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Chapter 8
Community Ecology: Structure,
Species Interaction, Succession,
and Sustainability
Key Concepts
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•
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Community Structure
Roles of Species
Species Interactions
Changes in ecosystems
Stability of ecosystems
Focus Questions
• What determines the number of species in a
community?
• How can we classify species according to
their roles?
• How do species interact with one another?
• How do communities change as conditions
change?
• Does high species diversity increase the
stability of ecosystems?
What is Community Structure?
4 characteristics:
1.
2.
3.
4.
Physical appearance
Species diversity
Species abundance
Niche structure
1. Physical Appearance
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Types of plants
Relative sizes of plants
Stratification of plants and animals
Aquatic life zones
Edge effects: differences in physical
properties (sun, temp, wind) at ecotones
2. Species Diversity
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1.
2.
3.
1.
2.
3.
4.
Highest biodiversity: rain forests, coral reefs,
deep ocean
Areas with high biodiversity may have low
species abundance (few members of each
species)
3 factors affect biodiversity:
Latitude (decreases with distance from equator)
Depth in aquatic systems (highest at surface and
bottom)
Pollution
Biodiversity increases with:
More solar radiation
More precipitation
Less elevation
Seasonal variations
Biodiversity decreases with
distance from equator
Species Diversity
Species Diversity
100
10
Latitude
0
30ºS
80ºN
60
40
20
0
3. Species Abundance
• Theory of Island Biogeography (aka
Species Equilibrium Model): number
of species on an island is determined by
a balance between the immigration rate
of new species and the extinction rate of
species already on the island
© 2004 Brooks/Cole – Thomson Learning
Rate of immigration
or extinction
High
Low
Equilibrium number
Number of species on island
(a) Immigration and extinction rates
Theory of Island
Biogeography, continued
Immigration and extinction rates are
affected by:
1. Size of the island
2. Distance from mainland
Small islands=lower biodiversity
Small islands=higher extinction rate
General Types of Species
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Native
Non-native
Indicator
Keystone
*Scientists apply labels to clarify
their niches
Types of Species
• Native: species that normally live in a
particular ecosystem
• Non-native (exotic, alien): species that
migrate into an ecosystem or are
accidentally introduced into an
ecosystem by humans
Non-native Species
• Example: 1957 Brazil imported wild
African bees to increase honey
production (aggressive and unpredictable)
• Displaced domestic honeybees and
reduced honey supply
Indicator Species
• Species that serve as early warnings
of damage to a community or
ecosystem
• Example: birds-found everywhere and
respond quickly to change
trout and amphibians-indicate
good water quality—they require clean
water with good oxygen supply
Keystone Species
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1.
2.
3.
4.
5.
Species with much more important
roles than their abundance suggests
Critical roles include:
Pollination by bees, hummingbirds
Dispersion of seeds by fruit-eating animals (bats)
Habitat modification (elephants, beaver dams)
Predation by top carnivores to control populations
(wolf, lion, alligator)
Recycling of animal wastes (dung beetle)
Species Interactions
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•
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Competition
Predation
Parasitism
Mutualism
Commensalism
Competition
• Intraspecific: competition between members
of the same species for the same resources
Example: territoriality (mark and defend
area against their own species)
• Interspecific: competition between different
species for resources
Occurs when niches overlap
Number of individuals
© 2004 Brooks/Cole – Thomson Learning
Species 1
Species 2
Region
of
niche overlap
Number of individuals
Resource use
Species 1
Species 2
Resource use
Other types of Competition
Interference Competition: one species
limits another’s access to resources
Example: hummingbird defends patches
of wildflowers (nectar) so that other
hummingbirds may not get to them
Exploitation Competition: species have
equal access to resources but differ in
how fast they exploit it
Competitive Exclusion
Principle
• One species eliminates another species
in an area through competition for
limited resources
Example: Paramecium
Can’t occupy the same niche
with limited resources
High
Relative population density
Paramecium
aurelia
Paramecium
caudatum
Low
0
2
4
6
8
10
12
Days
Both species grown together
14
16
18
Resource Partitioning
• Species avoid competition by dividing
scarce resources amongst them
Example: using resources at
different times, in different ways, or in
different places (warblers)
Predator-Prey Interaction
• Predation: members of one species
(predator) feed directly on another species
(prey)
-At the individual level, prey is harmed
-BUT, predators kill sick, weak individuals
(improves genetics)
-Remaining prey population has greater
access to resources (enough for
everyone)
How do prey defend
themselves against predators?
