Adaptations, interdependence and competition

7. Ecology

Ecology is about organisms and the environment they live in, and how the two affect each other. This subtopic gives you the vocabulary — habitat, population, community, ecosystem — and then two big ideas built on it: organisms compete for the resources they need, and they depend on each other.

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What adaptations, interdependence and competition covers

Every subtopic below has its own questions, mark schemes and notes.

4.7.1.1Communities and competition

4.7.1.2Abiotic factors

Abiotic factors are the non-living parts of the environment. When one of them changes, the populations that depend on it change too — and so do the populations that depend on those.

4.7.1.3Biotic factors

Biotic factors are the living parts of the environment. There are only four on the specification's list, and every exam question about them is built from one of the four.

4.7.1.4Adaptations

An adaptation is a feature that lets an organism survive in the conditions where it lives. Everything in this subtopic comes down to sorting adaptations into three types and explaining how each one helps.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Habitat — The place where an organism lives.

Population — All the organisms of one species living in a habitat.

Community — All the populations of the different species living in a habitat.

Ecosystem — A community of organisms interacting with the non-living parts of their environment.

Interdependence — The way each species in a community relies on other species for things such as food, shelter, pollination and seed dispersal.

Stable community — One in which the species and environmental factors are in balance, so population sizes stay roughly constant.

Abiotic factor — A non-living factor of the environment, such as temperature or light intensity.

Moisture level — How much water is present in the soil or the air.

Mineral content — The amount of mineral ions, such as nitrates, available in the soil.

Biotic factor — A living factor of the environment, such as food availability, predators, pathogens or competition.

Pathogen — A microorganism that causes disease.

Outcompete — To be better than another species at obtaining a shared resource, so that the other species gets less of it.

Adaptation — A feature that allows an organism to survive in the conditions where it normally lives.

Structural adaptation — A feature of an organism's body structure, such as its shape or colour.

Behavioural adaptation — A way in which an organism behaves that helps it survive.

Functional adaptation — A process inside the body, such as one involving metabolism or reproduction, that helps an organism survive.

Extremophile — An organism adapted to live in very extreme conditions such as high temperature, high salt concentration or high pressure.

Metabolism — All the chemical reactions taking place inside an organism's body.

Where marks get lost

The mistakes that come up again and again in this topic.

Population is one species; community is all the species. Muddling these two costs easy marks.
An ecosystem is not just the living things plus the weather — the mark is for the word interaction between the two.
When you trace a change through a food web, do it one step at a time and say which way each population moves. Jumping straight to the end loses the middle marks.
Mates is on the list of things animals compete for, and it is the one most often left out.
Answer these in a chain: what happens to the factor, what happens to the plant, what happens to the population that eats the plant. One sentence per mark.
Oxygen level is the aquatic one and carbon dioxide level is the plant one — don't swap them.
Wind is easy to overlook because it is not on the photosynthesis list. Its main effect is increasing water loss by transpiration.
If data show that two things change together, that is a correlation. Saying the abiotic factor definitely caused the change needs more evidence than one set of readings.
The specification's wording for competition is that numbers become too low to breed. Saying only that numbers fall misses the point being tested.
If a question says give other biotic factors, do not repeat the one already described in the question.
Check whether the factor involves a living organism. Disease and food supply are biotic; temperature and pH are abiotic.
A graph showing two populations changing at the same time never proves on its own that one caused the other.
Hibernation is classed as functional, not behavioural, because the key change is the lowered metabolic rate. Say the word metabolism in your answer.
Cold means keep heat in, so a low surface area to volume ratio; hot means lose heat, so a high one. Decide which you need before you start writing.
A cube has six faces, so surface area is 6 times side squared. Forgetting the 6 is the commonest slip in these calculations.
Never write that an animal changed itself because it needed to. Adaptations are features the organism already has, built up over many generations.
When an unfamiliar organism is described, take each characteristic in turn and link it to a condition the organism has to cope with.

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