Download Text 2 How Species Interact

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How Species Interact
with Each Other
Objectives
Focus and Motivate Explain the difference between
niche and habitat.
Objectives
Give examples of parts of a
niche.
Organisms in the wild interact with many different species. For example, in the
African savanna, lions hunt zebras, fight with hyenas, and are fed upon by fleas and
ticks. Negative interactions between species, like competition and predation, aren’t
the only type of interactions. Sometimes different species help each other. For
example, cleaner fish remove parasites from fish living on coral reefs.
Describe the five major types of
interactions between species.
An Organism’s Niche
Explain how symbiotic
relationships may evolve.
Many ecologists are interested in understanding species’ role in ecosystems and requirements for survival. To do this they measure a species’
niche. There are several ways to define a niche. A niche (NICH) can be
the range of conditions in which a species can survive. For example, a
plant may only be able to survive in a particular range of temperatures,
with a certain amount of rainfall, and with access to enough nutrients.
Other scientists are interested in the ecological role of a species. These
scientists define a species’ niche by the resources they use, or the species
they feed on. For example, zebras are large herbivores on African grasslands. Kangaroos occupy a similar niche in Australia. Herbivores often
interact with carnivores, like lions, if they both exist in the same habitat.
Some parts of a lion’s niche are shown in Figure 2.1. A niche is different
from a habitat. An organism’s habitat is a location. However, an organism’s pattern of use of its habitat is part of its niche.
(bc) ©Beverly Joubert/National Geographic Society; (bl) ©Y. Arthus-Bertand; (r) ©uwesSERENGETI/Alamy
SECTION 2
SECTION 2
Explain the difference between
parasitism and predation.
Before beginning this section, review
with your students the Objectives listed
in the Student Edition. This section
explores the many ways in which species
interact with each other and with their
habitat.
Classroom Catalyst
Key Terms
Ask students to write a short paragraph
describing a carnivore, such as a fox,
through the eyes of its intended meal,
such as a mouse.
niche
competition
predation
parasitism
mutualism
commensalism
symbiosis
Misconception Alert!
Niche Versus Habitat Ecologists differ
in how they define niche. Some people
focus primarily on the physical resources
and conditions needed for a species to
survive. This definition often works well
for plants. Others define niches based
on the ecological role of a species, or
the interactions (primarily feeding
interactions) it has with other organisms. The definition of niche used often
depends on the goal of a particular
question a scientist is trying to answer.
FIGURE 2.1
A Lion’s Niche Parts of a lion’s niche are shown here. Can you think of other parts?
Connect to
Geophysics
Differentiated Instruction
Group Activity
EV_CNLESE904016_C08S2.indd 203
Constructing a Personal Niche Map Invite
groups of students to think about where they
fit in their community. Where do they gather
their resources? What kinds of resources do
they prefer? Who do they compete or
cooperate with? Where do they live, and
where do they hang out? Do they spend
certain parts of the day in certain places? Tell
students that all organisms use their habitats
in a unique way, and this defines their niche.
Ask students to draw a “niche map” that
shows how they use their habitat. Ask
Chapter 8:
Understanding Populations
203
volunteers to share their niche maps with the
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class, and compare niches (no one will have
exactly the same niche).
Evolution and Empty Niches Around
5.5 million years ago, most of the
Mediterranean Sea dried up, and all
gobies (a family of fish) that were living
there disappeared. When the Straits of
Gibraltar opened, the Atlantic Ocean
flooded into the empty Mediterranean
basin, and gobies from the Atlantic were
able to move in. With no competition
from other gobies, they were able to
“fill” a wide variety of “empty” ecological niches. As a result, many unique
species of gobies evolved within the
Mediterranean Sea.
Chapter 8: Understanding Populations 203
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Types of Interactions Between Two Species
FIGURE 2.2
Teach
TEACH FROM VISUALS
Indirect Interactions Have students
look at the table within Figure 2.2 and
note that four of the five types of
interactions are shown in the picture.
Have students brainstorm other ways
that the organisms in the scene might
interact, either directly or indirectly.
