1. Studies that demonstrate that species living in an ecological community change
independently of one another in space and time—
a. support the individualistic concept of ecological communities
b. support the holistic concept of ecological communities
c. suggest species interactions are the sole determinant of which species
coexist in a community
d. none of the above
The correct answer is a—support the individualistic concept
A. Answer a is correct. If communities acted as integrated “wholes” (like a kind of
superorganism), we would expect communities to abruptly change in composition
as we moved along spatial gradients, or over long periods of time accompanied by
changes in habitat.
The correct answer is a—
B. Answer b is incorrect. The holistic concept of ecological communities predicts
that species will covary in concert with one another in space and time, leading to
abrupt and complete changes in species composition with changes in space and
time.
The correct answer is a—
C. Answer c is incorrect. Independent variation in species in response to change in
the habitat suggests that the presence and success of each species is determined
solely by its interactions with other components of its environment, whether biotic
or abiotic factors.
The correct answer is a—
D. Answer d is incorrect. A is the correct response to the question.
2. If two species have very similar realized niches and are forced to coexist and share
a limiting resource indefinitely—
a. both species would be expected to coexist
b. both species would be expected to go extinct
c. the species that uses the limiting resource most efficiently should drive the
other species extinct
d. both species would be expected to become more similar to one another
The correct answer is c—
A. Answer a is incorrect. By definition, only one of several competing species can
exist on a limiting resource. Any slight difference between the competing species
should ultimately be translated into increased success leading to more severe
limitations on the other species. Coexistence is only possible when the two
species are in fact not limited by the identical limiting resource(s).
The correct answer is c—
B. Answer b is incorrect. As long as the necessary requirements are provided, the
better competitor should be able to survive and maintain itself at the expense of
the second species.
The correct answer is c—the species that uses the limiting resource most efficiently
should drive the other species extinct
C. Answer c is correct. Whether by exploitation or interference, an advantage of one
species over another is all that is necessary for competitive exclusion to result
when both species are limited by a shared resource.
The correct answer is c—
D. Answer d is incorrect. Even when species do compete but are not limited by the
identical resource, they would be expected to become more different over time
(character displacement) in their use of potentially limiting resources that are
shared.
3. According to the idea of coevolution between predator and prey, when a prey
species evolves a novel defense against a predator
a. the predator is expected to always go extinct
b. the prey population should increase irreversibly out of control of the predator
c. the predator population should increase.
d. evolution of a predator response should be favored by natural selection
The correct answer is d—
A. Answer a is incorrect. Predators should be able to respond by natural selection to
challenges presented by novel traits appearing in their prey.
The correct answer is d—
B. Answer b is incorrect. Although prey might increase, unless the predator
population is completely incapable of responding through natural selection, the
predators should be able to respond. In many cases the rate of evolutionary
change in predators and their prey is commensurate and therefore lends itself to
an “arms race” that allows for control to return. Furthermore, during lags in
response, other factors can serve to limit a prey population.
The correct answer is d—
C. Answer c is incorrect. The evolution of an effective mechanism for evading
predators should have a negative influence on the number of predators.
The correct answer is d—evolution of a predator response
D. Answer d is correct. Just as any trait leading to escape from the predator would be
strongly favored, traits that enhance prey capture by predators would be favored
by natural selection.
4. In order for mimicry to be effective in protecting a species from predation, it
must—
a.
b.
c.
d.
occur in a palatable species that looks like a distasteful species
have cryptic coloration
occur such that mimics look and act like models
occur in only poisonous or dangerous species
The correct answer is c—
A. Answer a is incorrect. Although Batesian mimicry involves the evolution of a
palatable species to look like a distasteful species, thereby avoiding predation, this
isn’t the only type of mimicry. Müllerian mimicry involves the evolution of
different but poisonous or dangerous species to look like each other. None of
them are palatable.
