Adaptations, Interdependence and Competition » Ecosystems, Communities and Competition

Levels of organisation in an ecosystem

Living things never live alone. They share their surroundings with many other organisms. To study this, ecologists look at nature in levels, from the smallest group to the biggest.

1. Individual organism: a single living thing, such as one oak tree or one badger.
2. Population: all the organisms of one species living in the same place, such as all the badgers in a wood.
3. Community: all the populations of different species living in the same place at the same time.
4. Ecosystem: the community together with the non-living parts of the environment.

Key terms:

• Ecosystem: the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.
• Community: all the different species living in the same habitat.
• Population: all the organisms of one species in a habitat.
• Habitat: the place where an organism lives.

A pond ecosystem

A pond is a good example. The pond water, the mud and the rocks on the bottom are the non-living, or abiotic, parts. The plants, fish, insects, snails and bacteria are the living, or biotic, parts. All of these interact, so the whole pond is one ecosystem.

Ecosystems can be small, such as a pond or a rotting log. They can also be huge, such as a rainforest or an ocean.

What organisms need

To survive and reproduce, organisms need a supply of materials. These come from their surroundings and from the other living organisms there.

• Plants take light, water, carbon dioxide and mineral ions from their surroundings.
• Animals take oxygen and water from their surroundings and get food from other organisms, such as plants or other animals.

The supply of these materials is limited. When there is not enough to go round, the organisms in a community compete.

Competition

Competition happens when organisms need the same limited resource. The one that gets more of it is more likely to survive and reproduce.

Competition can be between members of the same species or between different species. Two oak saplings growing close together compete for light. Two stags compete for mates.

Interdependence

Within a community, each species depends on other species. It may need them for food, shelter, pollination or seed dispersal. This is called interdependence.

• Food: a hawk depends on the small birds and mice that it eats.
• Shelter: many insects and birds depend on trees to live in.
• Pollination: many flowering plants depend on insects such as bees to carry pollen between flowers.
• Seed dispersal: some plants depend on animals to carry their seeds away, so the seedlings do not compete with the parent plant.

Key term:

• Interdependence: each species in a community depends on other species for things such as food, shelter, pollination and seed dispersal.

Because of this, if one species is removed it can affect the whole community. If the bees in a meadow died out, flowers that depend on bees would not be pollinated. Fewer seeds would form, and the animals that feed on those plants would also be affected.

Stable communities

A stable community is one where all the species and environmental factors are in balance, so that population sizes remain fairly constant. Numbers may go up and down a little, but over time they stay about the same.

In a stable community the species depend on each other. Removing one species, or changing an environmental factor, can upset the balance and make the whole community change.

Reading data about a community

You may be given a chart, graph or table about the organisms in a community. You need to pull out the information and say what it shows.

Steps to follow: read the title and the axis labels or column headings, look for the pattern, then link it to a factor such as food or competition.

Exam-style question

A scientist studies a meadow. It contains grass, daisies, bees, rabbits and foxes.

(a) The scientist says that all the rabbits in the meadow are a population. Explain what is meant by a population. [1 mark]

(b) Name two factors the grass and daisies compete for. [2 marks]

(c) Bees visit the daisies. Explain why the bees and the daisies show interdependence. [2 marks]