Plants in a community or habitat often compete with each other for light…

The idea

Green plants are producers, so their growth sets the energy budget of a habitat. Because light, space, water and mineral ions are finite, individual plants that capture more of a resource leave more offspring. Light and space are contested above ground: a taller shoot or a wider leaf canopy shades neighbours, while a spreading root or stem system claims ground. Below ground, root networks draw water and dissolved mineral ions such as nitrate and magnesium from the same soil volume. A gardener thinning carrot seedlings sees the result: crowded seedlings stay pale and thin, whereas spaced plants grow larger because each has more light, water and ions. Competition is therefore a density-dependent limit on growth, and it shapes which species and individuals persist.

What earns the marks

• Competition occurs when a resource is in limited supply and more than one plant requires it.
• Light and space are contested mainly by shoots and leaves, so shading and crowding reduce photosynthesis and growth.
• Water and mineral ions are absorbed by roots from soil, so overlapping root systems deplete the same supply.
• Mineral ions such as nitrate and magnesium are needed for healthy growth, so shortage causes poor growth and yellowing.
• Successful competitors survive and reproduce, so competition acts as a selection pressure within the community.
• Thinning or spacing crop plants reduces competition and increases yield, which shows the effect in practice.

Common mistakes and how to put them right

• Treating competition as fighting or deliberate harm.

  Put it right by describing it as two or more organisms needing a resource that is in short supply.

• Assuming plants compete only above ground.

  Put it right by naming water and mineral ions taken in by roots as below-ground resources.

• Confusing a mineral ion shortage with a disease.

  Put it right by linking poor growth or yellowing to a lack of ions such as nitrate or magnesium.

Exam tips

• Name the resource and say why it is limiting, for example light is blocked by a taller neighbour.
• Link each resource to the plant part that obtains it: leaves for light, roots for water and ions.
• Use a before-and-after comparison, such as crowded seedlings against spaced seedlings, to show the effect of competition.