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JANUARY 2008
560
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Alison Rae
Primary succession – theory and case studies
Definition
A primary succession is one which ‘takes
place on a surface where no soil or
vegetation has formerly existed’
(Skinner, Redfern and Farmer (2003)
Complete A–Z Geography Handbook, 3rd
edn, p 228). An area of bare land is an
opportunity for the development of a
whole new ecosystem. Speed of
development can be extraordinarily fast,
as case studies in this Geofile will show.
Primary succession (or priseres) can be
divided into xeroseres, those in dry
environments, and hydroseres, those in
wet areas (Figure 1). This Geofile deals
with the two types of xerosere, lithoseres
and psammoseres.
Succession – the context
Succession in an ecosystem is the series
of changes which take place in the
community over time. A sere is a
particular type of plant succession.
Succession can be subdivided into
primary (or prisere) and secondary (or
subsere), according to where and when
it occurs. Primary succession happens
first because it takes place on a surface
where no soil or vegetation has
previously existed. Sand dunes, tidal
marshes, outwash plains, deltas,
landslips and areas which have
experienced a volcanic eruption or lava
flow or which have recently been
revealed by glacial melting all come into
this category. Prisere development can
happen after a major physical disaster.
Some human environments also class as
priseres – abandoned quarries, spoil
heaps from mines and some types of
cleared urban land. (Subseres occur on
land which has been previously
vegetated; soil already exists, so the
process is usually quicker.)
The pioneer community is the first
group of plants to colonize a newly
exposed land area. Typically these are
simple, hardy plants, often with
particular adaptations to their
challenging environment. They alter the
environment slightly, adding nutrients
when they die and perhaps some shelter
to allow less resistant species to cope.
Throughout the succession process the
characteristics and species of plants will
change and develop until a balance is
reached with the environmental
conditions. This climax community
will not change unless the
environmental conditions do so.
Geofile Online © Nelson Thornes 2008
Figure 1: Types of primary succession
Prisere
Xerosere
Hydrosere
develops in dry conditions
develops in wet conditions
Lithosere
Psammosere
Fresh hydrosere
Halosere
on bare rock
on sand dunes
in fresh water
in salt water
Figure 2: Succession on land exposed by retreating glacier – Iceland
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Fig 560_01 Mac/eps/illustrator 11 s/s
NELSON THORNES PUBLISHING
Artist: David Russell Illustration
Lithosere succession on bare
rock
Mosses and lichens are usually the first
species to colonise a bare rock surface,
wherever its location. These cling to
surfaces even in the most challenging of
climatic circumstances. In the very cool
wet coastal plain of southern Iceland this
pioneer community covers the most
recent lava flows, including some from
the Laki fissure eruption of 1783. Thick
cushions of mosses lie like an even
bright green blanket over the
hummocky lava flow, creating an eerie
and inhospitable scene.
Lichens take their nutrients directly
from the rock on which they grow. In so
doing they cause biological chemical
weathering because the chemical
reactions that take place break up the
rock structure. This is therefore the very
beginning of soil formation and the
development of a climax community.
Mosses live primarily on water – even
rainwater contains some useful
nutrients. Because they act as a sponge,
holding the water in, they keep the rock
surface wet, encouraging solution of
minerals and hydration processes, both
forms of weathering.
Bedding planes, joints, faults and small
cracks are all lines of weakness and as
such are vulnerable to weathering and
erosion processes which enlarge them.
Small pockets of soil are likely to
develop within the crack. A degree of
shelter from wind and salt is provided,
especially in coastal areas. Larger plants
are then able to take root in this slightly
more favourable environment,
continuing the succession and making it
more complex.
Some lithoseres are particularly
specialised. The surface of a limestone
pavement, such as that above Malham
Cove in the Yorkshire Dales, is a high,
exposed, inhospitable place. The surface
of the clints (blocks of limestone) is bare
January 2008 no.560 Primary succession – theory and case studies
of soil or vegetation, but down in the
grykes (weathered joints) a unique
small-scale ecosystem flourishes. The
highly localized microclimate –
sheltered, humid, alkaline – harbours
plants not found anywhere else.
Figure 3: Cliff top succession (photo)
Case studies
1. Land revealed by a melting glacier Iceland
Iceland’s glaciers have shown significant
melting over the 20th century and since.
