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Community Dynamics
 The changing nature of community structure
across the landscape reflects the shifting
distribution of populations in response to:
– Changing environmental conditions
– Interactions among species
 Community structure is dynamic
– Shifting pattern of species’ dominance and diversity
through time
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Subsets of species can be defined by:
• Taxonomic affinity (e.g., all bird
species in a community)
• Guild—group of species that use the
same resources
• Functional group—species that
function in similar ways
Species are not assembled randomly in nature.
Physical stress
3
Herbivory
2
1
0
Low
Mid
High
Disturbance level
(Sousa 1979 Ecology)
15
10
Nitrogen availability
5
0
0
100
200
300
Herbivore Density (snails/m2)
(Lubchenco 1978 Am Nat)
log # seaweed species
4
Nov-75
May-76
Oct-76
May-77
# seaweed species
Species richness
5
1.00
0.75
0.50
0.25
0.0
0.2
0.4
0.6
0.8
1.0
log ammonium loading
(micromol/L/h)
(Bracken & Nielsen 2004 Ecology)
~ 59 seaweed species
20 – 26 common species
2 species
30 cm
17 species
Community Dynamics
 Factors affecting changes in community structure
–
–
–
–
–
Intermediate Disturbance Hypothesis
Community Instability (Alternate stable states)
Keystone Species
Exotic (introduced species)
Ecological Succession
 Primary
 Secondary
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Intermediate Disturbance Hypothesis


Ecological Community stability is an uneasy
balance
Areas with moderate levels of disturbance have
greater species diversity
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Community Instability
Disturbances can cause a community
to change in ways that persist even
if the change is reversed. In some
cases these are considered
“alternate stable states”
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Keystone Species
 A species that has a disproportionately large
effect on community structure
 Removal of a keystone species can cause drastic
changes in a community
– can increase or decrease diversity
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sea star community
15 species coexist
- 1 sea star is removed
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8 remain in community
 Keystone species
exert an important
regulating effect
on other species
in a community.
Populations
are limited by
competition for
resources
Fig. 53.14
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 If Keystone Species
are removed,
community structure
is greatly affected.
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Interactions of Multiple Species
Trophic cascade:
A carnivore eats an herbivore (a direct
negative effect on the herbivore).
The decrease in herbivore abundance
has a positive effect on a primary
producer.
Interactions of Multiple Species
In kelp forests, sea otters feed on sea
urchins, which feed on the kelp.
Sea otters have a positive indirect effect
on kelp.
Kelp, in turn, can positively affect
abundance of other seaweeds, which
serve as habitat and food for marine
invertebrates and fishes.
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Copyright
© 2002
Pearson
Education,
Inc., publishing as Benjamin Cummings
Species Introductions
 Exotic species
– A species moves out of its home range and takes up
residence in a new place
– No natural enemies or controls
– Can outcompete native species
 Geographic dispersal
 slow or rapid movement
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Caulerpa taxifolia
•Native to Indian Ocean, used as
ornamental in aquaria
•Specific strain was found to thrive
in cold aquarium environments
•Selective breeding under exposure
to chemicals and ultra-violet light
made the strain even hardier
•Escaped from Oceanographic
Museum in Monaco (France)
•Smothering parts of Mediterranean
Sea
•Continues to spread
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Invading Seaweed
Spread of Caulerpa in the Mediterranean Sea
The introduction of Caulerpa to the
Mediterranean dramatically changed
native species interactions, and thus
community structure and function.
Seagrass meadows support a multitude
of species and were overgrown by
Caulerpa.
A Mediterranean Seagrass Meadow
The seagrass Posidonia and Caulerpa
have different growth cycles: Posidonia
loses blades in the summer when
Caulerpa is most productive.
This allows Caulerpa to overtop
Posidonia and dominate.
Caulerpa acts as an ecosystem engineer,
accumulating sediments around its
roots, which changes the invertebrate
community.
There is also a decrease in numbers and
sizes of fish after Caulerpa invades,
suggesting the habitat is no longer
suitable.
Connections in Nature:
Stopping Invasions Requires Commitment
In 2000, Caulerpa was discovered near
San Diego, California.
A team of scientists and managers was
immediately assembled to design an
eradication plan.
It eventually took six years and $7 million
to eradicate the alga.
