B15.5 Competition in Plants

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: competing for light, space, water and mineral ions; seed dispersal and other ways plants avoid competition. 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

• State the four resources that plants compete for and why each is needed
• Explain why seedlings growing close to the parent plant grow poorly
• Describe how seeds are dispersed by wind, animals, water and explosion
• Describe other ways plants avoid or win competition
• Analyse data from a seed-density investigation

Key words

• Competition: When plants need the same limited resource, such as light or water.
• Mineral ions: Dissolved substances from the soil, such as nitrate (for making proteins) and magnesium (for chlorophyll).
• Seed dispersal: Spreading seeds away from the parent plant.
• Canopy: The layer of leaves at the top of a wood or forest that blocks light from reaching the ground.
• Dry mass: The mass of a plant after all its water has been removed; a reliable measure of growth.

What plants compete for

• light; Why the plant needs it for photosynthesis, to make glucose
• water; Why the plant needs it for photosynthesis, to keep cells firm (turgid) and to carry dissolved substances
• mineral ions; Why the plant needs it e.g. nitrate to make amino acids and proteins; magnesium to make chlorophyll
• space; Why the plant needs it room for roots to take up water and ions, and for leaves to spread and catch light

Plants make their own food, so they do not compete for food. Plants growing close together shade each other, and their roots take water and mineral ions from the same soil. When a tall tree in a forest dies and falls, the gap lets light in: young trees race upwards, and the fastest-growing one fills the gap in the canopy.

Competition between two plants of the same species is intraspecific; between plants of different species it is interspecific.

Worked example: reading the seed-density graph

As seed density increases, mean dry mass per seedling decreases: 0.92 g at 5 seeds per pot but 0.21 g at 80.
With more seedlings, each gets less light, water and mineral ions, so each photosynthesises and grows less.
Percentage decrease from 10 to 40 seeds = (0.85 − 0.40) ÷ 0.85 × 100 = 52.9% (3 s.f.).

Answer: more competition → less growth per seedling

Seed dispersal

If seeds fell straight under the parent plant, the seedlings would compete with the parent and with each other. Most plants have adaptations that carry seeds away.

• wind; Adaptation light seeds with “parachutes” or “wings” carried on the breeze; Example dandelion, ash, thistle
• animals (outside); Adaptation hooks that catch on fur or clothing; Example burdock, cleavers
• animals (inside); Adaptation juicy fruits eaten by birds or mammals; seeds pass out in droppings; Example blackberry, rowan
• water; Adaptation fruits with air spaces that float; Example coconut, alder
• explosion; Adaptation pods dry out and burst, flinging seeds; Example gorse, broom

Key fact: Seed dispersal reduces competition between the parent plant and its seedlings, and between the seedlings themselves. It also lets the species colonise new areas.

Other ways to avoid competition

• Timing: bluebells and wood anemones grow early in spring, using food stored underground, before the tree canopy blocks the light.
• Roots: some plants have very deep roots, others a wide network of shallow roots, so they take water from different layers of soil.
• Height: tall plants and climbers such as ivy reach the light above their neighbours.
• Leaves: plants on a shady woodland floor often have large, thin leaves with lots of chlorophyll to absorb as much light as possible.

Exam tip: In data questions on woodland plants, link the time of growth to the light at the floor and to the leaves on the trees.

Common mistakes

• Saying plants compete for food. They compete for light, space, water and mineral ions.
• Saying a seedling under its parent “gets no sunlight” without linking it to less photosynthesis and slower growth.
• Saying seeds are dispersed “to make more plants”. The point is to reduce competition and colonise new areas.
• Comparing total mass instead of mean mass per seedling when pots have different numbers of seedlings.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these do plants not compete for? (Choose from: Territory, Light, Water, Mineral ions.)

Answer: Territory

Working: Plants compete for light, space, water and mineral ions from the soil.

Question 2. True or false? Bluebells avoid competing with trees for light by growing and flowering before the trees’ leaves are fully open.

Answer: True

Working: True. They grow in early spring using food stored in their bulbs, while light can still reach the ground.

Question 3. The graph shows how much light reaches the floor of a deciduous wood during the year. In which months should a woodland flower grow and flower to get the most light? (Choose from: February to April, June to August, September to October, May to July.)

Answer: February to April

Working: From February to April 65–80% of the light reaches the floor, because the trees have few leaves. From June to August only about 8–10% gets through the canopy.

Related lessons

• Previous lesson: B15.4 Competition in Animals
• Next lesson: B15.6 Adapt and Survive
• In this chapter: B15.1 The Importance of Communities
• In this chapter: B15.2 Organisms in Their Environment
• In this chapter: B15.3 Distribution and Abundance

Continue in the full course

This lesson is section B15.5 (AGCSC1505) of our free GCSE Combined Science 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 Combined Science course.