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Chapter 7
Community
Interactions
Let’s get
ready to
ruuuuumble!
Community
• Community: populations of all
species living and interacting in an
area at a particular time
Four Characteristics of Community Structure
• Physical Appearance: size and
distribution of its population and
species
• Species Diversity/Richness: number
of different species
• Species Abundance: number of
individuals of each species
• Niche Structure: number of niches,
how they compare, and how they
interact
Differing Physical Appearances
• Patch Effects: most large
communities usually consist
of a mosaic of vegetation
“patches”
• Edge Effects: differences in
physical appearance at
boundaries between
ecosystems
Species Diversity
• Species Rich Environments
–
–
–
–
–
Tropical Rain Forests
Coral Reefs
Deep Sea
Large Tropical Lakes
Tend to have high species diversity
but low species abundance
• Factors that Affect Diversity
– Latitude (terrestrial) – distance
from equator
– Depth (aquatic)
– Pollution (aquatic)
Species Abundance
• Determined by:
– Rate at which new species immigrate
– Rate at which species become extinct
Ecological Niche vs. Habitat
Niche: role an organism
plays in an ecosystem
•niche is like an
"occupation“ – a species’
interactions with habitat
and other organisms (their
role in food web)
Habitat: actual location
where an organism lives
•habitat is like an
"address"
7-2 GENERAL TYPES OF
SPECIES
GENERAL TYPES OF SPECIES
•Generalist Species
•Specialist Species
•Native Species
•Nonnative Species
•Indicator Species
•Keystone Species
Generalists vs. Specialists
•Generalist Species have broad
niches, can live many places, use a
variety of resources
e.g., dandelions, cockroaches, coyotes,
humans
•Specialist Species have narrow
niches, live only in specific places
e.g., spotted owls, giant pandas
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Native vs. Nonnative Species
• Native Species: species that
normally live & thrive in a
particular ecosystem
• Nonnative Species:
•also called exotic, invasive,
or alien species
•originate in other
ecosystem
•deliberate or accidental
introduction by humans
causes problems
•may thrive and crowd out
native species
Emerald Beetle
Larva under
bark of ash tree
Why
Purple???
The Case of the Killer Bees
• 1957 Brazil imported wild African
bees to help increase honey
production
• Displaced domestic honeybees
• Actually reduced honey
production
• Moved north in Central America
• Established populations in Texas,
Arizona, New Mexico, Puerto
Rico, and California
Stink Bugs!
Should we release Asian
Wasps to neutralize the
stink bug population???
Snakehead Fish
CANE TOADS!
The Simpsons!
• Bart vs. Australia
Indicator Species
• Indicator Species: species that
serve as early warnings that a
community or ecosystem is being
damaged
•Birds are good indicator species
•Found everywhere
•Respond to environmental
change quickly
•northern spotted owls are
indicators of healthy old–
growth forest
•Fish are good indicator species in
aquatic ecosystems (i.e. trout)
“The loss of a keystone species is like a
drill accidentally striking a power line.
It causes lights to go out all over.”
Keystone Species
– E.O. Wilson
Keystone Species: species that
play a critical role in an
ecosystem
•Ex: sea otters are keystone
species because they prevent
sea urchins from depleting kelp
beds
•Ex: flying foxes are keystone
species because they pollinate &
disperse tropical trees such as
durian
7-3 SPECIES INTERACTIONS:
COMPETITION AND
PREDATION
Species Interactions
• The effects of one species on another may be
negative, positive, or neutral
• Five kinds of interactions:
•Interspecific competition
•Predation
•Parasitism
•Mutualism
•Commensalism
Intra- vs. Inter- specific competition
• Intraspecific Competition –
members of the SAME species
compete for resources
•Interspecific Competition –
members from 2 or more
DIFFERENT species compete for
resources
•When two or more species use
the same limited resource
(food, space, etc.) they may
adversely affect each other
•niche overlap
•Ex: fire ants & native ants in
North America
No Competition
High
Relative population density
Paramecium
aurelia
Low
Paramecium
caudatum
0
2
4
6
8
10
12
Days
Each species grown alone
14
16
18
Interspecific Competition
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 with similar
resource
requirements can
coexist because
they use limited
resources:
•at different times
•in different ways
•in different places
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ITP
Resource Partitioning
Where are the 2
species competing?
Why is there
no
competition
here?