Prickly Pear Cactus in Australia
1916 – introduced to Australia
1925 – 30 million acres covered (500 ton/acre)
1926 – Cactus moth (Cactoblastis) introduced
1934 – only 1/10 of the cactus remained
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Kudzu in Georgia
 Imported for erosion control
 No natural herbivores, pathogens, or
competitors
 Grows over landscapes and cannot be dug up or
burned out
 May turn out to have some
commercial use
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Aquatic Invasives: Unintended Stow-aways
 Ballast water
has brought
many invaders
to Bay Area
 Great Lakes
affected along
with ALL other
shipping ports
world-wide
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Community Structure Changes through Time
 Community structure varies in time and space
– Across the landscape (zonation)
– In one position as time passes
 Succession is the gradual and (seemingly)
directional change in community structure
through time from field to forest
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Ecological Succession
 Over time,
one array
of species
is replaced
by another
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Community Structure Changes through Time
 The sequence of communities from grass to
shrub to forest has been called a sere, and each
of the changes is a seral stage
 Each seral stage has its characteristic structure
and species composition
– It may last only one to two years or for several
decades
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Types of Succession
 Primary succession: new environments
– Begins with “foundation species”
– Ends with “climax community”
 Secondary succession: communities
destroyed or displaced
– May not have “foundation species”
– Also ends with “climax community”
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Climax Community
 Stable array of species that does not change
over time
 In a particular habitat, succession produces the
same climax community
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Primary Succession Occurs on Newly Exposed
Substrates
 Primary succession begins on sites that have
never supported a community
– Rock outcrops and cliffs
– Sand dunes
– Newly exposed glacial till
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Primary Succession
 Primary succession begins on sites that have
never supported a community
– Rock outcrops and cliffs
– Sand dunes
– Sand is deposited by wind and water and sand particles
may be piled in long, windward slopes to form dunes
– Dune position can shift, and this often covers existing
vegetation
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Primary Succession
 Colonization of sand dunes
 The establishment and growth of plant cover
acts to stabilize the dunes
– Grasses are the most successful pioneering plants
 Once the dunes are stabilized, mat-forming
shrubs invade the area
 The vegetation eventually shifts to trees, first
pines and then oak
– Differences in tree species based on moisture
availability and region
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Sand Dune Succession
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Primary Succession Occurs on Newly Exposed
Substrates
 Newly deposited alluvial soil on a floodplain
represents another example of primary
succession
 Glacier Bay National Park, Alaska
– Over the past 200 years the glacier that once covered
the entire region has been retreating
– The newly exposed landscape is initially colonized by
a variety of species
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Glacial Succession
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Most communities are disturbed
by events like fire, weather, or human activities
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Marine communities are subject to disturbance by
tropical storms.
Any major changes in the living community has significant
Fig. 53.17
affects on local environmental conditions.
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Volcanic Island Succession
Foundation Species
Climax Community
a relatively stable complex community at the final stage of succession that is
in equilibrium with the current physical environment.
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 Secondary succession –
occurs where an existing community has been
cleared, but the soil is left intact.
 Relatively rapid succession
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Secondary Succession:
Disturbed Habitats
Along Roadsides
Abandoned Fields
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Cyclic Replacement
Disease Damage – Sudden Oak Death
Storm Damage
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Fire Damage
Cyclic Replacement:
Fire Climax
Chaparral –
fire adapted plant
community
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No fire –
no seedlings,
growth of white fir
Low intensity –
allows Giant
Sequoia to re-seed
Medium intensity –
kills competing
fir trees
Giant Sequoia
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High intensity –
crown fire
destroys forest
Community Structure Changes through Time
 The process of succession is common to all
terrestrial and aquatic environments
 W. Sousa examined the process of succession in
a rocky intertidal algal community
– Results show a pattern of colonization and extinction,
with other species displacing populations that initially
colonized as time progressed
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Community Structure Changes through Time
 Patterns of succession are not random
 Early successional species or pioneer
species are usually characterized by high
growth rates, smaller size, high degree of
dispersal, and high rates of per capita population
growth
 Late successional species have lower rates of
dispersal and colonization, slower per capita
growth rate, and they are larger and longerlived
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Community Structure Changes through Time
 Succession in terrestrial plant communities at
the Hubbard Brook Experimental Forest (New