The correct answer is c—
B. Answer b is incorrect. Although cryptic coloration is a defensive mechanism, it
usually is not involved in mimicry. Cryptic coloration, which involves coloring
of the animal so that it blends with its surroundings, is usually not the case with
Batesian or Müllerian mimicry. With mimicry, the models and mimics usually
have warning coloration.
The correct answer is c—occur such that mimics look and act like models
C. Answer c is correct. In Batesian mimicry, a harmless mimic looks and acts like its
dangerous model species and with Müllerian mimicry, all of the species are
dangerous and they all look and act like one another.
The correct answer is c—
D. Answer d is incorrect because although mimicry does occur in only poisonous or
dangerous species, such is the case in Müllerian mimicry, Batesian mimicry
involves harmless species. In Batesian mimicry, a harmless, palatable species
will look and act like a distasteful or dangerous species. Predators will avoid the
palatable species thinking it is the dangerous species.
Hint: In both Batesian and Müllerian mimicry, mimic and model must not only look
alike but also act alike if predators are to be deceived. For example, the members of
several families of insects that closely resemble wasps behave surprisingly like the
wasps they mimic, flying often and actively from place to place.
5. Which of the following is an example of commensalism?
a. A tapeworm living in the gut of its host
b. A clownfish living among the tentacles of a sea anemone
c. An acacia tree and acacia ants
d. Bees feeding on nectar from a flower
The correct answer is b—
A. Answer a is incorrect. This is an example of parasitism. In this relationship, the
tapeworm benefits from living inside the host because the host provides food and
protection for the tapeworm. In turn, the tapeworm harms the host by denying it
full nutritional benefit from the food it eats.
The correct answer is b—A clownfish living among the tentacles of a sea anemone
B. Answer b is correct. This type of relationship is commensalism because the sea
anemone provides the fish with food, the detritus left on its tentacles following a
meal. However, the sea anemone itself receives no real benefit from the
relationship, but also, it is not harmed.
The correct answer is b—
C. Answer c is incorrect. The relationship between acacia trees and the ants that
inhabit them is usually mutualistic, in which both benefit. Occasionally, however,
it becomes parasitic, in which the ants benefits while the trees are harmed.
The correct answer is b—
D. Answer d is incorrect. Bees feeding on nectar from a flower is an example of
mutualism. In this relationship, the bees obtain nutrients from the flowers but
they in turn help the flowers cross-pollinate by transporting pollen from one
flower to the next.
Hint: Commensalism is a symbiotic relationship that benefits one species and neither
hurts nor helps the other. One of the best-known examples of symbiosis involves the
relationship between certain small tropical fishes and sea anemones. These fish
evolved the ability to live among the stinging tentacle of the sea anemone, feeding off
of the detritus left on its tentacles. The sea anemone is not harmed by this
relationship nor does there seem to be a real benefit for it either.
6. Oxpeckers’ eating—
a. noninjurious insects off mammals is an example of commensalism
b. injurious ectoparasites off mammals is an example of mutualism
c. scabs to so they can feed on a mammal’s blood is an example of
parasitism
d. all of the above
The correct answer is d—
A. Answer a is incorrect. An oxpeckers’ eating insects that don’t affect the fitness of
the mammal is an example of a commensal relationship, but this is not the only
correct answer.
The correct answer is d—
B. Answer b is incorrect. An oxpeckers’ removing injurious ectoparasites is an
example of mutualism because both the bird and the mammals benefit. This is
not the only correct answer.
The correct answer is d—
C. Answer c is incorrect. The oxpeckers’ behavior harms the mammal (removal of
blood) and is a parasitic relationship.
The correct answer is d—all of the above
D. Answer d is correct. All of the relationships correctly describe their effects on
each member.
7. A species whose effect on the composition of a community is greater than
expected based on its abundance can be called a—
a. predator
b. primary succession species
c. secondary succession species
d. keystone species
The correct answer is d—
A. Answer a is incorrect. Not only predators can serve as keystone species.
The correct answer is d—
B. Answer b is incorrect. A primary succession species is a species that inhabits a
barren environment with no soil and usually doesn’t interact with many other
organisms.
