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Chapter 53
Community Ecology
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What Is a Community?
• Assemblage of populations of various species
living close enough for potential interaction
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• Savanna community in southern Africa
Figure 53.1
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• Community’s interactions include competition,
predation, herbivory, symbiosis, and disease
• Populations are linked by interspecific
interactions
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Interspecific interactions
Table 53.1
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Competition
• Species compete for a particular resource that
is in short supply
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Competitive Exclusion Principle
• Two species competing for the same limiting
resources cannot coexist in the same place
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Ecological Niches
• The total of an organism’s use of the biotic and
abiotic resources in its environment
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• Ecologically similar species can coexist in a
community if there are one or more significant
difference in their niches
EXPERIMENT
Ecologist Joseph Connell studied two barnacle
speciesBalanus balanoides and Chthamalus stellatus that have a
stratified distribution on rocks along the coast of Scotland.
RESULTS
When Connell removed Balanus from the lower
strata, the Chthamalus population spread into that area.
High tide
High tide
Chthamalus
Chthamalus
realized niche
Balanus
Chthamalus
fundamental niche
Balanus
realized niche
Ocean
Ocean
Low tide
In nature, Balanus fails to survive high on the rocks because it is
unable to resist desiccation (drying out) during low tides. Its realized
niche is therefore similar to its fundamental niche. In contrast,
Chthamalus is usually concentrated on the upper strata of rocks. To
determine the fundamental of niche of Chthamalus, Connell removed
Balanus from the lower strata.
Figure 53.2
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Low tide
The spread of Chthamalus when
CONCLUSIONwas removed indicates that
Balanus
competitive exclusion makes the realized
niche of Chthamalus much smaller than its
fundamental niche.
• As a result of competition
– A species’ fundamental niche may be different
from its realized niche
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Resource Partitioning
• Differentiation of niches that enables similar
species to coexist in a community
A. insolitus
usually perches
on shady branches.
A. ricordii
A. insolitus
A. distichus perches
on fence posts and
other sunny surfaces.
A. alinigar
A. christophei
A. distichus
A. cybotes
A. etheridgei
Figure 53.3
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Character Displacement
• There is a tendency for characteristics to be more
divergent in sympatric populations of two species
than in allopatric populations of the same two
species
G. fortis
G. fuliginosa
Beak
depth
Percentages of individuals in each size class
Santa María, San Cristóbal
40
Sympatric
populations
20
0
Los Hermanos
40
G. fuliginosa,
allopatric
20
0
Daphne
40
20
G. fortis, allopatric
0
8
10
12
Beak depth (mm)
Figure 53.4
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14
16
Predation
• One species, the predator, kills and eats the
other, the prey
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• Feeding adaptations of predators include
– Claws, teeth, fangs, stingers, and poison
• Animals also display
– A great variety of defensive adaptations
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• Cryptic coloration, or camouflage
– Makes prey difficult to spot
Figure 53.5
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• Aposematic coloration
– Warns predators to stay away from prey
Figure 53.6
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• Batesian mimicry
– A palatable or harmless species mimics an
unpalatable or harmful model
(b) Green parrot snake
Figure 53.7a, b
(a) Hawkmoth larva
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• Müllerian mimicry
– Two or more unpalatable species resemble
each other
(a) Cuckoo bee
Figure 53.8a, b
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(b) Yellow jacket
Herbivory
• Herbivore eats parts of a plant
–  evolution of plant mechanical and chemical
defenses and consequent adaptations by
herbivores
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Parasitism
• One organism, the parasite, derives its
nourishment from another organism, its host,
which is harmed in the process
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Disease
• Pathogens, disease-causing agents
– Are typically bacteria, viruses, or protists
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Mutualism (mutualistic symbiosis)
• Interspecific interaction that benefits both
species
Figure 53.9
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Commensalism