(Sample answers: All of the organisms
probably compete for water; the fox
and coyote compete for den space; the
yucca competes with other plants for
space and nutrients; parasites on the rat
may spread to its predators; the rat and
wren both may have other predators,
may eat and spread seeds of plants, and
may prey on the moths; they all provide
food for microbes.)
Species Interactions There are five
major types of species interactions.
COMMENSALISM Wren
makes nest without
affecting cactus
Species A
Species B Description
Competition
harmed
harmed
Each species negatively affects the other.
Predation and
parasitism
benefited
harmed
Species A feeds on species B.
Mutualism
benefited
benefited
Each species is helpful to the other.
Commensalism
benefited
unaffected
Species A benefits from species B, but B
is unaffected.
COMPETITION Fox and
coyote are predators of
same prey
MUTUALISM Yucca moth
pollinates and lays eggs on
yucca flowers; moth larvae
spread yucca seeds
PREDATION Kit fox
hunts and feeds on
kangaroo rat
Ways in Which Species Interact
Reading Toolkit
Brainstorming Have students look
at Figure 2.2 carefully, and copy the
table in their science journals. Ask them
to suggest and then write or draw
additional examples of each type of
interaction in their tables.
Interaction
CHECK FOR UNDERSTANDING
Identify What determines how a species
interaction is categorized?
The five major types of species interactions, summarized in Figure 2.2,
are competition, predation, parasitism, mutualism, and commensalism. These categories are based on whether each species causes benefit
or harm to the other species in a given relationship. Not all interactions
occur directly. For example, a tiger shark may cause sea turtles to not use
certain areas. Seagrasses in these areas are not grazed as heavily. Tiger
sharks have a positive indirect interaction with the seagrass!
Competition
For most organisms, competition is part of daily life. Seed-eating birds
compete with each other for seeds under a bush, and the bush competes
with the tree next to it for nutrients in the soil. Competition is a relationship in which different individuals or populations attempt to use the
same limited resource. Each individual has less access to the resource
and is harmed by the competition.
Answers
Check for Understanding
Species interactions are categorized
based on whether each species causes
harm or benefit to the other species in
the relationship.
FIELDSTUDY
Go to Appendix B to find the field study
Observing Competition.
204
Unit 3:
Competition can occur both within and between species. Members of
the same species compete with each other because they require the same
resources—they occupy the same niche. When members of different species have niches that overlap they may compete for some resources. If two
species have requirements that are too similar, one species may eliminate
the other from a habitat. This is called competitive exclusion.
Populations
Differentiated Instruction
english learners
EV_CNLESE904016_C08S2.indd 204
Species Interactions Skit Divide students
into five groups. Assign one of the different
types of species interaction from Figure 2.2
to each group, but tell students not to let the
other groups know which interaction they
have. Ask each group to plan a skit that
illustrates their assigned interaction. Tell them
to be creative—the skit does not have to
reflect a real habitat, it just needs to explain
the concept. Have each group perform their
skit for the class, and ask the audience to
guess the interaction.
204 Unit 3: Populations
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Types of Competition
ECOFACT
Species may compete in different ways. Sometimes, individuals try to
get as many resources as they can but do not fight over resources or get
in each other’s way. In this type of scramble competition, the winner is
the individual or species that gets the most resources the fastest. Some
examples of scramble competition are fish feeding on plankton or sea
turtles and sea cows feeding on seagrass. In interference competition,
individuals fight over resources or get in each others’ way when feeding.
For example, lions will steal kills from wild dogs and hyenas.
Adaptations to Competition
If two species have identical resource needs in the same ecosystem, the
more successful species might drive out the less successful species. The
individuals that do best and leave the most offspring will be those that either feed on slightly different resources or use resources in different ways.
One way competition can be reduced between species is by dividing
up the niche. Niche restriction occurs when each species uses less of the
niche than it is capable of using. Niche restriction is observed in closely
related species that use the same limited resources within a habitat. For
example, two similar barnacle species compete for space in the intertidal
zone of rocky shorelines. One of the species, Chthamalus stellatus, is
found only in the upper level of the zone when the other species is present. But when the other species is removed from the area, C. stellatus is
found at deeper levels, as shown in Figure 2.3. In the presence of competition, the actual niche used by a species may be smaller than the potential
niche. Ecologists have observed various other ways of dividing up a niche
among groups of similar species.
