Mastering Concepts
37.1
1. Distinguish among communities, ecosystems, and populations.
A community is all of the populations that live together in one habitat. An ecosystem is
all of the communities in an area plus the abiotic environmental components. Populations
are interbreeding individuals of one species living in an area.
2. Name some abiotic and biotic components of your environment.
Abiotic components of my environment include the nonliving substances such as air,
water, furniture, houses, streets, and cars. Biotic components include the people, animals,
plants, fungi, and bacteria that share my environment.
3. Distinguish between habitat and niche.
A habitat is where a species lives. A niche is all the resources and conditions a species
requires to survive, grow, and reproduce. The niche includes biotic and abiotic factors.
Habitat is just one facet of a species’ niche.
4. How do ecologists measure species diversity in a community?
One way is through species richness, the total number of species in a habitat. Another
method is relative abundance, which describes the proportion of the community that each
species represents.
5. What is the competitive exclusion principle?
The competitive exclusion principle states that two species cannot coexist indefinitely in
the exact same niche: sooner or later, the one with the greater reproductive rate will
replace the other.
6. List three examples of symbiotic relationships.
Three examples of symbiotic relationships include mutualistic relationships between
algae in coral animals, mycorrhizal fungi in plant roots, and humans and the bacteria that
live in their guts; commensal relationships such as moss living on tree bark; and parasitic
relationships such as ticks, fleas, and lice and the animals on which they feed.
7. Describe some adaptations that protect against herbivory and predation.
Thorns, milky sap, and poisonous or distasteful chemicals are protections against
herbivory; camouflage, warning coloration, weapons, and defensive behaviors are
protection against predation.
8. How are keystone species important in communities?
Keystone species are important in communities because they make up a small proportion
of the community’s biomass, yet they disproportionately promote species diversity.
9. Define coevolution and describe an example.
In coevolution, two species influence the directions of each other’s evolution and
subsequent adaptations. An example is the relationship between a flowering plant and its
pollinator.
37.2
1. What is succession?
Succession is a change in the species that make up a community.
2. Distinguish between primary and secondary succession.
Primary succession begins in a pristine habitat with no previous communities, and
secondary succession occurs in communities that are disturbed but not destroyed.
3. What processes and events contribute to primary and secondary succession?
Primary succession begins with the emergence of a new habitat; secondary succession
begins with disturbance to an existing community. In both cases, pioneer species are the
first to arrive. These species change the environment in ways that permit other species to
thrive. In theory, the community continues to change until a stable climax community is
in place.
4. How do disturbances prevent true climax communities from developing?
Pockets of local disturbance create patches in different successional stages throughout the
habitat. These disturbances (fires, trees falling, storm damage) happen frequently enough
to prevent a true climax community.
37.3
1. Identify the trophic levels of a food chain.
Primary producers are plants and other autotrophs that can produce their own organic
matter. Primary consumers eat primary producers, secondary consumers eat primary
consumers, and tertiary consumers eat secondary consumers. Decomposers eat dead
organisms and organic wastes.
2. What sources of energy sustain ecosystems?
The ultimate source of energy that sustains most ecosystems is the sun. Some
ecosystems are sustained by potential energy in inorganic chemicals.
3. What roles do primary producers and decomposers play in ecosystems?
Primary producers create the organic material an ecosystem needs, and decomposers
recycle the inorganic nutrients the primary producers need.
4. How efficient is energy transfer between trophic levels in food webs?
In general, about 2% to 30% of the energy available to a trophic level is fixed into tissue
and made available to the next higher trophic level. The rest is lost as heat.
5. Draw an energy pyramid for an ecosystem with three levels of consumers.
Answers may vary, but the pyramid might include producers (100 kcal), primary
consumers (10 kcal), secondary consumers (1 kcal), and tertiary consumers (0.1 kcal)
6. Explain how biomagnification disproportionately affects organisms at the top of a food
chain.
