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LESSON
Species Interactions
Guiding Question: How do species interact in nature?
• Discuss the factors that influence an organism’s
niche.
• Compare and contrast predation, parasitism, and
herbivory.
• Describe mutualism and commensalism.
2
Reading Strategy As you read, complete a vocabulary
word map for each of the boldface, highlighted words in the
lesson.
Vocabulary niche, tolerance, resource partitioning,
predation, coevolution, parasitism, symbiosis, herbivory,
mutualism, commensalism
How many species do you interact with each day? At first, you
might think the number is a low one. But if you really think about it, it is
probably quite high. Did you eat any plants today? Accidentally step on
any insects? Did you pet a dog or swat a fly? In nature, species interact
all the time. Sometimes, they interact in ways that benefit both species.
Other times, one species can harm the other. Species interactions form
the structure of communities and ecosystems.
The Niche and Competition
An organism’s niche is affected by both its tolerance and
competitive interactions.
Recall that an organism’s habitat describes the general place it lives.
Habitats provide organisms with all of the resources they need to survive.
Together, habitat and resource use define the concept of niche.
Defining the Niche An organism’s niche describes its use of
resources and its functional role in a community. The niche includes not
only the habitat where an organism lives, but also what food it eats, how
and when it reproduces, and what other organisms it interacts with. The
niche is therefore a kind of summary of everything an organism does and
when and where it does it.
5.2 LESSON PLAN PREVIEW
Real World Students make
analogies for niche concepts.
Differentiated Instruction Less proficient readers use
symbols to clarify species
interactions.
Inquiry Students research
examples of mutualism and
commensalism.
5.2 RESOURCES
Bellringer Video, The “Invisible Cloak”
of the Cuttlefish • Lesson 5.2 Worksheets • Lesson 5.2 Assessment •
Chapter 5 Overview Presentation
FOCUS Watch the ABC News
video The “Invisible Cloak”
of the Cuttlefish, which
demonstrates the amazing
ability of the cuttlefish to
blend in with its surroundings. Use this video to launch
a discussion of the role natural
selection likely played in the
development of cuttlefish
camouflage.
Figure 6 Niche A big part of a
spider’s role, or niche, is to prey on
insects caught in its web.
Evolution and Community Ecology 133
Tolerance Where and how an organism lives is influenced by its tolerance. Tolerance is the ability to survive and reproduce under changing
environmental conditions. Some organisms, such as panda bears, have very
restricted tolerance ranges and are called specialists. Organisms with wide
tolerance ranges, such as rats, are able to live in a wide array of habitats or
use a wide array of resources. These organisms are called generalists.
Specialist and generalist strategies each have advantages and disadvantages. Specialists can be very successful in their niche by being
extremely good at the things they do. However, they might not be able to
adapt when conditions change. Generalists succeed by being able to live
in many different places and variable weather conditions. However, they
may not be as successful as specialists in a given situation.
Competition When multiple organisms seek the same limited
ANSWERS
Figure 7 It means that zebra
mussels have been better able to
get and use resources than native
mussels.
Figure 7 Competition Competition
can occur between members of (a)
the same species or (b) different
species. Apply Concepts What does it
mean that zebra mussels have “outcompeted” native mussels in many
ways?
(a)
resource, such as food, light, water, or space, they compete. Competing
organisms do not always fight with one another directly and physically
like the foxes in Figure 7a. Flowers in a field, for example, do not physically fight to attract pollinators, but they are still in competition for
that valuable resource. Competitive interactions can take place among
members of the same species, called intraspecific competition, or among
members of two or more different species, called interspecific competition.
Interspecific competition can give rise to different types of outcomes.
Direct competition between species often
results in a winner and a loser. If one species is a very effective competitor, it may exclude another species from resource use entirely. This outcome, called competitive exclusion, can happen when two or more species
try to occupy the exact same niche. In 1986, ecologists with the National
Oceanic and Atmospheric Administration began monitoring mussel
and clam populations in Lake St. Clair. The lake connects Lakes Erie and
Huron. They found there were twenty native mussel species in Lake St.
Clair. By 1997, they were all gone—only the invasive zebra mussel occupied the lake’s waters. Figure 7b shows zebra mussels attached to a native
clam on the shores of Lake Erie. When colonized by zebra mussels, shellfish cannot open their shells. They eventually suffocate or starve.
▶ Competitive Exclusion (b)
134 Lesson 2
Realized Niche
Fundamental Niche
Species 1
Species 1 is limited in its
roles and/or uses only a
subset of resources.
Resource 2
Resource 2
Species 1 fulfills all its
roles and uses all the
resources it can.
