23.4 Succession

Ecosystems are dynamic (constantly changing) - one process in which they change in through Succession.

Succession occurs as a result of changes in the environment (abiotic factors), causing the plant and animal species present to change.

There are two types of succession

  • Primary succession - this occurs on an area of land that has been newly formed or exposed such as bare rock, there is no soil or organic material present to begin with.

  • Secondary succession - this occurs on areas no land where soil is present, but it contains no plant or animal species. E.g bare earth after forest fire.

Primary succession occurs when

  • volcanoes erupt, depositing lava cools and solidifies igneous rock is created.

  • sand is blown by the wind or deposited by the sea to create new sand dunes

  • silt and mud are deposited at river estuaries

  • glaciers retreat depositioning rubble and exposing rock

Stages of succession

Each step is known as seral stage, each can be identified by key species that change abiotic factors, especially the soil to make it more suitable for the subsequent existence of other species.

Main stages

  • Pioneer community

  • Intermediate community

  • Climax community

insert image

Pioneer community

Primary succession begins by the colonisation of an inhospitable environment, by organisms known as pioneer species, representing the first seral stage.

These species arrive as spores or seeds carried by the wind from nearby land masses or sometimes by droppings of the birds or animals passing through.

Examples include algae and lichen.

Pioneer species have a number of adaptations that include

  • The ability to produce large quantities of seeds or spores, which are blown by the wind and deposited on the new land

  • Seeds that germinate rapidly

  • The ability to photosynthesise to produce their own energy - light, rainfall and air are often the only abiotic factors present.

  • The ability to fix nitrogen from the atmosphere, so adding to the mineral content of the soil.

Intermediate community

Overtime weathering of bare rocks produces particles that form the basis of soil. On its own this cannot support other species. However, when organisms of the pioneer species die and decompose small organic products are released into the soil. → This organic product is known as humus.

The soil becomes able to support the growth of new species of plants, known as secondary colonisers as it contains minerals including nitrates and has an ability to retain some water. These secondary colonisers arrive as spores or seeds.

Mosses are an example of secondary coloniser species, in some cases pioneer species also provide a food source for consumers so some animal species will start to colonise the area.

As the environmental conditions continue to improve, new species of plant arrive such as ferns → these are known as tertiary colonisers. These plants have a waxy cuticle that protects them from water loss, these species can survive in conditions without an abundance of water - however they need to obtain most of their water and mineral salts from the soil.

At each stage the rock continues to be eroded and the mass or organic matter increases. When organisms decompose they contribute to a deeper, more nutrient-rich soil, which retains more water. This makes the abiotic conditions more favourable initially for small flowering plants like grasses, later shrubs and small trees.

At each seral stage different plant and animal species are better adapted to the current conditions in the ecosystem, they outcompete many species that were previously present and become the dominant species, these are the most abundant species (by mass) present in the ecosystem at a given time.

Climax community - final seral stage

Community is then in a stable state - will show little change overtime and which species make up this will depend on the climate.

Although biodiversity generally increases as succession takes place, the climax community if often not the most biodiverse. Biodiversity tends to reach a peak in mid-succession.

It then tends to decrease due to the dominant species out-competing pioneer and other species, resulting in their elimination, the more successful the dominant species, the less the biodiversity in a given ecosystem.

Deflected succession

Human activities can halt the natural flow of succession and prevent the ecosystem from reaching a climax community, known as plagioclimax.

Agriculture is one of the main reasons deflected succession occurs

  • Grazing and trampling of vegetation by domesticated animals - results in large areas remaining as grassland

  • Removing existing vegetation to plant crops - the crop becomes the final community

  • Burning as a means of forest clearance - this often leads to an increase in biodiversity as it provides space and nutrient-rich ash for other species to grow.

  • insert image

  • insert image