• One species benefits and the other is not
affected
Figure 53.10
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• Dominant and keystone species exert
strong controls on community structure
• In general, a small number of species in a
community
– Exert strong control on that community’s
structure
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Species Diversity
• Variety of different kinds of organisms that
make up the community
– 2 components 
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• Species richness
– total number of different species in the
community
• Relative abundance
– proportion each species represents of the total
individuals in the community
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• 2 different communities same species
richness, but a different relative abundance
A
B
C
D
Community 1
A: 25%
B: 25%
C: 25%
D: 25%
Community 2
Figure 53.11
A: 80%
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B: 5%
C: 5%
D: 10%
• A community with an even species abundance
– Is more diverse than one in which one or two
species are abundant and the remainder rare
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Trophic Structure
• Feeding relationships between organisms in a
community
– key factor in community dynamics
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• Food chains
– Link the trophic
levels from
producers to top
carnivores
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Zooplankton
Herbivore
Primary
producers
Plant
Figure 53.12
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A terrestrial food chain
Phytoplankton
A marine food chain
Food Webs
• branching food chain,
more complex
Humans
Smaller toothed
whales
Baleen
whales
Crab-eater seals
Birds
Sperm
whales
Elephant
seals
Leopard
seals
Fishes
Squids
Carnivorous
plankton
Copepods
Euphausids
(krill)
Phytoplankton
Figure 53.13
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• Food webs can be simplified
– By isolating a portion of a community that
interacts very little with the rest of the
community
Juvenile striped bass
Sea nettle
Fish larvae
Figure 53.14
Fish eggs
Zooplankton
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Dominant Species
• Species in a community that are most
abundant or have the highest biomass
• Exert powerful control over the occurrence and
distribution of other species
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• One hypothesis suggests that dominant
species
– Are most competitive in exploiting limited
resources
• Another hypothesis for dominant species
success
– Is that they are most successful at avoiding
predators
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Keystone Species
• Are not necessarily abundant in a community
• Exert strong control on a community by their
ecological roles, or niches
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With Pisaster (control)
20
present
Number of species
• Sea stars, keystone species in intertidal
communities
15
Without Pisaster (experimental)
10
5
0
1963 ´64
(a) The sea
star Pisaster ochraceous
feeds preferentially on mussels but
will consume other invertebrates.
Figure 53.16a,b
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´65
´66
´67
´68
(b) When
´69
´70
´71
´72
Pisaster was removed from an intertidal
zone, mussels eventually took over the rock face
and eliminated most other invertebrates and
algae. In a control area from which Pisaster was not removed,
there was little change in species diversity.
´73
• Observation of sea otter populations and their
predation
Otter number
(% max. count)
80
60
40
20
0
(a) Sea
otter abundance
400
Grams per
0.25 m2
– Shows the
effect the
otters have
on ocean
communities
100
300
200
100
0
Number per
0.25 m2
(b) Sea
urchin biomass
10
8
6
4
2
0
1972
1985
1989
Year
Figure 53.17
Food chain before
killer whale involvement in chain
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(c) Total kelp
density
1993
1997
after killer
whales started
preying
on otters
Food chain
Ecosystem “Engineers” (Foundation Species)
• Beaver dams
– Can transform landscapes on a very large
scale
Figure 53.18
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• Some foundation species act as facilitators
– That have positive effects on the survival and
reproduction of some of the other species in the
community
Number of plant species
8
6
4
2
0
Figure 53.19
Salt marsh with Juncus
(foreground)
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With
Juncus
Without
Juncus
Conditions
• Pollution
– Can affect community dynamics
• But through biomanipulation
– Polluted communities can be restored
Polluted State
Fish
Restored State
Abundant
Rare
Zooplankton
Rare
Abundant
Algae
Abundant
Rare
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What Is Disturbance?
• Changes a community
• Removes organisms from community
• Alters resource availability
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• Fire
– significant disturbance in most terrestrial
ecosystems
– often a necessity in some communities
Figure 53.21a–c
(a) Before a controlled burn.