Homework
A Wide Niche
Coyotes live in a wide range of habitats
and are willing to get close to human
settlements. Coyotes are also known
to eat a wide variety of animals and
plants, including food that humans
throw away. Thus, coyotes have a
broad and varied niche. Coyotes take
advantage of those parts of their niche
that are easiest to use and that present
less competition.
Artistic Interactions Ask students to
create a portrait, a comic strip, watercolor painting, or other piece of art
depicting one of the five types of
species relationships.
HMDScience.com
Populations and
Communities
FIGURE 2.3
Niche Restriction The barnacle species Chthamalus stellatus uses less of its potential
niche when competing for space with a similar barnacle species, Semibalanus balanoides.
HIGH TIDE
©Nature Picture Library/Britain On View/Getty Images
Chthamalus stellatus
Chthamalus niche
size when
competition
is present
Semibalanus balanoides
Chthamalus
niche size when
competition
is absent
LOW TIDE
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teach with technology
EV_CNLESE904016_C08S2.indd 205
Restricted and Potential Niches In a study
done by Robert MacArthur in 1958, a pattern of
division, similar to Figure 2.3, was found in
warblers that feed on insects in coniferous
trees (spruce, fir, pine). MacArthur observed
that each species of warbler foraged for insects
in a different part of the tree, thus reducing
competition among the different species. Ask
students to search the Internet for this example
of niche partitioning. Have them draw a
representative profile of a conifer. Have them
indicate, with different colors, the parts of the
tree that each warbler uses when the other
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warblers are present. Also, ask students3/15/2012
to try
to find out what each warbler’s potential niche
is (the parts of the tree each warbler might use
if the other species are absent). If they can find
this information, have them draw a dashed line
around those parts of the tree. Students could
also research and diagram other examples of
niche restriction and potential niches.
Chapter 8: Understanding Populations 205
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FIGURE 2.4
Predation This predatory bird had
Predation
to outrun its prey. Many organisms
are adapted to avoid predation.
Demonstration
Predator Adaptations Ask students to
think about the various special adaptations that a predator might have. Start
with the head, and ask students to name
all the possible features of a successful
predator. Draw, or have volunteers draw,
each feature on the board as it is
suggested. Allow the drawing to get as
wild as possible. Have the volunteers
label each feature as they draw. (Features
may include: a long, sticky tongue; sharp
claws, teeth, or bill; keen eyesight or
night vision; keen sense of smell; big ears
that can move independently; long,
muscular legs; long, broad wings; an
ejectable stomach or digestive fluids;
stealthy silent feathers or paws; camouflage; venom.)
Homework
Predatory Strategies Have pairs of
students research a specific predatorprey relationship of their choice.
Encourage them to consider a variety
of types of organisms. Have students
produce a poster or report describing
the effects of this relationship on each
of the species. Ask students to include
information related to concepts in this
chapter, such as the size, dispersal, or
habitat of each population, as well as
the special adaptations of each species.
Make it Relevant
Biomimicry The design principle of
biomimicry uses nature as a model in
order to come up with designs and
processes that will help solve human
problems. It is based on the premise that
millions of years of evolution have
produced useful innovations that should
be copied. Some examples of products
that have been inspired by nature
include Velcro (barbs on seed pods) and
a hay-baling machine called a Haying
Mantis™. Have students research
biomimicry or brainstorm their own
ideas.
206 Unit 3: Populations
An organism that feeds on another organism is called a predator, and the
organism that is fed upon is the prey. This kind of interaction is called
predation. Examples of predation include sharks eating fish, bats eating
insects, or wolves eating deer. Figure 2.4 shows a predatory bird with its
captured prey.
