Biodiversity: A great biodiversity ensures the stability of ecosystems by reducing the dependence of one species on another for food, shelter and the maintenance of the physical environment.

The idea

Biodiversity is the variety of species in an ecosystem. When many species are present, each one tends to rely less on any single other species for food, shelter and the maintenance of the physical environment, such as nutrient cycling, soil formation and water regulation. In a species-poor system, losing one species can remove a food source or shelter for others, so the whole community may collapse. In a species-rich system, alternative food sources and habitats exist, so the ecosystem can absorb change and remain stable. For example, a hedgerow with many plant species supports many herbivores, and predators can switch prey if one species declines. This reduces the dependence of one species on another and helps the ecosystem resist disturbance.

What earns the marks

• Biodiversity means the variety of different species living in an ecosystem.
• Greater biodiversity means species have alternative food sources, so a decline in one prey species does not cause starvation of a predator.
• Greater biodiversity provides a range of shelter and habitat types, so loss of one plant or structure does not remove all shelter.
• A diverse community maintains physical environmental processes such as nutrient cycling, decomposition and soil stability through the activities of many species.
• Stability means the ecosystem can resist or recover from change, such as disease, drought or the removal of one species.
• Reduced dependence on one species means the food web has many interconnections rather than a single critical link.

Common mistakes and how to put them right

• Thinking biodiversity only means the number of individuals, not the number of species.

  Put it right: Biodiversity is the variety of species, and often the range of habitats and genetic variation too.

• Assuming a stable ecosystem never changes.

  Put it right: Stability means the ecosystem can resist or recover from change, not that it is static.

• Believing one species can maintain the whole physical environment alone.

  Put it right: Many species together contribute to processes such as nutrient cycling and soil maintenance.

Exam tips

• Link each clause of the statement to a named example, such as a food web where a predator can switch prey.
• Use the phrase reduced dependence when explaining why a species-rich ecosystem is more stable.
• When asked to explain, give a cause and effect chain: many species → alternative food or shelter → less impact if one species declines → stable ecosystem.