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Learning Goals for Ch. 27
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?
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
Copyright © 2011 Pearson Education Inc. Biology: Life on Earth, 9e
Ecological hierarchy
Biosphere
Ecosystem
Community
Population
Organism
Table 27-1
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
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
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
Table 27-1
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
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
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
– Ex. Cattle and deer
– Ex.Wild horses and Elk
– Ex. Zebra mussels and native mussels
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
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
P. aurelia P. caudatum
(a) Grown in separate flasks
(b) Grown in the same flask
Competitive Exclusion
Fig. 27-1
Resource Partitioning
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
Resource Partitioning
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
Resource Partitioning
Fig. 27-2
Blackburnian
warbler
Black-throated
green warbler
Cape May
warbler
Bay-breasted
warbler
Yellow-rumped
warbler
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
Intraspecific 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
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
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
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
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
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
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
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
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
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
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
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
Predators and prey may engage in chemical
warfare
– Predators and prey use toxins for attack and
defense
–The bombardier beetle sprays boiling-hot
chemicals from its abdomen onto its attacker
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
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
Social parasites!
Animals that take advantage of the social
behavior of a host to complete their life cycle.
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
27.5 What Is Mutualism?
Mutualism refers to interactions between
species in which both benefit
–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
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
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
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
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
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
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
Ecological Succession
Change in the
composition of species
over time
Classical model
describes a
predictable sequence
with a stable climax
community
Pioneer
Species
Other
Species
Climax
Community
27.7 Succession: How Do Community Interactions
Cause Change Over Time?
There are two major forms of succession
– Primary succession
– Secondary succession
Primary succession
Changes an area lacking any community (no plants, animals, seeds, soil) to one with a functioning community of plants, animals, fungi etc.
Pioneer plants: lichen and mosses
Secondary Succession
Follows
disturbance of an
existing
community that
removes or
damages the
vegetation, but
does not remove,
destroy, or cover
the soil.
Secondary Succession
Pioneer plants of secondary succession start from roots or seeds remaining in the soil or from seeds carried in by wind or animals from surrounding communities.
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
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 nutrient-
rich ash and spares some trees and many healthy
roots
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
rock scraped
bare by a
glacier
lichens and
moss on
bare rock
bluebell,
yarrow
blueberry,
juniper
jack pine,
black spruce,
aspen
spruce-fir
climax forest:
white spruce,
balsam fir,
paper birch
0
1,000
Primary Succession
Fig. 27-16
plowed
field
ragweed,
crabgrass,
Johnson
grass
aster,
goldenrod,
Queen Anne's lace,
broom sedge grass
blackberry,
smooth sumac
Virginia pine,
eastern red
cedar
oak-hickory
climax forest:
white and black oak,
bitternut and
shagbark hickory
0
100
Secondary Succession
Fig. 27-17
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
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
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
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
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