Ecological Succession
• Ecological succession is the gradual process by
which ecosystems change and develop over
time.
• For example, 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 (in Britain).
The same ground 2 years
later, now covered in grasses
and low flowering plants.
What is ecological succession?
• "Ecological succession" is the observed process of
change in the species structure of an ecological
community over time. Within any community
some species may become less abundant over
some time interval, or they may even vanish from
the ecosystem altogether. Similarly, over some
time interval, other species within the
community may become more abundant, or new
species may even invade into the community
from adjacent ecosystems. This observed change
over time in what is living in a particular
ecosystem is "ecological succession".
Pioneer Species
The bare ground conditions favor pioneer plant species which are the first plants
to occupy these areas. These are often species which grow best where there is
little competition for space and resources. The pioneer community is formed of
species able to survive under hostile environments. The presence of these species
modifies the microenvironment generating changes in abiotic and biotic factors of
the ecosystem undergoing formation. Therefore they open the way to other
species to establish in the place by the creation of new potential ecological niches.
Two Types of Succession
• Primary succession is the changing sequence of
communities from the first biological occupation of a
place where previously there were no living beings. For
example, the colonization and the following succession
of communities on a bare rock.
• Secondary succession is the changing sequence of
communities from the substitution of a community by
a new one. For example, the ecological succession of
the invasion of plants and animals in an abandoned
crop or land.
Which type of succession is pictured above?
Why does ecological succession occur?
•
•
Every species has a set of environmental conditions under which it will grow and
reproduce most optimally. In a given ecosystem, and under that ecosystem's set of
environmental conditions, those species that can grow the most efficiently and
produce the most viable offspring will become the most abundant organisms. As
long as the ecosystem's set of environmental conditions remains constant, those
species optimally adapted to those conditions will flourish. The "engine" of
succession, the cause of ecosystem change, is the impact of established species
have upon their own environments. A consequence of living is the sometimes
subtle and sometimes overt alteration of one's own environment. The original
environment may have been optimal for the first species of plant or animal, but
the newly altered environment is often optimal for some other species of plant or
animal. Under the changed conditions of the environment, the previously
dominant species may fail and another species may become ascendant.
Ecological succession may also occur when the conditions of an environment
suddenly and drastically change. A forest fires, wind storms, and human activities
like agriculture all greatly alter the conditions of an environment. These massive
forces may also destroy species and thus alter the dynamics of the ecological
community triggering a scramble for dominance among the species still present.
How are humans affected by
succession?
•
Ecological succession is a force of nature. Ecosystems are in a constant process of change and
re-structuring. To appreciate how ecological succession affects humans and also to begin to
appreciate the incredible time and monetary cost of ecological succession, one only has to
visualize a freshly tilled garden plot. 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 adapted 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". Now "weeds" have very important ecological roles and
functions but weeds also compete with the garden plants for nutrients, water and physical
space. If left unattended, a garden will quickly become a weed patch in which the weakly
competitive garden plants are choked out and destroyed by the robustly productive weeds. A
gardener's only course of action is to spend a great deal of time and energy weeding the
garden. This energy input is directly proportional to the "energy" inherent in the force of
ecological succession. If you extrapolate this very small scale scenario to all of the agricultural
fields and systems on Earth and visualize all of the activities of all of the farmers and
gardeners who are growing our foods, you begin to get an idea of the immense cost in terms
of time, fuel, herbicides and pesticides that humans pay every growing season because of the
force of ecological succession.
Does succession ever stop?
• There is a concept in ecological succession called the "climax" community.
The climax community represents a stable end product of the successional
sequence. Its apparent species structure and composition will not
appreciably change over observable time. To this degree, we could say
that ecological succession has "stopped". We must recognize, however,
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 successional process. As long as these
random and potentially catastrophic events are possible, it is not
absolutely accurate to say that succession has stopped. 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.
These photos show __________ succession,
the development of a community where none was before.
The images were taken at Acadia National Park in Maine. The
first to appear on the bare rock are lichens and algae. These
secrete acids which begin to extract nutrients from the rock
and which form tiny cracks which are widened by freezing and
thawing. As the cracks widen they trap enough organic
material and moisture for mosses to take hold. Larger cracks have
enough soil to support grasses and small shrubs. The largest cracks
come together to form small basins where trees can take root,
although the tree in the photo below didn't make it too long; perhaps
a drought exhausted the water in the small basin. However, in the
background the climax coniferous forest is visible where enough soil
has accumulated to support the trees.
A successional story takes root on the west coast
at Mt. Rainier in Washington. Once again, the
lichens are the first to appear (although they are
hard to spot here). More obvious are the
mosses, grasses and small plants like
flowers. Each stage accumulates soil and organic
material that facilitates the growth of the next
stage. On this mountainside, the
coniferous forest is the climax community.