The correct answer is d—
C. Answer c is incorrect. A secondary succession species is a species that inhabits an
environment that has been cleared of plant life but the soil still remains. These
types of organisms usually don’t interact with many other organisms.
The correct answer is d—keystone species
D. Answer d is correct. A keystone species is a species that has particularly strong
effects on the composition of a community out of proportion to its abundance.
Predators often serve as keystone species because their presence restrains
competition among other species, maintaining high species richness. A keystone
species might also affect the environment by creating new habitats or niches for
other species. When the keystone species is removed from a community, the
community itself is disrupted.
Hint: Species that have particularly strong effects on the composition of communities
are termed keystone species. Predators, such as the starfish, can often serve as a
keystone species by preventing one species from outcompeting others, thus
maintaining high levels of species richness in a community.
8. When a predator preferentially eats the superior competitor in a pair of competing
species—
a. the inferior competitor is more likely to go extinct
b. the superior competitor is more likely to persist
c. coexistence of the competing species is more likely
d. none of the above
The correct answer is c—
A. Answer a is incorrect. Removal of the superior competitor should reduce the
negative effect on the inferior competitor by freeing up access to a limiting
resource.
The correct answer is c—
B. Answer b is incorrect. Predators can control and even drive their prey to
extinction, so the probability of extinction would increase.
The correct answer is c—coexistence of the competing species is more likely
C. Answer c is correct. Removal of the superior competitor should help to balance
the competitive abilities of the two species. That is, any advantage in resource use
efficiency is offset by removal of superior individuals such that the population
response is offset.
The correct answer is c—
D. Answer d is incorrect. One of the answers provided is correct.
9. Keystone species—
a. always tend to increase species diversity
b. always tend to decrease species diversity
c. are always predators
d. none of the above
The correct answer is d—
A. Answer a is incorrect. Although the effect of a keystone species might be to
increase species diversity in a community (by preying preferentially on a superior
competitor), that is not necessarily the case; for example, if the predator
preferentially preys on less superior competitors, species diversity will decrease.
The correct answer is d—
B. Answer b is incorrect. See the reasoning in answer a. A keystone species may
affect diversity of a community in more than one way depending on the particular
mechanism by which its effects are out of proportion to its abundance.
The correct answer is d—
C. Answer c is incorrect. Keystone species are defined by the magnitude of their
effects relative to their abundance, not by the mechanisms that result in particular
effects.
The correct answer is d—none of the above
D. Answer d is correct. None of the answers provided is correct.
10. Species that are the first colonists in a habitat undergoing primary succession—
a.
b.
c.
d.
are usually the fiercest competitors
help maintain their habitat constant so their persistence is ensured
may change their habitat in a way that favors the invasion of other species
must first be successful secondary succession specialists
The correct answer is c—
A. Answer a is incorrect. Early successional species are usually specialized for rapid
establishment and growth, not for competition since competition is usually weak
for the earliest colonists of any new habitat.
The correct answer is c—
B. Answer b is incorrect. Succession, by definition, involves the continual change of
habitat by the biotic members of a community.
The correct answer is c—may change their habitat in a way that favors the invasion of
other species
C. Answer c is correct. Succession is defined as the gradual and continual change in
a community over time. In most cases, species that become abundant can have
profound effects on their habitat, even leading to a change that may no longer suit
them. Think of trees thriving on high light levels in relatively open environments
growing fast to tall stature and producing shade that might inhibit the growth of
seedlings. Such a result could lead to the replacement of the initial tree species by
those more tolerant to shade.
The correct answer is c—
D. Answer d is incorrect. The characteristics that favor certain species in secondary
succession could be completely different from those favoring species in primary
succession, including dispersal, competitive ability, reproductive rate, or
competitive ability.
