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Chapter 17
Ecosystem Change
• LongLong-term changes in the physical
environment
• Changes in the genetic constitution of
organisms
Billings, W. D. 1938. The structure
and development of old-field shortleaf
pine stands and certain associated
physical
h i l properties
ti off the
th soil.
il
Ecological Monographs 8:437-499.
• Changes in the types, numbers and
groupings of organisms
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Nothing in biology makes sense
except in the light of evolution.
Ecological
Succession
Theodosius Dobzhansky
(1900-1975)
Pioneer of the use of fruit flies in genetics
experiments.
i
t
In 1937 he published one of the major works of
the modern evolutionary synthesis, the synthesis
of evolutionary biology with genetics, entitled
Genetics and the Origin of Species, which
amongst other things defined evolution as "a
change in the frequency of an allele within a
gene pool".
Succession
Succession
• Sequence of plant, animal, and
microbial communities that
successively occupy an area over a
period of time.
• Process of change by which biotic
communities replace each other and by
which the physical environment
becomes altered over a period of time.
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Sere
Seral Stages
• Product of succession.
• Product of succession.
• The characteristics sequence of biotic
communities that successively
y occupy
py
and replace each other in a particular
environment over time after either
disturbance of the original community or
the formation of a new, previously
uncolonized environment.
• Various communities that together
make up a sere.
Primary Succession
Example: plant seral stages
Seral Stage
Soil Modification
Bare rock or soil
Algal, lichen, moss community
Grass, forb, fern community
Small shrub community
Large shrub community
Small tree species community
Large tree species community
Final tree species community
little or no soil
create simple soil
accumulate and deepen soil
developing soil profile
accumulating organic material
deeper, more complex soils
fully developed soil profile
deep, stabilized soil profile
Secondary Succession
• Begins in environments that lack organic matter
• Not yet altered by living organisms
Dry (xeric) environments
Moist (mesic)
environments
Very wet (hydric)
environments
• Begins in environment that has already been
modified by living organisms
Abandoned Fields
Clearcuts
Succession: xerarch
mesarch
hydrarch
Seres:
mesoseres
hydroseres
xeroseres
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Why Change Occurs???
Why Change Occurs???
• Change in environment (physical, biotic)
caused by physical processes external to
the biotic community
y
• Change in environment (physical, biotic)
caused by resident organisms
=> Allogenic Succession
SEDIMENT
=> Autogenic Succession
Changes in biota
Lake
Changes in biota
Bog
Bog
Climax
Communities
Relatively stable communities,
representing either the final or an
indefinitely prolonged stage of a sere.
Vegetation Dynamics
Forest
Successional Retrogression
• So far, we have discussed forward
development of the ecosystem
toward a climax condition.
• Retrogression: effects of disturbance
changing the seral condition back to
an earlier stage (fire destroying a
climax community => pioneer
successional community).
Classical
concepts &
theories
• Some researchers prefer to use term
“Vegetation Dynamics” rather than
succession,, (see
(
pp.
pp 466 Kimmins).
)
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Classical concepts &
theories
1863 - American philosopher
and naturalist Thoreau
Monoclimax theory
• Composition and structure of species of
the climax is determined by regional
macroclimate.
• Plants in climax reproduce successfully
under their own shade.
• coined term “forest succession” (even-aged
hardwood stands replacing pine stands
after logging in N.E. US)
• Community is maintained indefinitely
under prevailing climatic conditions.
Ecological convergence
principle
• If succession is regulated mainly by
climate, all biotic community (in all
site types) within a climate region
will converge in structure and
species composition to one
eventual climax community - the
climatic climax
Kimmins, Fig. 17.2
Polyclimax theory
Many factors can intervene to prevent reaching the
climatic climax condition.
Pyral climax - frequent natural fires
Biotic climax - grazing animals
Edaphic climax - Serpentine soils (mg) and
limestone soils (calcium) in areas of acid igneous
rocks result in different local climaxes.
What is a climax community?
• Self-replacing, stable seral stage.
• Insufficient allogenic and autogenic agents to
promote change.
• Persists over several generations.
• If community is disturbed, replaced by individuals
of the same species.
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Approaching the Climax
• Succession continues until the addition of
new species and exclusion of established
species no longer changes the environment:
– at this point succession reaches climax:
• the community growth form has come into
equilibrium with its physical environment
Continuum concept
The Climax Pattern Hypothesis
• Monoclimax and polyclimax
theories assume discrete mutually
exclusive plant communities.
• But vegetation forms a continuum
wherein species are distributed and
replace each other independently
along environmental gradients.
The Climax Pattern Hypothesis
• Vegetation is seen as a complex
pattern of integrating communities
and not a mosaic of distinct
communities
communities.
• Conclusion: Highly unlikely that
one theory of succession will apply
equally well everywhere.
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