Many populations, living together: A community is a collection of interacting populations within an ecosystem. Each population is one species.
The points to remember
- A community is all the populations of different species living in the same area at the same time.
- The populations interact: they eat, compete with, help or infect each other.
- It is only the living part: plants, animals, fungi and microorganisms.
- It includes producers, consumers and decomposers.
- Add the physical environment (soil, water, air, light) and you have an ecosystem.
Remember it as: One species = population. Many species, interacting = community.
Community
- Living things only
- Many species, interacting
Ecosystem
- Community plus soil, water, air and light
- Living and non-living parts
Say 'interacting': A community is not just a list of species in a place: the populations must interact.
Real example: a single old oak tree in England supports a community of about 2,300 species, from caterpillars and beetles to birds, bats, mosses and fungi.
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The populations in a community depend on each other. Population interactions are studied in detail in 2.1.9.
How populations in a community interact
- Feeding: herbivores eat plants; predators eat other animals.
- Competition: populations need the same food, light or space.
- Mutualism: both gain, such as bees pollinating flowers.
- Parasitism and disease: one population lives on or harms another.
- A change in one population ripples through the rest of the community.
Remember it as: Eat, compete, help, infect: and every change ripples.
A local community: on an oak, caterpillars eat the leaves, blue tits eat the caterpillars, and fungi on the roots swap nutrients with the tree (mutualism).
Studying a community near you: List the species in one place, such as a hedge or a pond, then draw arrows for who eats, helps or competes with whom.
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Why carnivores keep a community stable: Carnivores hold the other populations in check, so the whole community stays in balance.
The points to remember: carnivores and stability
- Negative feedback: more herbivores, more carnivores; then fewer herbivores, so fewer carnivores.
- So carnivores regulate population sizes, and herbivores do not strip the plants bare.
- Their faeces and dead bodies feed the decomposers, which recycle nutrients.
- Carnivores compete with each other for prey and space, which limits their own numbers.
- They leave uneaten prey for scavengers, such as vultures and crows.
- A top carnivore can be a keystone species, holding many food chains together.
- They eat the most common herbivore, so rarer herbivore species can survive.
Remember it as: Feedback, regulate, feed decomposers, compete, scraps, keystone, eat the common one.
Real example: on Isle Royale, USA, only two wolves were left by 2016 and the moose rose to over 2,000, eating young trees. Wolves were brought back in 2018-19, and by 2023 the moose had fallen to about 970.
Link every point to stability: 'Carnivores eat herbivores' is not enough: say how it keeps the numbers in balance.
How this comes up: Paper 2, Section B (b): explain how a community of carnivores contributes to stability [7].
Wolves were brought back to Isle Royale, USA, in 2018 after they had almost died out.
Explain how a community of carnivores contributes to the overall stability of an ecosystem.
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