Disturbance & Succession
(Chapter 22)
1.
2.
3.
4.
5.
Disturbance
Mechanisms of succession
Patterns across successional stages
Alternatives to succession
Effects of disturbance
• Disturbance – marked change in a
community caused by an outside
influence
– displacing an ecological system from its
equilibrium
– only used where the habitat could
potentially recover
• Natural – fires, floods, grazing, glaciers,
storms
• Anthropogenic – logging, grazing,
agriculture
Succession – gradual change in plant
and animal communities following
disturbance
Sere – the series of stages in a
community during succession
weedy field → old field → shrubland →
pine forest → hardwood forest
• Primary succession – community
development in a newly exposed habitat
devoid of life
• Secondary succession – regeneration
of a previously existing community
following disturbance
Primary
glacial retreat
landslide
Secondary
intense fire
abandoned field
lava flow
logging
surface fire
storm
sand dune
bog
no soil
poor soils
good soils
no plants
few plants
existing plants
no seeds
some seeds?
seedbank
• In similar environments, succession
follows regular pathways
• Pioneers – first plant species to arrive
• Climax – final, stable community
following a successional sere
Mechanisms of Succession
1. Why do plant communities change
over time during succession?
2. Why do the changes over time follow
regular, repeated patterns?
Early-arriving species change the
environment for later-arriving species
Three processes determine the course of
succession
1. Facilitation – earlier-arriving species
enhance establishment of later arrivals
– nitrogen fixers
– lichens that break down rocks
– dune plants that stabilize soils
2. Inhibition – early species inhibit
establishment of later species
– due to allelopathy or competition for
space, light, nutrients
– succession only happens when
individuals die
– favors long-lived species
Precedence effects – the first-arriving species wins.
3. Tolerance – early species have no
effect on the establishment of later
species
– late-arriving species can become
established regardless of the existing
plants, but
– competition determines which species
survive
Pearly everlasting
Fireweed
lupine – nitrogen-fixing perennial
• Effects of lupine:
– increase soil nitrogen
– mulch from dead tissues on soil surface
• Experiment: grow pearly everlasting and
fireweed seedlings with and without
lupine nearby
Results:
• Survival rates
– seedlings survived best away from lupine
• Growth rates:
– both species grew faster near lupine
• Overall – blend of facilitation and
inhibition
• Successional mechanisms can be
complex
Characteristics of successional
species
• Pioneer species are determined by
dispersal – who can get there first
– wind-, animal-dispersed seeds
– rapid growth
– reproduce immediately
– require full sun
– small size
– typically annuals
• Middle-successional species
– more shade-tolerant
– biennial or perennial
– larger size
– more investment in roots, woody stems
• Climax species
– large size
– shade-tolerant
– well-developed roots
– long-lived
2nd Midterm – Wednesday!
Disturbance & Succession
(Chapter 22)
1. Patterns across successional stages
2. Intermediate-disturbance hypothesis
3. Disturbance-dependent ecosystems
Climax community is stable and selfregenerating
ADULT TREES
Beech
0.9
Red Oak
0.7
Red
Maple
0.6
Red Oak
0.05
0.2
0.1
0.1
Maple
0.05
0.1
0.3
0.1
Tulip
Poplar
0.0
0.0
0.0
0.0
SAPLINGS
Beech
Tulip
Poplar
0.8
beech
tree
abundance
maple
tulip poplar
time
red oak
Another model:
• Growth and survival rates of trees in
shade vs. sun
• How much shade each tree casts
• Seed dispersal distances
• Climax community depends on climate,
soils, topography
• Species composition varies from place
to place according to the continuum
concept
Changes in communities over time
during succession
• Resources are used more efficiently
• Mutualisms become more important
• Species richness increases
Animal communities become more
diverse during succession
Intermediate Disturbance Hypothesis
• Low levels of disturbance – competition
results in only a few species dominating
• High levels of disturbance – only early
successional species survive
• Intermediate disturbance
…is just right
Number of
species
Small boulders
Medium
Large
x
xx x
Other examples
• Gap formation in forests
• Prairie dogs in grasslands
• Fire in prairies
Disturbance-dependent Ecosystems
• Some ecosystems require regular
disturbance to maintains certain
vegetation types
– climatic climax – would occur without
disturbance
– fire climax – vegetation maintained by fire
• Disturbance keeps the vegetation in an
early successional stage
Tallgrass prairie in Illinois
– climatic climax = oak woodland
– fire climax = tallgrass prairie
Ponderosa pine forests in Arizona…
• Management of these ecosystems
requires regular disturbance
– how much? how often?
• Many animal species depend on
disturbed habitats