Explain how communities can be affected by abiotic and biotic factors,…

The idea

A community is all the populations of different species living together in a habitat. Abiotic factors are non-living: temperature sets enzyme-controlled rates and can limit which species survive; light intensity limits photosynthesis and so plant growth and the food available to consumers; water supply controls photosynthesis, transport and survival, with too little or too much both harmful; pollutants such as sulfur dioxide or excess fertiliser can kill sensitive species or cause eutrophication. Biotic factors come from other organisms: competition between individuals for light, water, space or food reduces the resources each obtains, and predation transfers energy from prey to predator and controls prey numbers. A change in any factor shifts population sizes, so the composition of the community alters.

What earns the marks

• Defines a community as all the populations of different species in a habitat, distinguishing it from a single population.
• Explains that temperature affects enzyme-controlled reactions and so growth and survival, changing which species thrive.
• Explains that light intensity limits photosynthesis, affecting plant growth and the food available to consumers.
• Explains that water availability affects photosynthesis, transport and survival, with deficiency or waterlogging both reducing growth.
• Explains that pollutants such as sulfur dioxide or excess fertiliser can kill sensitive species or cause eutrophication.
• Explains that competition for light, water, space or food reduces the resources available to each individual.
• Explains that predation transfers energy and controls prey numbers, altering community composition.

Common mistakes and how to put them right

• Calling a community a group of individuals of one species.

  Put it right by stating a community contains all the populations of different species in a habitat.

• Treating abiotic factors as living.

  Put it right by classifying temperature, light, water and pollutants as non-living factors.

• Assuming more water is always better.

  Put it right by noting that waterlogging can also reduce growth and survival.

Exam tips

• Read the stem carefully to decide whether the factor named is abiotic or biotic before choosing an option.
• Link each factor to its effect on population size or survival rather than just naming it.
• Eliminate options that describe a single organism's feature instead of a community-level effect.