Community at Higher Level: The same idea as SL: a community is the interacting populations in one place. HL uses the Serengeti grassland.
Practise this as you read
- Name the populations in a community and how they interact.
- Explain how carnivores keep it stable.
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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: in the Serengeti, Tanzania, the grassland community includes grasses, about 1.3 million wildebeest, zebra, lions, hyenas and dung beetles.
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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: in the Serengeti, wildebeest graze the grass, lions hunt the wildebeest, and dung beetles bury the dung, returning nutrients to the soil for the grass.
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.
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: Serengeti lions and hyenas mostly hunt whichever grazer is most common, often wildebeest or zebra. This stops one grazer taking over, so many grazer species share the grassland.
Who else the carnivores feed
- Vultures and jackals clear what the lions leave.
- Beetles, bacteria and fungi return the nutrients to the soil.
Link every point to stability: 'Carnivores eat herbivores' is not enough: say how it keeps the numbers in balance.
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How this comes up: Paper 2, Section B (b): explain how a community of carnivores contributes to stability [7].
The Serengeti grassland of Tanzania has large populations of lions, leopards, cheetahs and hyenas.
Explain how a community of carnivores contributes to the overall stability of an ecosystem.
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