Predators come in many shapes and sizes. Tiny spiders, lizards, lions,
and blue whales are all predators. Some predators eat only specific types
of prey. For example, the Canadian lynx feeds mostly on snowshoe hares
during the winter. In this kind of close relationship, the sizes of each
population tend to increase and decrease in linked patterns, as shown
in Figure 2.5. Other predators will feed on many types of prey or switch
between different types of prey.
Most predators are themselves prey of other predators. For example,
lizards are predators of insects, but are prey of hawks. Because almost
all species (including blue whales) have predators, most organisms have
evolved some mechanisms to avoid or defend against predators.
CASESTUDY
Predator-Prey
Adaptations
Understanding predator-prey interactions is very
important for predicting how ecosystems will respond
to human activities or environmental changes. Most
organisms, including blue whales, are at risk from
predators. That means there is strong selective
pressure for adaptations that help them stay safe. Some
adaptations are physical features of prey. Some are
behaviors that help them avoid running into predators or
get away when they have been spotted. But predators also
have physical features and behaviors that help them catch
their prey.
Many animals are camouflaged—disguised so that they
are hard to see. An animal’s camouflage usually disguises
its recognizable features. Most camouflage helps the
animal blend into its surroundings. Many predators are
camouflaged too. Wobbegong sharks look like a piece of
coral and lunge off the bottom at fish that don’t recognize
them. Lions blend into the grasslands where they hunt so
they can sneak up on their prey.
206
Unit 3:
Case Study
Some prey animals contain toxic chemicals that harm or
deter predators. Many animals that have chemical defenses
have a striking coloration. This warning coloration alerts
potential predators to stay away. Patterns with black stripes
and red, orange, or yellow are common in many species
of bees, wasps, skunks, snakes, and poisonous frogs.
Warning coloration works well against predators that can
learn and that have good vision.
During the course of evolution, members of several
well-protected species have come to resemble each other.
For example, both bees and wasps often have black and
yellow stripes. This is an example of mimicry of one species
by another. The advantage of mimicry is that the more
individual organisms that have the same pattern, the less
Populations
Have students play a predator-prey game. One
of the important decisions prey have to make
is when to run away from predators. Run too
soon and you don’t get enough food. Run too
late and you are a predator’s lunch. This
activity will teach students how prey optimize
their behavior to get enough food while
staying safe. Set up three cones. The first cone
is the “predator attack spot,” the second is the
“prey feeding spot,” the third is the “prey safe
spot.” The attack spot should be about 20
yards from the feeding spot and the feeding
spot should be 5–10 yards from the safe spot.
This is a lot like a squirrel looking for nuts and
EV_CNLESE904016_C08S2.indd 206
Patterns of black and
red, orange, or yellow are
common warning signs.
(tl) ©Tierbild Okapia/Photo Researchers, Inc.; (cr) ©Joseph Collins/Photo Researchers, Inc.
Teach continued
a cat attacking from bushes—the squirrel has
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to decide when to start running to make sure
it gets back to the tree in time.
Start with one “predator” student at the
attack spot and one “prey” student at the
feeding spot. When the teacher says “go” the
predator can run toward the prey to try to tag
them. The “prey” student gets 10 points for
every second they keep their foot on the
feeding spot before running for safety. If the
prey gets to the safe spot before being
tagged, they get to keep their points. If they
are tagged, they get 0 points and are out of
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FIGURE 2.5
TEACH FROM VISUALS
Predator-Prey Populations
Specialists Vs. Generalists Tell students
that some predators, such as the lynx,
are specialists and eat only a certain
type of prey. Other predators, such as
tiger sharks, are generalists, feeding on
the prey that is easiest to find and
capture. Specialized predators are
closely tied to the population of their
prey. This is not true of generalized
predators. Ask students how Figure 2.5
demonstrates this. (Each increase or
decrease in the hare population seems
to be matched by a similar change in the
lynx population.)
160
Snowshoe hare
Population (in thousands)
120
Lynx
80
40
0
1855
1865
1875
1885
1895
1905
1915
1925
Year
Populations of predators depend on populations of prey, so changes in one of these populations
may be linked to changes in the other. This graph shows population estimates over time for
Canadian lynx and their favorite food, snowshoe hares.