Biomagnified chemicals are stored in fatty tissues and are not biodegradable. All of the
molecules present in each trophic level are therefore concentrated into the much smaller
biomass that makes up the next highest level. Top consumers therefore acquire the
highest concentrations in their tissues.
37.4
1. What features do biogeochemical cycles share?
The elements in all biogeochemical cycles are stored in one of four reservoirs: organisms,
atmosphere, water, and soil/rock. The elements are transferred among all four reservoirs
and transformed in the process.
2. Describe the steps of the water, carbon, nitrogen, and phosphorus cycles.
Water cycle: In the atmosphere, condensation of water vapor causes precipitation that
falls to Earth. The water may then evaporate and return to the atmosphere, or it may run
off from land surfaces into bodies of water. Some rain water enters groundwater, which
eventually returns to the land surface.
Carbon cycle: CO2 from the atmosphere is incorporated into tissues of autotrophs. Their
tissues are consumed by multiple levels of consumers and, eventually, by decomposers.
Meanwhile, both autotrophs and heterotrophs respire, returning CO2 to the atmosphere.
Burning fossil fuels and wood also returns CO2 to the atmosphere. CO2 in the atmosphere
may also dissolve into oceans and become part of ocean sediments.
Nitrogen cycle: Nitrogen-fixing bacteria transform N2 into ammonium, and lightning
transforms N2 into nitrates. Plants incorporate ammonium and nitrate into organic
molecules, which enter food chains and are passed up trophic levels. Decomposers
release ammonium ions as they decompose organic wastes and the remains of animals
and plants. Nitrifying bacteria transform the ammonium into nitrates and nitrites, which
denitrifying bacteria transform into N2.
Phosphorus cycle: Rock that contains phosphorus erodes. Plants absorb the phosphorus,
which subsequently passes up food chains. Decomposers return the inorganic phosphorus
to soil. Some phosphates enter waterways and become deposited in new sediments that
eventually form new layers of rock.
3. Describe how a terrestrial ecosystem can interact with a faraway aquatic ecosystem.
A terrestrial ecosystem can interact with a faraway aquatic ecosystem through the action
of the atmosphere, through circulation of ocean currents, and through runoff from land to
water. In some cases, migratory animals can carry nutrients from the land to the ocean
and back.
37.5
1. Describe the relationship between panic grass, the fungus, and the virus.
The virus infects the cells of the fungus, which, in turn, infects the grass. The grassfungus-virus relationship allows for all three species to colonize warm habitats that they
could not otherwise occupy.
2. The fungus–virus partnership helped young grass plants survive at very high
temperatures. What would be the benefits of inoculating all of our food crop plants with
the fungus–virus team? What else would you need to know before recommending that
strategy? Can you think of any possible drawbacks?
Food crop plants might better withstand heat stress, possibly increasing yields. To
determine whether to inoculate a plant, you would need to know whether the fungus or
virus cause disease in the plant or whether they would change the food crops in ways that
might harm humans. Drawbacks could include unintended large-scale change to
ecosystems. Also, growing crops in hotter areas might increase irrigation demands in
areas where water is already scarce.
Write It Out
1. How does a community differ from an ecosystem?
A community is a group of interacting populations that inhabit the same region. An
ecosystem includes a community plus the nonliving environment within a defined area.
2. How does a habitat differ from a niche? Describe your own habitat and niche.
A habitat is a physical place where members of a population typically live. A niche is all
the resources a species exploits for its survival, growth, and reproduction. My habitat is
my home and the other physical locations where I spend time; my niche includes the
habitat plus the food I eat, the water and air I breathe, the other resources I exploit, and
the job I do.
3. How can two species share the same habitat without one driving the other to
extinction?
Two species can share the same habitat if they use a different subset of the available
resources (i.e. they have different niches).
