1. Succession Syllabus
2.3.5 – Describe the concept and processes of succession in a named habitat.
2.3.6 – Explain the changes in energy flows, gross and net productivity, diversity and mineral cycling in different stages of succession.
2.3.7 – Describe factors affecting the nature of climax communities.

2. Ecological succession
The gradual process by which the species population of a community changes is called ecological succession.
A forest following a disturbance such as a fire.
Succession takes places as a result of complex interactions of biotic and abiotic factors.
Early communities modify the physical environment causing it to change.
This in turn alters the biotic community which further alters the physical environment and so on.
Long ago it was noticed that following the destruction of most of the plants on a site by a disturbance such as a fire or landslide, the first invading plants differed from those that colonized the site later.

3. Succession – What happens?
Each successive community makes the environment more favourable for the establishment of new species.
A succession (or sere) proceeds in seral stages, until the formation of a climax community is reached.

4. Primary Succession
Refers to colonization of regions where there is no pre-existing community.
Can you think of examples where this would occur?
After a volcano
Newly formed glacial moraines
Newly formed volcanic islands
Bare rock

5. Primary Succession Community changes on a glacial moraines
After a volcanic eruption

6. Questions – Glacial Moraines
During succession there is a change in species composition of a community. There are also changes in species diversity, stability of the ecosystem, and in gross and net production until a climax community is reached.
Explain what is meant by a climax community.
Explain each of the following changes which occur during succession:
Species diversity increases
Gross production increases
Stability of the ecosystem increases
Give two reasons why farmland in the UK does not reach a climax community.

7. Primary and Secondary Succession
Primary Succession – occurs on newly formed habitats that have not previously supported a community.
Examples?
Secondary Succession – occurs on sites that have been disturbed.
Primary succession = bare rock, sand dunes, volcanic islands, new ponds and lakes
Secondary Succession – farm land

8. Primary Succession – Bare Rock
Mosses, Grasses and small shrubs
Lichens, bryophytes and annual herbs
Bare Rock
After years
Fast growing trees e.g. Ash
Slower growing broadleaf species e.g. oak
Lichens are the first organisms to colonise these harsh areas perhaps only briefly at first when the rocks are wet from rain. The lichens are carried by spores in the wind. They are well adapted for colonising exposed bare rock surfaces because the hyphae can penetrate tiny cracks to absorb inorganic nutrients. Lichens have a symbiotic partnership between an algae and a fungus.
Lichens release acids which corrode the rocks, thus creating tiny crevices that trap dust and other debris. In this way they make the rock face more hospitable for other species that arrive later.
Mosses are usually the next to colonise the rocky habitat – they also have spores that can be carried in the wind. Mosses also have unicellular root like hairs which can bind to tiny crevices. They grow in tight clumps that can trap water and dust.
This eventually creates a thin layer of soil.
Plants such as clover are capable of nitrogen fixation and flourish on the thin layer of still nutrient poor soil.
The decomposing remains of the clover enrich the soil with nitrates thus making it more fertile.
Drought resistant ferns and in turn grasses will be the next to colonize. Because they are taller than the mosses, they will competitively exclude the mosses and suppress their growth.
The dead, decomposing remains of the grasses and ferns will add to the ever growing layer of soil. As this soil layer builds up, taller herbaceous plants and shrubs. In time the shrubs will give way to trees and in time, perhaps a thousand years, what was once a habitat consisting of pristine rocks becomes covered in a layer of soil deep enough to support a forest community.
Complex Community
Example for a Northern Hemisphere lithosere: a succession on bare rock

9. In Summary - the 1st Invaders!
These are usually fast growing plants that photosynthesize well in full sunlight.
We call these pioneer species making up the pioneer community
Examples = lichens, grasses, herbs
As these species begin to grow well, they produce shade. Their own seedlings grow more poorly than shade-adapted plants.
Plants that grow well under full sun are replaced by plants that germinate and grow better in deeper shade.

