16.1 The Importance of Communities

A free GCSE Biology lesson for AQA (8461) from OBA School: habitats, populations, communities and ecosystems; interdependence, competition and stable communities. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define habitat, population, community and ecosystem, and put them in order of size
• Describe how organisms in a community depend on each other (interdependence)
• List what plants and animals compete for
• Use a food web to explain how a change in one population affects others
• Describe what makes a community stable

Key words

• Habitat: The place where an organism lives, for example a pond, a hedge or a rock pool.
• Population: All the organisms of one species living in a habitat at the same time.
• Community: All the populations of different species living and interacting in a habitat.
• Ecosystem: The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.
• Inter- dependence: Species in a community depend on each other for food, shelter, pollination, seed dispersal and more.
• Competition: Organisms trying to get the same limited resource, such as light, water, food or mates.
• Stable community: A community where all the species and environmental factors are in balance, so population sizes stay roughly constant.

From individual to ecosystem

To survive and reproduce, organisms need materials from their surroundings and from other organisms. Ecologists study this at different levels; each level contains the one before it.

• Individual; Meaning One organism; Example in a garden pond one great diving beetle
• Population; Meaning All the organisms of one species; Example in a garden pond all the great diving beetles in the pond
• Community; Meaning All the populations of all species; Example in a garden pond beetles, frogs, snails, pondweed, algae, bacteria …
• Ecosystem; Meaning The community plus the non-living environment; Example in a garden pond the community plus the water, mud, light, temperature and dissolved oxygen

Key fact: Population = one species. Community = many species. Ecosystem = community + abiotic factors.

Interdependence

No species lives on its own. In a community, species depend on each other for:

• food – animals eat plants or other animals;
• shelter – birds nest in trees, insects hide under bark;
• pollination – many flowering plants need bees, moths or other insects to carry pollen;
• seed dispersal – birds and mammals eat fruits or carry seeds to new places.

A food web shows many linked food chains. The arrows point from the food to the organism that eats it, so they show the direction in which energy is transferred.

Worked example: a change in the food web

A disease kills most of the rabbits in the wood. Suggest what happens to the voles and the owls.
Foxes have fewer rabbits to eat, so they eat more voles. The vole population falls.
Owls also eat voles, so they now have less food. The owl population is likely to fall too.
With fewer rabbits eating it, there may be more grass, which could later help the voles recover.

Answer: voles fall (eaten more by foxes), so owls also fall

Exam tip: In “what happens if…” questions, follow every arrow into and out of the organism that changes. Give a reason for each step: “…because…, so…”.

Competition

Resources are limited, so organisms compete for them. Competition can be between members of the same species or between different species.

• light (for photosynthesis); Animals compete for food
• space; Animals compete for mates
• water; Animals compete for territory
• mineral ions from the soil; Animals compete for

Plants make their own food, so they do not compete for food. Animals do not need light to make food, so “light” is not on the animal list.

Stable communities

In a stable community all the species and the environmental factors are in balance. Population sizes go up and down a little but stay roughly constant over many years.

Examples include tropical rainforests, ancient oak woodlands and coral reefs. They have many species and many links between them, so they take a long time to develop. If they are damaged, they can take hundreds of years to recover, or may never recover.

Worked example: is the population stable?

A wildlife group counts the badgers in a wood each spring: 28, 31, 29, 30, 27.
Mean = (28 + 31 + 29 + 30 + 27) ÷ 5 = 145 ÷ 5 = 29. Range = 31 − 27 = 4.
The numbers vary only a little around the mean, so the population looks stable.

Answer: mean 29 badgers; small range of 4, so roughly constant

Removing even one species can upset the balance. If a top predator is removed, its prey may increase and overgraze the plants, which then affects every species that relies on those plants.

Common mistakes

• Mixing up population (one species) and community (all the species).
• Saying an ecosystem is “just the plants and animals”. It includes the non-living parts too.
• Drawing food-web arrows from the eater to the food. Arrows point towards the eater.
• Saying plants compete for food, or explaining a food-web change in one step with no reasons.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the name for all the populations of different species living and interacting in one habitat? (Choose from: A community, A population, An ecosystem, A habitat.)

Answer: A community

Working: A community is all the populations of different species in a habitat. An ecosystem also includes the non-living (abiotic) parts.

Question 2. True or false? In a stable community, the sizes of the populations stay roughly constant over time.

Answer: True

Working: True. In a stable community the species and environmental factors are in balance, so population sizes stay roughly constant (they rise and fall only a little).

Question 3. Which of these resources do plants compete for? (Choose from: Light, Mates, Territory, Food.)

Answer: Light

Working: Plants compete for light, space, water and mineral ions. Food, mates and territory are what animals compete for.

Related lessons

• Previous lesson: 15.10 New Systems of Classification
• Next lesson: 16.2 Organisms in Their Environment
• In this chapter: 16.3 Distribution and Abundance
• In this chapter: 16.4 Competition in Animals
• In this chapter: 16.5 Competition in Plants

Continue in the full course

This lesson is section 16.1 (AGCBH1601) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.