4. Describe and give examples of three types of symbiotic relationships.
In a symbiotic relationship, one species lives in or on another. Three types include
mutualism, commensalism, and parasitism. Both partners benefit from a mutualistic
relationship; an example is a cow plus the cellulose-digesting microbes in its rumen. In
commensalism, one partner benefits and the other neither benefits nor is harmed by the
relationship; an example is the tiny mites that live, eat, and breed in a human’s hair
follicles. In parasitism, one partner benefits and the other is harmed. A tree that has
mistletoe growing on it illustrates parasitism because the mistletoe lives on water and
nutrients that the tree has acquired.
5. List examples of adaptations that enable an organism to compete with other species,
live inside another species, find food, and avoid herbivory or predation. How does each
adaptation contribute to the organism’s reproductive fitness?
An example of an adaptation that enables an organism to compete is the rapid growth of a
seedling, which may shade out others as it acquires light energy. More photosynthesis
means more energy for reproduction. An adaptation that enables an organism to live
inside another is the scolex of a tapeworm, which keeps the worm from being flushed out
of an animal’s digestive tract. A worm with a scolex acquires more food than one
without, which means more energy is available for reproduction. An example of an
adaptation that helps an animal find food is an eagle’s keen eyesight. An eagle that can
spot small animals from above acquires food than one with poor eyesight. An example of
an adaptation that helps an organism avoid predation is camouflage. An animal that can
avoid being eaten (at least before reproductive age) has a chance at reproductive success.
6. In one type of mimicry, a harmless species such as a jumping spider physically
resembles a noxious species such as an aggressive type of ant. Explain why this type of
mimicry can exist only if the spiders are less abundant than the ants.
If a harmless species were more abundant than the noxious one it is mimicking, predators
would not be fooled. Mimicry works because a predator learns by experience whether or
not a species is harmful. If the harmless species is more abundant, the predator will learn
that the majority of its encounters with similar-looking creatures are not painful.
7. What is a keystone species? How do researchers design experiments that help them
identify keystone species?
Keystone species greatly influence species diversity, even though their biomass makes up
only a small proportion of the community. One mechanism to identify keystone species
is to remove one species at a time and monitor the effects on diversity.
8. How is natural selection apparent in ecological succession?
Succession is a change in species composition over time, and it occurs because one set of
species alters the environmental conditions in such a way that other species can begin to
exploit the habitat. In changing the conditions, they are altering the selective forces that
determine whether or not an organism can survive and reproduce in the habitat.
9. Why are true climax communities rare?
True climax communities are rare because communities are always changing. Natural
disasters and human interference create pockets of disturbance that make a landscape a
patchwork of different successional stages.
10. After fires destroyed much of Yellowstone National Park in 1988, forest managers
suggested humans could help the areas recover by feeding deer, bringing in plants, and
planting trees. What are some advantages and disadvantages of intervening in recovery
from a disaster?
Advantages include more rapid recovery of the ecosystem, which would help ensure the
survival of deer and other wildlife. The disadvantage to such direct involvement would be
that humans sometimes might skip stages that are required for the overall health of the
ecosystem. In addition, the wildlife might get accustomed to the presence of humans,
and they may lose the ability to fend for themselves.
11. When Mount St. Helens erupted in 1980, areas nearest the volcano were totally
devastated. Farther away from the blast site, however, the disturbance was milder. Predict
how the rate of forest succession would change with distance from the volcano, and
explain your prediction.
Further from the blast site, secondary succession would be at work since the habitat was
not completely devastated. Therefore rates of succession would be much faster than near
the blast site, where most if not all life was destroyed.
12. Can you think of an example of an organism that is both a producer and a consumer?
Examples could include carnivorous plants or photosynthetic protists like Euglena.
13. Identify a consumer in an ecosystem not mentioned in the text.
[Many answers are possible]
14. Krill are tiny, shrimplike plankton that are abundant in Antarctic waters. Some people
have suggested that we “farm’’ the krill in Antarctic waters and use it to feed people who
are starving. Predict the effects of krill farming on the Antarctic ecosystem.