Species 2
Species 1
Species 3
Resource 1
Figure 8 Fundamental
and Realized Niche
(a) Without competitors,
an organism can use its
entire fundamental niche.
(b) Competitors, however,
limit an organism to
a realized niche. The
realized niche represents
only a portion of what
an organism can do and
what resources it can use.
Resource 1
The zebra mussel’s total exclusion
of its competitors in Lake St. Clair is unusual. Usually, neither competing species fully excludes the other. Instead, competing species tend to
adjust to each other, minimizing competition. Individuals can do this by
changing their behavior or using only a portion of the resources they are
capable of using. In such cases, individuals do not fulfill their entire niche,
as shown in Figure 8. The full niche of a species is called its fundamental
niche. A niche restricted by competition is called a realized niche.
▶ Fundamental and Realized Niche ANSWERS
Reading Checkpoint Resource
partitioning decreases competition
between species.
Figure 9 Resource Partitioning
When species compete, they tend
to divide resources. Many types of
birds—including the woodpeckers,
creeper, and nuthatch shown here—
feed on insects from tree trunks. By
each specializing in particular insects
on particular parts of the tree, the birds
minimize competition.
Over time, competing species may evolve to
occupy only their realized niches. In this way, they adapt to competition
by using slightly different resources or their shared resources in different ways. If two bird species eat the same type of seeds, for example, one
might come to specialize on larger seeds and the other to specialize on
smaller seeds. Or one bird may become more active in the morning and
the other more active in the evening. This process is
called resource partitioning because the species partition, or divide, the resource they use in common by
specializing in different ways, as shown in Figure 9.
▶ Resource Partitioning Sometimes, resource
partitioning can lead to the evolution of physical characteristics among the competing species that reflect
their specialized role in the environment. Ecologists
call this character displacement. Through increased
differences, two species can reduce competition. For
example, through natural selection, birds that specialize in eating larger seeds may evolve larger bills that
enable them to make the best use of the resource.
Similarly, birds specializing in eating smaller seeds
may evolve smaller bills. This is precisely what extensive research has revealed about the finches from
the Galàpagos Islands that were first described by
Charles Darwin.
▶ Character Displacement Reading
Checkpoint
White-breasted
nuthatch climbs
down trunk looking
for insects. 
Pileated
woodpecker digs
deeply into wood
to find large
insects. 
 Yellow-bellied
sapsucker drills
rows of holes
and consumes
sap and insects
stuck in sap.
 Brown
creeper climbs
up trunk
looking for
tiny insects.
ow does resource partitioning affect
H
competition between species?
Evolution and Community Ecology 135
BIG QUESTION
How do organisms affect one another’s survival and environment?
Perspective After students have read
about predation, have them work in
pairs. Ask one member of the pair
to describe a scenario involving a
change in a predator population.
For example, the predator population is struck by disease. His or her
partner should then predict how this
would impact the prey population.
Have students trade roles and repeat
the activity, this time focusing on a
change in the prey population.
ANSWERS
Reading Checkpoint An increase
in a prey population can create more
food for predators, which can then
increase the predator population. As
the predator population increases,
the population of prey decreases.
Decreases in the number of prey can
result in a decline in the predator
population.
Figure 10 Predator-Prey Cycles
Predator-prey systems sometimes
show paired cycles. In this system,
the number of moose is affected by
the number of wolves and vice versa.
Notice, too, that the wolf population
was also affected by a disease
outbreak.
Predation, Parasitism, and
Herbivory
Predation, parasitism, and herbivory are interactions in which
one species benefits, while the other is harmed.
When organisms interact, they can affect each other in different ways.
Throughout this lesson, the symbols “+”, “–”, and “0” are used to indicate
how each interaction affects the success of the organisms involved. A “0”
symbolizes a relationship in which there is no effect or the effect is neutral. Competition is a (–/–) relationship, because there is a negative effect
on both organisms as each takes resources the other could have used.
Other types of interactions are beneficial for one participant, but harmful
for the other (+/–). Three examples of this type of species interaction are
predation, parasitism, and herbivory.
Predation Every living thing needs food, and for many animals that
means eating other living organisms. Predation is the process by which
an individual of one species, a predator, hunts, captures, kills, and consumes an individual of another species, the prey. Interactions between
predators and prey influence community structure by helping determine
the relative numbers of predators and prey.
Predation can sometimes cause cycles in population sizes. An increase in the population size of prey creates more food for
predators, which may survive and reproduce more effectively as a result.
