Communities: suggest the factors for which organisms are competing in a given habitat

The idea

Organisms in a community share a habitat, so when a resource is in short supply they compete for it. Plants compete for light, water, mineral ions and space; animals compete for food, water, mates and territory. In a given habitat you should identify which of these is likely to be limiting by reading the description: a dense woodland suggests light is scarce for ground plants, while a dry desert suggests water is the key factor. The command word 'suggest' means you apply your knowledge to the context rather than recall a list. Name the factor, then justify it from the habitat details, for example 'the shrubs compete for light because the tall trees cast shade'. Competition between members of the same species is intraspecific; between different species it is interspecific.

What earns the marks

• Name a specific resource, such as light, water, mineral ions, space, food, mates or territory, rather than writing the vague phrase 'resources'.
• Match the named factor to the habitat described, for example water in a dry environment or light in a shaded woodland.
• Justify the choice by referring to a feature of the habitat, such as dense canopy, low rainfall or a crowded rock pool.
• Distinguish between intraspecific competition within one species and interspecific competition between different species where the question allows.
• Recognise that competition occurs when a resource is limited, and that the organism best adapted to obtain it is more likely to survive and reproduce.
• Give more than one factor where the habitat description supports it, since real habitats usually have several limiting resources.

Common mistakes and how to put them right

• Writing only 'food' for plants.

  Put it right: Plants make their own food by photosynthesis, so they compete for light, water, mineral ions and space.

• Naming a factor without linking it to the habitat.

  Put it right: Add a clause explaining why that resource is scarce, such as 'because the canopy blocks sunlight'.

• Confusing competition with predation.

  Put it right: Competition is a struggle for the same limited resource; predation is one organism eating another.

Exam tips

• Use the habitat words in the question as your evidence, quoting them briefly to justify each factor.
• Aim for two or three different factors so you cover more of the available credit.
• Write the factor first and the reason second, keeping each point to one clear sentence.
• Check whether the question asks about plants, animals or both before choosing your factors.