Chapter 53 Lecture
CHAPTER 53
COMMUNITY ECOLOGY
Section A: What Is a Community?
1. Contrasting views of communities are rooted in the individualistic and
interactive hypotheses
2. The debate continues with the rivet and redundancy models
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Introduction
• What is a Community?
• A community is defined as an assemblage of
species living close enough together for potential
interaction.
• Communities differ
in their species
richness, the
number of species
they contain, and
the relative
abundance of
different species.
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1. Contrasting views of communities are
rooted in the individualistic and interactive
hypotheses
• An individualistic hypothesis depicts a
community as a chance assemblage of species
found in the same area because they happen to
have similar abiotic requirements.
Fig. 53.1a
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• An interactive hypothesis depicts a
community as an assemblage of closely linked
species locked in by mandatory biotic
interactions.
Fig. 53.1b
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• These two very different hypotheses suggest
different priorities in studying biological
communities.
– In most actual cases, the composition of
communities does seem to change
continuously.
Fig. 53.1c
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2. The debate continues with the
rivet and redundancy models
• The rivet model of communities is a
reincarnation of the interactive model.
• The redundancy model states that most species
in a community are not closely associated with
one another.
• No matter which model is correct, it is important
to study species relationships in communities.
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CHAPTER 53
COMMUNITY ECOLOGY
Section B1: Interspecific Interactions and
Community Structure
1. Populations maybe linked by competition, predation, mutualism, and
commensalism
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Introduction
• There are different interspecific interactions,
relationships between the species of a
community.
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1. Populations may be linked by
competition, predation, mutualism
and commensalism
• Possible interspecific interactions are introduced in
Table 53.1, and are symbolized by the positive or
negative affect of the interaction on the individual
populations.
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• Competition.
– Interspecific competition for resources can
occur when resources are in short supply.
• There is potential for competition between any
two species that need the same limited resource.
– The competitive exclusion principle: two
species with similar needs for same limiting
resources cannot coexist in the same place.
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– The ecological niche is the sum total of an
organism’s use of abiotic/biotic resources in
the environment.
• An organism’s niche is its role in the
environment.
• The competitive exclusion principle can be
restated to say that two species cannot coexist in
a community if their niches are identical.
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• Classic experiments confirm this.
Fig. 53.2
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– Resource partitioning is the differentiation
of niches that enables two similar species to
coexist in a community.
Fig. 53.3
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Fig. 53.2
• Character displacement is the tendency for
characteristics to be more divergent in sympatric
populations of two
species than in
allopatric
populations
of the same two
species.
– Hereditary changes
evolve that bring
about resource
partitioning.
Fig. 53.4
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• Predation.
– A predator eats prey.
– Herbivory, in which animals eat plants.
– In parasitism, predators live on/in a host and
depend on the host for nutrition.
– Predator adaptations: many important feeding
adaptations of predators are both obvious and
familiar.
• Claws, teeth, fangs, poison, heat-sensing organs,
speed, and agility.
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– Plant defenses against herbivores include
chemical compounds that are toxic.
– Animal defenses against predators.
• Behavioral defenses include fleeing, hiding, selfdefense, noises, and mobbing.
• Camouflage includes cryptic coloration, deceptive
markings.
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Fig. 53.5
• Mechanical defenses include spines.
• Chemical defenses include odors and toxins
• Aposematic coloration is indicated by warning
colors, and is sometimes associated with other
defenses (toxins).
Fig. 53.6
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• Mimicry is when organisms resemble other
species.
– Batesian mimicry is where a harmless species mimics
a harmful one.
Fig. 53.7
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• Müllerian mimicry is where two or more
unpalatable species resemble each other.
Fig. 53.8
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– Parasites and pathogens as predators.
• A parasite derives nourishment from a host,
which is harmed in the process.
• Endoparasites live inside the host and
ectoparasites live on the surface of the host.
• Parasitoidism is a special type of parasitism
where the parasite eventually kills the host.
• Pathogens are disease-causing organisms that can
be considered predators.
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• Mutualism is where
two species benefit
from their
interaction.
• Commensalism is
where one species
benefits from the
interaction, but other
is not affected.
– An example would
be barnacles that
attach to a whale.
Fig. 53.9
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• Coevolution and interspecific interactions.
– Coevolution refers to reciprocal evolutionary
adaptations of two interacting species.
• When one species evolves, it exerts selective
pressure on the other to evolve to continue the
interaction.
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CHAPTER 53
COMMUNITY ECOLOGY
Section B2: Interspecific Interactions and
Community Structure (continued)
2. Trophic structure is a key factor in community dynamics
3. Dominant species and keystone species exert strong controls on community
structure
4. The structure of a community may be controlled bottom-up by nutrients or
top-down by predators
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2. Trophic structure is a key factor
in community dynamics
• The trophic structure of a community is
determined by the feeding relationships between
organisms.
• The transfer of food energy from its source in
photosynthetic organisms through herbivores and
carnivores is called the food chain.
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• Charles Elton first
pointed out that the
length of a food
chain is usually
four or five links,
called trophic
levels.