Most of the newly exposed rock is from
previous lava flows, so it weathers to
form high quality soil. It is an uneven
surface where sheltered hollows exist,
allowing plants an extra advantage.
Figure 2 shows a new ecosystem after 60
years of development. There are many
species, including some shrubs reaching
over a metre high.
2. Coastal lithoseres
Cliff coastlines provide new bare rock
surfaces for vegetation to try to take
hold. Iceland probably has more bare
rock surface per unit area than any other
European country, due to its location
along the Mid-Atlantic Ridge and the
consequent lava flows. Its coastline has
many cliff areas where lithosere
succession is in progress.
The southern Icelandic coast has high
vertical cliffs, home to many sea birds; it
is a bleak and desolate place, except
perhaps on an unusually sunny day as in
Figure 3). On the cliff tops the hard
basaltic lava flows are gradually
weathering and resistant plants can use
small patches of regolith starting to
become soil. A few plants take root in
cracks on the cliff face, giving the birds
some limited shelter. In turn, the birds
defecate on and around the cliffs,
providing nutrients for the developing
ecosystem.
3. Volcanic eruptions
a) Krakatoa (Indonesia – 1883)
Krakatoa, a well-known volcanic island
in Indonesia, is a caldera so its eruptions
are particularly violent. Its last huge
eruption in 1883 was so explosive that it:
Figure 4: Primary succession, Krakatoa, 1983
Krakatoa
800
fe rn s, mosses , Cyr tandra
shrubs and orchids
Height (m )
600
Neonauclea trees
400
200
Casuarina with dense
grass on steepest slopes
Neonauclea trees
with fig and
macarandra trees
Terminalia trees ,
beach plants, macarandra and
Barringtonia, fig
Terminalia,
Casuarina
beach plants,
Barringtonia,
coconut
rainforest
cliffs
0
Climate
Temperatures are high and constant.
Most months a verage 2 8°C, giving a ver y
low annual range . Rain is heavy, falling in convec tional storms most afternoons
throughout the year .
800
800
Cyrt andr a shrubs ,
fe rn s, mosses an d orchids , mosses , fe rn s,
fe rn s, Cyr tand ra
orchid s, Cyr tand ra small trees
shrubs , mosses and
shrubs, woodland in
orchid s
ravines
increasing number
600
of Neonauclea t rees
mi
xe
d
woodland
savanna grassland,
coarse grassland
400
fe rn s, shrubs,
dense grass, some
fe rns growing an d macarandra and
blue-green bacteri a fig s
200
beach plants,
Barringtonia
0
Number
of plan t
species
1883
0
1886
26
grass 3 m high
400
rain fo rest climax:
increasing number Neonauclea trees Neonauclea with fig,
macarandra and
of macarandra and tak ing ov er from
Neonauclea trees , macarandra and figs Terminalia
200
fig s
beach plants, coconuts beach plants ,
coastal woodland
Barringtonia,
Barringtonia ,tussock climax (types as 1918)
tussock grass
grass , coconut
1908
115
1918
132
1933
271
Barringtonia, beach
plants , Casuarina
Height (m )
Height (m )
600
Year
Note : The rainforest climax vegetation here
does not contain as man y species as the
rainf orests on surrounding islands .
0
1983
Year
?
Number
of plan t
species
Source: D. Waugh (2000) Geography: An Integrated Approach, Nelson Thornes, p.289 Fig 11.8
Geofile Online © Nelson Thornes 2008
Fig 11.7
January 2008 no.560 Primary succession – theory and case studies
• was heard right across the Indian
Ocean
• reduced the island to one third of its
previous size
• covered the area in 50m of ash,
destroying all previous vegetation
and wildlife
• killed all life in the surrounding sea
• set up a tsunami which reached all
shores of the Indian Ocean, killing
30,000 people and destroying
innumerable other ecosystems.
In the hundred years following, the
return of soil and vegetation was
amazingly quick. Obviously the
equatorial climate with its typical high
temperatures and rainfall helped the
processes involved. Figure 4 shows that
after 50 years vegetation had developed
to such a degree that true rainforest
could start to develop. By 1983, 100
years after the catastrophic eruption,
rainforest had developed up to 400 m
above sea level around the whole
mountain, and as high as 500 m in parts.
