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Biology
A Guide to the Natural World
Chapter 35 • Lecture Outline
An Interactive Living World 2: Communities in Ecology
Fifth Edition
David Krogh
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35.1 Structure in Communities
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Structure in Communities
• An ecological community is all the
populations of all species that inhabit a
given area.
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Structure in Communities
• Many communities are dominated by only a
few species.
• The few species that are abundant in a given
area are called ecological dominants.
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Ecological Dominants
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Figure 35.1
Keystone Species
• A keystone species is a species whose
absence from a community would bring
about significant change in that community.
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Keystone Species
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Figure 35.2
Biodiversity
• Biodiversity, defined as variety among
living things, takes three primary forms:
• A diversity of species in a given area.
• A geographic distribution of species
populations.
• Genetic diversity within species populations.
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High biodiversity
Low biodiversity
many different species
few species
broad distribution of species
narrow distribution of species
high genetic diversity
within population
low genetic diversity
within population
(a) Species
diversity
(b) Geographic
diversity
(c) Genetic
diversity
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Figure 35.3
Biodiversity
• Recent research indicates that species
diversity tends to enhance a community’s
productivity and stability.
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35.2 Types of Interaction Among
Community Members
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Modes of Interaction
• There are four primary types of interaction
among community members:
• competition
• predation (and a special variety of it,
parasitism)
• mutualism
• commensalism
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Habitat
• Habitat can be thought of as the physical
surroundings in which a species normally
can be found.
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Niche
• Niche can be defined metaphorically as an
organism’s occupation, meaning what the
organism does to obtain the resources it
needs to live.
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35.3 Interaction through
Competition
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Competition for Resources Among
Species
• The competitive exclusion principle states
that when two populations compete for the
same limited, vital resource, one always
outcompetes the other and thus brings about
the latter’s local extinction.
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(a) Competitive exclusion
P. aurelia
Population size
When two species compete for
the same limited, vital resource,
one will always drive the other
to local extinction—as the
paramecium P. aurelia did to the
paramecium P. caudatum. This
is the competitive exclusion
principle at work.
P. caudatum
(b) Resource partitioning
Conversely, when Gause put
P. aurelia together with another
paramecium, P. bursaria, the
two species divided up the
habitat, and both survived. This
is a demonstration of resource
partitioning.
Population size
Time (days)
X
P. aurelia
P. bursaria
Time (days)
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Figure 35.4
Competition for Resources Among
Species
• Coexistance through resource
partitioning refers to instances in which
two similar species use the same kinds of
resources from the same habitat over an
extended period of time but will divide the
resources up such that neither of the species
undergoes local extinction.
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Resource Partitioning
Resource Partitioning
Cape May
warbler
Bay-breasted
warbler
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Myrtle
warbler
Figure 35.6
35.4 Interaction through Predation and
Parasitism
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Predation
• Predation is defined as one free-standing
organism feeding on parts or all of a second
organism.
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Parasitism
• Parasitism is a variety of predation in which
the predator feeds on prey but does not kill
it immediately and may not kill it ever.
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Plants Parasitizing Plants
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Figure 35.7
Predator–Prey Dynamics
• Predator and prey population sizes can
move up and down together in a fairly tight
linkage, but predator–prey interaction
generally is only one of several factors that
control the population level of either
predators or prey.
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10
10
Lemming
population
0.1
1
0.001
0.1
Stoat
population
0.01
1988
Lemmings’ winter nests
occupied by stoats
per 2.5 acres
Lemmings per 2.5 acres
Predator–Prey Dynamics
1990
1992
0.00001
1994
1996
1998
2000
2002
Year
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Figure 35.8
Predator–Prey Dynamics
• Predator–prey interactions have spurred the
evolution of physical modifications in both
predator and prey species.
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Avoiding Predation Through
Camouflage
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Figure 35.10
Mimicry
• One form that such modifications take is
mimicry: a phenomenon in which one
species has evolved to assume the
appearance of another.
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Mimicry
• Batesian mimicry, occurs when one species
evolves to resemble a species that has
superior protective capability.
• Batesian mimicry always includes three
players: a mimic, a model, and a dupe.
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Batesian Mimicry
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Figure 35.11
Mimicry
• Müllerian mimicry occurs when several
species that have protection against
predators come to resemble each other.
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Müllerian Mimicry
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Figure 35.12
35.5 Interaction through Mutualism
and Commensalism
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Mutualism
• Mutualism is an interaction between
individuals of two species that is beneficial
to both individuals.
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Mutualism
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Figure 35.13
Commensalism
• Commensalism is an interaction in which an
individual from one species benefits while
an individual from another species is neither
harmed nor helped.
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Coevolution
• Coevolution is the interdependent evolution
of two or more species.
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Coevolution
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Figure 35.15
Community Interactions
Table 35.1
Community Interactions
In this type of interaction . . .
One organism . . .
While the other . . .
Competition
Is harmed
Is harmed
Predation and parasitism
Gains
Is harmed
Mutualism
Gains
Gains
Commensalism
Gains
Is unaffected
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Table 35.1
35.6 Succession in Communities
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Succession
• Parcels of land or water that have been
abandoned by humans or devastated by
physical forces will almost always be
reclaimed by nature to some degree.
• This process is called succession: a series of
replacements of community members at a
given location until a relatively stable final
state is reached.
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Succession
• Primary succession proceeds from an
original state of little or no life and soil
that lacks nutrients.
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Primary Succession
ponds and
bogs
eventually?
spruce
spruce-hemlock
alder bush
exposed
till
pioneer species:
lichens, bacteria,
horsetails, and
liverworts
Dryas shrub
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Figure 35.17
Succession
• Secondary succession occurs when a final
state of habitat is first disturbed by some
outside force, but life remains, and the soil
has nutrients.
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Succession
• The final community in any process of
succession is known as the climax
community.
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Succession
• A common set of developments occurs in
most instances of primary succession.
• These include the arrival of “pioneer”
photosynthesizers, facilitation of the growth
of some later species through the actions of
earlier species, and the competitive driving
out of some earlier species by the actions of
later species.
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Succession
• As succession proceeds, species diversity
tends to increase within communities and
smaller, shorter-lived species tend to be
replaced by larger, longer-lived species.
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Succession
• The rejuvenation of the Mount St. Helens
area that has occurred since 1980 has
provided ecologists with a wealth of
information regarding both primary and
secondary succession.
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Mount St. Helens
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Figure 35.16
Succession
• One of the chief lessons learned concerns
the degree to which succession can be
facilitated by biological legacies, defined as
living things, or products of living things,
that survive a major ecological disturbance.
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Survivor on Mount St. Helens
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Figure 35.18