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Succession: How communities come to be
Primary succession:
New space becomes available for occupation and is colonized by
species that can cope with the extreme conditions an void of soil
and organisms. The colonizers alter the environment to make it
more livable for later successional species (new sand dunes, lava
flows, areas exposed after glacial retreat).
Secondary succession:
A previously occupied space was cleared of vegetation by
disturbance and is reoccupied. Soil and seed banks were left in
place. Early colonizers must be able to cope high exposure to
light, temperature and moisture change (burnt, plowed or clear-cut
areas).
Primary Succession
Rock lichen facilitate the transformation of bedrock to soil.
Lupines: early successional species.
An example of primary succession:
vegetation recovery on the mudflows of Mount St. Helens
At 8:32 Sunday morning, May 18, 1980, Mount St. Helens erupted.
The eruption latest 9 hours, but changed 200 square miles of forest into a lifeless,
ash covered moonscape. Thousands of game animals died, 12 million salmon, and
uncounted millions of birds and small mammals. 57 people were killed.
The largest landslide in recorded history swept down the mountain at speeds of 70 to
150 miles per hour and buried rivers under an average of 150 feet of debris. Some
areas were covered by as much as 600 ft of debris.
Forest, before
Same spot, after
Fireweed, seen just weeks after eruption, is an example of a “pioneer species”.
Secondary Succession
An example of secondary succession:
The Yellowstone fire of 1988.
The fire burnt 1.4 million acres (36% of the park) during a summer that was the
driest in the Park's recorded history.
$120 million was spent and 25,000 people participated in saving people and
property. The fire itself was finally stopped by rain and snow in September.
Disturbance:
Fire
Parasites
a destructive event that removes all or a
large part of the community.
Wind
Overgrazing
Water
Human activities
Gap models of community dynamics:
After disturbance
Early successional stage
Mid successional stage
Late successional stage
Some silviculture methods rely on natural forest succession
Seed trees are left
standing inside clear cut
patches.
A major question of the theory of succession:
How is the succession of communities on the road to the
“climax” community controlled?
 Is succession an orderly, predictable process that selects the most
appropriate species for every stage of succession and ends in a
fully predestined climax community?
 Is succession a chance-driven process that depends on what
species are available and tolerate living there and ends in one of
possibly many climax communities?
Is community composition “predestined”?
Yes (Frederic Clements)
(1874-1945)
Community as
“super-organism”
No (Henry Allen Gleason)
(1882-1975)
Community assembled by
chance and opportunity
Frederic Clements
"Succession . . . must be regarded as the
development or life-history of the climax
formation. . . . Succession is preeminently a
process the progress of which is expressed in
certain initial and intermediate structures or
stages, but is finally recorded in the structure of
the climax formation.“
Plant Succession: An Analysis of the Development of Vegetation.
Washington, D.C.: Carnegie Institution of Washington, 1916.
(1874-1945)
Henry Allen Gleason
"The vegetation of an area is merely the resultant of
two factors, the fluctuating and fortuitous immigration
of plants and an equally fluctuating and variable
environment. As a result, there is no inherent reason
why any two areas of the earth's surface should bear
precisely the same vegetation. . . . Every species of
plant is a law unto itself, the distribution of which in
space depends upon its individual peculiarities of
migration and environmental requirements. . . . The
species disappears from areas where the environment
is not longer endurable. It grows in company with any
other species of similar environmental requirements..",
The Individualistic Concept of the Plant Association, Bulletin of the Torrey
Botanical Club, 53 (1926), pp. 7-16, 23-26.
(1882-1975)
Clement’s succession model:
Community 1:
Earliest
successional
stage
Community 2:
Seres
Climax
community
Gleason’s community concept:
Community 2
Species abundance
Community 1
Some environmental gradient (altitude, soil moisture, soil fertility)
The role of species interactions in succession:
Facilitation model
Earlier species alter the physical environment to pave the
way for later species.
Inhibition model
Initial colonists hang on to the space they captured and
exclude other colonists.
Tolerance model
Species neither facilitate not inhibit, but they simply get
there at different times due to differences in life history.
Facilitation model
Lichen growing on lava
Nitrogen fixing plants
Rock weathering and soil building species pave the way for species that require
soil. Important in the early stages of primary succession.
Inhibition model
Initial coral colonizers may hang on to their space until the next disturbance.
Tolerance model
Early colonists:
long-distance dispersal
high seed production
high germination rate
fast growth
Late arrival:
high competitiveness
slow growth
large size
Invasions can alter the course of succession:
Seres
Land use changes
Invaders
Earliest
successional
stage
Climax
community 2
Climax
community 1
The sagebrush steppe (Great Basin Desert)
perennial
grasses & shrubs
Invasion by Bromus tectorum:
Before 1900
Mack, 1981
1930
Exotic plants can interfere with the succession process:
perennial
Low disturbance
grasses & shrubs
regime
overgrazing
+ Bromus seed
Annual grasses
fire
Summary:
•
Communities are dynamic species associations.
•
Species co-occur because they tolerate the same sort of
environment, but…
•
Species also alter the environment.
•
Succession refers to the dynamics of species assembly on either
new terrain (primary succession) or disturbed terrain (secondary
succession).
•
Primary succession takes much longer because soil needs to be
formed.
•
Invaders can alter successional events.