Biotic (living) factors which can affect a community are: • availability…

The idea

Biotic factors are living influences on a community. Food availability sets the energy supply: if a plant's pollinators decline, seed set falls and the whole food web is affected. A new predator can reduce prey numbers directly, and may also alter prey behaviour, such as feeding less or hiding more. A new pathogen can spread rapidly if the population has little immunity, reducing numbers before resistance builds. Competition matters because if one species outcompetes another for food, light, space or mates, the weaker population may fall below the number needed to breed successfully. For example, grey squirrels outcompeting red squirrels for food and habitat can leave red squirrel numbers too low to sustain breeding. These factors interact, so a community's size and diversity depend on the balance between births, deaths, immigration and emigration.

What earns the marks

• Define biotic factors as living influences on a community, distinguishing them from abiotic factors such as temperature or light intensity.
• Explain that food availability controls the energy available to consumers, affecting growth, survival and reproductive success.
• Describe how a new predator can reduce prey numbers by direct predation and can also change prey behaviour, for example reducing feeding time.
• Explain that a new pathogen can cause disease and reduce population size, especially where the population has little immunity.
• Explain that interspecific competition can lead to one species outcompeting another for resources such as food, light, space or mates.
• State that if numbers fall too low, individuals may not find mates or may not breed successfully, so the population may decline further or disappear.
• Use a named example, such as grey squirrels outcompeting red squirrels, to show how a biotic factor affects a community.

Common mistakes and how to put them right

• Classifying temperature or rainfall as a biotic factor.

  Put it right: These are abiotic; biotic factors are living, such as predators, pathogens and competitors.

• Thinking a new predator only kills prey.

  Put it right: It can also alter prey behaviour, for example causing prey to spend more time hiding and less time feeding.

• Assuming a new pathogen always wipes out a population.

  Put it right: Some individuals may have resistance, so the population may survive and recover.

• Believing competition always kills the weaker species immediately.

  Put it right: It may reduce its numbers until breeding is no longer successful, leading to local extinction over time.

• Describing a factor without linking it to the community.

  Put it right: State the effect on population numbers, breeding or community diversity.

Exam tips

• When asked to identify a biotic factor, name a living factor and then state its effect on the community, for example 'a new predator reduces prey numbers'.
• Use the phrase 'numbers are no longer sufficient to breed' when explaining the final consequence of outcompetition, as this directly matches the specification.
• If a question gives a scenario, link each factor to a specific organism and a measurable change, such as a fall in population size or breeding success.
• Avoid listing factors without explanation; marks are usually awarded for the effect on the community, not just the name of the factor.