Communities and Succession
Communities and Succession
A community consists of all the plants and
animals that occupy a particular area.
The individual populations within a community
interact with each other.
The community is a constantly changing and
dynamic unit, which passes through a number of
stages from its origin to its climax.
The transition from one stage to the next is
called succession.
nothing
Climax
community
Have you ever seen something
like this?
How does it happen?
This is bare
rock.
Nothing lives
here.
Why?
•Exposed.
•No soil.
•Absorbs and reflects heat.
•Dry.
•No nutrients.
How does succession work?
A bare patch of ground will not stay bare. It will rapidly be
colonized by a variety of plants.
A recently cleared patch of ground
The same ground 2 years later,
How Does Succession Work?
There are 2 types of succession:
PRIMARY- Plants grow where NO PLANTS HAVE GROWN
BEFORE.
SECONDARY- Plants grow where there has been a previous
population. These will have been destroyed, eg: by fire
The 1st species to colonise are called PIONEER SPECIES.
The “ultimate” species to colonise are called the CLIMAX
COMMUNITY.
Sand dune succession.
The pioneer species is
marram grass.
Pioneer species
Pioneer species are often 'opportunist' species which are able to
rapidly exploit a sudden new opening in ground plant cover.
They must be able to grow quickly. They are usually short-lived.
EXAMPLE: Mosses
They are low growing, carpeting the
ground and with little height.
They then provide a microhabitat
equivalent to a miniature forest for a
variety of invertebrates such as mites
and spiders.
The moss also acts like a sponge when
wet, in some cases providing a semiaquatic microhabitat.
Looking again at our example….
Bare Ground
No plant competition for light,
space, nutrients or water.
Soil mobile and liable to erosion
and loss.
A more extreme surface
microclimate because the bare soil
both absorbs and reflects heat
more than soil covered in
vegetation.
A drier environment because there
is no plant cover to hold moisture
above ground and little humus to
hold it in the soil.
Lower nutrient levels in the soil.
Two Years Later
Intense plant competition for
space and other resources.
Soil bound by roots and plant
cover.
The plant cover provides a certain
amount of ground insulation from
extremes of temperature. There
are now also a variety of
microclimates within the vegetation.
Plant cover and increasing humus
levels help to retain water.
The nutrient levels in the soil will
have increased.
Bare Ground
No plant competition for light,
space, nutrients or water.
Soil mobile and liable to erosion
and loss.
A more extreme surface
microclimate because the bare soil
both absorbs and reflects heat
more than soil covered in
vegetation.
A drier environment because there
is no plant cover to hold moisture
above ground and little humus to
hold it in the soil.
Lower nutrient levels in the soil.
Two Years Later
Intense plant competition for
space and other resources.
Soil bound by roots and plant
cover.
The plant cover provides a certain
amount of ground insulation from
extremes of temperature. There
are now also a variety of
microclimates within the vegetation.
Plant cover and increasing humus
levels help to retain water.
The nutrient levels in the soil will
have increased.
Over time…
• Undisturbed, an area will pass through a number
of further different successional stages, each
with its own characteristic mix of species.
• Each new community will be better adapted to
the changed environment which has been provided
by the previous community.
•Eventually, a climax or 'final' community is
reached. At this point, the succession will not go
any further. However, this does not imply that
there will be no further change.
Succession Stages
Grassland, small flowering plants.
Grass, daisies
Taller herbaceous plants.
Willow herb, foxglove
Bushes and shrubs.
Hawthorn, bramble
Fast growing trees.
birch
Larger, slow growing, stronger trees.
oak
Application Task:
Mount St Helens
Succession on a Hydrosere:
Succession =
change in the composition or structure of
an ecological community.
A sere is a stage in the process of succession.
There are several examples of this:
•bare rock - lithosere
•bare sand - psammosere (as found on sand dunes).
•Hydroseres – water
If the pond in your back garden is not dredged
occasionally it will happen there in a few years.
Succession on a Hydrosere:
CASE STUDY: Loch A’Mhuillin
This is an ancient lake on the Isle of Arran that
is gradually being colonised by vegetation.
Why is it being colonised?
Inputs:
Outputs:
Groundwater flow
River
Hydrological Rivers
Evaporation
Precipitation
Transpiration
Surface run off
seepage
Through flow
Vegetation
Sediment
The hydrological outputs outweigh the
inputs= lake is drying up.
Succession on a hydrosere will happen
relatively quickly.
Loch A’Mhuillin has
become overgrown
with a succession of
plant species.