11. Species diversity of very early and late successional stages—
a. is expected to be low
b. is expected to be high
c. is always at equilibrium
d. depends only on the physical characteristics of the habitat
The correct answer is a—is expected to be low
A. Answer a is correct. Species diversity should be low at early and late stages, but
for different reasons. In early stages, conditions may be harsh enough that only a
few species can be pioneers and survive. At later stages, a small subset of species
that persist could represent the most formidable competitors.
The correct answer is a—
B. Answer b is incorrect. Not many species are good at invading new habitats that
might have limited nutrients and offer harsh biophysical conditions. Similarly,
competition among species is expected to be strong in late successional stages
because competitive interactions (with the possibility of competitive exclusion)
have long periods of time to work themselves out.
The correct answer is a—
C. If a community in an early successional stage was at equilibrium, how would it
advance to later successional stages? Species diversity in late successional stages
may remain at equilibrium until the next disturbance starts the process anew.
The correct answer is a—
D. Answer d is incorrect. Turnover of species in communities does not just depend
on the physical characteristics. The types of organisms that live together in a
community determine the kinds of interactions and outcomes of interactions.
12. The fundamental niche of an organism—
a. is always more restricted than its realized niche
b. is usually less restricted than its realized niche
c. takes into account actual environmental conditions and the presence of
other species
d. none of the above
The correct answer is b—
A. Answer a is incorrect. The fundamental niche makes the least restrictive
assumptions about the potential of a species to exist in particular environments.
For example, it does not include information about competitors or predators.
The correct answer is b—is usually less restricted than its realized niche
B. Answer b is correct. The realized niche is usually smaller because it accounts for
potentially negative effects on the ability of an organism to persist in a particular
environment (both biotic and abiotic factors). In some cases, the realized and
fundamental niches may be the same.
The correct answer is b—
C. Answer c is incorrect. The fundamental niche takes into account only the
physiological tolerance limits and resource needs. It is a very general statement,
primarily useful in understanding the factors that limit species in real-world
habitats.
The correct answer is b—
D. Answer d is incorrect. One of the preceding answers provided is correct.
13. Resource partitioning—
a. reduces niche overlap
b. increases competition
c. increases niche overlap
d. all of the above
The correct answer is a—reduces niche overlap
A. Answer a is correct. Resource partitioning, like that seen in the study of warblers
by Robert MacArthur, is a way that potentially competing species can subdivide a
niche axis such that they do not compete. In a way, it is a mechanism that spaces
out individuals to use the full range of a resource available so that their individual
needs are exclusive of the needs of other potentially competing species.
The correct answer is a—
B. Answer b is incorrect. By subdividing a resource, competitive interactions are
reduced.
The correct answer is a—
C. Answer c is incorrect. Once again, partitioning reduces competition by reducing
overlap.
The correct answer is a—
D. Answer d is incorrect. One of the preceding answers provided is correct.
14. Lichen growing on the surface of rocks provides an example of—
a. facilitation
b. tolerance
c. inhibition
d. secondary succession
The correct answer is b—
A. Answer a is incorrect. Facilitation is a process in succession whereby the original
species that move into a newly opened habitat alter the environment in such a way
as to allow new species to move in. Lichens are not part of this step in the
process of succession.
The correct answer is b—tolerance
B. Answer b is correct. Succession happens because species alter the habitat in ways
that permit other species to enter it. Tolerance is the first step in this process,
where hardy, more weedlike species that are tolerant of harsh, lifeless conditions
become established in barren areas. Lichens are able to grow on rocks or other
abiotic substrates and so are usually a species that appears during the tolerance
phase of succession.
The correct answer is b—
C. Answer c is incorrect. Inhibition is a process in succession whereby changes in the
habitat caused by one species actually favor another species, which inhibits the
growth of the original species. Lichens are not part of this step in the process of
succession.
The correct answer is b—
D. Answer d is incorrect. Secondary succession involves succession, the changing of
a community from simple to more complex—in areas where an existing
community has been disturbed but soil still remains. Lichens are involved in
primary succession, not secondary succession.
