Succession (AQA A-Level Geography): Revision Notes

Succession

What is succession?

Ecological succession describes how ecosystems progressively develop and transform over extended time periods. Nothing in nature stays the same – environments and the communities of organisms living within them are in constant flux.

The process occurs when conditions become more suitable for certain species, allowing them to thrive, while the die-off happens when conditions become less favourable. This results in a shift in the dominant plant species present at different points in time.

Types of succession

There are two main categories of succession, determined by the initial state of the area where the process begins:

Primary succession

This type occurs on completely fresh surfaces that have never previously supported life. The area starts as a lifeless, barren environment with no soil or organic matter present.

Secondary succession

This type takes place on ground that previously supported living communities but has experienced disturbance or damage. Unlike primary succession, some soil and organic matter typically remain, allowing for faster recovery.

Types based on starting surface

Ecologists classify succession based on the initial surface conditions where the process begins. Each type has its own characteristics:

Lithosere

A succession beginning on newly exposed bare rock surfaces. This might include freshly erupted volcanic lava or rock revealed as glaciers retreat.

Psammosere

A succession starting on bare sand environments, commonly found in coastal sand dunes.

Hydrosere

A succession that begins in freshwater environments, such as ponds or lakes.

Halosere

A succession that starts in saltwater conditions, for example in salt marshes.

Key concepts in succession

Seres and seral stages

When succession progresses, it moves through several distinct phases. The complete sequence of all stages is called a sere, whilst the individual phases within this sequence are known as seral stages.

Alternatively, the entire progression may be referred to as a prisere.

As succession advances through each new stage, several changes occur:

• The structure becomes increasingly complex
• The total amount of living matter (biomass) increases
• The variety of different species grows
• Net primary productivity rises
• Greater flows of nutrients and energy occur

Pioneer species

The first organisms to colonise a barren area are called pioneer species. These remarkable organisms possess special adaptations that enable them to survive in extremely harsh conditions where most species cannot establish themselves.

Species diversity

Throughout succession, species diversity (the range and variety of different species present) steadily increases. Early stages feature only hardy pioneer species, but as conditions improve, more species can colonise and establish themselves.

The succession process

How succession works

Succession is both directional and usually predictable. Seres gradually transition from one stage to another following a recognisable pattern.

As each plant dies, it contributes organic material to the surface. This material combines with weathered rock fragments and windblown dust to create increasingly complex soil. Better soil conditions then enable taller, more demanding plants to invade and become established. Each stage essentially creates an environment that eventually leads to its own replacement.

As soil quality improves and nutrients accumulate, wind-dispersed seeds can germinate in the more sheltered, damper areas.

How organisms interact with their environment

The physical characteristics of plants directly affect the type of micro-organisms and animals that can inhabit an area. For example, the plant structure determines what microhabitats are available.

As plant communities transform, this causes corresponding changes in:

• Herbivores (plant-eating animals)
• Carnivores (meat-eating animals that feed on herbivores)
• Fungi and decomposers

The species present in a location gradually change over time, and as they do, the physical and chemical characteristics of the environment itself also transform.

Stages of succession: lithosere example

Let's examine the detailed progression of a lithosere succession, which demonstrates the general principles applicable to all succession types.

Bare rock surface

The starting point is bare rock – sand, rock, water, or salt – depending on the succession type. The surface is initially colonised by bacteria and simple single-celled photosynthetic organisms. These pioneers can survive with very few nutrients and obtain their energy directly from sunlight. Surface conditions are often very dry, with minimal moisture retention in the weathered rock.

Seral stage 1: Colonisation

The first plant species capable of colonising the area are pioneers – typically lichens adapted to survive severe dry, windy, and soil-free conditions. They begin breaking up the rock surface to form a thin layer of proto-soil.

As pioneers die, they add dead organic matter to weathered rock and windblown dust. This creates basic soil that improves water retention. Mosses can then develop in these slightly improved conditions.

Seral stage 2: Establishment

As soil develops further, ferns and small herbaceous plants and grasses begin to grow. Species diversity increases at this stage. More invertebrates start living in the soil, which increases the organic content and enables the soil to hold more water.

Seral stage 3: Competition

Larger plants begin establishing themselves, including shrubs and small trees. These use much of the available water and shade the ground, reducing light for smaller plants. Some earlier species are unable to compete and die out. Herbivores become more established, and predators begin moving into the area.

Seral stage 4: Stabilisation

Fewer new species colonise as competition intensifies. Complex food webs develop during this stage. The area becomes dominated by larger, fast-growing trees such as birch and rowan. Top predators are found at this stage.

Seral climax

This represents the final seral stage – the maximum possible development that a community can achieve under the prevailing climatic conditions (temperature, light, and rainfall). This is also called the climatic climax community.

The total number of larger plant species decreases as a few large species come to dominate the area. In southern England, these are typically broad-leaved deciduous trees such as oak and ash.

Climatic climax and related concepts

Climatic climax

A useful way to describe the final outcome of succession is the concept of a climatic climax community. This is the ultimate stage in succession – an association of plants and animals that represents the maximum development possible under the environmental conditions of that location.

Climate, microclimate, geology, soil type, and other environmental factors determine what the actual climax community will be in any given area. It can progress no further.

Secondary succession and disturbance

Secondary succession can occur if the climatic climax community experiences disturbance. The succession sequence essentially restarts, though often more rapidly than primary succession since soil and organic matter already exist.

Time scales

Time scales vary enormously – from hundreds to thousands of years. Climate, microclimate, geology, soil type, and other environmental factors all determine how quickly succession progresses.

Predictability

Large numbers of species are involved, but each sere is typically named after the dominant plant species present at that particular stage. Succession is directional and generally predictable, with seres gradually merging from one stage into another.

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