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Ecological Succession
 Progressive change in species composition, ecosystem
function and structure following a disturbance
 Minor changes in structure and function accumulate over time
 Ecological function of ecosystem changes
 Initiated by a disturbance
 Directional change in structure
 Follows a predictable pattern
Successional Classifications (seres)
 Primary succession: establishment of plant communities on
newly formed habitats lacking plants
 Lava flows, sand dunes, landslides, etc.
 Secondary succession: return to vegetation following a
disturbance
 There is some blurring of the classifications
Some examples
 Tornado levels a strip of forest
 Trees are all gone, but seed bank is still
there.
 Secondary succession will follow
 Severe fire burns through organic layer of
soil and destroys the seed bank and
nutrients
 Primary succession would proceed even
though plants were there before
 Disturbances vary along with their impact
Succession as a
Deterministic Process
 Frederick Clements (1916)
 Distinct steps…ends in a climax community
Clementsian Succession
 Established species alter the environment
 Allow new species to move in and establish
 Directional change in composition – aka. Community
structure – is maintained by the continuous
alteration of the environment
 Within the community (specified):
 Succession always follow the same pattern as it
develops to climax community
 If interrupted, it will follow the path again.
 This is why considered deterministic
Stability and Climax Community
 In Clement’s view,
 The concept of climax community assumes:
 The species colonizing and establishing themselves in a
given region can achieve stable equilibrium
 Stable equilibrium – forces to change system = forces to
keep system the same…no change results
 Therefore, the climate community = stable equilibrium
Clementsian Example
 Hardwood trees, a climax community, has a
disturbance in the form of deforestation.
 The community will always proceed this way
 The transition will occur in predictable manner.
 Barring any further disturbances, the hardwood
community will be reached and will be stable.
Is deterministic pathway the only way?
 Idea of stable community fell out of favor
 Individualistic Perspective (Gleason, 1926)
 The relationship between coexisting species (communities) as the
result of similarities in their requirements and tolerance to the
environment.
 Partly result of chance
 Succession is not deterministic
The challenge
 Gleason argued that Clements explanation of
succession could not explain retrogressive
successions
 Plant community simplifies and loses biomass over
time
Clements vs. Gleason
Clements
 Assume long term stability
 Deterministic
 Interdependence among
species
Gleason
 Assume environment can
deteriorate over time
 Random
 No relationship between
species – together because
of similar env. requirements
Stability Refuted
 Succession viewed as a phenomenon that rarely
attains equilibrium
 Equilibrium related to nature of disturbance
 Disturbance acts at variety of scales
 Magnitude of disturbance varies
 Many disturbances remove only part of the previous
plant community
Disturbance
 Variations in the definition (general agreement):
 Any relative discrete event in space and time that
disrupts an ecosystem, community, or population
structure and changes resources, substrate, or the
physical environment – Pickett and White, 1985
 Discrete in time (as opposed to chronic stress or
background environmental variability)
 Cause a notable change (perturbation) in the state of the
system
Consequences of
Disturbance
 Total habitat destruction
 Creation of new habitat (transformation)
 Fragmentation (loss of certain habitat, isolation of
habitats)
 Increase patch number, isolation, edge
 Decrease patch size, connectivity, interior
 Alter local climate/microclimate, hydrology, biota
diversity, behavior, health, persistence)
The good in disturbances
 Dependent on temporal scale of analysis
 Example:
 Forest fire
 Short term = disturbance
 Long term = required to maintain seed bank and
regeneration of intermediate tree growth (pines)
• Intermediate disturbance
hypothesis
• Highest diversity when
disturbances occur at
certain time intervals.
Succession Related to Biodiversity
An example
 Mt. St. Helens eruption – May 1980
 Good place to see the different aspects of
succession and disturbance at work.
The disturbance
Eruption Survival
 Survival of organisms was strongly influenced by
characteristics of disturbance processes, local site
conditions, and biological factors
 Pyroclastic flow and avalanche debris:
 Almost no organisms survived the blast
 Those that survived:
 Plants with underground buds, burrowing animals, and
organisms protected by snow, topography, or other features
 Having diverse refuges facilitated survival of some
organisms
Eruption Survival
 Life history attributes:
 Many organisms not present yet
 Anadromous fish – still at sea
 Migratory birds
 Larger, local populations suffered higher mortality than
smaller species or migratory species
 Surviving groups included all of the primary trophic levels –
herbivores, predators, scavengers and decomposers
 Complex food webs quickly developed in the emerging
ecosystems
 Surviving species established new interactions and began to
process the dead organisms from the pre-eruption system.
Timing
 Eruption in early morning
 Allowed nocturnal animals to be protected in
subterranean burrows
 Eruption in early spring
 Snow and ice created refuges and many plants had not
broken out of winter dormancy at higher elevations
 Early successional stage of many recently harvested
forest sites
 Profusion of wind dispersed seeds of pioneer plant
species