Hint: Succession happens because species alter the habitat and resources in ways that
favor other species. Three dynamic concepts are of critical importance in the process:
tolerance, facilitation, and inhibition. Tolerance involves the early successional
stages where weedy r-selected species that are tolerant of the harsh, abiotic conditions
are able to grow in barren areas. Abiotic barren areas include rocks. Lichens are the
first vegetation able to grow under such conditions.
Challenge Questions
1. Competition is traditionally indicated by documenting the effect of one species on
the population of another. Are there alternative ways to study the potential effects
of competition on organisms that are impractical to study with experimental
manipulations because they are too big or live too long?
Answer—Experiments are useful means to test hypotheses about ecological limitations,
but they are generally limited to rapidly reproducing species that occur in relatively small
areas. Alternative means of studying species’ interactions include detailed studies of the
mechanisms by which species might interact; sometimes, for long-lived species, instead
of monitoring changes in population size, which may take a very long time, other indices
can be measured, such as growth or reproductive rate. Another means of assessing
interspecific interactions is to study one species in different areas, in only some of which
a second species occurs. Such studies must be interpreted cautiously, however, because
there may be many important differences between the areas in addition to the difference
in the presence or absence of the second species.
2. Refer to Figure 56.9.
If the single prey species of Paramecium was replaced by several different
potential prey species that varied in their palatability or ease of subduing by the
predator (leading to different levels of preference by the predator), what would
you expect the dynamics of the system to look like (for example, would the
system be more or less likely to go to extinction that is?).
Answer—Adding differentially preferred prey species might have the same effect as
putting in a refuge for prey in the single species system. One way to think about it is that
if a highly preferred species becomes rare due to removal by the predator, then a predator
might switch to a less desirable species, even if it doesn’t taste as good or is harder to
catch, simply because it is still provides a better return than chasing after a very rare
preferred species. While the predator has switched, there might be enough time for the
preferred species to rebound. All of these dynamics will depend upon the time it takes a
predator to reduce the population size of its prey relative to the time it takes for those
prey populations to rebound once the predator pressure is removed.
3. Refer to Figure 56.22.
Are there alternative hypotheses that might explain the increase followed by the
decrease in ant colony numbers subsequent to rodent removal in the experiment
described in Figure 56.22? If so, how would you test the mechanism
hypothesized in the figure?
Answer—Although the mechanism might be known in this system, hidden interactions
might affect interpretations in many ways because ecological systems are complex. For
example, what if some other activity of the rodents besides their reduction of large seeds
leading to an increase in the number of small seeds was responsible for the positive effect
of rodents on ants? One way to test the specific mechanism would be to increase the
abundance of small seeds experimentally independent of any manipulation of rodents.
Under the current hypothesis, an increase in ant population size would be expected and
should be sustained, unlike the initial increase followed by a decrease seen when rodents
are removed.
4. Refer to Figure 56.7.
Examine the pattern of beak size distributions of two species of finches on the
Galápagos Islands. One hypothesis that can be drawn from this pattern is that
character displacement has taken place. Are there other hypotheses? If so, how
would you test them?
Answer—By itself, the pattern shown in Figure 56.7 suggests character displacement,
but alternative hypotheses are possible. For example, what if the distribution of seeds
available on the two islands where the species are found alone is different from that seen
where they are found in sympatry? If there were no large and small seeds seen on Los
Hermanos or Daphne, just medium-sized ones, then it would be hard to conclude that the
bill size on San Cristobal has diverged relative to the other islands just due to
competition. This is a general criticism of inferring the process of character displacement
with just comparing the size distributions in allopatry and sympatry. In this case,
however, the Galápagos system has been very well studied. It has been established that
the size distribution of seeds available is not measurably different. Furthermore, natural
selection-induced changes seen in the bill size of birds on a single island, in response to
drought-induced changes in seed size lend further support to the role of competition in
establishing and maintaining these patterns.