• He also recognized
that food chains are
not isolated units
but
are hooked together
Fig. 53.10
into food webs.
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• Food webs.
– Who eats whom in a
community?
– Trophic relationships
can be diagrammed
in a community.
– What transforms
food chains into
food webs?
– A given species may
weave into the web
at more than one
trophic level.
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Fig. 53.11
– What limits the length of a food chain?
• 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 states that long food
chains are less stable than short chains.
Fig. 53.13
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3. 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
(the sum weight of all individuals in a
population).
– If we remove a dominant species from a
community, it can change the entire community
structure.
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• Keystone species
exert an important
regulating effect
on other species
in a community.
Fig. 53.14
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• If they are removed, community structure is greatly
affected.
Fig. 53.15
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4. The structure of a community
may be controlled bottom-up by
nutrients or top-down by predators
• Simplified models based on relationships
between adjacent trophic levels are useful for
discussing how communities might be organized.
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– Consider three possible relationships between
plants (V for vegetation) and herbivores (H).
• VH
VH
V  H
– Arrows indicate that a change in biomass of one trophic level
causes a change in the other trophic level.
• The bottom-up model postulates V  H linkages,
where nutrients and vegetation control community
organization.
• The top-down model postulates that it is mainly
predation that controls community organization
V  H.
• Other models go between the bottom-up and top-down
extreme models.
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CHAPTER 53
COMMUNITY ECOLOGY
Section C1: Disturbance and Community Structure
1. Most communities are in a state of nonequilibrium owing to disturbances
2. Humans are the most widespread agents of disturbance
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Introduction
• Disturbances affect community structure and
stability.
– Stability is the ability of a community to
persist in the face of disturbance.
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1. Most communities are in a state of
nonequilibrium owing to disturbances
• Disturbances are events like fire, weather, or
human activities that can alter communities.
– Some are routine.
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Fig. 53.16
– Marine communities are subject to
disturbance by tropical storms.
Fig. 53.17
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Fig. 53.17
• We usually think that disturbances have a
negative impact on communities, but in many
cases they are necessary for community
development and survival.
Fig. 53.18
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2. Humans are the most widespread
agents of disturbance
• Human activities cause more disturbance than
natural events and usually reduce species
diversity in communities.
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CHAPTER 53
COMMUNITY ECOLOGY
Section C2: Disturbance and Community Structure
(continued)
3. Ecological succession is the sequence of community changes after a
disturbance
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3. Ecological succession is the sequence of
community changes after a disturbance
• Ecological succession is the transition in species
composition over ecological time.
• Primary succession begins in a lifeless area
where soil has not yet formed.
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Fig. 53.19
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– Mosses and lichens colonize first and cause
the development of soil.
• An example would be after a glacier has retreated.
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• Secondary succession occurs where an existing
community has been cleared by some event, but
the soil is left intact.
– Grasses grow first, then trees and other
organisms.
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• Soil concentrations of
nutrients show changes
over time.
Fig. 53.20
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CHAPTER 53
COMMUNITY ECOLOGY
Section D: Biogeographic Factors Affecting the
Biodiversity of Communities
1. Community biodiversity measures the number of species and their relative
abundance
2. Species richness generally declines along an equatorial-polar gradient
3. Species richness is related to a community’s geographic size
4. Species richness on an island depends on island size and distance from the
mainland
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Introduction
• Two key factors correlated with a community’s
biodiversity (species diversity) are its size and
biogeography.
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1. Community biodiversity
measures the number of species and
their relative abundance
• The variety of different kinds of organisms that
make up a community has two components.
– Species richness, the total number of species in the
community.
– Relative abundance of the different species.
– Imagine two small forest communities with 100
individuals distributed among four different tree
species.
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• Species
richness
may be equal,
but relative
abundance
may
be different.
Fig. 53.21
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– Counting species in a community to
determine their abundance is difficult,
especially for insects and smaller organisms
Fig. 53.22
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2. Species richness generally declines along
an equatorial-polar gradient
• Tropical habitats support much larger numbers of
species of organisms than do temperate and polar
regions.
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Fig. 53.23
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• What causes these gradients?
– The two key factors are probably evolutionary
history and climate.
– Organisms have a history in an area where they
are adapted to the climate.
• Energy and water may factor into this phenomenon.
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Fig. 53.24
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3. Species richness is related to a
community’s geographic size
• The species-area curve quantifies what may
seem obvious: the larger the geographic area, the
greater
the number
of species.
Fig. 23.25
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4. Species richness on islands depends on
island size and distance from the mainland
• Because of their size and isolation, islands
provide great opportunities for studying some of
the biogeographic factors that affect the species
diversity of communities.
– Imagine a newly formed island some distance
from the mainland.
• Robert MacArthur and E. O. Wilson developed a
hypothesis of island biogeography to identify the
determinants of species diversity on an island.
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• Two factors will determine the number of species
that eventually inhabit the island.
– The rate at which new species immigrate to the island.
– The rate at which species become extinct.
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Fig. 53.26
• Studies of plants on
many island chains
confirm their
hypothesis.
Fig. 53.27
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