As the predator population rises, additional predation drives down the
population of prey. Less prey can then cause some predators to starve, so
that the predator population declines. This allows the prey population to
begin rising again, restarting the cycle. Most natural systems involve so
many factors that such cycles don’t last long, but in some cases, like the
one shown in Figure 10, cycles can continue for a long time.
▶ Population Cycles Reading
Checkpoint
escribe how predator and prey populations can affect each
D
other.
Wolf and Moose Populations on Isle Royale, Michigan
50
2400
Disease outbreak
2000
Wolves
Moose
40
1600
30
1200
20
800
10
400
0
1955
0
1960
Data from Isle Royale Wolf Project
136 Lesson 2
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Number of moose
Number of wolves
60
Find Out
More
(a)
(b)
▶ Predation and Evolution Individual predators that are better at
capturing prey will likely be more successful than less skilled predators.
Thus, natural selection often leads to the evolution of adaptations that
enable predators to be better hunters. Prey face an even stronger selective
pressure—the risk of immediate death. In response to these pressures,
prey organisms have evolved an elaborate array of defenses against being
eaten, as shown in Figure 11.
Some predator–prey
relationships are examples of coevolution. Coevolution is the process by
which two species evolve in response to changes in each other. A change
in one species, therefore, is usually followed by a change in the other. A
newt, for example, might evolve toxins that kill animals that prey on it.
Some predators, however, might evolve immunity to the toxins. Then,
in turn, the newts evolve stronger toxins. Ecologists refer to this kind of
coevolution as an arms race. Each species develops stronger and stronger
“weapons” in response to the other.
Ecologists think that the rough-skinned newt, found on the west
coast of the United States, and its predator, the common garter snake,
have been locked in an arms race for a long time. Shown in Figure 12,
the newt is one of the most poisonous animals in nature. A single newt
has enough poison to kill more than 100 people. However, the common
garter snake can still eat them! In this arms race, the newt’s weapon is its
toxin, and the predator’s weapon is resistance to the toxin. As the newts
evolved stronger toxins, the snakes evolved stronger resistance. It is
important to remember, however, that natural selection does not produce adaptation for a purpose. The newts did not evolve toxins to kill the
snakes, and the snakes did not evolve immunity to respond to the newts.
Rather, these adaptations enabled the newts and snakes to better survive
and reproduce in their environments.
▶ Coevolution and Evolutionary “Arms Races” (c)
Figure 11 Prey Defenses Natural
selection to avoid predation has
resulted in many dramatic adaptations.
(a) Some prey hide from predators
by camouflage, such as this leafy
seadragon in the seaweed.
(b) Other prey are brightly colored
to warn predators that they are toxic
or distasteful, such as this poisonous
redtail coral snake. (c) Still others fool
predators with mimicry—imitation of
something else. This katydid does an
excellent leaf impression.
Find Out
More
Have you heard about antibiotic
resistance in the news? It’s the
result of an evolutionary arms race:
bacteria vs. antibiotics. Look on
the Internet or ask a doctor about
antibiotic resistance in your area.
ANSWERS
Find Out More Students’ responses
should indicate that they have
researched antibiotic resistance
using reliable sources.
Figure 12 Fighting an Arms Race
The rough-skinned newt has evolved
incredible toxicity. Yet, the common
garter snake can still make a meal out
of it.
Evolution and Community Ecology 137
Parasitism Parasitism is a relationship in which one
organism, the parasite, depends on another, the host, for
nourishment or some other benefit. In the process, the host is
harmed. However, unlike predation, parasitism usually does
not result in an organism’s immediate death. Examples of
parasites include tapeworms that live in the digestive tract of
their host and ticks that attach themselves to their host’s skin.
A close, long-term association between organisms—such
as between a tapeworm and its host—is called symbiosis, literally “living together.” Ecologists debate the exact definition
of symbiosis. Some argue that symbiosis is any long-lasting
relationship between species, regardless of distance. Further,
some ecologists think that both organisms must benefit for
a relationship to be considered symbiotic. Here, however, we
define symbiosis as a long-lasting and physically close relationship in which at least one organism benefits.
Herbivory The interaction in which an animal feeds on
Figure 13 Parasitism Unfortunately for this
western long-eared myotis bat, ear mites get
everything they need from their host.
a plant is called herbivory. Insects that feed on plants are
the most common type of herbivore. In most cases, herbivory does not kill a plant directly but may affect its growth
and reproduction. Because of natural selection, plants have
evolved a wide array of defenses against the animals that feed
on them. Many plants produce chemicals that are toxic or distasteful to herbivores. Others have thorns, spines, or irritating
hairs. In response, herbivores may evolve ways to overcome
these defenses.
