Biotic factors: Students should be able to explain how a change in a biotic factor might affect a given community given appropriate data or context.

The idea

A community is all the interacting populations in a habitat. Biotic factors are living influences: food availability, new predators, new pathogens, and competition between species. To explain a change, identify the factor, name the affected population, then trace the effect through feeding relationships and numbers. For example, if a new predator arrives, prey numbers fall; predators may then compete, and plants the prey ate may grow more. If a pathogen spreads, susceptible species decline while resistant ones may increase. If a competitor is removed, the remaining species may gain resources and increase. Always link cause to effect using data such as population counts, then state the resulting change in the community.

What earns the marks

• Identifies the specific biotic factor that has changed, such as a new predator, pathogen, competitor or change in food availability.
• Names the population directly affected and states whether its numbers rise or fall.
• Explains the mechanism, for example predation reduces prey numbers or competition reduces shared resources.
• Traces the effect to at least one other population through a feeding relationship or shared resource.
• Uses data or context provided, such as population sizes or dates, to support the explanation.
• Concludes with the overall change to the community, such as a fall in biodiversity or a shift in species present.

Common mistakes and how to put them right

• Treating a biotic factor as abiotic, for example calling temperature a biotic factor.

  Put it right: Biotic factors are living influences such as predators, pathogens and competitors.

• Stating only that numbers change without explaining why.

  Put it right: Give the mechanism, such as predation or competition for food.

• Assuming every population in the community changes in the same direction.

  Put it right: Trace each effect separately, as some populations may rise while others fall.

Exam tips

• Underline the factor named in the question and the population it acts on before writing.
• Use connectives such as therefore and as a result to show a clear chain of reasoning.
• Refer to figures from the data, for example a fall from 200 to 120, rather than saying numbers decreased.