AQA A Level Biology复习笔记7.4.6 Succession

• 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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