Harvesting krill would leave less food for animals such as baleen whales, whale sharks,
seals, penguins, fish, and squid. Their populations would almost certainly decline. In
addition, the krill are consumers as well, so krill farming could also cause a decline in the
species the krill rely on for food.
15. In a long-term study of a tropical rain forest in Puerto Rico, researchers studied the
aftermath of hurricanes Hugo and Georges. Tree debris supported the growth of
detritivores. Beetles and flies ate the detritivores. Propose a food chain and trace how the
booming insect populations might eventually benefit top predators such as birds and
snakes in the forest. What do you predict happened to fruit- and-nectar-eating animals
immediately after the storm?
Detritivores feed beetles and flies, which are in turn eaten by reptiles and small
mammals. These small animals are prey for snakes and birds, so the populations of top
predators should grow. On the other hand, populations of fruit and nectar-eating animals
should decline; robbed of their resources, they would starve or migrate to new habitats.
16. As the human population in the United States has increased, we have reduced or
eliminated the populations of top predators such as wolves and coyotes. How might this
history relate to the recent explosion of deer populations? Using the ecological principles
you have learned in this chapter and chapter 36, what are some possible ways to reduce
deer populations? What are the pros and cons of each approach?
Without the top predators, death rates in deer populations declined, leading to an increase
in population size. To reduce the population you could increase death rates by controlled
hunting, or you could decrease birth rates with birth control or sterilization measures.
Hunting puts humans in the role of top predator and this re-establishes a balance. Unlike
wolves, however, humans do not cull the sick and old deer. Instead, they attempt to
remove the prime specimens, which could alter the gene pool of the population.
Sterilization or birth control methods are awkward, time-consuming and expensive.
17. Pigs and chickens in commercial farms may eat food made from herrings, anchovies,
and other fish. What are some possible consequences to marine food webs?
Removing fish from the marine web removes food sources for marine organisms higher
in the web, and also reduces the predation pressures on lower levels of the web. Both
disturbances could cause the marine food web to crash.
18. In the United States alone, tens of millions of domesticated cats consume hundreds of
millions of songbirds. Use the Internet to learn more about songbird diets, then predict
how predation by cats might affect the rest of the food web.
Song birds eat insects and/or seeds, and as young they are almost exclusively fed insects.
Loss of songbirds from the food web could have multiple effects. For one example,
plants that rely on songbirds for dispersal of their seeds would suffer. In addition, the
increase in insects could increase the destruction of vegetation or promote the spread of
disease in humans.
19. Some farmers and ranchers kill the prairie dogs on their land. Use the Internet to
research the ecology of prairie dogs and some of the techniques used to control prairie
dog populations. How does each technique affect the prairie ecosystem?
Nonlethal techniques include preventing overgrazing, since prairie dogs prefer short
grass. Another possibility is trapping and relocating the prairie dogs. Lethal techniques
include shooting or poisoning the animals or destroying their burrows. Consequences of
prairie dog removal may include disruption to the ecosystem, since prairie dogs churn the
soil and are eaten by secondary consumers. Additionally, poisoning may affect other
animals in the ecosystem.
20. Use the second law of thermodynamics and a pyramid of energy to explain why most
food chains have four or fewer levels.
The second law of thermodynamics says that energy lost as heat every time it is
converted from one form to another. As a result, only a small proportion of the energy
available at one level of a food chain is available to be transferred to the next level.
These energy constraints keep most food chains constrained to four or fewer levels.
21. Some forms of mercury biomagnify in aquatic food chains. High concentrations of
mercury in fish are prompting health advisories that warn pregnant women not to eat tuna
and some other fish species. From this information, do you think tuna is an herbivore or a
carnivore? Explain your reasoning. What properties do you predict that mercury shares
with DDT and other pollutants that biomagnify as they ascend food chains?
