Succession and Conservation

Ecological Succession

Succession is the gradual, directional change in the species composition and community structure of an ecosystem over time. It proceeds through a series of stages, from initial colonisation to a stable climax community.

Types of Succession

Primary Succession

Primary succession occurs on a previously uncolonised substrate — bare rock, volcanic lava, sand dunes, or glacial deposits. There is no existing soil or seed bank.

Stages of primary succession (a lithosere — on bare rock):

1. Pioneer stage: The first organisms to colonise are pioneer species — organisms adapted to harsh conditions with minimal nutrients. On bare rock, these include lichens (mutualistic associations of fungi and photosynthetic algae/cyanobacteria) and some mosses.

• Lichens can grow on bare rock, obtaining nutrients from rainfall and the rock surface
• They begin to weather the rock (physically by root-like structures penetrating cracks; chemically by producing acids)
• When they die, their remains mix with weathered rock particles to form the very beginnings of soil

2. Early colonisers: As a thin layer of soil accumulates, mosses, ferns, and small herbaceous plants can establish. Their roots hold soil together; their organic matter adds to it when they die.

• Soil depth increases, water retention improves, nutrient content rises
• Conditions improve for more demanding species

3. Intermediate stages: Grasses, then shrubs, then small trees colonise as soil deepens. Each stage modifies the environment:

• Increased humus → better water and nutrient retention
• Greater root networks → stabilise soil
• Shade from taller plants → changes in light availability
• Increased biodiversity as more niches become available

4. Climax community: A stable, self-sustaining community develops that is in equilibrium with the prevailing climatic conditions. In the UK, the climatic climax is typically deciduous oak woodland. The climax community remains relatively unchanged unless disturbed.

Secondary Succession

Secondary succession occurs on a substrate where a community previously existed but was removed or destroyed (e.g. after a fire, flood, deforestation, or abandonment of agricultural land). Soil is already present, often containing a seed bank, roots, and nutrients. Secondary succession is therefore faster than primary.

Example: Abandoned farmland → grassland → scrub → woodland (over decades to centuries).

Key Concepts in Succession

Changes During Succession

Feature	Early (pioneer) stages	Late (climax) stages
Species diversity	Low	High
Biomass	Low	High
Soil depth and quality	Thin, nutrient-poor	Deep, nutrient-rich
Food web complexity	Simple	Complex
Productivity	High relative to biomass	Lower relative to biomass (more respiration)
Stability	Low (vulnerable to disturbance)	High (resilient)

How Species Change the Environment

At each stage, the existing species modify the abiotic environment in ways that:

• Make conditions more favourable for new species (e.g. deeper soil, more shelter, greater humidity)
• Make conditions less favourable for themselves (e.g. shade-intolerant pioneers are outcompeted by taller plants)

This process of environmental modification is why succession is directional — each stage facilitates the next.

Deflected Succession and Plagioclimax

Human activities can arrest succession at an intermediate stage, creating a plagioclimax — a community maintained by management rather than natural processes:

• Mowing grasslands prevents scrub and woodland from developing
• Grazing by livestock maintains grassland or heathland
• Burning of heather on moorland (controlled burning/muirburn) maintains heather-dominated heathland for grouse management
• Coppicing of woodland — cutting trees to ground level on a rotation; encourages regrowth from stumps and allows light to reach the woodland floor, promoting ground flora diversity

If the management activity stops, succession resumes toward the climatic climax.

Conservation

Conservation is the management of ecosystems and Earth's resources to maintain or restore biodiversity. It is not the same as preservation (leaving an ecosystem completely undisturbed).

Why Conserve Biodiversity?

Reason	Explanation
Ecological	Every species has a role in its ecosystem (niche); loss of one can destabilise food webs and nutrient cycles
Economic	Biodiversity provides ecosystem services — pollination, water purification, flood control, fisheries, timber, medicines
Ethical	Many people believe all species have an intrinsic right to exist
Aesthetic	Natural habitats have recreational, cultural, and spiritual value
Genetic resource	Wild populations are a reservoir of genetic diversity for crop breeding and drug discovery. Once lost, a species' unique genetic information is gone forever

Conservation Methods

In-situ conservation — protecting species in their natural habitats:

• Nature reserves and protected areas (e.g. National Parks, SSSIs — Sites of Special Scientific Interest, marine protected areas)
• Wildlife corridors — connecting fragmented habitats to allow gene flow between populations
• Habitat management — mowing, grazing, controlled burning, coppicing, removing invasive species to maintain desired successional stages
• Legislation — laws protecting endangered species (e.g. Wildlife and Countryside Act 1981, CITES)
• Reintroduction programmes — returning species to areas they have disappeared from (e.g. red kites, beavers in the UK)

Ex-situ conservation — protecting species outside their natural habitats:

• Seed banks (e.g. Millennium Seed Bank at Kew) — storing seeds at low temperature and humidity for future use
• Zoos and captive breeding programmes — maintaining viable populations of endangered species with the aim of reintroduction (e.g. Arabian oryx, California condor)
• Botanic gardens — cultivating rare plant species
• Cryopreservation — freezing gametes, embryos, or tissue samples
• Gene banks — storing DNA or tissue samples for future research

Challenges of ex-situ conservation:

• Small populations suffer from inbreeding and loss of genetic diversity
• Animals may develop behavioural changes in captivity that reduce their fitness in the wild
• Maintaining captive populations is expensive
• Does not protect the habitat — if the habitat is destroyed, there is nowhere to reintroduce the species

Sustainable Management

Sustainable management aims to meet current needs without compromising the ability of future generations to meet theirs:

• Sustainable forestry — replanting harvested trees; selective logging rather than clear-felling; FSC certification
• Sustainable fishing — quotas, minimum net mesh sizes (allowing juveniles to escape and breed), closed seasons, marine protected areas
• Agri-environment schemes — farmers receive payments for managing land in ways that benefit biodiversity (e.g. leaving field margins uncultivated, maintaining hedgerows, reducing pesticide use)

The Conflict Between Conservation and Human Activity

Conservation often conflicts with economic development:

• Protecting habitats may restrict agriculture, mining, logging, or urban development
• Sustainable practices may be more expensive or produce lower yields in the short term
• In developing countries, the immediate need for food, fuel, and income can outweigh long-term conservation goals
• Solutions require balancing ecological, economic, and social needs — often through international cooperation and financial support

Exam Tips

• AQA expects you to describe the stages of succession in order and explain how each stage modifies the environment to facilitate the next
• Know the difference between primary and secondary succession — the key is whether soil is already present
• Plagioclimax is commonly tested — always name the human activity and explain what would happen if it stopped (succession would resume)
• When discussing conservation, always consider both in-situ AND ex-situ methods, with advantages and limitations of each
• Link conservation to the maintenance of genetic diversity — this is a key point in AQA mark schemes