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Community Ecology
What Is a Community?
• biological community
• an assemblage of populations of various species living close enough for
potential interaction
• The various animals and plants surrounding this watering hole are all members of
a savanna community in southern Africa
Figure 53.1
• A community’s interactions include:
•
•
•
•
•
competition
predation
herbivory
symbiosis
disease
• Populations are linked by interspecific interactions that
affect the survival and reproduction of the species
engaged in the interaction
• Interspecific interactions can have differing effects on the
populations involved
Table 53.1
• Interspecific competition
• Occurs when species compete for a particular resource that is in
short supply
• Strong competition can lead to competitive exclusion
• The local elimination of one of the two competing species
The Competitive Exclusion Principle
• States that two species competing for the same
limiting resources cannot coexist in the same
place
• Is the total of an organism’s use of the biotic and
abiotic resources in its environment
• The niche concept allows restatement of the competitive
exclusion principle
• Two species cannot coexist in a community if their niches are
identical
• Ecologically similar species can coexist in a community If there are
one or more significant differenc.e in their niches
e 53.2
RESULTS
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.
When Connell removed Balanus from the
lower strata, the Chthamalus population spread into that area.
High tide
High tide
Chthamalus
Balanus
Chthamalus
realized niche
Chthamalus
fundamental niche
Balanus
realized niche
Ocean
Ocean
Low tide
Low tide
CONCLUSION
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.
The spread of Chthamalus when Balanus was
removed indicates that competitive exclusion makes the
realized niche of Chthamalus much smaller than its
fundamental niche.
Resource Partitioning
• the differentiation of niches that enables similar species to coexist
in a community
A. insolitus
usually perches
on shady branches.
A. ricordii
A. distichus perches
on fence posts and
other sunny
surfaces.
A. insolitus
A. alinigar
A. christophei
A. distichus
A. cybotes
A. etheridgei
Figure 53.3
• 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
Figure 53.4
10
12
Beak depth (mm)
14
16
• Interaction where one species, the predator,
kills and eats the other, the prey
• Feeding adaptations of predators include
• Claws, teeth, fangs, stingers, and poison
• Animals also display
• A great variety of defensive adaptations
• Feeding adaptations of predators include
Claws, teeth, fangs, stingers, and poison
• Animals also display
A great variety of defensive adaptations
• Cryptic coloration, or camouflage
• Makes prey difficult to spot
Figure 53.5
• Aposematic coloration
• Warns predators to stay away from prey
Figure 53.6
• In some cases, one prey species may gain significant protection by
mimicking the appearance of another
•
Batesian mimicry:
A palatable or harmless species mimics an unpalatable
or harmful model
(b) Green parrot snake
Figure 53.7a, b
(a) Hawkmoth larva
• In Müllerian mimicry
• Two or more unpalatable species resemble each other
(a) Cuckoo bee
Figure 53.8a, b
(b) Yellow jacket
• the process in which an herbivore eats parts of a
plant.