A prairie that has not burned for
several years has a high proportion of detritus (dead grass).
(b) During the burn. The detritus
serves as fuel for fires.
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(c) After the burn. Approximately one
month after the controlled burn,
virtually all of the biomass in this
prairie is living.
• The large-scale fire in Yellowstone National
Park in 1988
– Demonstrated that communities can often
respond very rapidly to a massive disturbance
(a)
Soon after fire. As this photo taken soon after the fire shows, ( b) One year after fire. This photo of the same general area taken
the burn left a patchy landscape. Note the unburned trees in the
distance.
Figure 53.22a, b
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the following year indicates how rapidly the community began to
recover. A variety of herbaceous plants, different from those in the
former forest, cover the ground.
Human Disturbance
• most widespread agents of disturbance
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Ecological Succession
• Sequence of community and ecosystem
changes after a disturbance
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• Primary succession
– no soil exists when succession begins
• Secondary succession
– where soil remains after a disturbance
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•
Early-arriving species
1. May facilitate the appearance of later species
by making the environment more favorable
2. May inhibit establishment of later species
3. May tolerate later species but have no impact
on their establishment
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• Retreating glaciersfield-research opportunity on
succession
Canada
Grand
Pacific Gl.
1940
Alaska
0
1912
1948
1879
1935
Miles
1941
1 899
1907
5
1879
1948
1931
1911
1900
1892
1879
1913
1949
1860
Reid Gl.
Johns Hopkins
Gl.
1879
Glacier
Bay
1830
1780
1760
Pleasant Is.
Figure 53.23
McBride glacier retreating
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10
• Succession on the moraines in Glacier Bay, Alaska
– Follows a predictable pattern of change in vegetation
and soil characteristics
(a) Pioneer
stage, with fireweed dominant
(b) Dryas
60
Soil nitrogen (g/m2)
50
40
30
20
10
0
Figure 53.24a–d
Pioneer Dryas Alder Spruce
Successional stage
(d) Nitrogen fixation by Dryas and alder
increases the soil nitrogen content.
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(c) Spruce
stage
stage
• Biogeographic factors affect community
diversity
• 2 key factors correlated with a community’s
species diversity
– Are its geographic location and its size
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• Species richness generally declines along an
equatorial-polar gradient
• The greater age of tropical environments
greater species richness
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• Climate
– Is likely the primary cause of the latitudinal
gradient in biodiversity
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• 2 main climatic factors correlated with
biodiversity
– solar energy input and water availability
180
200
140
Vertebrate species
richness
(log scale)
Tree species richness
160
100
120
100
50
80
60
40
20
0
100
(a) Trees
10
1
300
Actual
900
1,100
evapotranspiration (mm/yr)
500
700
Figure 53.25a, b
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(b) Vertebrates
500
1,000
1,500
2,000
Potential evapotranspiration (mm/yr)
Area Effects
• All other factors being equal, the larger the
geographic area of a community, the greater
the number of species
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• A species-area curve of North American
breeding birds
Number of species (log scale)
1,000
100
10
1
1
10
100
Figure 53.26
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103
104
105
Area (acres)
106
107
108
109
1010
Island Equilibrium Model
• Species richness on islands
– Depends on island size, distance from the
mainland, immigration, and extinction
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• Studies of species richness on the Galápagos Islands
– Support the prediction that species richness
increases with island size
FIELD STUDY
Ecologists Robert MacArthur and E. O. Wilson studied the
number of plant species on the Galápagos Islands, which vary greatly in size, in
relation to the area of each island.
RESULTS
400
Number of plant species (log scale)
200
100
50
25
10
5
0
0.1
10
1
100
1,000
Area of island (mi2)
(log scale)
CONCLUSION
Figure 53.28
The results of the study showed that plant species
richness increased with island size, supporting the species-area theory.
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