stay in balance (for example a tropical rainforest).

Worked examples

Example 1

Sort these into a population, a community and an ecosystem: all the frogs in a pond; all the plants and animals in the pond; the pond with its living things, water, mud and light.

  1. A population is one species only: all the frogs in the pond.
  2. A community is all the species living together: every plant and animal in the pond.
  3. An ecosystem adds the non-living parts: water, mud, light and temperature, as well as the community.

Answer: population: all the frogs; community: all the plants and animals; ecosystem: the pond with its living and non-living parts

Example 2

Which of these are abiotic factors and which are biotic: temperature, a new predator, soil pH, a disease, light intensity?

  1. Abiotic means non-living. Temperature, soil pH and light intensity are physical and chemical conditions.
  2. Biotic means living. Predators and disease-causing organisms are living things.
  3. So the abiotic factors are temperature, soil pH and light intensity; the biotic factors are the predator and the disease.

Answer: abiotic: temperature, soil pH, light intensity; biotic: a new predator, a disease

Example 3

The graph shows the numbers of hares (prey) and lynx (predators) over many years. Explain the pattern.

  1. When there are many hares, the lynx have plenty of food, so more lynx survive and breed.
  2. As the lynx increase, they eat more hares, so the hare population falls.
  3. With fewer hares, the lynx run short of food, so their numbers fall. Then the hares can increase again.
  4. The lynx peaks come a little after the hare peaks, because it takes time for predators to breed.

Answer: more hares means more food for lynx, so lynx increase; then more hares are eaten, so hares fall; with less food, lynx fall; then hares recover

Example 4

Bees pollinate apple trees. Explain how a fall in the number of bees could affect an orchard and the animals that live there.

  1. Apple flowers need insects such as bees to carry pollen from flower to flower.
  2. With fewer bees, fewer flowers are pollinated, so fewer apples (and seeds) form.
  3. Birds, insects and mammals that feed on the apples have less food, so their populations may fall too.

Answer: fewer flowers are pollinated, so fewer apples form; animals that eat the apples have less food and may decrease