4.1.5Community Level Systems

Interdependence and Competition — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 6

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7What organisms compete for

Resources in an ecosystem are limited. When two or more organisms need the same resource and it is in short supply, they compete. Organisms that compete more successfully survive and reproduce more.

Type of organism	Resources they compete for
Plants	Light, water, mineral ions (such as nitrates), space, carbon dioxide
Animals	Food, water, mates, territory (space), shelter

Competition can be interspecific (between different species) or intraspecific (between members of the same species). Intraspecific competition is usually more intense, because members of the same species need exactly the same resources. Competition among seedlings under a parent tree, for example, is intraspecific.

Part 2 of 7Interdependence and food webs

Species in a community rely on one another. A food chain shows one path of energy, for example grass, rabbit, fox. A food web shows many connected chains and gives a more realistic picture. The arrows point in the direction of energy flow, from the organism that is eaten to the organism that eats it.

Examples of interdependence include:

predator and prey: both populations depend on the other;

pollinators such as bees and flowering plants: the bee gains nectar, the plant gains pollen transfer;

seed dispersers, such as birds that eat fruits;

decomposers and plants: decomposers return mineral ions that plants need.

Because of these links, removing or adding one species can change the numbers of many others. If a key species is lost, the effect can spread through the web.

Part 3 of 7Predator-prey cycles

In a stable community, the populations of predators and prey tend to rise and fall in linked cycles. The pattern on a graph is two wavy lines, with the predator line lagging slightly behind the prey line.

Prey numbers rise when there is plenty of food and few predators.

Predators now have more food, so their numbers increase after a delay.

More predators eat more prey, so prey numbers fall.

Predators now have less food, so they starve or fail to breed and their numbers fall.

With fewer predators, prey numbers can recover and the cycle repeats.

When describing such a graph, always say which peaks first, and mention the time lag. The classic data for lynx and snowshoe hare in Canada is often used as an example.

Part 4 of 7Changes in abiotic and biotic factors

Population size is affected by changes in the environment.

Abiotic changes

Abiotic changes a cold winter can kill insects, so insect-eating birds lose food. A drought reduces plant growth, so herbivores decline. Pollution can reduce oxygen in a river and kill fish.

Biotic changes

Biotic changes a new predator or disease, a new competitor (for example an invasive species), or loss of a food source can alter populations. The grey squirrel, introduced to Britain, competes with the native red squirrel and also carries a virus that harms reds.

When predicting effects, trace the chain

change in X leads to change in Y because of the feeding or competition link.
For example, if the number of an insect pest rises, the plants it eats are reduced, but the birds that feed on the insect are likely to increase.

Part 5 of 7Adaptations that help in competition

Species survive competition by being adapted to their niche.

Adaptations that help in competition

Plants in a dense woodland may have large, thin leaves that capture the little light available, while desert plants have deep or wide roots to compete for water.

Predators have adaptations such as speed or camouflage, and prey have adaptations such as warning colours or sharp senses.

Natural selection favours these features because those with them gain more resources and leave more offspring, which links this topic with evolution.

Part 6 of 7Worked interpretation: reading a population graph

Exam questions often give a graph of two populations over time.

Describe it in a fixed order.

First, state the overall pattern, for example 'both populations rise and fall in cycles'.

Second, quote data: 'prey peaked at 3000 in year 4 while predators peaked at 400 in year 5'.

Third, link the change to a cause: 'predators increase because more prey are available'.

Finally, comment on stability: if the peaks stay about the same height over many cycles, the community is stable.

A sudden drop in one line that is not matched by the other suggests an outside cause, such as disease, hunting or a hard winter, rather than the predator-prey relationship.

Part 7 of 7Exam-style questions

In a food web, grass is eaten by rabbits and voles; foxes eat both rabbits and voles. Disease kills most rabbits. Predict and explain what happens to the vole and fox populations. [3]3 marks
A predator-prey graph shows prey peaks at 20 years and predator peaks at 22 years. Explain why the predator peak comes later. [2]2 marks
Name three resources that plants compete for and suggest one adaptation that helps a plant in a shaded woodland compete successfully. [4]4 marks

Key terms

Term	Meaning
Competition	Two or more organisms trying to obtain the same limited resource.
Interdependence	Species in a community relying on each other.
Food web	A diagram of linked food chains in a community.
Predator	An animal that kills and eats other animals.
Prey	An animal that is eaten by a predator.
Interspecific competition	Competition between different species.
Intraspecific competition	Competition between members of the same species.
Stable community	A community where population sizes stay roughly constant over time.

Avoid these mistakes

Drawing food web arrows from predator to prey. Arrows show the transfer of energy, from the food to the feeder.

Describing a predator-prey graph without mentioning the lag between the two peaks.

Confusing intraspecific (same species) and interspecific (different species) competition.

Saying that removing a predator always benefits the prey. Other species and competition can change the outcome.

Forgetting that animals compete for mates and territory as well as food.

Last updated: 20 October 2026