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Chapter 27
Community Interactions
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 27 At a Glance
 27.1 Why Are Community Interactions Important?
 27.2 What Is the Relationship Between the Ecological
Niche and Competition?
 27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 27.4 What Is Parasitism?
 27.5 What is Mutualism?
 27.6 How Do Keystone Species Influence Community
Structure?
 27.7 Succession: How Do Community Interactions
Cause Change Over Time?
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27.1 Why Are Community Interactions Important?
 An ecological community consists of all the
interacting populations within an ecosystem
– A community can encompass the entire biotic,
or living, portion of an ecosystem
– Interactions between populations in a community
help limit their size
–Populations maintain a balance between
resources and the numbers of individuals
consuming them
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27.1 Why Are Community Interactions Important?
 An ecological community consists of all the interacting
populations within an ecosystem (continued)
– The process by which two interacting species act as
agents of natural selection on one another is called
coevolution
– For example, when killing prey that is easiest to catch,
individuals with the best defenses against predation
remain
– These individuals produce the most offspring and,
over time, their characteristics increase within the prey
population
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27.1 Why Are Community Interactions Important?
 The most important community interactions are:
– Competition, which harms both species
– Predation, which benefits the predator but harms
the prey
– Parasitism, which benefits parasite but harms the
host
– Mutualism, which benefits both species
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Type of
Interaction
Effect on
Species A
Effect on
Species B
Competition
between A and B
Harms
Harms
Predation
by A on B
Benefits
Harms
Parasitism
by A on B
Benefits
Harms
Mutualism
between A and B
Benefits
Benefits
Table 27-1
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Each species occupies a unique ecological niche that
encompasses all aspects of its way of life
– These include:
– Its physical home or habitat
– The physical and chemical environmental factors
necessary for its survival, such as nesting sites,
climate, and the type of nutrients it needs
– The role that the species performs within an
ecosystem, such as what it eats and the other species
with which it competes
– Although different species share aspects of their niche
with others, no two species ever occupy exactly the
same ecological niche within a community
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Competition occurs whenever two organism
attempt to use the same, limited resources
– Interspecific competition occurs between
members of different species, if they feed on the
same things or require similar breeding areas
–This type of competition is detrimental to all of
the species involved as it reduces their access
to resources in limited supply
–The greater the overlap of ecological niches,
the more intense the interspecific competition
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Adaptations reduce the overlap of ecological
niches among coexisting species
– The competitive exclusion principle states that
if two species occupy exactly the same niche
with limited resources, one will outcompete the
other
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Author Animation: Competitive Exclusion
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 The competitive exclusion principle was formulated by
microbiologist G. F. Gause, who performed laboratory
experiments using two species of protists, Paramecium
aurelia and P. caudatum
– Both species thrived on the same bacteria and fed in the
same region of their laboratory flasks
– When put into the same flask, P. aurelia always
eliminated P. caudatum
– Gause repeated the experiment, replacing P. caudatum
with P. bursaria, which fed in a different part of the flask
– In that case, both species could coexist because they
occupied different niches
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Competitive Exclusion
P. aurelia
P. caudatum
(a) Grown in separate flasks
(b) Grown in the same flask
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Fig. 27-1
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Adaptations reduce the overlap of ecological
niches among coexisting species (continued)
– When species with largely similar ecological
niches coexist and compete, each species
occupies a smaller niche than it would by itself, a
phenomenon called resource partitioning
–This reduces interspecific competition, and is
the outcome of the coevolution of species with
extensive niche overlap
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Author Animation: Resource Partitioning
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Ecologist Robert MacArthur explored the competitive
exclusion principle by carefully observing five species
of North American warbler
– These birds all hunt for insects and nest in the same type
of eastern spruce tree
– MacArthur found that each species concentrates its
search for food in specific regions within spruce trees,
employs different hunting tactics, and nests at a slightly
different time
– By dividing up the resources provided by the spruce
trees they share, the warblers minimize the overlap of
their niches and reduce interspecific competition
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Resource Partitioning
Yellow-rumped Bay-breasted
warbler
warbler
Cape May
warbler
Black-throated
green warbler
Blackburnian
warbler
Fig. 27-2
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Interspecific competition may reduce the
population size and distribution of each species
– Although natural selection can reduce niche
overlap, interspecific competition may still restrict
the size and distribution of competing
populations
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Interspecific competition may reduce the population
size and distribution of each species (continued)
– For example, consider Chtlamalus and Balanus
barnacles of the Scottish intertidal zone
– Chthamalus occupies the upper shore, while Balanus
occurs in the lower shore
– Ecologist J. Connell scraped off Balanus and the
Chthamalus population increased, spreading downward
into the area that its competitor had once inhabited
– Where the habitat is appropriate for both genera,
Balanus conquers because it is larger and grows faster;
however, Chthamalus tolerates drier conditions, giving it
a competitive advantage on the upper shore
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Author Animation: Resource Partitioning in
Barnacles
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27.2 What Is the Relationship Between the
Ecological Niche and Competition?