• Has led to the evolution of plant mechanical and
chemical defenses and consequent adaptations by
herbivores
• one organism, the parasite derives its nourishment from
another organism, its host, which is harmed in the process
• Parasitism exerts substantial influence on populations
and the structure of communities
• The effects of disease on populations and communities is
similar to that of parasites
• Pathogens, disease-causing agents
are typically bacteria, viruses, or protists
• Mutualistic symbiosis, or mutualism is an interspecific
interaction that benefits both species
Figure 53.9
• One species benefits and the other is not affected
Figure 53.10
• Commensal interactions have been difficult to
document in nature because any close association
between species likely affects both species
Species Diversity
• 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
• The species diversity of a community
Is the variety of different kinds of organisms that
make up the community
Has two components
• Species richness
•Is the total number of different species in the
community
• Relative abundance
•Is the proportion each species represents of the
total individuals in the community
• Two different communities can have the same species
richness, but a different relative abundance
A
B
C
D
Figure 53.11
A: 25%
Community 1
B: 25%
C: 25%
D: 25%
A: 80%
Community 2
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
• the feeding relationships between organisms in
a community
• a key factor in community dynamics
• 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
A terrestrial food chain
Phytoplankton
A marine food chain
Humans
– a branching
food chain with
complex
trophic
interactions
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
• 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
• Each food chain in a food web is usually only a few links
long
• There are two hypotheses that attempt to explain food
chain length
• The energetic hypothesis suggests that the length
of a food chain
• Is limited by the inefficiency of energy
transfer along the chain
The dynamic stability hypothesis
Proposes that long food chains are less stable than
short ones
• Most of the available data supports the energetic
hypothesis
6
No. of species
5
No. of trophic
links
4
4
3
3
2
2
1
1
0
0
High
(control)
Medium
Productivity
Figure 53.15
5
Low
Number of trophic links
Number of species
6
• Certain species have an especially large impact on the
structure of entire communities
• Either because they are highly abundant or because
they play a pivotal role in community dynamics
• Are those species in a community that are most
abundant or have the highest biomass
• Exert powerful control over the occurrence and
distribution of other species
• 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
• Are not necessarily abundant in a community
• Exert strong control on a community by their ecological
roles, or niches
• Field studies of sea stars
Number of species
present
• Exhibit their role as a keystone species in intertidal
communities
20
With Pisaster (control)
15
10
Without Pisaster (experimental)
5
0
1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73
(a) The sea star Pisaster ochraceous feeds
preferentially on mussels but will
consume other invertebrates.
Figure 53.16a,b
(b) When 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.
• 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
1993
1997
Year
Figure 53.17
Food chain before
killer whale involvement in chain
(c) Total kelp density
Food chain after killer
whales started preying
on otters
• Some organisms exert their influence by causing physical changes
in the environment that affect community structure
• Beaver dams can transform landscapes on a very
large scale
Figure 53.18
• 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)
With
Juncus
Without
Juncus
Conditions
• The bottom-up model of community organization:
• Proposes a unidirectional influence from lower to higher trophic
levels
• In this case, the presence or absence of abiotic nutrients
• Determines community structure, including the abundance of
primary producers
Bottom-Up and Top-Down
Controls
• The top-down model of community organization
• Proposes that control comes from the trophic level
above
• In this case, predators control herbivores
• Which in turn control primary producers
• Long-term experiment studies have shown
• That communities can shift periodically from bottom-up to top-down
Percentage of
herbaceous plant cover
100
75
50
25
0
0
Figure 53.20
100
200
Rainfall (mm)
300
400
• 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
• Disturbance influences species diversity and
composition.
• Decades ago, most ecologists favored the
traditional view
• That communities are in a state of equilibrium
• However, a recent emphasis on change has led to a
nonequilibrium model
• Which describes communities as constantly changing after
being buffeted by disturbances
•Is an event that changes a community
•Removes organisms from a community
•Alters resource availability
• Fire
• Is a significant disturbance in most terrestrial ecosystems
• Is 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.
(c) After the burn. Approximately one
month after the controlled burn,
virtually all of the biomass in this
prairie is living.
• The intermediate disturbance hypothesis
• Suggests that moderate levels of disturbance can foster higher species
diversity than low levels of disturbance
• 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,
the burn left a patchy landscape. Note the unburned trees in the
distance.
Figure 53.22a, b
( b) One year after fire. This photo of the same general area taken 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.
• most widespread agents of disturbance
• Human disturbance to communities usually
reduces species diversity
• Humans also prevent some naturally occurring
disturbances which can be important to
community structure
• Is the sequence of community and ecosystem
changes after a disturbance
Primary succession
Occurs where no soil exists when succession
begins
Secondary succession
Begins in an area where soil remains after a
disturbance
• Early-arriving species
•May facilitate the appearance of later species
by making the environment more favorable
•May inhibit establishment of later species
•May tolerate later species but have no impact
on their establishment
• Retreating glaciers
• Provide a valuable field-research opportunity on
succession
Canada
Grand
Pacific Gl.
1940
Alaska
0
1912
1948
1879
1949
1935
Miles
1941
1 899
1907
5
1879
1948
1931
1911
1900
1892
1879
1913
1860
Reid Gl.
Johns Hopkins
Gl.
1879
Glacier
Bay
1830
1780
1760
Pleasant Is.
Figure 53.23
McBride glacier retreating
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 stage
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.