Figure 14 Herbivory Herbivores like this giraffe get all of their
nutrients and energy from plants.
138 Mutualism and Commensalism
Mutualism and commensalism are relationships in which
neither participant is harmed.
Not every interaction between species results in one being harmed.
Instead, both species can benefit. Or, one species can benefit without
affecting the other one at all.
Mutualism A relationship in which two or more species benefit is
called mutualism (+/+). Many mutualisms are symbiotic. For example, plant roots and some fungi together form symbiotic associations
called mycorrhizae, shown in Figure 15. In these relationships, the
plant provides energy and protection to the fungus, while the fungus
assists the plant in absorbing nutrients from the soil.
Not all mutualists live in close proximity. One of the most important mutualisms, pollination, involves free-living organisms that may
encounter each other only once in their lifetimes. Bees, birds, bats,
and other creatures transfer pollen (male sex cells) from one flower
to another, fertilizing eggs that become embryos within seeds. Most
pollinating animals visit flowers for their nectar, a reward the plant
uses to attract them. The pollinators receive food, and the plants are
pollinated and reproduce. Some plant-pollinator relationships are so
close that they are examples of coevolving species.
One classic example of coevolution, shown in Figure 16, is that
of orchid flowers and the African moths that pollinate them. The
proboscis of the Darwin’s hawk moth is an impressive 20–35 cm
(8–14 in.) long—just long enough to reach the nectar at the bottom
of the comet orchid’s flower. Although not discovered and described
until 1903, Darwin predicted the existence of the moth when he
was shown the comet orchid in 1862. If such a flower exists, Darwin
reasoned, so must an organism that pollinates it. There are many
such seemingly perfect pairs of pollinators and plants in nature, each
resulting from mutualistic, coevolutionary relationships.
Reading
Checkpoint
Figure 15 Mutualism Together, the
hyphae of a fungus (white) and the
roots of a strawberry tree (brown) form
a mutualistic association known as a
mycorrhiza. Both the fungus and the
plant benefit from their association.
ANSWERS
Reading Checkpoint Mycorrhizae
are long-lasting, physically close relationships between plants and fungi
in which both organisms benefit.
Figure 16 A Perfect Fit Darwin’s
hawk moth (Xanthopan morganii)
and the comet orchid are an example
of species that coevolved through a
mutualistic relationship.
ow do mycorrhizae fit the definition of a symbiotic
H
relationship?
139
Figure 17 Commensalism Plants
grow in the shady, moist, and nutrientrich area beneath the palo verde
“nurse” tree in the Sonoran Desert.
ANSWERS
Lesson 2 Assessment
1. Competition can cause species to
divide common resources and specialize in using only part of their
possible resources. This results in a
species occupying only a smaller,
realized niche, not its entire fundamental niche.
2. They all result in benefit to one
species and harm to another.
Predation involves one organism hunting, killing, and eating
another. In parasitism, a parasite
depends on a host organism for
some benefit, and harms the host
in the process. In herbivory, the
organism harmed is the plant that
is eaten.
3. Mutualism; It is symbiotic because
it is a long-lasting, physically close
relationship in which both organisms benefit.
4. Commensalism (+/0); competition
(–/–); herbivory (+/–); mutualism
(+/+); parsitism (+/–); predation
(+/–). (Note that answers may be
reversed and still be correct.)
Commensalism Commensalism, describes a relationship in which
one species benefits and the other is unaffected (+/0). For instance, palo
verde trees in the American Southwest’s Sonoran Desert create shade
and leaf litter that allow the soil beneath them to hold moisture. The area
around palo verde trees becomes cooler and moister than the surrounding ground, making it easier for young plants to germinate and grow.
Seedling cacti and other desert plants generally grow up directly beneath
“nurse” trees such as palo verde, as shown in Figure 17.
2
1. Relate Cause and Effect Explain how competition
can affect an organism’s niche.
2. Compare and Contrast How are predation,
parasitism, and herbivory similar? How are they
different?
3. Apply Concepts The human digestive tract is filled
with bacteria. The bacteria live in the body and get
nutrients while helping to digest food. What kind
of species interaction is this—mutualism or commensalism? Is it symbiotic? Explain.
4. Explore the BIGQUESTION Copy the chart below
into your notebook. For each species interaction,
indicate whether it has a positive (+), negative (–),
or neutral (0) effect on each species.
Interaction
Commensalism
Competition
Herbivory
Mutualism
Parasitism
Predation
140 Lesson 2
Effect on Species A
Effect on Species B