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Ecological Succession
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Ecological Succession is…
•  The observed process of change in the species structure
of an ecological community over time.
•  The community begins with relatively few pioneering
plants and animals and develops through increasing
complexity until it becomes stable or self-perpetuating as
a climax community.
•  Succession occurs in all natural environments. Each
environment has a particular name that expresses the
nature of their ecological succession: Primary and
Secondary
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Ecological Succession
1st year
Horseweed
dominant;
crabgrass,
pigweed
2nd year
Asters
dominant;
crabgrass
3rd to 18th
year
Grass scrub
community;
broomsedge
grass, pines
coming in
during this
stage
19th to 30th
year
Young pine
forest
30th to 70th
year
Mature pine
forest;
Understory
of young
hardwoods
70th to 100th
year
Pine to
hardwood
transition
100th year
plus
Climax oakhickory forest
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There are two main types of
Ecological Succession
•  Primary Succession: The process of
creating life in an area where no life
previously existed.
•  Secondary Succession: The process of restabilization that follows a disturbance in
an area where life has formed an
ecosystem.
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Examples of Succession
Primary
Secondary
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Primary Succession
•  The development of an
ecosystem in an area that
has never had a
community living within it
occurs by a process
called PRIMARY
SUCCESSION.
•  An example of an area in
which a community has
never lived before, would
be a new lava or rock
from a volcano that
makes a new island.
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Primary Succession
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Secondary Succession
•  SECONDARY SUCCESSION begins in habitats where
communities were entirely or partially destroyed by some
kind of damaging event.
•  When an existing community has been cleared by a
disturbance such as a fire, tornado, etc...and the soil
remains intact, the area begins to return to its natural
community. Because these habitats previously supported
life, secondary succession, unlike primary succession,
begins on substrates that already bear soil. In addition,
the soil contains a native seed bank.
•  Since the soil is already in place, secondary succession
can take place five to ten times faster than primary
succession.
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The Circle of Life in
Secondary Succession
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Why Does Ecological Succession
Occur?
•  Because it is the process of life for plants,
soil and other living organisms.
•  Because organisms alter soil structure,
chemistry, and microclimates, the species
composition of ecological communities
constantly changes over time.
•  Succession will continue until the
environment reaches it s final stage…the
Climax Community.
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The Climax Community
•  A climax community is a mature, stable community
that is the final stage of ecological succession. In an
ecosystem with a climax community, the conditions
continue to be suitable for all the members of the
community.
•  Any particular region has its own set of climax
species, which are the plants that are best adapted
for the area and will persist after succession has
finished, until another disturbance clears the area.
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These are Climax Communities
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•  Two main physical factors determine the nature of
the community that develops in an area. These are
temperature and the amount of rainfall.
•  If we place the amount of rainfall on a graph s x
axis, from 0-10, 10-20,and 20-30+ inches and the
temperature along the y axis from hot, moderate,
to cold, the various types of ecosystems will fit into
the graph based on the conditions that they require.
Temperature
Cold
Cold desert
Tundra
Taiga
Moderate
Temperate forest
Grassland
Deciduous forest
Hot
Hot desert
Savanna
Tropical forest
Rainfall (inches)
0-10
10-20
20-30+
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A summary of changes that occur during
succession:
•  Pioneer species colonize a bare or disturbed site. Soil building.
•  Changes in the physical environment occur (e.g., light,
moisture).
•  New species of plants displace existing plants because their
seedlings are better able to become established in the changed
environment.
•  Newly arriving species alter the physical conditions, often in
ways that enable other species to become established.
•  Animals come in with or after the plants they need to survive.
•  Eventually a climax community that is more or less stable will
become established and have the ability to reproduce itself.
•  Disturbances will start the process of succession again.
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Threats to Succession
•  The grasses that move
in as pioneer species
are often thought of as
weeds.
•  The subsequent growth
of shrubs are
considered undesirable
"brush".
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•  Without these intermediate stages, the
disturbed habitat can't return to a natural
forest.
•  The fragility and stability of the ecological
community are dependent upon several
factors.
•  For example, in temperate forests, if the
shrubs are not allowed to grow, insect pests
begin to feed on young trees instead. This has
happened in many places where trees are
replanted after an area is clear-cut. Large
quantities of pesticides are then brought in,
polluting the soil and water and altering the
natural ecosystem even further.
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How Do Humans Affect
Ecological Succession?
•  Clearing the land for the garden and preparing the soil
for planting represents a major external event that
radically re-structures and disrupts a previously
stabilized ecosystem. The disturbed ecosystem will
immediately begin a process of ecological succession.
•  Plant species that adapt to the sunny conditions and
the broken soil will rapidly invade the site and will
become quickly and densely established. These
invading plants are what we call "weeds in which we
consider a nuisance or invaders.
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•  A gardener's only course of action is to spend a
great deal of time and energy weeding the garden or
using chemicals to infringe upon the weeds and the
ecosystem around it.
•  The farmers and gardeners who are growing our
foods incur an immense cost in terms of time, fuel,
herbicides and pesticides that humans pay every
growing season because of the force of ecological
succession.
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Does Ecological Succession
Ever Stop?
•  We must recognize that any ecosystem, no
matter how inherently stable and persistent,
could be subject to massive external
disruptive forces (like fires and storms) that
could re-set and re-trigger the success ional
process.
•  As long as these random and potentially
catastrophic events are possible, it is not
absolutely accurate to say that succession
has stopped.
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•  Also, over long periods of time ( geological
time ) the climate conditions and other
fundamental aspects of an ecosystem
change.
•  These geological time scale changes are not
observable in our ecological time, but their
fundamental existence and historical reality
cannot be disputed.
•  No ecosystem, then, has existed or will exist
unchanged or unchanging over a geological
time scale.
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