Pioneer Species
These colonise where no species has previously been.
The first plant species
to colonise Loch
A’Mhuillin were floating
aquatics.
These simple plants can
grow relatively easily in
any pond or lake.
They do not require
stable soil.
Since uncolonised land usually has thin, poor quality soils with few
nutrients, pioneer species are typically very hardy plants, with
adaptations such as long roots, root nodes containing nitrogen fixing
bacteria, and leaves that reduce transpiration.
What happens next?
The population of pioneer plants will
increase, and some of them will die.
Dead plants will begin to decompose
and form soil.
More complex plants (ie with simple
roots) can then grow.
A characteristic of these plants is that they
produce huge numbers of seeds.
They may live only one year, and spend the
greatest part of their energy ensuring that
offspring will arise the following year.
Species of this kind are known as opportunist
species. Grasses are a common example of
opportunist species.
…and then?
Plants continue to live and
die, contributing to the soil
that is forming.
Eventually, the soil is able
to support more complex
plants
Sun dew
Bell Heather
Woody stemmed plants
gorse
bracken
rhododendrons
As the soil becomes deeper and more
stable, more complex plants with
deeper roots become established.
Climax Community
In the final stages of
succession, taller trees
begin to grow. They, in
turn, block out the
sunlight needed by
smaller trees and
replace them.
The final stage of
ecological succession is
known as a climax
community.
In Scotland, near the lake the typical climax community would
be Scot’s Pine.
Gorse, rhododendron
Mosses, sundew, bell heather
grasses
Floating
aquatics
bracken
An oak wood is a climax community
About 4000 years
ago much of Britain
was a climax
community of oak
woodland.
Most of this forest
was cleared to
allow for grazing
and cultivation.
Questions.
1. What kind of problems would a pioneer species
encounter?
2. Describe the kind of adaptations you might expect to
see in a pioneer species in order to overcome these
challenges.
3. Explain the difference between primary and secondary
succession.
4. Early stages of ecological succession contain short-lived
plants which devote enormous amounts of energy
producing seeds. More mature communities do not need
to do this. Why?
The diagram shows a profile along a transect
showing succession on some coastal sand dunes.
Point
A
B
C
D
Index of
diversity
1
2.3
5.4
6.1
10
B
A
C
Seashore
0
Distance inland/m
Height
above
seashore
/m
D
0
300
a) What data must be collected in order to be able to
calculate an index of diversity of plants at one of the
points along the transect? (2)
b) Suggest an explanation for the trend in diversity along
the transect. (3)
Have you driven past areas like this?
What is this type of community called?
Heathland
The many heaths and grasslands in Britain that
we refer to as “natural” are in fact the result of
clearance and grazing.
These are not true climax communities but subclimax ones resulting from human activities.
Because the normal succession has been
artificially changed it is often referred to as a
deflected succession and the resultant subclimax is a plagioclimax.
In the process of succession, land on a newly formed
volcanic island will be gradually colonised. Over time,
changes in the composition of the plant and animal
communities will occur until eventually no further
change takes place.
(a) What term is used to describe the final stable
community at the end of a succession? (1)
(b) Explain why one community is gradually replaced by
another during succession. (2)
(c) Describe how each of the following farming practices
influences the process of succession. (total 3)
i) regular grazing by sheep
ii) ploughing fields each year
The niche concept was investigated in some classic
experiments in the 1930s by Gause. He used flasks of
different species of the protozoan Paramecium, which
eats bacteria.
Conclusion: These two species of Paramecium share the same
niche, so they compete. P. aurelia is faster-growing, so it outcompetes P. caudatum.
In a second experiment:
Conclusion: These two species of Paramecium have slightly different
niches, so they don't compete and can coexist.
It is important to understand the distribution:
P. caudatum lives in the upper part of the flask because only it is adapted
to that niche and it has no competition.
In the lower part of the flask both species could survive, but only P.
bursaria is found because it out-competes P. caudatum. If P. caudatum was
faster-growing it would be found throughout the flask.
Rock Shore Ecology
Discuss rocky shores as an example ecosystem.
Explain how organisms have become adapted to
suit the environmental conditions and to occupy a
particular niche.
Rock Shore Ecology
1.
What are the physical problems with living on
an exposed rocky shore?
2. What are the “pay-offs”?
3. Rocky shores have zoned communities.What
are the zones on a rocky shore? What kinds of
organisms live in each of these zones?
4. Sketch a diagram showing the zonation and
examples of the typical organisms that live
there.