Tuna must be a carnivore; the high levels of mercury suggest the fish is high on the food
chain. Biomagnification concentrates mercury and certain other chemicals in the highest
trophic levels because the chemical passes on to the next consumer rather than being
excreted as waste. Mercury is stable and fat-soluble, like other pollutants that
biomagnify.
22. How do organisms return water, carbon, nitrogen, and phosphorus to the abiotic
environment?
Plants absorb water from the soil and release through transpiration. Animals and humans
drink water and consume it, returning it to the environment through evaporation and
urination. Respiration returns C to the atmosphere, and N returns to the environment in
metabolic wastes. In addition, all organisms return C, N, and P to the abiotic
environment when they die and are decomposed.
23. Use the carbon cycle to trace a hypothetical path of a carbon atom from Abraham
Lincoln’s body into your own.
Abraham Lincoln may have exhaled a carbon atom as CO2. That gas may have circulated
in the atmosphere for a long time, until a plant absorbed the carbon atom and
incorporated it into its own tissues. The carbon atom may have circulated within a food
web, including animals and decomposers, until it was finally absorbed as CO2 and
incorporated into the tissues of a plant that I ate.
24. Suppose your friend says “I hate germs! I wish we could kill all the bacteria in the
world!” What would happen to your friend if she didn’t harbor bacteria in her own body?
What would happen to nutrient cycles without bacteria?
Without her bacterial companions, your friend would be much more vulnerable to
infection, and she would lose the ability to digest some components of her food. Nutrient
cycles would collapse without bacteria, which are important decomposers and autotrophs.
In addition, only bacteria can fix nitrogen, so ecosystems would eventually collapse in
their absence.
25. Several sites on the Internet offer water- or carbon-footprint calculators. Choose one
and compute your footprints. What are the assumptions that go into these calculations?
What strategies could you use to reduce either footprint?
Calculations of carbon and water footprints are based on measures of home energy and
water use, travel, diet, shopping, and recycling habits. Both footprints are estimates based
on average lifestyles and behaviors. Some strategies to reduce your carbon footprint
include reusing materials, biking or walking, buying locally, bringing your own bags to
the grocery store, and adjusting your thermostat to save energy. Some strategies to
reduce your water footprint include reducing water use in the home and yard, eating less
meat, and purchasing less stuff.
26. Use the Internet to look up the components of a so-called “low-carbon” diet. How
does each element of a low-carbon diet help reduce CO2 emissions into the atmosphere?
Eating less meat and dairy reduces CO2 from livestock methane emissions. Eating locally
produced, seasonal food cuts down on CO2 emissions produced in transportation.
Reducing the use of packaged materials and buying used goods helps reduce the use of
plastics and reduces CO2 emissions from transportation.
27. Review the structures of organic molecules in chapter 2. How do the molecules in
living cells create interconnections between the carbon, nitrogen, and phosphorus cycles?
Carbon is part of all organic molecules, while nitrogen occurs in proteins and nucleic
acids, and phosphorus occurs in nucleic acids, ATP, and phospholipids. Molecules that
contain combinations of C, N, and/or P represent intersections of the nutrient cycles.
Pull it Together
1. Where do mutualism, commensalism, and parasitism fit into this concept map?
All are examples of symbiotic interactions in a community.
2. How are decomposers and autotrophs essential to ecosystem function?
Decomposers release inorganic nutrients from organic wastes. Autotrophs incorporate
these inorganic nutrients into organic molecules, which sustain all food webs.
3. Distinguish between primary and secondary succession.
Primary succession occurs in new habitats, whereas secondary succession occurs in areas
that are damaged but that retain intact soil. Relative to primary succession, secondary
succession is quite fast.
4. List some additional components of your abiotic environment.
[Answers will vary.]
5. Where do primary, secondary, and tertiary consumers fit into this concept map?
All would fit under the entry for “Herbivory/Predation.” Primary consumers are
herbivores; secondary consumers eat primary consumers, and tertiary consumers eat
secondary consumers.