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Community Change – Chapter 21
Community Change
• Sit in an open field or wooded lot, and you will
see the community change
• If we designate a prairie as a protected area and
keep out grazers and fire, the grassland will go
from prairie  shrubland  forest
• Succession – the development of the community
by the action of the vegetation on the
environment, which leads to additional species
being established
Two Types of Succession
• Primary succession – occurs on new sterile land,
such as that uncovered by a retreating glacier or
created by an erupting volcano
• Secondary succession – the recovery of
disturbed sites
Mount St. Helens – Primary Succession
Mount St. Helens – Primary Succession
Chance events tend to
drive early primary
succession – little
biological interaction
Concepts of Succession
• Relay floristics – an orderly hierarchical system
of change in the community
Monoclimax hypothesis – the
development of the community is a
gradual and progressive change from
pioneer species to the climatic climax
(community determined by the climate).
Subclimax community – results from
particular soil types, fire, or grazing
(refers to the climatic climax)
Facilitation model – early species
facilitates the arrival of the later
species.
Concepts of Succession
• Initial floristic composition – development at any
one site depends on who gets there first
First species there either inhibits or
has no effect on the next species.
Eventually, longer lived species will
become the stable community.
Concepts of Succession
• Random Colonization Model – succession only
involves the chance survival of different species
and the random colonization of new species
No facilitation or inhibition.
Succession can move in any direction
The Climax State
• The final or stable community in a successional
series
– Self-perpetuating and in equilibrium with the
physical and biotic environment
• Monoclimax – only one possible climax state in
each region
– Take away time and disturbance, will always end up with
the same community
• Polyclimax – many different climax states can be
found in the same area
– Climaxes controlled by soil moisture, nutrients, animal
activity, or some other factor
The Climax State
• Criteria is a steady state over time
– Abstract idea, climate is always changing on a
geologic time scale
Patch (Gap) Dynamics
• Space in a forest = light
– A large tree falls in the forest and a ‘gap’ in the
canopy opens up and light floods the floor
(Even if no one hears it!)
• A stable community at a regional level
may be a mosaic of patches at a local level