Chapter 23 P3

Question 1

Succession occurs as a

Answer 1

result of changes to the environment (the abiotic factors), causing the plant and animal species present to change.
There are two types of succession:

Question 2

There are two types of succession:

Answer 2

1 Primary succession - this occurs on an area of land that has been newly formed or exposed such as bare rock. There is no soil or organic material present to begin with.
2 Secondary succession - this occurs on areas of land where soil is present, but it contains no plant or animal species. An example would be the bare earth that remains after a forest fire.

Although much of the natural landscape has taken hundreds of years to reach its existing form, primary succession is still taking place.

Question 3

Primary succession occurs when:

Answer 3

• volcanoes erupt, depositing lava - when lava cools and solidifies
Igneous rock is createdi
• sand is blown by the wind or deposited by the sea to create new sand dunes
• silt and mud are deposited at river estuaries
• glaciers retreat depositing rubble and exposing rock.

Question 4

Stages of succession

Answer 4

Succession takes place in a number of steps, each one known as a seral stage (or sere). At each seral stage key species can be identified that change the abiotic factors, especially the soil, to make it more suitable for the subsequent existence of other species.
The main seral stages are pioneer community, intermediate community, and climax community, these are summarised in Figure 1.

Question 5

Summary of the main stages in succession

Answer 5

Question 6

Pioneer community

Answer 6

Primary succession begins by the colonisation of an inhospitable environment, by organisms known as pioneer species (or pioneer colonisers). This represents the first seral stage. These species arrive as spores or seeds carried by the wind from nearby land masses or sometimes by the droppings of birds or animals passing through.

Question 7

Examples of pioneer species

Answer 7

algae and lichen.

Question 8

Pioneer species have a number of adaptations that enable them to colonise this bare environment, including:

Answer 8

• the ability to produce large quantities of seeds or spores, which are blown by the wind and deposited on the ‘new land’
• seeds that germinate rapidly
• the ability to photosynthesise to produce their own energy - light, rainfall, and air (and so carbon dioxide) are often the only abiotic factors present
• tolerance to extreme environments
• the ability to fix nitrogen from the atmosphere, so adding to the mineral content of the soil.

Question 9

Intermediate community p1

Answer 9

• Over time weathering of the bare rock produces particles that form the basis of a soil.
• On its own this cannot support other species, However, when organisms of the pioneer species die and decompose small organic products are released into the soil.
• This organic component of soil is known as humus.
• The soil becomes able to support the growth of new species of plant, known as secondary colonisers, as it contains minerals including nitrates and has an ability to retain some water.
• These secondary colonisers arrive as spores or seeds.
• Mosses are an example of a secondary coloniser species.
• In some cases, pioneer species also provide a food source for consumers, so some animal species will start to colonise the area.
• As the environmental conditions continue to improve, new species of plant arrive such as ferns.
• These are known as tertiary colonisers.
• These plants have a waxy cuticle that protects them from water loss.
• These species can survive in conditions without an abundance of water - however, they need to obtain most of their water and mineral salts from the soil.

Question 10

Intermediate community p2

Answer 10

• At each stage the rock continues to be eroded and the mass of organic matter increases.
• When organisms decompose they contribute to a deeper, more nutrient-rich soil, which retains more water.
• This makes the abiotic conditions more favourable initially for small flowering plants such as grasses, later shrubs, then finally small trees.
• This period of succession is known as the intermediate community and in many cases multiple seral stages evolve during this period until climax conditions are attained.
• At each seral stage different plant and animal species are better adapted to the current conditions in the ecosystem.
• These organisms outcompete many of the species that were previously present and become the dominant species.
• These are the most abundant species (by mass) present in the ecosystem at a given time.

Question 11

Climax community

Answer 11

• The final seral stage is called the climax community.
• The community is then in a stable state - it will show very little change over time.
• There are normally a few dominant plant and animal species.
• Which species make up the climax community depends on the climate.
• For example, in a temperate climate where the temperatures are mild and there is plenty of water, large trees will normally form the climax community.
• By comparison in a sub-arctic climate, herbs or shrubs make up the climax community as temperature and water availability are low.
• Although biodiversity generally increases as succession takes place, the climax community is often not the most biodiverse.
• Biodiversity tends to reach a peak in mid-succession.
• It then tends to decrease due to the dominant species out-competing pioneer and other species, resulting in their elimination.
• The more successful the dominant species, the less the biodiversity in a given ecosystem.

