Succession from pioneer species to climax community. At each stage in succession, certain species may be recognised which change the environment so that it becomes more suitable for other species with different adaptations. The changes in the abiotic environment result in a less hostile environment and changing diversity. Conservation of habitats frequently involves management of succession. Students should be able to evaluate evidence and data concerning issues relating to the conservation of species and habitats and consider conflicting evidence; and use the concept of succession to explain the management of an ecosystem.

What this dot point is asking

AQA wants you to describe succession from pioneer species to climax community, explain how each stage changes the abiotic environment and diversity, distinguish primary from secondary succession, and use the concept of succession to explain conservation and the management of ecosystems, evaluating conflicting evidence.

From pioneer species to climax community

Succession proceeds through stages called seres:

1. Pioneer species colonise the bare, hostile environment. They are adapted to extreme abiotic conditions: lichens and mosses can survive with little water, no soil and exposed conditions, and some fix nitrogen.
2. Each community changes the abiotic environment. Pioneers weather the rock and, when they die, decompose to add humus, beginning to form soil. The soil retains water and nutrients, so conditions become less hostile.
3. The improved conditions let new species with different adaptations colonise and outcompete the previous community. Grasses, then herbs, then shrubs, then trees establish in turn.
4. Diversity increases through the intermediate stages as more niches appear and more species can survive.
5. A stable climax community finally develops: a self-sustaining community dominated by the species best suited to the stable conditions (for example, oak woodland in much of the UK), which does not change significantly over time.

Diversity through succession

Species diversity is low at the pioneer stage (few species can tolerate the harsh conditions), rises through the intermediate stages, and may fall slightly at the climax community, where a few dominant competitors (such as tall trees) outcompete and shade out smaller species.

Conservation and managing succession

Conservation is the protection and management of habitats and species to maintain biodiversity. Because succession naturally tends toward a single climax community, conserving the diversity of earlier stages often means deliberately preventing succession from reaching its endpoint.

Management techniques that stop or set back succession:

• Grazing or mowing keeps grassland (such as chalk grassland or heathland) open by removing tree seedlings, maintaining a high diversity of low-growing plants and the animals that depend on them.
• Controlled burning removes accumulated growth and resets succession to an earlier, more diverse stage.
• Coppicing of woodland lets light reach the woodland floor, supporting ground-layer diversity.

Without management, these habitats would undergo succession into woodland and lose their characteristic species. This is why a managed ecosystem is often a plagioclimax: a community held below the natural climax by human activity.

Evaluating conservation evidence

Conservation involves conflicting evidence and competing interests. AQA expects you to weigh data and viewpoints, for example:

• Economic versus ecological priorities (development and jobs versus protecting a habitat).
• The interests of different stakeholders (farmers, conservationists, local communities, tourism).
• Uncertainty in data (population estimates have sampling error; the effects of an intervention may take years to show).

A good evaluation states what the data show, identifies limitations or conflicts, and reaches a justified conclusion rather than simply listing arguments.

Try this

Q1. Distinguish between primary and secondary succession and explain why secondary succession is faster. [3 marks]

• Cue. Primary starts on bare land with no soil; secondary starts where soil already exists after disturbance; secondary is faster because the soil, nutrients and seed bank are already present.

Q2. Explain how a pioneer species changes a bare-rock environment so that other species can colonise. [3 marks]

• Cue. Pioneers weather the rock and add organic matter (humus) when they die, forming soil that retains water and nutrients, making conditions less hostile for larger plants.

Q3. Explain, using the concept of succession, why a nature reserve manages chalk grassland by grazing it with sheep. [3 marks]

• Cue. Without management, succession would proceed to scrub and woodland; grazing removes tree and shrub seedlings, holding the community at an earlier, more diverse plagioclimax stage and conserving grassland species.