Transcript Community Ecology

Community Ecology
BCOR 012
Chapter 54
March 2 and 4, 2009
CHAPTER 54
COMMUNITY ECOLOGY
Introduction - A community is defined as an assemblage of species
living close enough together for potential interaction.
I. What Is a Community?
A.Contrasting views of communities are rooted in the
individualistic and interactive hypotheses
II. Interspecific Interactions and Community Structure
A. Populations may be linked by competition, predation, mutualism
and commensalism
B. Trophic structure is a key factor in community dynamics
C. Do dominant species and keystone species exert strong
controls on community structure?
III. Disturbance and Community Structure
A.Most communities are in a state of nonequilibrium owing to
disturbances
B.Humans are the most widespread agents of disturbance
C.Ecological succession is the sequence of community changes
after a disturbance
IV. Biogeographic Factors Affecting the Biodiversity of Communities
A.Community biodiversity measures the number of species and
their relative abundance
B. Species richness generally declines along an equatorial-polar
gradient
C. Species richness is related to a community’s geographic size
D. Species richness on islands depends on island size and
distance from the mainland
A community is defined as an assemblage of species
living close enough together for potential interaction.
Clements was a proponent of the interactive hypothesis of community
structure, while Gleason held with the individualistic hypothesis.
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Frederick E. Clements, (1874 - 1945)
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Henry Allen Gleason, (1882-1975)
Individualistic hypothesis of community structure (Gleason): The
community is a chance assemblage of species.
Interactive hypothesis (Clements): The plant community is a tight
association of closely linked species
Figure 53.1 Testing the individualistic and interactive hypotheses of communities
CHAPTER 54
COMMUNITY ECOLOGY
Introduction - A community is defined as an assemblage of species
living close enough together for potential interaction.
I. What Is a Community?
A.Contrasting views of communities are rooted in the
individualistic and interactive hypotheses
B.The debate continues with the rivet and redundancy models
II. Interspecific Interactions and Community Structure
A. Populations may be linked by competition, predation, mutualism
and commensalism
B. Trophic structure is a key factor in community dynamics
C. Do dominant species and keystone species exert strong
controls on community structure?
Competition
The ecological niche is the sum total of an organism’s use of
biotic and abiotic resources in the environment.
An organism’s niche is its role in the environment.
Photo courtesy of Dan Wells
The competitive exclusion principle: two species with similar needs
for the same limiting resources cannot coexist in the same place
Another way to say this is that two species cannot coexist if their
niches are identical.
Resource partitioning is the differentiation of niches that
permits similar species to coexist in a community
Predation
Plant defenses against herbivory:
• Chemical compounds such as alkaloids and
glycosides
• Mechanical features such as thorns and spines
• Anatomical features such as raphides (calcium
oxalate crystals)
• “Behavioral” features such as the
sensitive response in sensitive plants
Animal defenses against predators:
• Behavioral defenses, including fleeing, hiding,
self-defense, noises, and mobbing.
• Camouflage, including cryptic coloration,
deceptive markings.
• Mechanical defenses, including spines.
• Chemical defenses, including odors and toxins
• Aposematic, or warning coloration
• Mimicry
Aposematic (warning) coloration in a poisonous blue frog
Deceptive coloration: moth with "eyeballs"
Mimicry:
Batesian mimicry is where a harmless species mimics a harmful one.
Monarch butterfly
Viceroy butterfly
Mimicry:
Müllerian mimicry is where two or more unpalatable species resemble
one another
Cuckoo bee
Yellow jacket
Mutualism
Mutualism is a relationship between two species, both of which
benefit from the interaction.
Ants and bullhorn acacia: the
acacia provides food and shelter
to the ants; ants defend the acacia.
Ant acacia
Commensalism is a relationship in which one species benefits from
the interaction but other is not affected.
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Trophic Structure
The trophic structure of a community is determined by
the feeding relationships among organisms. Trophic
structure is a key factor in community dynamics.
Food chain - the transfer of energy from
primary producers through herbivores and
carnivores. Most food chains include only
four or five trophic levels, presumably
because the length of the chain is limited
by the inefficiency of energy transfer along
the chain.
Food chains are not isolated
units but are hooked together
into food webs.
Do dominant species and keystone species
exert strong controls on community structure?
Dominant species are those in a community that have the highest
abundance or highest biomass.
American chestnut, formerly a dominant species of eastern North
American forests.
Keystone species exert an important regulating effect on other
species in a community. If they are removed, community
structure is greatly affected.
