• Ecosystems are?dynamic, meaning that they are constantly?changing
• Sometimes, ecosystems change from being very?simple?to being relatively?complex
  • This process is known as?succession
  • During succession, the?biotic?conditions (i.e. the living factors) and the?abiotic?conditions (i.e. the non-living factors)?change?over time

   

• Primary?succession is the process that occurs when?newly formed?or?newly exposed?land (with no species present) is gradually?colonised?(inhabited) by an increasing number of species
• This new uninhabited land can be created in several ways. For example:
  • The magma from erupting?volcanoes?cools and often leads to the formation of?new rock?surfaces or even new rocky?islands?in the sea
  • Another way new land can be exposed is by sea-level dropping or the drying up of a lake, leaving areas of?bare rock

   

• Primary succession does not only occur on bare rock. Any?barren terrain?that is slowly being colonised by living species is undergoing primary succession. For example:
  • Sand dunes?in coastal areas (marram grasses?are the pioneer species in these environments as they have?deep roots?to access water that other plants can't reach and are able to?tolerate the salty environment?i.e. the high concentrations of sodium and calcium ions caused by sea spray)

   

Primary Succession Occurs in a Series of Stages

• Firstly,?seeds?and?spores?that are carried by the?wind?land on the exposed rock and begin to grow
  • These first species to colonise the new land (often?moss?and?lichens) are known as?pioneer species
  • As these pioneer species die and decompose, the dead?organic?matter (humus) forms a?basic soil

   

• Seeds of?small plants and grasses, sometimes also carried in the wind or sometimes transported other ways (e.g. in bird faeces) land on this basic soil and begin to grow (these smaller plants are adapted to survive in?shallow, relatively?nutrient-poor?soils)
  • As these small plants and shrubs die and decompose, the new soil becomes?deeper?and more?nutrient-rich
  • The?roots?of these small plants and shrubs also form a network that helps to?hold the soil in place?and prevent it from being washed away

   

• Larger plants and shrubs, as well as?small trees, that require deeper, more nutrient-rich soil, can now begin to grow
  • These larger plants and small trees also require?more water, which can be stored in?deeper soils

   

• Finally, the soil is sufficiently deep, contains enough nutrients and can hold enough water to support the growth of?large trees
  • These final species to colonise the new land become the?dominant species?of the now relatively?complex ecosystem
  • The?final community?formed, containing all the different plant and animal species that have now colonised the new land, is known as the?climax community

   

An example of primary succession occurring on a newly formed rock surface

• At each?stage?in?succession, there are certain species that gradually?change?the?local environment?so that it becomes?more suitable?for?other species?(with different adaptations) that have not yet colonised the new land
  • For example, pioneer species change the abiotic conditions so that they are less hostile to new colonising species

   

• Often, these new colonising species then change the environment in such a way that it becomes?less suitable?for the?previous species

Examples of a Changing Environment During Succession

• The processes described above are demonstrated in the example of primary succession occurring on a newly formed rock surface. For example:
  • Pioneer species that first colonise and grow on the bare rock, such as?lichens, help to slowly?break apart?the top surface of the rock. This fragmented rock, along with the dead organic matter (humus) left behind when the lichens die and are broken down, forms a?basic soil. In this way, the lichens gradually?change?the?local environment?so that it becomes?more suitable?for other species, such as?mosses
  • As mosses grow, the basic soil continues to build up?until small plants and grasses can colonise the new land. Eventually, these species result in the formation of a?thin soil layer?that?covers?the newly formed rock surface completely. As lichens cannot grow on soil, they now?disappear?from the?ecosystem. In this way, the new species that arrived after the lichens have changed the environment in such a way that it becomes?less suitable?for the lichens
  • Finally, as the soil deepens further and?trees?are able to grow, they may then?out-compete?certain shrubs and other smaller plant species, which may no longer be able to grow beneath the trees due to a?lack of light

   

• Human activities?often?prevent?or?interrupt?the process of?succession
• As a result, this stops a?climax community?from developing. For example:
  • If?left?alone, a grassy field would eventually develop into an area of?shrubs?and?trees?due to the process of succession. However, regular?mowing?prevents these larger, more woody plants from?establishing?themselves, so succession can't occur (only the small grasses can?survive?being regularly mowed)
  • Similarly, in areas where?livestock?such as sheep, horses or cattle are kept, succession is halted by the?grazing?activity of these animals, which eat any?new plant shoots?trying to grow. Again, grasses are the main plant species that can remain established in these areas

   

Managing Succession

• Conservation?involves the?protection?and?management?of?ecosystems
• In conservation terms, preventing an area from reaching its climax community can sometimes be a?good?thing
  • This is because ecosystems at the?intermediate?stages of succession, where small plants, grasses, ferns or shrubs are present, often hold a?distinct diversity?of plant species (some of which may be of?conservation importance) that would no longer exist if the climax community was reached
  • These diverse plant species also provide food and habitat for a?high diversity of animal species, some of which may also be of conservation importance (e.g. species that are?rare?or?threatened, or species that have important?ecosystem functions, such as?pollinators?like bees, which are also of great importance to humans due to their role in pollinating the crops we consume)

   

• As a result, some conservation projects require the?deliberate,?artificial?prevention of succession in order to?preserve?an ecosystem in its current stage of succession. For example:
  • Scottish?moorlands?provide habitats for many species of plants and animals
  • If succession was allowed to occur, this valuable moorland would be replaced by a climax community dominated by spruce forest, which?cannot support the same species?as the moorlands
  • This would mean?losing?these important species
  • By having some areas where the climax community is allowed to develop and other areas where succession is prevented so that the moorland remains?intact, both ecosystems can be?maintained, giving a?higher overall species diversity

   

Preventing Succession

• There are a few different ways that succession can be deliberately prevented for conservation purposes. For example:
  • Grazing animals?can be?introduced?temporarily. As they eat the growing shoots of shrubs and trees, this stops these plants from establishing themselves and prevents succession
  • Managed burning?can be used, during which?controlled fires?are deliberately lit and allowed to?burn away?the shrubs and trees. Species such as heather (a key moorland plant) grow back quickly in their place. This?resets?the process of succession, meaning the larger, woody plants will take a long time to grow back, at which point the burning can be repeated

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