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•
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Ability to run, swim, or fly fast
Very developed sense of sight or smell
Protective shells (turtle, armadillo)
Thick bark
Spines or thorns (porcupines, cacti)
Chemical warfare (poison)
Camouflage/Mimicry (owl butterfly)
Parasitism
One organism benefits and the
other is harmed.
Example:
Fleas
Mutualism
• Both species benefit
Example: clownfish
and anemone
Commensalism
• One species
benefits and the
other is neither
harmed nor benefits
Example: bird in
tree
How do ecosystems respond
to change?
• Ecological succession: gradual
change in species composition in a
given area
Primary succession: establishment
of biotic communities on a lifeless
ground
Secondary succession:
reestablishment of biotic community
where some type of community is
already present
What is Primary Succession?
• Begins with a lifeless area where there is no
soil
Examples: bare rock exposed by
retreating glacier
cooled lava
abandoned highway or parking lot
newly created shallow pond
Primary Succession, continued
• First, there must be SOIL
• Soil formation begins when pioneer species
attach themselves to bare rock
Trap wind-blown soil particles
Secrete acids that break down rock
Produce tiny bits of organic matter
• Perennial plants (live for 2 years without being
reseeded) and herbs replace lichens and mosses
• Early successional plant species grow close to
the ground, can tolerate harsh conditions
• Midsuccessional plant species are herbs,
grasses and low shrubs
• Late successional plant species (mostly trees)
Primary Succession
Lichens
Exposed
and mosses
rocks
Small herbs
and shrubs
Heath mat
Time
Balsam fir,
paper birch, and
Jack pine,
white spruce
black spruce, climax community
and aspen
Ecological Succession
© 2004 Brooks/Cole – Thomson Learning
Early Successional
Species
Rabbit
Quail
Ringneck pheasant
Dove
Bobolink
Pocket gopher
Ecological succession
Midsuccessional
Species
Late Successional
Species
Wilderness
Species
Elk
Moose
Deer
Ruffled grouse
Snowshoe hare
Bluebird
Turkey
Martin
Hammond’s
flycatcher
Gray squirrel
Grizzly bear
Wolf
Caribou
Bighorn sheep
California condor
Great horned owl
What is Secondary
Succession?
• Begins in area where community has been
disturbed, destroyed, or removed
• Some soil remains
Examples: abandoned farmlands
burned or cut forests
heavily polluted streams
flooded land
Secondary Succession
Mature oak-hickory forest
Annual
weeds
Perennial
weeds and
grasses
Shrubs
Young pine forest
Time
How do species replace one
another in ecological
succession?
• Facilitation: one set of species makes
an area suitable for other species
• Inhibition: early species hinder the
growth of other species
• Tolerance: late successional plants are
unaffected by earlier succession plants
(thrive in mature communities without
having to eliminate earlier species)
Intermediate Disturbance
Hypothesis
• Communities that experience
frequent disturbances have the
greatest biodiversity
• Disturbances create openings for
new species
Species diversity
0
100
Percentage disturbance
What is Stability?
• Complex networks of negative and positive
feedback loops that interact to provide stability
and sustainability
• Stability is maintained by constant change in
response to changing environmental conditions
• 3 aspects of stability/sustainability
1. Inertia: ability of a system to resist
disturbance
2. Constancy: ability of a system to keep its
numbers within the limits (resources available)
3. Resilience: ability of a system to bounce
back after a disturbance
Precautionary Principle
• Alternative view: biodiversity does not
necessarily lead to more stability, and if nature
is unpredictable, there is no point in trying to
manage and preserve old-growth forests and
ecosystems. We should convert grasslands to
cropfields, drain and develop wetlands, and not
worry about extinction
• Precautionary Principle: when evidence
indicates that an activity can harm the
environment, we should take precautionary
measures to prevent harm, even if the causeand-effect relationships have not been fully
established