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Succession Syllabus
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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.
Ecological succession
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The gradual process by which the species population
of a community changes is called ecological
succession.
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A forest following a disturbance such as a fire.
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Succession takes places as a result of complex
interactions of biotic and abiotic factors.
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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.
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Succession – What happens?
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Each successive community makes the
environment more favourable for the
establishment of new species.
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A succession (or sere) proceeds in seral
stages, until the formation of a climax
community is reached.
Primary Succession
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Refers to colonization of regions where there
is no pre-existing community.
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Can you think of examples where this would
occur?
Primary Succession
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Community changes on a glacial moraines
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After a volcanic eruption
Questions – Glacial Moraines
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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.
1.
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.
2.
a)
b)
c)
3.
Primary and Secondary Succession
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Primary Succession – occurs on newly
formed habitats that have not previously
supported a community.
Examples?
Secondary Succession – occurs on sites that
have been disturbed.
Examples?
Primary Succession – Bare Rock
Lichens,
bryophytes
and annual
herbs
Bare Rock
Mosses,
Grasses and
small
shrubs
After 100-200 years
Slower growing
broadleaf species
e.g. oak
Fast
growing
trees e.g.
Ash
Complex Community
Example for a Northern Hemisphere lithosere: a succession on bare rock
In Summary - the 1st Invaders!
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These are usually fast growing plants that photosynthesize
well in full sunlight.
We call these pioneer species making up the pioneer
community
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Examples = lichens, grasses, herbs
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As these species begin to grow well, they produce shade.
Their own seedlings grow more poorly than shade-adapted
plants.
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Plants that grow well under full sun are replaced by plants that
germinate and grow better in deeper shade.
Secondary Succession
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This type of succession takes place after a land
clearance (e.g. from fire or landslide).
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These events do not involve loss of the soil.
Secondary succession therefore occurs more rapidly
than primary succession.
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Humans may deflect the natural course of succession
in these circumstances (e.g. by mowing or farming).
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This leads to the development of a different climax
community than would otherwise develop naturally.
Secondary Succession – Cleared
Land
Primary Bare
Earth
Open pioneer
community
(annual grasses)
Time to develop: Years
Grasses and low
growing
perennials
1-2
3-5
Young broad
leaved woodland
31-150
Mature
woodland:
mainly oak
150+ = climax community
Scrub: shrubs
and small trees
16-30
Succession Continues
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As the plant community changes, the soil will also undergo
changes (abiotic factors will change).
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Decomposers will join the community as well as animal
species.
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Animal species have a profound affect on the plant species
occurring within a habitat.
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Changing conditions in the present community allows for new
species to become established (the future community).
Succession continues until the climax community is reached.
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Wetland Succession
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Wetland areas present a special case of
ecological succession.
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Wetlands are constantly changing:
Open water
Plant invasion
Siltation and
Infilling
• Wetland ecosystem may develop in a variety of ways:
Wetland Succession
• In well drained areas, pasture or heath may
develop as a result of succession from fresh
water to dry land.
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In non-acidic, poorly drained areas, a swamp
will eventually develop into a fen.
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In special circumstances, a an acid peat bog
may develop. (may take 5000+ years).
Productivity
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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.
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This is called productivity
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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.
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Productivity
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The area is normally one square metre and the
time is usually one year.
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It is therefore measured in units of kilojoules
per square metre per year (kJ m- year )
2
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-1
The rate at which producers convert light
energy into chemical energy is called primary
productivity.
Gross Productivity
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Gross Productivity (GP) – is the total gain in energy or
biomass per unit time.
This is sometimes shown as GPP – Gross Primary Productivity
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It is related to the total amount of chemical energy
incorporated into the producers.
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The producers use some of this energy during respiration and
energy needs which is eventually lost to the environment as
heat.
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The remaining energy is available to the herbivores and is
known as net primary productivity (NPP)
Remember
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NPP = GPP – R
Net Primary Productivity = Gross Primary
Productivity - Respiration
NPP and GPP in Succession
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The NPP and GPP of any ecosystem is going to
fluctuate. This is especially the case during each
seral stage.
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As ecosystems become more diverse, the overall GPP
is also going to increase.
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This is because climax communities are better
adapted to an efficient rate of utilisation of their
resources.
The Early Stages
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Gross Productivity = Low
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This is due to the initial conditions and the relatively low
density of producers.
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Net Productivity = High
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This is due to low respiration rates of the initial producers and
therefore a lot of energy available to be passed on.
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This allows the system to grow and biomass to accumulate.
The Later Stages
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Gross Productivity = High
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This is due to an increase in the consumer community
who can synthesise a lot of energy from the food they
eat.
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Net Productivity = Low
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Increased rates of respiration and other energy
sapping activities by consumers means that NP will
begin approaching zero.
Climax community
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The P:R ratio approaches zero.
The productivity : respiration ratio approaches
zero
Think about it:
NPP = GPP – R
“0” = high value – high value
The Climax
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Succession comes to an end with the establishment of
a mature, relatively stable community – the climax
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Climax communities are more stable that the seral
stages that preceded them.
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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.
Climax Communities
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The major factor affecting the formation of the
climax community is the climate: temperature
and humidity.
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Edaphic Factors, such as components of the
soil or the presence of toxic substances also
affect the formation of the climax community.