Secondary Succession - Lake Succession: In this
image, a former bog in Maine has almost
completely filled. A former lake, formed originally
when a large piece of ice broke off a retreating
glacier, is now well along the transition to dry
land. These kettlehole lakes often support an
extensive mat of floating vegetation. The bog in
the picture above left (from Minnesota) is a good
example. You can still see open water, but a mat of
floating vegetation (the material between the
foreground and the open water) has developed.
Fire and Succession: Fire plays a complex role in succession. Usually, a fire stops the
progression of succession and sets the stage for new, secondary succession as plants take
root and grow in the soil enriched by the mineral ashes of their predecessors. In some
cases, however, fire plays an even more important role. It maintains the climax community
by removing competitors that would otherwise move the climax to a different type of
community.
Two examples are shown here. The shrubland habitat of southern California is maintained
by fire, which eliminates large trees. The pictures above and to the left show a large fire
burning a hillside; any trees that had been growing there would be killed and the shrubs and
other plants adapted to fire would spring back quickly and reclaim the habitat.
Above left, an area at the Archbold Biological Station, just a few days after it was burned.
The plant in the foreground is a palmetto; after the fire it was able to resprout quickly from
the prostrate stalk, which has a thick covering which is able to resist the fire. The picture
below shows an area on the other side of the road which had not burned for a long time,
and which is full of small trees such as scrub oaks and scrub hickories.
In the drought year of 1988, huge fires swept through Yellowstone National Park (above
and left). After years of fire suppression in the park, the forest floor was loaded with
potential fuel, and the resulting fire burned hotter than it would have if more frequent,
patchy fires had been allowed to take their natural course. The pictures above were taken
in 1999; 11 years after the fires. The slow regrowth is in part due to the cold temperatures
there. To the left, a pine sapling struggles to regrow after the fire. In Yellowstone, the fires
moved the community away from its climax and set secondary succession into motion.
Below: since the Yellowstone fires of 1988, the National Park Service has adopted more
enlightened fire practices. Among these are prescribed burning. The Park Service actually
sets small fires to approximate the frequency of burning that would occur under natural
conditions. This creates smaller, patchier fires that kill only the species least tolerant of
fires, and tend to increase overall diversity as burned and unburned areas are in close
proximity. The pictures below are from Everglades National Park in Florida. The
everglades are a fire-dependent ecosystem; spring rains come in the form of
thunderstorms and lightning strikes any trees that are growing there. The resulting fire
kills the trees and allows the native sedges like sawgrass to reclaim the landscape. Here,
fire maintains an unstable climax.
Clear cutting: When forests are clear-cut, secondary
succession begins on the deforested plots. To speed
the process, and to ensure that the resulting forest
will be the same age and easy to harvest, timber
companies usually replant the forest. This, however,
results in a forest of trees which are often
genetically very similar, and of course, of the same
age. This creates a patchwork landscape as you can
see in these pictures. Because these forests do not
retain any of the old trees which provide food and
nest sites for many animals, they are not very
diverse in terms of wildlife either. Wildlife depends
on a diversity of trees which themselves vary in age
from saplings to mature and even dead trees.
Above: The valley you see in the photo was created by a retreating glacier (it's camera
shy and hiding behind the bend in the valley). Initially, the steep slopes are subject to
slides, and not much will be able to grow there. As the land stabilizes, however,
succession will begin to take hold and, if the climate does not cool and allow the glacier
to grow again, the forests will cover the slopes.
Old-field Succession: The abandoned pasture is slowly reverting to forest. Grasses
are gradually replaced by other perennials such as milkweeds, goldenrod, and
shrubs. Next come small trees adapted to this habitat. These would include
sassafras, hawthorns and the like. Larger trees such as oaks, maples, hickories and
eventually beeches will begin to come into the picture, and eventually a mature
forest such as the old-growth forest will come into being after hundreds of
years. Note the diversity of tree sizes in the mature forest.
In 1980, Mt. St. Helens in Washington erupted,
essentially destroying all life in a large blast
zone. Trees were killed (above left) and the ground
was covered with ash (left). In some places, such as
the protected side of a knob (below left), while the
mature trees were killed the soil was not sterilized as
it was in other places. This meant that seeds could
germinate and begin to replace the forest more
rapidly than was possible in areas where no seeds
survived.
The photo to the left shows a lava flow from
Costa Rica. This lava was deposited during an
eruption in 1992, 15 years before the picture
was taken. As you can see, plant life is trying
to re-establish itself on the lava flow, which is a
very inhospitable environment as it is hot and
dry, with little soil. On the other hand, the soil
that is present is rich in nutrients. Because the
existing community and the soil are so
thoroughly destroyed by volcanic eruptions the
succession that proceeds is called Primary
Succession (as opposed to the secondary
succession occurring after a forest fire).
Below Left: Orchids are actually among the
plants best adapted to live in such extreme
environments as lava flows.
Long growing seasons and plenty of rain help
accelerate succession in Costa Rica.