Succession

The definition of primary succession in biology

Primary succession occurs when new species gradually colonise a newly exposed or formed terrain. A new terrain can be formed in many ways.

Fig. 1 - Volcanic landscape

The definition of secondary succession in biology

Secondary succession occurs when an environment that already supports a stable ecosystem is suddenly and drastically altered. This phenomenon presents an opportunity for new species to come and recolonise the area.

Secondary succession occurs in the same way as primary succession, with the main difference being that it usually occurs much more quickly. Newly cleared environments, while much sparser than they were before, will still have some of the resources they could accumulate over many years of succession. For instance, they might still have a thick layer of soil full of seeds, spores, water, and nutrients. Surrounding areas that were not cleared might provide an influx of species, some of whom will be entirely new to the environment. Over time, the ecosystem may regain its original levels of diversity and support a new stable community.

What are the stages of succession?

Succession does not typically happen overnight; it occurs as a series of stages over a long period. At every stage, a new wave of species comes to colonise the area and makes additional changes to the environment, affecting the next wave of colonising species.

Fig. 2 - Stages of succession

Primary colonisation (pioneer species)

Succession begins when the new, inhospitable environment is colonised by primary colonisers or pioneer species. At this stage, the environment is empty, barren, and not suited to support most forms of life. However, pioneer species will have features that allow them to colonise the area successfully. For instance, pioneer species might:

• Reproduce asexually, allowing a single individual to multiply into a large population rapidly.

• Be able to photosynthesise and fix nitrogen (N) from the atmosphere, allowing them to produce their food independently.

• Produce lots of seeds or spores that can quickly disperse and germinate.

• Tolerate extreme conditions (these organisms are known as extremophiles).

Organisms like lichens generally grow on rocks. Lichens are composite organisms that arise from the symbiotic relationship between cyanobacteria and fungi. They are mainly self-sustaining and can survive in a wide range of temperatures and humidity levels. For this reason, lichens often become pioneer species.

Figure 3. Primary colonisers

Secondary colonisation

Over time, the chemical action of the lichens can contribute to the weathering of the rock, producing sand or soil. As lichens die, they also decompose and release nutrients to support communities of small plants like mosses and ferns. These abiotic changes to the environment help make the environment less hostile and more hospitable for new species to colonise (secondary colonisers).

Tertiary colonisation

As more small plants decompose and rocks continue to erode, the layer of soil and organic matter grows even thicker. Fungal spores and seeds of small plants like grasses are carried in by:

• the wind,

• water,

• or animals

And can now grow in the environment and support herbivores. These animals will, in turn, become sources of food for even larger animals.

Shrubland

As the new plants and animals die and decompose, they deepen the layer of soil and enrich it with more nutrients. The plants and fungi also develop extensive root systems that help to keep the soil in place. Eventually, when the soil is deep and nutrient-rich enough, it can store more water and grow larger plants like shrubs and small trees.

Climax community

Now, the ecosystem has become much more complex than it initially was, allowing large species to come and colonise the area. The final species to colonise the ecosystem will play a dominant role in the area, bringing in new resources and habitats and allowing a diverse range of species to live together.

The final, stable community formed is known as the climax community.

What are the characteristics of ecological succession?

We can see several common themes emerging at each stage of succession. Firstly, the abiotic environment usually becomes less hostile to organisms. The environment collects and produces more nutrients, the soil is formed, water is retained, and plants and other structures provide shelter. These create new habitats and niches for organisms to fill, which allow the environment to support more biodiversity and more complex food webs.

Ecosystems are particularly diverse and have very high biomasses during the intermediate or middle stages of succession. However, as they approach climax, dominant species can bring about changes in the abiotic features of the environment that make it less hospitable to the previous species.

These are thus eliminated from the community, reducing the overall biodiversity but leading to a more stable ecosystem overall.

Succession and conservation

Understanding the stages of succession is very important for the conservation of habitats, particularly for maintaining high levels of biodiversity in a habitat.