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Succession

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Biology

Succession describes the changes of a population in an ecosystem over a period and within a particular area. It is the process by which communities colonise an ecosystem and then, over time, are replaced by other communities. Ecosystems are dynamic, meaning they are constantly changing. Succession occurs in a series of stages. At each stage, the environment becomes more suitable for certain species. These environmental changes make the area less suitable for one species and more suitable for the other, resulting in one community succeeding against another.

Succession: the process by which communities colonise an ecosystem and are then replaced over time by other communities.

Community: a collection of all different populations of different species in a given area.

What is primary succession?

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

New rock surfaces can be formed by cooling magma from volcanic eruptions. The ocean floor or the bed of a river can be revealed when the water level drops. Even seemingly barren places like deserts and rocky coasts can also be sites of primary succession as long as new species are slowly colonising them.

What is secondary succession?

Succession can also occur 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 can occur when land is suddenly cleared and converted for agriculture, when forest fires occur, or when a serious disease destroys fundamental species in an ecosystem.

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 in biology?

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.

Succession Stages StudySmarterFigure 2. Stages of succession. Source: Finty Royle - StudySmarter originals

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 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).

As their name suggests, extremophiles live in very extreme conditions largely inhospitable for most organisms. These include halophiles (organisms that live in areas with high salt concentrations), radiophiles (organisms that live in areas with high levels of radiation) and acidophiles (organisms that live in areas with an acidic pH).

A rock is an example of a reasonably inhospitable environment. Rocks do not usually harbour nutrients or hold significant usable quantities of water. However, 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.

Succession primary colonisers StudySmarterFigure 3. Primary colonisers

Symbiotic relationship: a mutual relationship between two organisms in which both benefit from the association.

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).

Do not confuse secondary succession with secondary colonisation!

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 carried in by the wind, water, or animals (all of these being tertiary colonisers) 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; large species of trees, such as oaks in deciduous woodlands or dipterocarps in rainforests, will often fill this niche. The final, stable community formed is known as the climax community.

What is the significance 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, 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 towards the previous species. For instance, large, dominant trees can block out sunlight and prevent smaller trees and shrubs from getting enough sunlight to survive. 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. Read our article on Conservation to find out more!

Succession - Key takeaways

  • Succession refers to the changes in the species that occupy a particular area over time. Succession makes habitats more hospitable to organisms.

  • Primary succession occurs when new species gradually colonise a newly exposed or formed terrain. Secondary succession occurs when land undergoes drastic changes, and new species can recolonise.

  • Succession occurs in a series of stages. These include the primary colonisation by pioneer species, secondary colonisation, tertiary colonisation, shrubland and the climax community.

  • Pioneer species have traits that allow them to colonise not hospitable land to life. These traits include being able to reproduce asexually and fix nitrogen.

  • The final, stable community formed after the stages of succession is known as the climax community.

Succession

Succession refers to the changes in the species that occupy a particular area over time. Succession makes habitats more hospitable to organisms.

Succession occurs in a series of stages. These are:


  • Primary colonisation by pioneer species

  • Secondary colonisation

  • Tertiary colonisation

  • Shrubland

  • Climax community

Secondary succession can occur when a stable habitat is suddenly cleared and new species recolonise the land. Secondary succession occurs at a faster rate than primary succession because the cleared habitat usually still has some of the resources it accumulated during primary succession.

Primary succession occurs when a newly exposed or formed terrain is gradually colonised by new species.

Final Succession Quiz

Question

Define succession.



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Answer

Succession refers to the changes in the species that occupy a particular area over time.



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Question

Define primary succession.

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Answer

Primary succession occurs when a newly exposed or formed terrain is gradually colonised by new species.



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Question

Which of the following traits is not likely to describe a pioneer species?


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Answer

Carnivorous



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Question

Identify which stage of succession lichen is likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).



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Answer

Pioneer species

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Question

Identify which stage of succession very large trees are likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).



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Answer

Climax community



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Question

Identify which stage of succession bryophytes are likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).



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Answer

Secondary colonisers



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Question

Identify which stage of succession shrubs are likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).


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Answer

Shrubland

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Question

Identify which stage of succession field mice are likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).


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Answer

Tertiary colonisers

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Question

Identify which stage of succession thermophylic bacteria are likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).


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Answer

Pioneer species

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Question

Identify which stage of succession mushrooms are likely to belong to (pioneer species, secondary colonisers, tertiary colonisers, shrubland, climax community).


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Answer

Tertiary colonisers 

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Question

True or false. Secondary succession usually occurs at a faster rate than primary succession.



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Answer

True

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Question

Why does secondary succession occur faster than primary succession?


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Answer

Newly cleared environments still have some of the resources they were able to accumulate over many years of succession. Surrounding areas that were not cleared provide an influx of species.



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Question

Which of the following is not a result of succession?

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Answer

The environment loses nutrients.

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Question

Which of the following stages of succession has the highest species richness?



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Answer

Tertiary succession 

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Question

Why are climax communities usually less diverse than intermediate communities?



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Answer

Because dominant species begin to outcompete and eliminate other species.



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