Above this altitude other factors are
involved; Krakatoa is over 800 m high,
even after the 1883 event, so cooler
temperatures due to altitude and other
climatic factors inevitably limit
rainforest succession.
Today, the number of species within the
Krakatoa rainforest are too numerous to
be counted accurately, though there may
not yet be as many species as in the
original forest. Moreover, average tree
height in the canopy is not as high as it
will become in a few more decades.
b) Surtsey (Iceland – 1963)
The shield volcano, Surtsey, is located
18 km beyond Heimaey on the
periphery of the Vestmann Islands off
south west Iceland. This extremely
active area along the Mid-Atlantic Ridge
has some of the newest land and
lithoseres in the world. Surtsey was an
undersea volcano until the 1963
eruption increased its height sufficiently
to break the ocean surface and form the
world’s newest island.
One of the most researched places on
Earth, only scientists can go there and
they must wear specialist protective
clothing. The development of this new
lithosere is of huge interest to Icelandic
and other scientists; any risk of
contamination (bringing in any seed or
bacteria alien to this ecosystem) would
alter the natural succession. One
researcher made the mistake of arriving
on the island with a tomato sandwich
and seeds dropped to the ground as he
ate it. Later, tomato plants were found
Geofile Online © Nelson Thornes 2008
growing from a crevice in the lava!
They were uprooted and destroyed, as
plants alien to that ecosystem.
c) Mt St Helens (USA – 1980)
Similarly, there are areas around Mt St
Helens where the public are not
allowed to go; it is maintained as a
National Monument. This is a much
larger area than either Krakatoa or
Surtsey and not an island, so keeping
this lithosere ‘unpolluted’ is
impossible. Today, 27 years after the
famous eruption, its ecosystem
development is closely monitored and
will continue to be so.
Scientists have developed some
interesting theories on how ecosystems
respond to large-scale disturbances like
this massive eruption. The area around
Mt St Helens is a relatively simple
system and gives a great opportunity to
investigate developing habitat
relationships. It seems that animals from
the tiniest insect to the largest elk have a
crucial influence on the developing
vegetation and plants from all major
stages of forest development appear to be
establishing themselves simultaneously.
Neither of these phenomena had been
observed so clearly before. Seeds of all
levels of species are coming in from the
surrounding area and even some of the
larger plants manage to take hold in
situations of limited nutrients. This
contradicts classic ecological theory
stating that species establish themselves
in a certain order, that one species gives
way to another (i.e. that mosses are
followed by grasses, grasses by shrubs
and shrubs by trees). This is very
different from studies undertaken in
glacial terrain (such as in front of the
retreating glacier in Iceland, above),
where true succession does seem to take
place in terms of number and size of
species. The key factor here seems to be
whether the lithosphere area is
surrounded by other, thriving
ecosystems.
Psammosere primary
succession
Sand dunes may seem one of the most
alien environments as far as colonizing
plants are concerned. Such an
environment is:• Arid: any rainfall drains away
quickly through the course sand
with large pore spaces in between
• Salty: most plants cannot cope with
a saline environment
• Exposed: there is nothing to protect
plants from strong winds sweeping
in across the sea.
Nevertheless, primary succession does
take place and often remarkably
quickly. The plants which succeed in
such a difficult situation are those
adapted to the particular conditions:
• To combat aridity plants must be
xerophytic (drought resistant)
• To combat salinity plants must be
halophytic (salt tolerant)
• To combat strong winds plants can
have a variety of adaptations, which
will be described below.
The Studland Bay dune
system, eastern Dorset
Much of the land at Studland Bay has
been built up by longshore drift and
accumulation by constructive waves
over the last 500 years. There is a wide
sandy beach. Winds are generally
onshore so sand is blown inland. If it is
caught by an obstacle (driftwood, stone,
even litter!), it builds into an embryo
dune which is so small that a misplaced
foot can destroy it. Grasses such as sea
lyme and sea couch can take hold.
These are both halophytic and
xerophytic and, as they grow, they trap
more sand, building the dune until it is
so large these pioneer grasses are
overcome by the larger, tougher marram
grass.
The relationship between sand dunes
and marram grass is truly symbiotic, i.e.
each relies on the other and contributes
to the welfare of the other. The roots of
marram grass grow down through the
dune and help hold it together, allowing
it to grow. In turn, the enlarging dune
encourages the growth of the marram.
