Ecological Succession

Leave a patch of bare rock alone for long enough and it turns into a forest. Nobody plants it. The whole thing happens because each set of species that arrives changes the place enough to let the next set in – and, usually, changes it enough to push itself out.

What succession is

Ecosystems are dynamic, meaning they are constantly changing, and the name for that process of change is ecological succession.

What can trigger it

Primary succession

Primary succession is succession that occurs when newly formed or newly exposed land is inhabited by an increasing number of species.

• Newly formed land can be created by magma from erupting volcanoes cooling to form new rock surfaces, or new rocky islands in the sea.
• Newly exposed land can form when a landslide exposes bare rock, or when a glacier retreats to reveal bare rock.

The arrival of organisms on bare land is called colonisation, and the land is said to be colonised. Primary succession can happen on any type of bare land, including sand dunes at the edge of the ocean as well as exposed rock.

The stages, on bare rock

Working through it properly:

• Seeds and spores carried by the wind land on exposed rock and begin to grow. The first species to colonise, often mosses and lichens, are the pioneer species. They can germinate easily and withstand harsh conditions such as low nutrient and water availability.
• As these organisms die and decompose, the dead organic matter forms soil.
• Seeds of small plants and grasses land on this soil and grow. These early-stage plants are adapted to survive in shallow, nutrient-poor soils. Their roots form a network that holds the soil in place and stops it being washed away.
• As those small plants die and decompose, the soil becomes deeper and more nutrient-rich.
• Larger plants and shrubs, and then small trees, can now grow. These need more water, which the deeper soil can store.
• Eventually the soil is deep enough, nutrient-rich enough and able to hold enough water to support large trees. The final species to colonise become the dominant species of the now complex ecosystem.

Two details about the climax community are easy marks. First, the type of climax community depends on the location of the original bare land – in the tropics it would be rainforest, while in temperate regions it might be deciduous woodland. Second, a climax community is not always the most biodiverse stage of succession, but it is a stable community.

How succession changes the conditions

At each stage, the newly arriving species change the local environment so that it becomes more suitable for other species that have not yet colonised. Two examples:

• Pioneer species such as lichens slowly break apart the top surface of bare rock. That fragmented rock, together with the dead organic matter left when the lichens die and are broken down, forms a basic soil.
• Grasses grow roots that stabilise the soil, enabling it to hold more moisture and nutrients.

Now the twist, and it is the part examiners like most. Often the new colonising species change the environment in a way that makes it less suitable for the previous species:

• Lichens cannot grow on soil, so they disappear from the ecosystem once soil begins to form. The species they made way for have changed the environment so that it no longer suits them.
• Pioneer species may not be found in a climax community at all, because they are out-competed for light and other resources by species arriving in the later stages. Pioneers are well adapted to harsh conditions but are often poor competitors.
• As soil deepens and trees grow, they block out light to shrubs and smaller plants, out-competing them and causing them to die.

The general principles

Whatever the specific example, the following can always be observed as succession progresses:

• Larger plant species can be supported.
• There is an increase in the primary production of the community.
• Species diversity increases.
• The complexity of food webs increases.
• Nutrient cycling increases.

Secondary succession

Secondary succession takes place on previously occupied land, for example after a wildfire or after deforestation. It is very similar to primary succession, with one crucial difference: soil is already present, so the process begins at a later stage. There is no need to wait for lichens to make soil out of rock.

Cyclic succession

The succession described so far ends in a stable climax community that, if left undisturbed, remains unchanged over time. In some ecosystems that stable equilibrium never arrives. Instead there is a changing cycle of communities rather than a single unchanging climax, and this is known as cyclical, or cyclic, succession.

These cycles run over long periods – the lifetime of an oak tree, for example, which can be hundreds of years. The example to learn is grazed woodland, which contains a changing mosaic of grassland, woodland and scrub. This habitat is described as wood pasture.

In words, the cycle runs as follows. Grazers such as cattle graze on pockets of open grassland within a woodland, where they consume tree seedlings and prevent the growth of trees. Fast-growing thorny species such as brambles arise in some areas within the grassland, creating a mosaic of thorny scrub. Tree seedlings can survive within the thorny vegetation, which deters grazers. Those trees then grow up through the thorny scrub, which eventually fails to get enough light through the tree canopy and dies off. At the end of its life the tree dies and falls, creating an open area where grass can grow – and the cycle restarts.

Worked examples