What is a community?
One kind of living thing in one place is a population, for example all the deer in a meadow. A community is all the populations of different species living together in one place: the grass, the deer, the birds, the insects, the fungi and the tiny microbes in the soil.
- Population = one species.
- Community = many species together (only living things).
- Ecosystem = community plus the non-living things (sun, water, soil, air).
The members of a community depend on each other for food and shelter.
Why do communities differ?
Plants are the base of every community, so the plants that can grow decide what animals can live there. Plants need water, warmth and soil. The biggest difference between a desert, a grassland and a forest is the yearly rainfall. Temperature and soil also matter. A very cold place with some rain can form a different community, like tundra.
Desert community
A desert gets very little rain, usually less than about 25 cm in a year, and the sun is strong.
- Plants are spread far apart. They store water in thick stems (cactus), have tiny leaves or spines, and have long roots.
- Animals save water too: lizards, snakes, camels and desert rats. Many come out at night when it is cooler.
- Few species live here, and the food chains are short.
Grassland community
A grassland gets medium rain, roughly 25 to 100 cm a year, with a dry season. Too little rain for thick forest, too much for desert.
- Plants are mostly grasses with a few shrubs and scattered trees. Grass grows from the base, so grazing and even fire do not kill it.
- Animals are grazers such as deer, antelope, zebra and bison, their hunters (lions, wolves) and many insects and birds.
- Grass roots are deep and thick and make very rich soil.
Forest community
A forest gets plenty of rain, often over 100 cm a year, and is full of trees. A tropical rainforest has the most kinds of life of any land community.
- Layers: the tall canopy catches most sunlight; below it are the understorey and shrubs, and then the dim forest floor with ferns, fungi and fallen leaves.
- Animals live at every layer: monkeys and birds in the canopy, deer and tigers on the ground, insects and worms in the leaf litter.
- Dead leaves are broken down fast by decomposers, so nutrients are recycled quickly.
Comparing the three
| Desert | Grassland | Forest | |
|---|---|---|---|
| Yearly rain | very low | medium | high |
| Main plants | cactus, shrubs | grasses | trees |
| Plant cover | sparse | carpet of grass | dense, layered |
| Kinds of life | few | many | most |
Communities are not shut boxes. Between them there are in-between places (savanna, scrub) and, over long times, one community can change into another. This slow change is called ecological succession.
Try it: three places
Choose a dry, a grassy and a tree-filled place. List three plants and two animals for each. Say which plant trait helps it live there, such as thick stems, grass leaves that regrow, or tall trunks that reach the light.
Key formulas and definitions
- Population = one species in a place
- Community = all populations (living things only) in a place
- Ecosystem = community + non-living things
- Desert: very low rain; grassland: medium rain; forest: high rain (rough guide)
- Forest layers: canopy, understorey, forest floor
Worked examples
1. A pond has frogs, fish, reeds, algae and snails. Is this a population or a community? Why?
A community. It has many different species living together. A population would be only one species, such as only the frogs.
2. A place gets 15 cm of rain a year and has cacti and lizards. Which community is it?
A desert community. Very low rain and plants that store water point to a desert.
3. Why do deer live mostly in grasslands and not in deserts?
Deer eat grass and other plants. A grassland has plenty of grass, but a desert has too few plants to feed many grazers.
4. Why does a rainforest have more kinds of animals than a desert?
It has lots of water and many kinds of plants in many layers. Each layer gives food and shelter to different animals. A desert has few plants, so fewer animals can live there.
5. A grass plant is nibbled to the ground by cattle and grows back. Why does this suit a grassland?
Grass grows from the base of the leaf, near the ground, so grazers cannot easily kill it. This is why grass survives in grazed or burnt grasslands.
Common mistakes
- Saying community and ecosystem mean the same thing. A community has only living things; an ecosystem also has the non-living things.
- Saying a population and a community are the same. A population is one species; a community is many species together.
- Thinking every desert is hot and sandy. A desert is defined by very little rain; some are cold and rocky.
- Thinking only rain matters. Temperature and soil also help decide the community, rain is just the biggest factor in this lesson.