This grass protects itself from wind and
high consequent rates of transpiration
by having all its stomata (pores) on the
inside of leaves which curl in on
themselves. The tough stringiness of
this plant resists the wind, even wind
carrying sand, which makes its erosive
power so much greater.
As these grasses die they rot down into
organic material; this is the beginning
of humus for the next generation of
more complex colonizing plants. The
sand turns greyer in colour due to the
addition of this material. This is the
start of it becoming soil. Other plants
can now cope with what has become a
slightly less challenging environment.
At Studland Bay sand dunes this group
of plants include:
• herbs, i.e. soft-stemmed green plants
• rosette plants, which grow flat to the
ground to protect themselves from
January 2008 no.560 Primary succession – theory and case studies
wind, sandblasting and, in areas of
human use, from human feet;
plantains are an example
• tough, woody-stemmed plants like
heather, ling and bramble.
These plants thrive on the back of the
larger dunes and in the slacks, the dips
between lines of growing dunes. Here,
the environment is more sheltered, less
saline and has humus available from
earlier plants, providing nutrients. Also,
because slacks are lower lying, plant
roots may be able to reach the freshwater
table, solving the issue of effective
drought.
As more lines of dunes have developed
at Studland (Figure 5), the older ones
show:
Figure 5: Cross-section through Studland dunes to show the development of the
psammosere
Development of sand to soil
Key
Rosette plants
Sea lyme and sea couch
Marram grass
Herbs
Yellow sand
Humus/grey sand
Podsol (acid soil)
East
Sea
pH
8
Shelly sand
1
8–7
West
2
Wasting dunes (grey dunes)
Slack
B
1
Slack
0
Embryo
dunes
Gorse
Brambles
Heather and ling
Other shrubs, various trees
Yellow dunes
and ridges
Foredunes
(newly
developing
ridge 0)
Open
beach
G
B
H
S
B
H
7–6
H
Slack
2
G
6
GH
5
G H G
S S
S
4.5 – 4
Figure 6: Studland dunes climax vegetation
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Artist: David Russell Illustration
• a greyer colour due to increasing
humus content
• a decreasing pH value, as more
humus increases acidity levels
• a greater % of surface covered with
vegetation
• greater average height of vegetation
• increase in number of vegetation
species (apart from the climax
vegetation – see below).
Eventually this primary succession
reaches a climax ecosystem with
interesting characteristics. Light
woodland of alder and willow (waterloving species), silver birch and stunted
conifers is found at the back of the
oldest, most decayed dunes (Figure 6).
Here, pH is as low as 4, showing
incredibly high acidity; this is about as
acidic as a growing medium can be and
have plants survive in it. Fieldwork over
a number of years shows this low
reading to be consistent and accurate.
Shrubs, mainly gorse, brambles and
large heathers, cover almost every part of
the ground. Measurements using
quadrats show the percentage of ground
covered by vegetation increases steadily
inland from the beach along a transect
through Studland’s dunes. Here, the
larger species dominate and so drive out
the smaller plants. As a result, for the
first time in this succession process, the
number of species in the ecosystem
actually reduces.
Conclusion
Lithospheres make fascinating studies.
Depending on the environment,
development can be rapid in terms of
time and space.
Perhaps this Geofile will open up some
fieldwork or coursework ideas for you!
Geofile Online © Nelson Thornes 2008
Focus Questions
1. What makes a lithosere succession distinctive?
2. What makes a psammosere succession distinctive?
3. Using the information at the bottom of Figure 4, draw a line graph to
show the changes in species number between 1883 and 1933 in the lithosere
succession on Krakatoa. What is the best way to describe the shape of this
curve? What does this mean in terms of the development of the succession?
4. Comment on the fact that, by 1933, there were already 720 species of
insects identified on Krakatoa.
5. Seeds reached the island of Krakatoa to re-colonize it by wind, drifting
in the sea or by being carried by birds. What effects might this relatively
random situation have had on the succession?
6. (a) Explain what makes the succession observed at Mt St Helens different
from what is considered a ‘normal’ succession.
(b) The succession on Krakatoa developed as a result of seeds and insects
being blown to the island or carried in the sea. Why did it follow a more
normal pattern of succession when Mt St Helens did not do so?
7. Write an essay to compare prisere development with subsere
development. Refer to examples and case studies you have studied in detail
wherever possible.