 Competition within a species is a major factor
controlling population size
– Intraspecific competition, competition between
individuals of the same species, is the most
intense form of competition
–If resources are limited, this is a major factor
controlling population size
–The evolutionary result of intraspecific
competition is that the individuals better suited
to survive are more likely to reproduce and
pass their traits to the offspring
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Predator–prey interactions shape evolutionary
adaptations
– Predators eat other organisms; these include
herbivores (animals that eat plants) as well as
carnivores (animals that eat other animals)
–Predators include a grass-eating pika, a bat
hunting a moth, and the more familiar example
of a hawk eating a bird
– Predators tend to be less abundant than their
prey
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Author Animation: Ecosystem Roles
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Forms of Predation
Fig. 27-3
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Predator–prey interactions shape evolutionary
adaptations (continued)
– Predator and prey populations exert intense
selective pressure on one another, resulting in
coevolution
–As prey become more difficult to catch,
predators must become more adept at hunting
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Some predators and prey have evolved counteracting
behaviors
– Bat and moth adaptations provide excellent examples of
how body structures and behaviors are molded by
competition
– Bats emit high-pitch sound pulses that bounce off their
surroundings, allowing them to navigate and detect
prey
– Moths (their prey) have evolved ears sensitive to the
pitch of sounds the bats emit, and they take evasive
actions in response
– The bats, in turn, counter by switching the frequency
of their sound pulses away from the moth’s sensitivity
range
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Camouflage conceals both predators and their
prey
– Camouflage renders animals inconspicuous
even when in plain sight
–Predators and prey have evolved colors,
patterns, and shapes that resemble their
surroundings
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Camouflage by Blending In
Fig. 27-4
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Camouflage conceals both predators and their
prey (continued)
– To avoid detection by predators, some animals
have evolved to resemble objects, such as
leaves, twigs, seaweed, thorns, or even bird
droppings
– Some plants have evolved to resemble rocks to
avoid detection by herbivores
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Camouflage by Resembling Specific Objects
Fig. 27-5a, b
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Camouflage by Resembling Specific Objects
Fig. 27-5c, d
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Camouflage conceals both predators and their
prey (continued)
– Camouflage also helps predators ambush their
prey
–Examples include the cheetah blending with
tall grass and the frogfish resembling a rock
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Camouflage Assists Predators
Fig. 27-6
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Bright colors often warn of danger
– Some animals have evolved bright warning
coloration that attracts the attention of potential
predators
–Warning coloration advertises that the animal
is bad-tasting or poisonous before the
predator attacks
–Examples include poison arrow frogs, coral
snakes, and honey bees
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Warning Coloration
Fig. 27-7
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Some prey organisms gain protection through mimicry
– Mimicry refers to when members of one species have
evolved to resemble another species
– Two or more distasteful species may each benefit from a
shared warning coloration pattern (Müllerian mimicry)
– Predators need only experience one distasteful
species to learn to avoid all with that color pattern
– For example, toxic monarch and viceroy butterflies
have similar wing patterns; if a predator becomes ill
from eating one species, it will avoid the other
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Mullerian Mimicry
Fig. 27-8
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Some prey organisms gain protection through
mimicry (continued)
– Some harmless organisms can gain a selective
advantage by resembling poisonous species
(Batesian mimicry)
–For example, the harmless hoverfly avoids
predation by resembling a bee
–The harmless mountain king snake is
protected by a warning coloration that
resembles the venomous coral snake
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Batesian Mimicry
Fig. 27-9a, b
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Batesian Mimicry
Fig. 27-9c, d
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Some prey organisms gain protection through
mimicry (continued)
– Some animals deter predators by employing
startle coloration
–These animals may have spots that resemble
the eyes of a larger animal
–If a predator gets close, the prey will flash its
eyespots, startling the predator and allowing
the prey to escape
–Examples include the peacock moth and the
swallowtail caterpillar
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Startle Coloration
Fig. 27-10
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 A sophisticated variation on prey mimicking
dangerous animals is seen in the snowberry fly
– Snowberry flies avoid predation by jumping
spiders by mimicking them both visually and
behaviorally
–When the spider approaches, the fly spreads
its wings, jerking them back and forth
–This movement resembles the behavior of a
jumping spider driving another spider from its
territory, and so the attacking spider flees
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A Prey Mimics Its Predator
Fig. 27-11
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Predators may use mimicry to attract prey
– In aggressive mimicry, a predator resembles a
harmless animal or part of the environment, to
lure prey within striking distance
–For example, a frogfish dangles a wriggling
lure that attracts a curious fish that is then
eaten
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Aggressive Mimicry
Fig. 27-6b
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Predators and prey may engage in chemical
warfare
– Predators and prey use toxins for attack and
defense
–Spiders and poisonous snakes use venom to
paralyze their prey and to deter predators
–The bombardier beetle sprays boiling-hot
chemicals from its abdomen onto its attacker
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Chemical Warfare
Fig. 27-12a
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27.3 What Are the Results of Interactions Between
Predators and Their Prey?