Sea otter, a keystone predator in the northern Pacific
Sea otters as keystone predators in the northern Pacific
III. Disturbance and Community Structure
A.Most communities are in a state of nonequilibrium owing to
disturbances
B.Humans are the most widespread agents of disturbance
C.Ecological succession is the sequence of community changes
after a disturbance
IV. Biogeographic Factors Affecting the Biodiversity of Communities
A.Community biodiversity measures the number of species and
their relative abundance
B. Species richness generally declines along an equatorial-polar
gradient
C. Species richness is related to a community’s geographic size
D. Species richness on islands depends on island size and
distance from the mainland
Most communities are in a state of nonequilibrium owing to
disturbance
Q. What is disturbance?
A. An event or events, natural or anthropogenic, that alters the community
in terms of composition, species richness, resource availability, etc.
The ice storm of 1998
Forest fire
Wind-throw
Charlotte, Vermont
Grassland communities are adapted to periodic burning
Communities are often in some state of
recovery from disturbance
• Small-scale: fallen trees, flooding creeks, etc.
• Large-scale: large forest fires, volcanic
explosions, etc.
The intermediate disturbance hypothesis proposes that communities
may be most diverse at moderate levels of disturbance.
Small-scale disturbances, such as this fallen tree provides, promote
environmental patchiness. Patchiness is important to the maintenance
of species diversity.
Patchiness and recovery following a large-scale disturbance, the
Yellowstone fire of 1988
Another large-scale disturbance: Mount St. Helens, 1980
Human activities cause more disturbances than natural events and
usually reduce species diversity in communities.
Ecological succession is the transition in species
composition over ecological time. It is a response to
disturbance.
Primary succession
begins in a lifeless area
where soil has not yet
formed.
The Pattern of Primary Succession on Glacial Moraines in
Glacier Bay, Alaska
Figure 53.20 Change in soil nitrogen concentration during succession after glacial retreat in Glacier Bay, Alaska
Figure 53.20 Alders and cottonwoods covering the hillsides
Figure 53.20 Spruce coming into the alder and cottonwood forest
Figure 53.20 Spruce and hemlock forest
Ecological Monographs: Vol. 64, No. 2, pp. 149-175. Mechanisms of
Primary Succession Following Deglaciation at Glacier Bay, Alaska.
F. Stuart Chapin, Lawrence R. Walker, Christopher L. Fastie, and
Lewis C. Sharman
Secondary succession occurs where an existing community
has been cleared by some event, but the soil is left intact.
III. Disturbance and Community Structure
A.Most communities are in a state of nonequilibrium owing to
disturbances
B.Humans are the most widespread agents of disturbance
C.Ecological succession is the sequence of community changes
after a disturbance
IV. Biogeographic Factors Affecting the Biodiversity of Communities
A.Community biodiversity measures the number of species and
their relative abundance
B. Species richness generally declines along an equatorial-polar
gradient
C. Species richness is related to a community’s geographic size
D. Species richness on islands depends on island size and
distance from the mainland
Community biodiversity measures the number of species
and their relative abundance
Species
richness
- the
total
number
species
in the
area
Species
richness
- the
total
number
of of
species
in the
community
of interest (or, in the community)
Relative abundance - Differences in the abundance of species within a
community
Figure 53.21 Which forest is more diverse?
Which community is more diverse?
Most species in a community are relatively rare.
In a survey of a moth community in England, 197 species
were trapped, but only 6 species made up most of the biomass.
Tropical habitats support
much larger numbers of
species than do temperate
and polar regions.
Costa Rica and Panama each support
about 500-600 bird species, whereas
polar regions of northern Canada have
only 30 or 40 species.
19th century naturalist Alexander von Humboldt
first described the relationship between species
richness and geographic area.
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Friedrich Heinrich Alexander,
Baron von Humboldt (1804)
Imagine a newly formed island some distance from the mainland…
Two factors will determine the number of species that eventually
inhabit that island:
• The rate at which new species immigrate to the island
• The rate at which species become extinct
In turn, two physical factors affect immigration and extinction
rates:
• The size of the island
• Its distance from the mainland
A third factor influencing the immigration and extinction rates:
•
The number of species already present on the island
The Theory of Island Biogeography (1967)
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Robert MacArthur
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E. O. Wilson
The theory of island biogeography
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Number of plant species on the Galápagos Islands in
relation to island area.
Endemic Galapagos prickly
pear cactus tree
Good luck on the exam!