Download Chapter 53 communities

Chapter 53
Community Ecology
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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• Overview: What Is a Community?
• A biological community
– Is an assemblage of populations of various
species living close enough for potential
interaction
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• The various animals and plants surrounding
this watering hole
– Are all members of a savanna community in
southern Africa
Figure 53.1
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• Concept 53.1: A community’s interactions
include competition, predation, herbivory,
symbiosis, and disease
• Populations are linked by interspecific
interactions
– That affect the survival and reproduction of the
species engaged in the interaction
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• Interspecific interactions
– Can have differing effects on the populations
involved
Table 53.1
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Competition
• 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
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The Competitive Exclusion Principle
• The competitive exclusion principle
– States that two species competing for the
same limiting resources cannot coexist in the
same place
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Ecological Niches
• The ecological niche
– Is the total of an organism’s use of the biotic
and abiotic resources in its environment
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• The niche concept allows restatement of the
competitive exclusion principle
– Two species cannot coexist in a community if
their niches are identical
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• However, 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
Figure 53.2
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.
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Ocean
Low tide
CONCLUSION
The spread of Chthamalus when Balanus was
removed indicates that 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
• Resource partitioning is 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
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Character Displacement
• In 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
Figure 53.4
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10
12
Beak depth (mm)
14
16
Predation
• Predation refers to an interaction
– Where 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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• In some cases, one prey species
– May gain significant protection by mimicking
the appearance of another
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• In 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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• In 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
• Herbivory, 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
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Parasitism
• In parasitism, one organism, the parasite
– Derives its nourishment from another
organism, its host, which is harmed in the
process
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• Parasitism exerts substantial influence on
populations
– And the structure of communities
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Disease
• The effects of disease on populations and
communities
– Is similar to that of parasites
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• Pathogens, disease-causing agents
– Are typically bacteria, viruses, or protists
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Mutualism
• Mutualistic symbiosis, or mutualism
– Is an interspecific interaction that benefits both
species
Figure 53.9
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Commensalism
• In commensalism
– One species benefits and the other is not
affected
Figure 53.10
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• Commensal interactions have been difficult to
document in nature
– Because any close association between
species likely affects both species
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Interspecific Interactions and Adaptation
• Evidence for coevolution
– Which involves reciprocal genetic change by
interacting populations, is scarce
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• However, generalized adaptation of organisms
to other organisms in their environment
– Is a fundamental feature of life
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• Concept 53.2: 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
• The species diversity of a community
– Is the variety of different kinds of organisms
that make up the community
– Has two components
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• 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
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• 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%
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• 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
• Trophic structure
– Is the feeding relationships between organisms
in a community
– Is a 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
• A food web
Humans
– Is 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
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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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Limits on Food Chain Length
• 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
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• The energetic hypothesis suggests that the
length of a food chain
– Is limited by the inefficiency of energy transfer
along the chain
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• The dynamic stability hypothesis
– Proposes that long food chains are less stable
than short ones
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• Most of the available data
– Support the energetic hypothesis
6
No. of species
5
No. of trophic
links
4
5
4
3
3
2
2
1
1
0
0
High
(control)
Medium
Productivity
Figure 53.15
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Low
Number of trophic links
Number of species
6
Species with a Large Impact
• 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
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Dominant Species
• Dominant species
– 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
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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
• 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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• 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
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(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
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(c) Total kelp density
Food chain after killer
whales started preying
on otters
Ecosystem “Engineers” (Foundation Species)
• Some organisms exert their influence
– By causing physical changes in the
environment that affect community structure
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• 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
Bottom-Up and Top-Down Controls
• 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
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• 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
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• 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
100
Figure 53.20
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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
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• Concept 53.3: Disturbance influences species
diversity and composition
• Decades ago, most ecologists favored the
traditional view
– That communities are in a state of equilibrium
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• However, a recent emphasis on change has
led to a nonequilibrium model
– Which describes communities as constantly
changing after being buffeted by disturbances
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What Is Disturbance?
• A disturbance
– Is an event that changes a community
– Removes organisms from a community
– Alters resource availability
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• 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.
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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 intermediate disturbance hypothesis
– Suggests that moderate levels of disturbance
can foster higher species diversity than low
levels of disturbance
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• 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
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( 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.
Human Disturbance
• Humans
– Are the most widespread agents of
disturbance
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• Human disturbance to communities
– Usually reduces species diversity
• Humans also prevent some naturally occurring
disturbances
– Which can be important to community
structure
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Ecological Succession
• Ecological succession
– Is the sequence of community and ecosystem
changes after a disturbance
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• Primary succession
– Occurs where no soil exists when succession
begins
• Secondary succession
– Begins in an area where soil remains after a
disturbance
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• 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
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• 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
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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 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.
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(c) Spruce stage
• Concept 53.4: Biogeographic factors affect
community diversity
• Two key factors correlated with a community’s
species diversity
– Are its geographic location and its size
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Equatorial-Polar Gradients
• The two key factors in equatorial-polar
gradients of species richness
– Are probably evolutionary history and climate
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• Species richness generally declines along an
equatorial-polar gradient
– And is especially great in the tropics
• The greater age of tropical environments
– May account for the greater species richness
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• Climate
– Is likely the primary cause of the latitudinal
gradient in biodiversity
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• The two main climatic factors correlated with
biodiversity
– Are solar energy input and water availability
180
200
Vertebrate species richness
(log scale)
Tree species richness
160
140
100
120
100
80
60
40
20
0
100
(a) Trees
50
10
1
900
500
700
300
Actual evapotranspiration (mm/yr)
1,100
Figure 53.25a, b
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(b) Vertebrates
1,500
1,000
500
Potential evapotranspiration (mm/yr)
2,000
Area Effects
• 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
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• A species-area curve of North American
breeding birds
– Supports this idea
Number of species (log scale)
1,000
100
10
1
1
10
100
103
Figure 53.26
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104
105
106
Area (acres)
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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• 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.
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.
Figure 53.27a–c
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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
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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• Concept 53.5: Contrasting views of community
structure are the subject of continuing debate
• Two different views on community structure
– Emerged among ecologists in the 1920s and
1930s
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Integrated and Individualistic Hypotheses
• The integrated hypothesis of community
structure
– Describes a community as an assemblage of
closely linked species, locked into association
by mandatory biotic interactions
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• The individualistic hypothesis of community
structure
– Proposes that communities are loosely
organized associations of independently
distributed species with the same abiotic
requirements
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• 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.
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• 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.
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• 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.
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Rivet and Redundancy Models
• 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
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• The redundancy model of communities
– Proposes that if a species is lost from a
community, other species will fill the gap
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• It is important to keep in mind that community
hypotheses and models
– Represent extremes, and that most
communities probably lie somewhere in the
middle
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