Succession

• the plants and animals in an environment gradually change over long periods of time - and the environment itself changes too - due to a natural process called succession
• ecosystems are dynamic - constantly changing
• succession is the process by which an ecosystem changes over time

  • happens in a series of stages
  • at each stage the plant and animal communities in an area slowly change the environmental conditions
  • e.g. by making the soil more fertile, making the conditions more suitable for other species with different adaptations
  • this means that the biotic conditions change as the abiotic conditions change, causing one community of organisms to be succeeded (replaced) by another
• two main types of succession:

  • primary
  • secondary
• primary succession:

  • primary succession happens on land that's been newly formed or exposed
  • e.g. where a volcano has erupted to form a new rock surface, or where sea level has dropped, exposing a new area of land
  • no soil or organic material to start with i.e. just bare rock
• pioneer stage of succession:

  • primary succession starts when species colonise a new land surface
  • seeds and spores are blown by the wind and begin to grow
  • the first species to colonise the area are called pioneer species
  • the abiotic conditions are hostile (harsh) and only pioneer species can grow bc they are specially adapted to cope with the harsh conditions
• hostile abiotic conditions:

  • there is limited water available bc there's no soil to retain water
  • there are few minerals or nutrients be there's no soil
  • there may be high light intensity, exposure to wind and rain, and fluctuating temperatures bc the area is directly exposed to the Sun and the elements
• pioneer species:

  • marram grass - can grow on sand dunes near the sea bc it has deep roots to get water and can tolerate the salty environ
  • lichens are organisms usually made up of a fungus and an alga - able to survive in rocky conditions bc the fungus secretes acids which erode the rock, releasing minerals
  • shrubs of the Calligonum genus are pioneer species that can grow in areas that experience periodic drought
• pioneer species change the abiotic conditions - they die and microorganisms decompose the dead organic material (humus), which forms a basic soil. this makes conditions less hostile e.g. the basic soil helps to retain water so new organisms with different adaptations can move in and grow
• • the new organisms then die and are decomposed, adding more organic material, making the soil deeper and richer in minerals such as nitrates
  • nitrogen-fixing bacteria turn nitrogen from the atmosphere into ammonia - forms ammonium ions in solution that can then be used by plants
  • means that larger plants like shrubs can start to grow in the deeper soil - retains even more water and contains more nutrients
• • some new species may change the environment so that it becomes less suitable for the previous species
  • e.g. sand sedge stabilises the sand through the growth of rhizomes (underground stems)
  • this makes the conditions less suitable for marram grass, which needs constant reburial by sand in order to grow healthily
• later stages of succession:

  • at each stage, different plants and animals that are better adapted for the improved conditions move in, out-compete the plants and animals that are already there and become the dominant species in the ecosystem
  • the dominant species are the ones which cause the most change to the abiotic environment, making it more suitable for other species
• • as succession goes on - the ecosystem becomes more complex
  • new species move in, alongside existing species, which means that biodiversity increases
  • plants create more habitats for animals, the abiotic conditions become less hostile and the amount of biomass increases
  • eventually these changes result in a climax community - they ecosystem is supporting the largest and most complex community of plants and animals it can - it won't change much more - it is in a steady state
• 1. bare rock lacks soil, is exposed to strong winds and has periods of drought. lichens (pioneer species) are able to survive bc they can grow in cracks to avoid the wind, break down rock to release minerals and are adapted to survive periods of drought
  2. the lichens die and are decomposed helping to form a thin soil, which thickens as more organic material is formed - other species such as mosses can grow
  3. larger plants that need more water can move in as the soil deepens, e.g. grasses and small flowering plants. the soil continues to deepen as the larger plants die and are decomposed
• 4. shrubs, ferns and small trees begin to grow, out-competing the grasses and smaller plants to become the dominant species - diversity increases

  5. finally the soil is deep enough and rich enough in nutrients to support large trees - these become the dominant species and the climax community is formed
• secondary succession:

  • happens on land that has been cleared of all the plants, but where soil remains e.g. after a forest fire or where a forest has been cut down by humans
  • the established community of species is usually destroyed, but without too much disturbance to the soil
  • it can occur during any stage (including the climax community) after the pioneer stage
  • the process of secondary succession is similar to primary - but bc there is already a soil layer - it starts at a later stage - and the pioneer species are larger plants e.g. shrubs
• human impact on succession:

  • human activities can prevent succession, stopping a climax community from developing
  • when succession is stopped artificially like this the climax community is called a plagioclimax
• e.g. a regularly mown grassy field won't develop shrubs and trees (woody plants) even if the climate of the ecosystem could support them

  • the growing points of the woody trees are cut off by the lawnmower so larger plants can't establish themselves
  • the longer the interval between mowing, the further succession can progress and the more diversity increases
  • but with frequency mowing, succession can't progress and diversity will be lower - only the grasses can survive being mowed
• • mowing doesn't just affect plants - can affect the wider biodiversity of the area e.g. removing the woody plants destroys habitats for insects, decreasing number of insect species
• climatic climax communities: - which species make up the climax community depends on what the climate is like in an ecosystem - the climax community for a particular climate is its climatic community:

  • e.g. in a temperate climate e.g. UK - plenty of available water, mild temps and not much change between seasons - climatic community will contain large trees bc they can grow in these conditions once deep soils have developed
• • in a polar climate - not much available water - temps are low - massive changes between the seasons - large trees won't ever be able to grow - climatic climax contains only herbs/ shrubs
• biomass is the mass of living material in an ecosystem
• a community is all the populations of different species found in a habitat
• primary succession also happens on sand dunes, salt marshes and even in lakes
• tall plants can reduce the light available to shorter plants and can help stabilise fluctuating temps e.g. trees increase air humidity, provide shade and reduce wind - makes moderate temps
• the main difference between the 2 types of succession is that soil is present at the start of secondary but not primary - therefore tends to reach the climax community more quickly