 Predators and prey may engage in chemical
warfare for attack and defense (continued)
– Many plants have evolved chemical adaptations
that deter their herbivore predators, such as the
milkweed
–In the case of the milkweed, however,
monarch butterfly caterpillars have evolved to
tolerate the toxins and store them in their
tissues as a defense against predation
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Chemical Warfare
Fig. 27-12b
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27.4 What Is Parasitism?
 Parasites live in or on their prey, which are
called hosts, usually harming or weakening
them but not immediately killing them
– Parasites are generally much smaller and more
numerous than their hosts
–Examples include tapeworms, fleas, ticks, and
many types of disease-causing protists,
bacteria, and viruses
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27.4 What Is Parasitism?
 Parasites and their hosts act as agents of
natural selection on one another
– The powerful forces of coevolution between
parasitic microorganisms and their hosts can be
seen in the variety of infectious bacteria and
viruses, and the precision of the immune system
that counters their attacks
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27.4 What Is Parasitism?
 Parasites and their hosts act as agents of
natural selection on one another (continued)
– Nagana, a disease in cattle caused by a parasitic
protist, kills cattle imported into areas of Africa,
but some African breeds of cattle have evolved
an immunity to it and survive
– In some regions of Africa, 20-40% of the human
population carries a sickle-cell gene that confers
protection against the malaria protist parasite
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27.5 What Is Mutualism?
 Mutualism refers to interactions between
species in which both benefit
– Many mutualistic relationships are symbiotic and
involve a close, long-term physical association
between the participating species
–For example, lichens form a mutualistic
relationship between a fungus and an algae
–The fungus provides support and protection
while obtaining food from the
photosynthetic alga
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Mutualism
Fig. 27-13a
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27.5 What Is Mutualism?
 Mutualism refers to interactions between
species in which both benefit (continued)
– Another example of mutualism is the clownfish
and sea anemones
–The clownfish takes shelter from predators
among the venomous tentacles of an
anemone, while in turn cleaning it, providing it
with scraps of food, and defending it from
predators
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Mutualism
Fig. 27-13b
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27.6 How Do Keystone Species Influence
Community Structure?
 In some communities, a keystone species plays a
major role in determining community structure
– A keystone species role is out of proportion to its
abundance in the community
– If a keystone species is removed from the community,
normal community interactions are significantly altered
and the relative abundance of other species changes
dramatically
– Keystone species need to be identified and protected so
that human activities do not lead to the collapse of entire
communities and ecosystems
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27.6 How Do Keystone Species Influence
Community Structure?
 In some communities, a keystone species
plays a major role in determining community
structure (continued)
– An example of a keystone species is the
predatory sea star Pisaster ochraceous from
Washington’s rocky intertidal coast
–When removed from their ecosystem, their
favored prey, native mussels, became so
abundant that they outcompete other
invertebrates and algae
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Keystone Species
Fig. 27-14a
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27.6 How Do Keystone Species Influence
Community Structure?