Resource Partitioning
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Resource Partitioning
Five species of insect–eating warblers coexist in
spruce forests of Maine:
•feed in different portions of trees
•consume somewhat different insects
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Warblers
Predation
Members of one species (predator) feed on
another species (prey);
•+ / –
•Ex: lion feeding on zebra
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Predator-Prey Relationship
• What do predators do to increase
their chances of getting a meal?
• What do prey do to avoid being
eaten?
Predator-Prey Relationship
• Predators get better at
catching prey
• Prey get better at
avoiding capture
• Similar to an “arms
race”
– Ex: During the Cold War,
the US and the USSR tried
to intimidate the other
with bigger and better
weapons
Predators
• Pursuit
– Faster (cheetahs)
– Better eyesight (eagles)
– Hunting in packs (wolves)
• Ambush
– Camouflage (praying mantis)
– Mimicry (alligator snapping turtles)
Prey
• Camouflage
• Ex: walking stick
• Warning colors
• Ex: poison dart frog
• Mimicry
• Ex: king snake
• Behavior
• Ex: blowfish
• Chemical warfare
• Ex: skunk
7-4 SPECIES INTERACTIONS:
PARASITISM, MUTUALISM,
AND COMMENSALISM
Parasitism
One organism (parasite) lives on part of another
organism (host)
•+ / –
•Ex: flea living on a dog
•Ex: tongue eating parasite
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ITP
Mutualism
Two species interact in a way that benefits both
•+ / +
•Ex: lichens (algae & fungi)
•Ex: clownfish & anemones
•Ex: ants & acacias
WITH FRONDS
LIKE THESE, WHO
NEEDS
ANEMONES???
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ITP
Commensalism
One organism benefits from another, but neither
helps nor harms the other organism
•+ / 0
•Ex: epiphyte growing on a tree
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ITP
Species Interactions
• the effects of one species on
another may be negative, positive, or
neutral
• five kinds of interactions:
POPULATION A
COMPETITION
PREDATION
PARASITISM
COMMENSALISM
MUTUALISM
POPULATION B
7-5 ECOLOGICAL SUCCESSION:
COMMUNITIES IN TRANSITION
Ecological Succession
Succession: gradual & fairly predictable
change in species composition over time
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ITP
Primary Succession
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ITP
Primary Succession
• Type of succession
that occurs where
there was no
ecosystem before
• Occurs on rocks, cliffs,
and sand dunes
• Pioneer species: the
first organism to
colonize any newly
available area and
begin the process of
ecological succession
Primary Succession
Primary Succession: gradual establishment of
biotic communities in an area where no
life existed before
•Ex: succession on newly formed islands &
after the retreat of a glacier
•Early Communities: lichens & mosses
colonize bare rock
•Mid Communities: small herbs & shrubs
colonize
•Late Communities: tree species colonize
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ITP
Lichens
Mosses
Parking Lot
Secondary Succession
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Secondary Succession
Secondary Succession: gradual
reestablishment of biotic communities in an
area where a biotic community was previously
present
•Ex: "old field succession"
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Secondary Succession:
Mount St. Helens
• Erupted in 1980
• 44,460 acres were
burned and
flattened
• After the eruption,
plants began to
colonize the debris
Disturbance
Disturbance: a distinct event that disrupts an
ecosystem or community
•disturbance initiates secondary
succession
•Natural disturbances: fires, hurricanes,
tornadoes, droughts, & floods
•Human-caused disturbances: deforestation,
overgrazing, plowing
Intermediate Disturbance
Hypothesis:
Moderate disturbances in communities
promote greater species diversity than small
of major disturbances
Climax Community
• Climax community: the
final and stable
community in an
ecosystem
• Will continue to
change in small ways,
but will remain primarily
the same over time
(unless disturbed)
The Circle of Life in
Secondary Succession
7-6 ECOLOGICAL STABILITY
AND SUSTAINABILITY
Stability - complex networks of positive
and negative feedback loops
•
maintained only by constant dynamic
change in response to changing
environmental conditions
Ecosystem Stability
• Inertia: the ability of an ecosystem to resist being
disturbed (also called persistence)
• Constancy: ability of a living system (population)
to keep its numbers w/in the limits imposed by
available resources
• Resilience: the ability of an ecosystem to
“bounce back” after it has been disturbed
Precautionary Principle
• We should try to prevent potential
harm to an ecosystem even though
we don’t understand all of the
cause-and-effects
• “better safe than sorry”