(c) Spruce stage
• Biogeographic factors affect community
diversity.
• Two key factors correlated with a
community’s species diversity
•Are its geographic location and its size
• The two key factors in equatorial-polar gradients
of species richness
• Are probably evolutionary history and climate
• Species richness generally declines along an equatorialpolar gradient
• especially great in the tropics
• The greater age of tropical environments
• May account for the greater species richness
• Climate is likely the primary cause of the latitudinal
gradient in biodiversity
• The two main climatic factors correlated with
biodiversity
• Are solar energy input and water availability
180
140
120
100
80
60
200
Vertebrate species
richness
(log scale)
Tree species richness
160
100
50
40
20
0
900
500
700
1,100
300
100
Actual evapotranspiration (mm/yr)
(a) Trees
Figure 53.25a, b
10
1
(b) Vertebrates
1,500
2,000
1,000
500
Potential evapotranspiration (mm/yr)
• The species-area curve quantifies the idea that all
other factors being equal, the larger the geographic
area of a community, the greater the number of species
• A species-area curve of North American breeding birds
• Supports this idea
Number of species (log scale)
1,000
100
10
1
1
Figure 53.26
10
100
103
104
105
106
Area (acres)
107
108
109
1010
• Species richness on islands
• Depends on island size, distance from the mainland,
immigration, and extinction
• The equilibrium model of island biogeography maintains that
Equilibrium number
Number of species on island
(a) Immigration and extinction rates. The
equilibrium number of species on an
island represents a balance between the
immigration of new species and the
extinction of species already there.
Figure 53.27a–c
Rate of immigration or extinction
Rate of immigration or extinction
Rate of immigration or extinction
• Species richness on an ecological island levels off at some dynamic
equilibrium point
Small island
Large island
Number of species on island
(b) Effect of island size. Large islands may
ultimately have a larger equilibrium number of species than small islands because
immigration rates tend to be higher and
extinction rates lower on large islands.
Far island
Near island
Number of species on island
(c) Effect of distance from mainland.
Near islands tend to have larger
equilibrium numbers of species than
far islands because immigration rates
to near islands are higher and extinction
rates lower.
• 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
1
10
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.
Integrated and Individualistic Hypotheses
• Two different views on community structure
• Emerged among ecologists in the 1920s and 1930s
• The integrated hypothesis of community structure
• Describes a community as an assemblage of closely
linked species, locked into association by mandatory
biotic interactions
• The individualistic hypothesis of community structure
• Proposes that communities are loosely organized
associations of independently distributed species with
the same abiotic requirements
• The integrated hypothesis
Population
densities of
individual
species
• Predicts that the presence or absence of particular
species depends on the presence or absence of other
species
Environmental gradient
(such as temperature or moisture)
Figure 53.29a
(a) Integrated hypothesis. Communities are discrete groupings
of particular species that are closely interdependent and nearly
always occur together.
• The individualistic hypothesis
Population
densities of
individual
species
• Predicts that each species is distributed according to its
tolerance ranges for abiotic factors
Environmental gradient
(such as temperature or moisture)
Figure 53.29b
(b) Individualistic hypothesis. Species are independently
distributed along gradients and a community is simply the
assemblage of species that occupy the same area because of
similar abiotic needs.
• In most actual cases the composition of
communities
Number of
plants
per hectare
• Seems to change continuously, with each species more
or less independently distributed
600
400
200
0
Wet
Figure 53.29c
Moisture gradient
Dry
(c) Trees in the Santa Catalina Mountains. The distribution of tree species at one
elevation in the Santa Catalina Mountains of Arizona supports the individualistic
hypothesis. Each tree species has an independent distribution along the gradient,
apparently conforming to its tolerance for moisture, and the species that live
together at any point along the gradient have similar physical requirements.
Because the vegetation changes continuously along the gradient, it is impossible to
delimit sharp boundaries for the communities.
• The rivet model of communities
• Suggests that all species in a community are linked
together in a tight web of interactions
• Also states that the loss of even a single species has
strong repercussions for the community
Rivet and Redundancy Models
• The redundancy model of communities
• Proposes that if a species is lost from a community,
other species will fill the gap