Interpret Data
(r) ©Digital Vision/Getty Images; (l) ©Borut Furlan/WaterFrame/Getty Images
Both predators and prey may exhibit
an adaptation such as camouflage or
mimicry. The wobbegong (left) sits in
wait for its prey. The protective quills
of this porcupine (right) are a simple
but effective way to repel predators.
chance any one individual has of being killed. Predators
learn to avoid all animals that have similar warning patterns.
Occasionally, a harmless species is a mimic of a species
that has chemical protection. You have probably tried to get
away from insects that you thought were wasps or bees. In
fact, some of them were probably flies. Several species of
harmless insects have evolved to mimic wasps and bees.
On the other hand, sometimes a predator may look like
another, less threatening species. Some species of spiders
may be mistaken for ants or other types of insects.
Other prey defenses include protective coverings. The
quills of a porcupine, the spines of a cactus, and the shell
of a turtle are all examples of protective covering. Some
defenses, including spines of stingrays and horns of buffalo,
are very dangerous for predators.
Anti-predator behaviors are an important way animals
stay safe. Prey may form large groups, move away from
dangerous
habitats,
use impressive escape behavior
Critical
Thinking
when attacked, or even attack a predator! To counter prey
defenses, predators may work together to catch prey,
carefully attack dangerous prey, or change how they hunt to
be more successful.
Critical Thinking
1. Making Comparisons For each of these types
of adaptations, give an additional example.
2. Determining Cause and Effect Write
a paragraph to explain how one of these
adaptations might have evolved.
Chapter 8:
the game. Change the distances between the
attack and feeding spots and between the
feeding and safe spots. Have students graph
how long prey can “feed” before having to run
away for the different combinations. How did
changes in the “landscape” affect the behavior
of prey, their feeding success, and the success
of predators?
Predators will be more successful the farther the
prey are from safety and the closer the attack is
to the feeding spot. Prey will stop feeding and
run sooner the farther they are from safety, and
this will result in less feeding success.
EV_CNLESE904016_C08S2.indd 207
207
Understanding Populations
Answers to Case Study
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1. Answers may vary. Students should give
examples of camouflage, warning coloration,
mimicry, and protective covering.
2. Answers may vary.
Graphing Simulate the effect of prey
population density on the success of a
predator. Blindfold a volunteer “predator.” Loosely tape 25 poker chips or
beans to a large piece of cardboard.
Have another student keep time, and
allow the predator 30 seconds to pick
up as many “prey” as possible. (Adjust
the time limit if appropriate.) Conduct
five more trials, increasing the amount
of prey by 15 each time. Have students
record the predator’s success in a table,
plot the data on a graph, and write an
explanation of the results.
EcoSmart
Feature Recognition Experiments have
shown that most animals recognize
other animals by three main features:
the eyes, the silhouette, and the body’s
bulk. Most forms of camouflage disguise
one of these features. Have students
design an experiment to test their own
abilities to recognize other animals.
Seductive Sirens In the normal firefly
(or “lightning bug”) courtship ritual, a
male advertises for a mate by flashing a
certain pattern and watching for a
female to flash that pattern back.
However, there are some species of
fireflies in which a predatory female
(so-called “Femme Fatale”) will mimic the
flash pattern of another species, and eat
the male of the other species when he
approaches.
Chapter 8: Understanding Populations 207
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FIGURE 2.6
Teach continued
Parasitism Parasites such as ticks (left) and intestinal worms (right) could be harmful to you.
People try to avoid these parasites, almost as if they were predators. In what ways are parasites
like predators?
Connect to Botany
Interactions with Plants Plants may
also act as predators, prey, parasites,
commensals or mutualists. Some
examples are: Venus Fly-Trap (predatory),
any plant that is eaten whole by
herbivores (prey), mistletoe (parasite),
bromeliads (commensal epiphytes, like
Spanish moss), and legumes (mutual with
bacteria in root nodules). Have students
think of examples of mimicry or
protective covering in plants.