Hampshire) after clear-cutting
– Beech and sugar maple replaced by raspberry thickets
and seedlings of sun-adapted, fast-growing species
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Ecological Issues: American Forests
 Old-field communities commonly occur in the
eastern United States and represent the early
stages of secondary succession
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Ecological Issues: American Forests
 When European colonists first arrived on the
eastern shores of North America (1600s), the
landscape was dominated by forest
– Native Americans had used fire to clear areas for
planting crops, but this had minimal impact on the
landscape
 The clearing of forest by settlers was driven by
the need for agricultural lands and forest
products
– By the 1800s, most of the forest in eastern North
America had been felled
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Ecological Issues: American Forests
 The trend was reversed in the Dust Bowl period
in the 1930s
 With the advent of mechanization of agriculture,
small family farms gave way to large commercial
farms
 By the 1930s, the amount of agricultural land in
the east had peaked, and it has been declining
ever since
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Secondary Succession Occurs After Disturbances
 Old-field succession in the Piedmont region of
North Carolina
– Annual crabgrass is the first species to grow after a
crop field has been abandoned
– In the second years, horseweed quickly claims the
field, white aster and ragweed also invade
– By the third summer, broomsedge invades and soon
dominates
– Pine seedlings establish and within 5 to 10 years, the
pines are tall enough to shade the broomsedge
– Eventually, hardwood species take over the field
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Secondary Succession Occurs After Disturbances
 Studies of physical disturbance in marine
environments have demonstrated secondary
succession in various communities
–
–
–
–
–
Seaweed
Salt marsh
Mangrove
Seagrass
Coral reef
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Secondary Succession Occurs After Disturbances
 D. Duggins (University of Washington) examined
the process of succession in the subtidal kelp
forests of Torch Bay, Alaska
– One year after the removal of kelp forest, a mixed
canopy and understory of kelp species formed
– During the second and third years, the community
returned to the original composition
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Secondary Succession Occurs After Disturbances
 J. Zieman (University of Virginia) described the
secondary succession of seagrass communities in
Florida Bay
 Wave action and heavy grazing creates openings in
the grass cover, exposing underlying sediments —
subsequent erosion results in blowouts
– Rhizophytic macroalgae  contributes to sediment
– Halodule wrightii colonizes  stabilizes the sediment
surface
– Thalassia testudinum eventually colonizes the area and
again resembles the surrouding seagrass community
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The Study of Succession Has a Rich History
 E. Warming (Denmark) and H. Cowles (United
States) developed the concept of ecological
succession
 Several hypotheses have addressed the
processes that drive succession
–
–
–
–
–
Monoclimax hypothesis (F. Clements)
Initial floristic composition (F. Egler)
Facilitation model (J. Connell and R. Slatyer)
Inhibition model (J. Connell and R. Slatyer)
Tolerance model (J. Connell and R. Slatyer)
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The Study of Succession
 Monoclimax hypothesis: the community is a
highly integrated superorganism
– The process of succession represents the gradual and
progressive development of the community to the
climax stage
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The Study of Succession
 Initial floristic composition: the process of
succession is individualistic
– Depends on the particular species that colonize the site
and the order in which they arrive
– No species is competitively superior to the other
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The Study of Succession
 Facilitation model: Early successional species
modify the environment so that it becomes more
suitable for successive species to invade and
grow
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The Study of Succession
 Inhibition model: The first species to arrive
hold the site against all invaders — they make
the site less suitable for both early and late
successional species
– The initial species hold their position as long as they
live and reproduce
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The Study of Succession
 Tolerance model: Later successional species
are neither inhibited nor aided by species of
earlier stages
– Communities are composed of species most efficient in
exploiting available resources
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Succession Is Associated with Autogenic Changes
in Environmental Conditions
 Autogenic environmental change is a direct
result of the presence and activities of organisms
within the community
– Vertical profile of light in a forest is a direct result of
the vegetation structure
 Allogenic environmental change is governed by
physical processes
– Decline with average temperature with elevation
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Succession Is Associated with Autogenic Changes
in Environmental Conditions
 Though a general model of plant succession has
emerged, no single cause of succession fits all
the examples
– In both primary and secondary succession, plant
colonization alters the environment (e.g., light, soil)
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Succession Is Associated with Autogenic Changes
 Alteration of light environment
– In the early stages of plant succession, shadeintolerant species can dominate
– Shade-intolerant species grow above and shade the
slower-growing, shade-tolerant species
– Eventually, shade-intolerant species cannot grow and
