Competition and Adaptation

In any habitat, resources are limited. There is only so much light falling on a patch of woodland floor, only so much water in the soil, only so much food in a meadow. Because organisms need these resources to survive and reproduce, they compete for them — both with members of their own species and with other species. Over very long periods, the organisms that are best suited to their environment tend to survive and breed, so populations become adapted — they develop features that fit them to their way of life. This lesson, part of Topic B4 of OCR Gateway Science A, examines what plants and animals compete for, the difference between competing within and between species, and the three kinds of adaptation (structural, behavioural and functional), finishing with the remarkable extremophiles that survive where almost nothing else can.

By the end of this lesson you should be able to list what plants and animals compete for, distinguish intraspecific from interspecific competition, classify and give examples of structural, behavioural and functional adaptations, and describe how extremophiles are adapted to extreme conditions.

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What Organisms Compete For

Competition happens when two or more organisms need the same limited resource. The organism that obtains more of the resource is more likely to survive and reproduce; the one that loses out may grow poorly, fail to breed, or die. Importantly, plants and animals compete for different things, because they live in different ways.

Plants compete for:

• Light — needed for photosynthesis; a plant overshadowed by taller neighbours cannot photosynthesise well.
• Water — needed for photosynthesis, support and transport.
• Space — room to grow roots and leaves.
• Mineral ions in the soil — such as nitrate, needed to make proteins for growth.

Animals compete for:

• Food — their source of energy and materials.
• Water — needed for all life processes.
• Mates — to reproduce and pass on their genes.
• Territory — an area that provides food, shelter and a place to raise young.

Resource	Competed for by plants?	Competed for by animals?
Light	Yes	No
Water	Yes	Yes
Space	Yes	(as territory)
Mineral ions	Yes	No
Food	No (they make their own)	Yes
Mates	No	Yes
Territory	No	Yes

Exam Tip: A frequent slip is to say plants compete for "food". Plants make their own food by photosynthesis, so they compete for the raw materials and conditions for photosynthesis and growth — light, water, space and mineral ions — never for food itself. Animals, which cannot make food, compete for food, water, mates and territory.

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Intraspecific and Interspecific Competition

Competition comes in two forms, depending on who is competing.

• Intraspecific competition is competition between members of the same species (intra- means "within"). Two robins competing for the same territory, or two oak seedlings competing for the same patch of light, are competing intraspecifically. This is often the most intense form of competition, because members of the same species need exactly the same resources.
• Interspecific competition is competition between members of different species (inter- means "between"). A red squirrel and a grey squirrel competing for the same nuts, or different plants competing for the same light, are competing interspecifically.

Type	Between whom	Memory hook	Example
Intraspecific	Same species	intra = within	Two stags competing for the same mate
Interspecific	Different species	inter = between	Red and grey squirrels competing for food

When two different species compete for very similar resources, one is often better adapted and gradually out-competes the other, whose population then declines. This is exactly what has happened in much of Britain, where the introduced grey squirrel competes with — and largely displaces — the native red squirrel.

Exam Tip: Keep the prefixes straight: intraspecific = within one species; interspecific = between different species. If you struggle, remember "international" means between nations — so interspecific is between species.

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Adaptations: Fitting Organisms to Their Environment

An adaptation is a feature that helps an organism survive and reproduce in its environment. Adaptations arise over many generations by natural selection: individuals with helpful features are more likely to survive, breed and pass those features on. OCR groups adaptations into three types, and you should be able to classify any example into one of them.

Type of adaptation	What it is	Examples
Structural	A feature of the body's structure — its shape, size or parts	A polar bear's thick fur and fat for insulation; a cactus's spines (reduced leaves) to cut water loss; a camel's large feet to spread its weight on sand
Behavioural	The way an organism behaves	Migration of birds to warmer regions in winter; animals basking in the sun to warm up; nocturnal desert animals being active in the cool night
Functional	A feature of the internal workings / biochemistry of the body (how it functions)	Producing little, concentrated urine to conserve water (camel, kangaroo rat); a bear's slowed metabolism during hibernation; venom or antifreeze chemicals in the blood

The boundaries can overlap, but a reliable test is: Structural = something you could point to in the body; behavioural = something the animal does; functional = something happening inside the body (its chemistry or physiology).

Worked example — classifying adaptations

A desert kangaroo rat (1) has very long back feet, (2) is active only at night, and (3) produces extremely concentrated urine. Classify each adaptation.

Step 1 — feature (1), long back feet. This is a feature of the body's structure you could point to, so it is structural (it helps the rat move quickly across sand to escape predators).

Step 2 — feature (2), active only at night. This is something the animal does, so it is behavioural (being active in the cool night reduces water loss and avoids the daytime heat).

Step 3 — feature (3), concentrated urine. This is an internal, physiological process, so it is functional (it conserves water in the dry desert).

Answer: (1) structural, (2) behavioural, (3) functional.

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Adaptations to Cold and to Dry Conditions

Two environments come up so often that it is worth knowing a worked set of adaptations for each.

Adaptations to cold (e.g. a polar bear in the Arctic):

• Thick layer of fat (blubber) and dense fur — insulation to reduce heat loss (structural).
• Small surface-area-to-volume ratio (a large, rounded body) — reduces the proportion of the body losing heat (structural). This links back to the surface-area-to-volume idea from Topic B2.
• White fur — camouflage against the snow when hunting (structural).
• Large feet to spread weight on snow and ice (structural).

Adaptations to dry, hot conditions (e.g. a cactus in a desert):

• Spines instead of leaves — a very small surface area to reduce water loss by transpiration, and protection from herbivores (structural).
• A thick, waxy cuticle and few stomata — further cut water loss (structural/functional).
• A swollen, fleshy stem that stores water (structural).
• Widespread shallow roots to absorb rain quickly, or very deep roots to reach underground water (structural).

Exam Tip: When asked to explain an adaptation, always link the feature to the benefit: not just "the cactus has spines" but "the cactus has spines, which have a small surface area to reduce water loss". The mark is for the reason, not just the feature.

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Extremophiles