Herbivores are Like Both Predators
and Parasites An herbivore is in a way
like a parasite because it feeds on a plant
without killing it (usually), but they
sometimes kill certain plants. Simple
categories cannot fully capture the
diversity of all inter-species relationships. For example, many small wasps act
like parasites until they are ready to
emerge as adults. At that time their host
is killed by such “parasitoids.”
FIGURE 2.7
Mutualism These acacia trees in
Central America have a mutualistic
relationship with these ants. The trees
provide food and shelter to the ants,
and the ants defend the tree from
herbivores.
CRITICAL THINKING
Describe Why is the relationship between ants and
acacia trees categorized as
mutualistic?
Connect to
Language Arts
Parasite Rex Have students read
passages from Carl Zimmer’s Parasite
Rex: Inside the Bizarre World of Nature’s
Most Dangerous Creatures. The book
offers a guided tour to the hidden,
fascinating world of parasites, and shows
how parasites can control the fate of
entire ecosystems and even steer the
course of evolution. After reading, ask
students if their understanding of the
significance of parasites in ecosystems
has changed.
Critical Thinking
The relationship between ants and
bull-thorn acacia trees is mutualistic
because it is beneficial to both species.
The trees provide shelter and food for
the ants, while the ants defend the trees
against herbivores and other threats.
208 Unit 3: Populations
Unlike predators, parasites usually do not kill their hosts. Therefore,
some people consider vampire bats to be parasites. In fact, a parasite can
have an evolutionary advantage if it allows its host to live longer. However, the host is often weakened or exposed to disease by the parasite.
Mutualism
Many species depend on another species for survival. In some cases,
neither organism can survive alone. A close relationship between two
species in which each species provides a benefit to the other is called
mutualism. Certain species of bacteria in your intestines form a mutualistic relationship with you. These bacteria help break down food that you
could not otherwise digest or produce vitamins that your body cannot
make. In return, you give the bacteria a warm, food-rich habitat.
Another case of mutualism happens in the bull-thorn acacia trees of
Central America. Most acacia trees have spines that protect them against
plant-eating animals, but the bull-thorn acacias have an additional
protection—an ant species that lives only on these trees. The trees
provide these ants shelter within hollow thorns. The trees also supply
sugary nectar glands, shown in Figure 2.7, and nutrient-rich leaf tips.
In turn, the ants defend the tree against herbivores.
208
Unit 3:
Populations
Differentiated Instruction
group activity
below level
Evolving Relationships Describe some
examples of mutualism to students, or have
them research examples. Divide the class into
groups, and have each group choose one
example. Have the groups hypothesize as to
how a parasitism or commensalism might have
evolved into this mutualism. Ask each group
to present their ideas to the class.
Writing Have students write a humorous
advertisement for a newspaper or a commercial for television about a species seeking a
new relationship with another species. The
written ads can be combined in a newspaper
format, or students can act out their television ads for the class.
EV_CNLESE904016_C08S2.indd 208
Answers
An organism that lives in or on another organism and feeds on it is
a parasite. The organism the parasite takes its nourishment from is
known as the host. The relationship between the parasite and its host
is called parasitism. Examples of parasites are ticks, fleas, tapeworms,
bloodsucking leeches, and mistletoe. Some photos of parasites are shown
in Figure 2.6.
(tr) ©SPL/Photo Researchers, Inc.; (tl) ©John Shaw/Bruce Coleman, Inc./Photoshot; (b) ©Carol Farneti-Foster/Oxford Scientific/Getty Images
Parasitism
Misconception Alert!
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CorrectionKey=B
FIGURE 2.8
Commensalism Some orchids have a
commensal relationship with certain species of
trees. The orchid benefits from growing on the
tree, because the orchid is exposed to more rain
and sunlight than if it grew on the ground. The
tree is not harmed or helped by the orchid.
Assess and Reteach Quiz
1. Contrast a mutualistic relationship
that you know about with one that
seems to be a commensalism.
(Answers may vary. Mutualism is
beneficial to both parties; commensalism only benefits one but does not
harm the other.)