10. Secondary Succession
This type of succession takes place after a land clearance (e.g. from fire or landslide).
These events do not involve loss of the soil.
Secondary succession therefore occurs more rapidly than primary succession.
Humans may deflect the natural course of succession in these circumstances (e.g. by mowing or farming).
This leads to the development of a different climax community than would otherwise develop naturally.
A climax community arising from a deflected succession (caused by human disturbance) is called a plagioclimax

11. Secondary Succession – Cleared Land
Open pioneer community (annual grasses)
Primary Bare Earth
Grasses and low growing perennials
Time to develop: Years
1-2
3-5
Young broad leaved woodland
Scrub: shrubs and small trees
Characterised initially by a rapid turnover of opportunistic species that move in to exploit what is already a hospitable environment.
First colonizers = wind-dispersed common garden weeds. They grow very quickly, gaining early advantage from the relative absence of competition for light and nutrients. The annual weeds are gradually crowded out by tall pereninials and various types of grasses.
16-30
31-150
Mature woodland: mainly oak
150+ = climax community

12. Succession Continues
As the plant community changes, the soil will also undergo changes (abiotic factors will change).
Decomposers will join the community as well as animal species.
Animal species have a profound affect on the plant species occurring within a habitat.
Changing conditions in the present community allows for new species to become established (the future community).
Succession continues until the climax community is reached.
Some animal species feed on early successional plants only. They therefore leave the later species allowing them to flourish within the habitat. Sometimes animals can accelerate the successional change of an ecosystem.

13. Siltation and Infilling
Wetland Succession
Wetland areas present a special case of ecological succession.
Wetlands are constantly changing:
Open water
Plant invasion
Siltation and Infilling
Wetland ecosystem may develop in a variety of ways:

14. Wetland Succession
In well drained areas, pasture or heath may develop as a result of succession from fresh water to dry land.
In non-acidic, poorly drained areas, a swamp will eventually develop into a fen.
In special circumstances, a an acid peat bog may develop. (may take years).

15. Productivity Think back to the work on food webs/chains
It is often useful to know how much energy is passing through a trophic level over a period of time.
This is called productivity
Productivity is a measure of the amount of energy incorporated into the organisms in a trophic level, in an area, over a certain period of time.

16. Productivity
The area is normally one square metre and the time is usually one year.
It is therefore measured in units of kilojoules per square metre per year (kJ m-2 year-1)
The rate at which producers convert light energy into chemical energy is called primary productivity.

17. Gross Productivity
Gross Productivity (GP) – is the total gain in energy or biomass per unit time.
This is sometimes shown as GPP – Gross Primary Productivity
It is related to the total amount of chemical energy incorporated into the producers.
The producers use some of this energy during respiration and energy needs which is eventually lost to the environment as heat.
The remaining energy is available to the herbivores and is known as net primary productivity (NPP)

18. Remember
NPP = GPP – R
Net Primary Productivity = Gross Primary Productivity - Respiration

19. NPP and GPP in Succession
The NPP and GPP of any ecosystem is going to fluctuate. This is especially the case during each seral stage.
As ecosystems become more diverse, the overall GPP is also going to increase.
This is because climax communities are better adapted to an efficient rate of utilisation of their resources.

20. The Early Stages Gross Productivity = Low Net Productivity = High
This is due to the initial conditions and the relatively low density of producers.
Net Productivity = High
This is due to low respiration rates of the initial producers and therefore a lot of energy available to be passed on.
This allows the system to grow and biomass to accumulate.

21. The Later Stages Gross Productivity = High
This is due to an increase in the consumer community who can synthesise a lot of energy from the food they eat.
Net Productivity = Low
Increased rates of respiration and other energy sapping activities by consumers means that NP will begin approaching zero.

22. Climax community The P:R ratio approaches zero.
The productivity : respiration ratio approaches zero
Think about it:
NPP = GPP – R
“0” = high value – high value

23. The Climax
Succession comes to an end with the establishment of a mature, relatively stable community – the climax
Climax communities are more stable that the seral stages that preceded them.
Ultimately, the climate will be responsible for affecting the nature of the climax community unless human or other factors maintain an equilibrium at a sub-climax community.
Deflected succession – repeated disturbances such as grazing by cattle or sheep, floods or human activity can act as arresting factors to halt or set back the process of succession.
A succession which is held in check for prolonged periods is said to have reached a sub-climax stage. Heather Moorlands are an example of this. Heavy grazing and frequent accidental or deliberate firers keep the community at a sub-climax stage.
Such stable environments are maintained by persistent human interference or by the activities of domestic animals.

24. Climax Communities
The major factor affecting the formation of the climax community is the climate: temperature and humidity.
Edaphic Factors, such as components of the soil or the presence of toxic substances also affect the formation of the climax community.