 In some communities, a keystone species
plays a major role in determining community
structure (continued)
– Another example of a keystone species is the
African elephant, which, by grazing on small
trees and bushes, prevents the encroachment of
forests
–This activity helps maintain the grass savanna
that supports grazing mammals and their
predators
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Keystone Species
Fig. 27-14b
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Most communities do not emerge fully formed
from bare rock or naked soil
– Instead, they arise through succession, where
the community and its nonliving environment
change structurally over time
– Succession is usually preceded by a
disturbance, an event that disrupts the
ecosystem either by altering the community, its
abiotic (nonliving) structure, or both
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 During succession, most terrestrial communities
go through stages
– Succession begins with arrival of a few hardy
plants, called pioneers
–The pioneers alter the ecosystem in ways that
favor competing plants, which eventually
displace the pioneers
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 During succession, most terrestrial communities
go through stages (continued)
– Succession often progresses to a relatively
stable and diverse climax community
– Recurring disturbances can set back the
progress of succession
–The continuous disturbances maintain
communities in earlier, or subclimax, stages
of succession
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 There are two major forms of succession
– Primary succession
– Secondary succession
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Primary succession occurs “from scratch,”
where there is no trace of a previous community
– This process may take thousands or even tens of
thousands of years
– The disturbance that sets the stage for primary
succession may be a glacier scouring the
landscape to bare rock, or a volcano
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Succession in Progress
Fig. 27-15a
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Secondary succession occurs after a disturbance
changes, but does not obliterate, an existing
community, leaving remnants such as soil and seeds
– This type of succession often takes just hundreds of
years
– An example is Mount St. Helens, which erupted in
1980 and left a thick layer of nutrient-rich ash that
encouraged new growth
– Another example is fire, which also produces nutrientrich ash and spares some trees and many healthy
roots
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Succession in Progress
Fig. 27-15b
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Succession in Progress
Fig. 27-15c
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Primary succession can begin on bare rock
– Isle Royal, Michigan, is an example of primary
succession
– This island in Lake Superior was scraped down
to bare rock by glaciers
– The bare rock provided a place for pioneer
species, such as lichen and mosses
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Isle Royal, Michigan, is an example of primary
succession (continued)
– Lichens and mosses were eventually replaced by larger
plants, such as bluebell and yarrow, which were in turn
replaced by woody shrubs such as blueberry and juniper
– Eventually, trees such as jack pine, black spruce, and
aspen took root, and the sun-loving shrubs were shaded
out
– Faster growing trees such as balsam fir, paper birch, and
white spruce towered over and replaced the original trees
– A tall spruce-fir climax forest was eventually established
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Primary Succession
lichens and
rock scraped moss on
bare rock
bare by a
glacier
0
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bluebell,
yarrow
blueberry,
juniper
spruce-fir
climax forest:
jack pine,
black spruce, white spruce,
balsam fir,
aspen
paper birch
1,000
Fig. 27-16
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 An abandoned farm will undergo secondary succession
– First to arrive are the fast-growing pioneer species, such
as crabgrass, ragweed, and Johnson grass
– Perennial plants, such as asters, goldenrod, and
perennial grasses, then take over, followed by woody
shrubs such as blackberry and smooth sumac
– Pine and cedar trees become established, until a climax
forest of oak and hickory replace all other species after
about a century
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Secondary Succession
plowed
field
ragweed,
crabgrass,
Johnson
grass
Virginia pine,
eastern red
blackberry,
smooth sumac cedar
aster,
goldenrod,
Queen Anne's lace,
broom sedge grass
oak-hickory
climax forest:
white and black oak,
bitternut and
shagbark hickory
100
0
Fig. 27-17
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Succession also occurs in ponds and lakes
– This occurs not only through changes within the body of
water, but also through an influx of nutrients from outside
the ecosystem
– Sediments and nutrients carried in by runoff from the
surrounding land have a large impact on small
freshwater lakes, ponds, and bogs, which gradually
undergo succession to dry land
– In forests, meadows may be produced by lakes filling
in from around the edges; grasses colonize the newly
formed soil
– As the lake shrinks and the area of the meadow
expands, trees will encroach around the meadow’s
edges
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Succession in a Small Freshwater Pond
Fig. 27-18
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Succession culminates in a climax community
– Succession ends with a relatively stable climax
community, which perpetuates itself if not
disturbed by outside forces, such as fire
–The populations within a climax community
have ecological niches that allow them to
coexist without replacing one another
–Climax communities have more species and
more types of community interactions than do
earlier stages of succession
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Succession culminates in a climax community
(continued)
– Climax species tend to be larger and longer-lived
than pioneer species
– The exact nature of the climax community at a
site reflects the local geological and climatic
conditions, such as temperature, rainfall, and
elevation
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Some ecosystems are maintained in a
subclimax stage
– Frequent disturbances maintain subclimax
communities in some ecosystems
– A subclimax community example is the tallgrass
prairies that once covered northern Missouri and
Illinois
–Periodic fires maintained the grasses and
prevented forests from encroaching
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Some ecosystems are maintained in a
subclimax stage (continued)
– Another example of a subclimax community is
suburban lawns
–Mowing and use of herbicides keep weeds
and woody species in check
– A further example of a subclimax community is
agriculture
–Plowing and pesticides keep competing
weeds and shrubs from replacing grains
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27.7 Succession: How Do Community Interactions
Cause Change Over Time?
 Climax communities create Earth’s biomes
– The climax communities that form during
succession are strongly influenced by climate
and geography
– Extensive areas of characteristic climax plant
communities are called biomes, and include
deserts, grasslands, and forests
– These biomes dominate broad geographical
regions with similar climates
Biology: Life on Earth, 9e
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