A great biodiversity ensures the stability of ecosystems by reducing the…

The idea

Stable ecosystems keep functioning when conditions change. High biodiversity contributes to this stability because many species perform similar roles, so the loss of one species has less impact. If a woodland has several plant species providing food and shelter, a disease affecting one tree species does not remove all food and nesting sites. Similarly, several decomposers and pollinators may maintain nutrient cycling and reproduction. Where biodiversity is low, species depend heavily on one another: a single crop species, one pollinator or one decomposer may be essential, so its decline can collapse the whole community. Reducing dependence on any one species for food, shelter and maintaining the physical environment therefore buffers the ecosystem against change.

What earns the marks

• High biodiversity means many species are present, so several species can carry out similar roles.
• If one species declines, others can still provide food, shelter or nutrient cycling, so the ecosystem continues to function.
• Low biodiversity creates heavy dependence on a few species, so the loss of one species can have a large effect on the community.
• Maintenance of the physical environment includes processes such as nutrient cycling, soil formation and water retention carried out by living organisms.
• Stability means the ecosystem can resist or recover from change, such as disease, drought or the arrival of a new predator.
• Reducing dependence between species is the mechanism linking high biodiversity to ecosystem stability.

Common mistakes and how to put them right

• Saying high biodiversity means no species can ever be lost.

  Put it right: It means the ecosystem is more likely to keep functioning when a species is lost, not that loss is impossible.

• Treating stability as the same as no change.

  Put it right: Stable ecosystems still change, but they continue to support a community of organisms.

• Forgetting the physical environment.

  Put it right: Decomposers and plants maintain soil and nutrient cycles, which is part of the stability described.

Exam tips

• Use the phrase 'reduces dependence on one species' and then give a concrete example such as several pollinators or several decomposers.
• Structure your answer as cause and effect: high biodiversity leads to several species per role, which leads to continued food, shelter and nutrient cycling.
• When asked about a low-biodiversity example such as a wheat field, explain that a single pest or disease could remove the dominant species and disrupt the whole community.