2.If competition is a double-negative
relationship, why is there so much
competition? Why don’t all species
evolve to cooperate? (Answers may
vary. Because species produce more
offspring than can survive, individuals
of the same species must compete
for limited resources. The niches of
many species overlap, so they must
also compete.)
Connect to BIOLOGY
Commensalism
A relationship in which one species benefits and the other species is
neither harmed nor helped is called commensalism. An example is the
relationship between certain orchids and trees, shown in Figure 2.8. The
orchid grows around the tree’s branches without harming the tree. The
height exposes the orchid to rain and sunlight. Another example of commensalism is pilot fish swimming in front of a shark or sea turtle. The
small fish get protection and access to food. The shark is not affected by
the fish, and they are too small to bother eating.
©Kenneth W. Fink/Photo Researchers, Inc.
Symbiosis and Coevolution
A relationship in which two species live in close association is called
symbiosis. Many types of species interactions are considered symbiotic.
In some cases a symbiotic relationship is parasitic. In some cases, it is
commensalism or mutualism. For example, the symbiotic mutualism
between corals and the small algae living inside them that make food for
corals allows huge reefs to form!
Over time, species in close relationships may coevolve. These species
may evolve adaptations in response to one another. For example, coevolution can be seen in the relationships of flowering plants and their pollinators. Many types of flowers seem to match the feeding habits of certain
species of insects or other animals that spread pollen. Predators and prey
also can coevolve, with prey evolving better ways to escape and predators
evolving to be better hunters.
Connect to BIOLOGY
An Ecosystem in Your Body
Our health is affected by our
relationships with microorganisms in
our digestive system, skin, and other
parts of our body. For example, liveculture yogurt is considered a healthy
food because the kinds of bacteria
it contains are beneficial to us. The
bacteria assist our digestion of dairy
products and also compete with other
microorganisms, such as yeast, that
might cause infections.
Section 2 Formative Assessment
Reviewing Main Ideas
1. List as many parts as you can of the niche of an
organism of your choice.
2. Give examples of species that have the same
habitat but not the same niche that a lion has.
3. Describe the five types of species interactions.
Answers to Formative Assessment
1. Answers may vary. Students should mention
habitat traits, food sources, and other species
with which the organism interacts, and
consider interactions with abiotic factors
and with members of each biological
kingdom.
2. Answers may vary. Any prey organism or any
plant would qualify. Predators that do not eat
the same food would qualify. Even predators
that eat the same food, but also eat different
foods, would qualify. Examples: zebras, grass,
anteaters, hyenas.
3. Competition harms both organisms;
predation and parasitism harms one and
benefits one; mutualism benefits both;
EV_CNLESE904016_C08S2.indd 209
Critical Thinking
4. Making Comparisons Read the definition of
parasites and predators, and then explain how
parasites differ from predators.
5. Analyzing Relationships Choose an example
of mutualism, and then describe the long
process by which the relationship could have
evolved.
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Understanding Populations
209
commensalism benefits one and has no
10/30/2012 11:36:20 PM
effect on the other.
4. Unlike predators, a parasite spends some of
its life in or on the host, and generally does
not kill the host.
5. Answers may vary. A mutualistic relationship
may evolve from a close relationship that did
not originally benefit both parties. As two
species interact closely over time, either
organism may evolve methods that take
better advantage of the relationship with the
other. However, organisms cannot evolve a
trait on purpose.
Alternative
Assessment
Parasitic Lifestyles Ask students to
create a poster or a report on a parasitic
interaction. The project should describe
how the interaction may have evolved,
the parasite’s life cycle and host needs,
and what role the parasite plays within
its ecosystem (its niche).
Reteach
Guess the Relationship Have students
write several short descriptions of pairs
of organisms that exhibit one of the five
types of species interactions. Ask each
student to read one of these descriptions to the class, and have other
students guess which of the five types is
represented.
Reading Toolkit
Vocabulary The word parasite
comes from the Greek parasitos,
meaning “one who eats at someone
else’s table;” commensal is from Latin,
and means “together at the same table;”
symbiosis means “living together.”
Chapter 8: Understanding Populations 209