Question 12

Animal succession

Answer 12

• Alongside the succession of plant species, animal species undergo similar progression.
• Primary consumers such as insects and worms are first to colonise an area as they consume and shelter in the mosses and lichens present.
• They must move in from neighbouring areas so animal succession is usually much slower than plant succession, especially if the
  ‘new land’ is geographically isolated, for example, a new volcanic island.
• Secondary consumers will arrive once a suitable food source has been established and the existing plant cover will provide them with suitable habitats.
• Again these species must move in from neighbouring areas.
• Eventually larger organisms such as mammals and reptiles will colonise the area when the biotic conditions are favourable.

Question 13

Deflected succession

Answer 13

Human activities can halt the natural flow of succession and prevent the ecosystem from reaching a climax community. When succession is stopped artificially, the final stage that is formed is known as a plagioclimax. Agriculture is one of the main reasons deflected succession occurs. For example:

Question 14

Agriculture is one of the main reasons deflected succession occurs. For example:

Answer 14

• grazing and trampling of vegetation by domesticated animals - this results in large areas remaining as grassland
• removing existing vegetation (such as shrub land) to plant crops - the crop becomes the final community
• burning as a means of forest clearance - this often leads to an increase in biodiversity as it provides space and nutrient-rich ash for other species to grow, such as shrubs.

Question 15

Conservation

Answer 15

• Deflected succession is an important conservation technique.
• To ensure the survival of certain species, it is important to preserve their habitat in its current form.
• This may require ecological land management to prevent further succession from occurring.
• An example of this is at the National Trust’s Studland Heath nature reserve in Dorset, England.
• Studland Heath is home to a number of British reptiles, including the UK’s rarest reptile, the smooth snake.
• This region is heathland - if succession were allowed to occur, woodland would develop as the climax community.
• Other animal species would then inhabit the area, ultimately leading to the replacement of the smooth snake by other animal species.
• This would risk the elimination of this reptile species from the UK.
• So the heathland is managed by the National Trust to ensure that the precious ecosystem is preserved.
• A range of conservation techniques are being used:

Question 16

A range of conservation techniques are being used:

Answer 16

• Physical removal of established bracken, and saplings such as birch and pine.
Gorse, which is a legume, adds to the nutrient value of the soil. Heathlands are traditionally nutrient-poor areas of land, and so areas of gorse are also removed through physical means (cutting, crushing, and controlled burning).
These techniques are used to restore the heathland to its former state.
• Mimicking the controlled grazing to limit the spread of bracken, gorse, and tree saplings. Livestock crush and nibble new growth, therefore limiting the spread of these species. This technique is used to maintain the heathland in its current state.
Through using a combination of these techniques, low nutrient levels are maintained in the soil. This produces a varied vegetation structure, thus supporting a biodiverse heathland community.

Question 17

Succession on a sand dune p1

Answer 17

One of the few examples where all the stages of succession can often be seen clearly in one place is when a series of sand dunes form on a beach. The youngest dunes will be found closest to the shore and the oldest furthest away.

Question 18

Succession on a sand dune p2

Answer 18

• Seeds are blown onto the dunes or washed onto the sand by the sea.
• At this stage the rooting conditions are poor due to drought, strong winds, and salty seawater immersion.
• The presence of a large number of sea shells also makes the environment very alkaline.
• As the wind blows across the dunes it moves the sand, constantly changing the profile of the dunes and allowing rainwater to soak through rapidly.
• However, some species such as sea couch grass are able to survive these harsh conditions.
• Marram grass becomes the next dominant species.
• As these plants grow sand is trapped between the roots helping to stabilise the dunes, and the formation of soil begins.
• Minerals released from decaying pioneer plants create more fertile growing conditions, and the soil becomes less alkaline as pioneer plants grow and trap rainwater.
• The grasses also reduce salt spray into the hind dunes, and act as a partial wind barrier.
• Less hardy plants can now grow - over time, these species start to shade out the pioneers.
• As more species of plants colonise the dunes, the sand disappears and the dunes change colour - from yellow to grey.
• Finally taller plants, such as sea buckthorn, or more complex plant species like moorland heathers can grow.
• Plants from earlier stages are no longer present.
• When the water table reaches or nearly reaches the surface, dune slacks can occur.
• Plants that are specially adapted to be water-tolerant grow here adding to the biodiversity of the sand dunes.