survive in the shaded conditions
 Succession results from changes in the relative
tolerances and competitive abilities of species
under autogenically changing environmental
conditions
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Succession Is Associated with Autogenic Changes
 Autogenic changes in soil development —
stabilization and addition of organic matter
– Seagrass communities
– Succession on glacial sediments — changes in soil
nitrogen during primary succession
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Species Diversity Changes during Succession
 Chronosequences (chronoseres) are groups
of sites used to compare patterns of diversity
through succession
– Some studies have shown that plant diversity increases
with site age (old-field communities)
 R. Whittaker (Cornell University) found a different
pattern
– Species diversity increases with herbaceous stages,
decreases into shrub stages, increases again in young
forest, and finally decreases as the forest ages
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Species Diversity Changes during Succession
 Species colonization and replacement drive
succession
– Colonization by new species increases local species
richness
– Species replacement, as a result of competition or lack
of tolerance, acts to decrease species richness
 The peak in diversity during the middle stages of
succession corresponds to a transition period
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Species Diversity Changes during Succession
 Disturbance can have a similar effect to that of
reduced growth rates by extending the period
over which the species coexist
 Disturbance acts to “reset the clock” in
succession
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Species Diversity Changes during Succession
 Patterns of diversity depend on the frequency of
disturbance
– High  diversity will remain low because later
successional species never have a chance to colonize
– Low  diversity will decline as later successional
species displace earlier species
– Intermediate  colonization can occur but competitive
displacement is held to a minimum
 Intermediate disturbance hypothesis (M.
Huston and J. Connell)
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Succession Involves Heterotrophic Species
 Succession also occurs in heterotrophic
communities
 The decomposition of fallen trees, animal
carcasses, and droppings by fungi and
invertebrates recycle nutrients and alter the
substrate
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Succession Involves Heterotrophic Species
 As the structure of the plant community changes
during succession, so do the habitats available to
heterotrophs
 As the vertical structure becomes more complex,
new species appear
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Systematic Changes in Community Structure Are
a Result of Allogenic Environmental Change
at a Variety of Timescales
 Allogenic environmental change is governed by
physical (abiotic) processes
– Annual fluctuations in temperature and precipitation
have little influence on successional patterns
– Fluctuations or shifts in environmental conditions that
occur at periods longer than the organism’s life span
are likely to result in successional shifts
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Systematic Changes in Community Structure Are
a Result of Allogenic Environmental Change
at a Variety of Timescales
 Seasonal succession of phytoplankton (Lawrence
Lake, Michigan) is correlated with species’
optimal temperature, nutrient, and light
requirements
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Systematic Changes in Community Structure Are
a Result of Allogenic Environmental Change
at a Variety of Timescales
 Seaward expansion of marshlands (River Fal,
Cornwall, England) is a result of silt deposition
lowering water depths over the past century
 Sediment deposition in freshwater environments
can lead to infilling and a transition from an
aquatic environment  marsh  grassland,
swamp woodland, peat bog  meadow
 Shifting distribution of tree species in North
America following the last glacial maximum
18,000 years ago
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Primary succession –
begins in a new or lifeless area where soil has not yet formed.
Lake Succession
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Community Structure Changes Over Geologic
Time
 Earth’s surface has changed profoundly over the
past 4.6 billion years
 Paleoecology is the study of the distribution
and abundance of ancient organisms and their
relationship to the environment
– Reconstruction of plant distribution after the last
glacial maximum (Laurentian ice sheet) of the
Pleistocene
– Climatic oscillations between cold and temperate
– Historic occurrence and distribution of plant species is
indicated by the pollen record of sediment cores
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The Concept of Community Revisited
 Clements viewed the community as a quasiorganism made up of interdependent species
 Gleason saw the community as an arbitrary
concept and stated that each species responds
independently to the underlying features of the
environment
– Individualistic or continuum concept
 The reality of what a community is probably lies
somewhere in the middle
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The Concept of Community Revisited
 The organismal community is a spatial concept
 The continuum view is a population concept
 Each view can be applied to a given community
and though different, are consistent
– Patterns of co-occurrence along a gradient of altitude
– Patterns of forest communities in Great Smoky
Mountains National Park
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The Concept of Community Revisited
 The structure of communities is the product of a
complex interaction of pattern and process
 Species respond to an array of environmental
factors that vary over space and time
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