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Answers to Thinking Critically Questions
Mader: Inquiry into Life, Twelfth Edition
Chapter 34
1. A large forest has been removed by timber harvest (clear-cutting) and the land has
not been replanted. After several years, humidity and rainfall in the area seem to
have decreased. How can your knowledge of the water cycle be used to explain
these observations?
Explanation/Answer: Removal of trees changes the balance of evaporation,
condensation, and precipitation in an ecosystem. Removal of trees eliminates
shade, causing increases in evaporation relative to condensation and precipitation.
In addition, removal of trees causes erosion (due to reduction of tree roots that
hold the soil together), which causes more water to runoff into the ocean. This
causes precipitation patterns to shift from overland where the forest was, to over
the ocean since the water balance is shifting there.
2. Explain how human use of excess nitrogenous fertilizers and laundry detergents
containing phosphorous can disrupt aquatic ecosystems.
Explanation/Answer: Excess inputs of both of these chemicals disrupt the
nitrogen and phosphorus cycles, respectively. In aquatic systems, excess inputs of
these chemicals via runoff increases the levels in water above natural levels. This
can cause excess algal blooms (cultural eutrophication), which can lead to
increased phytoplankton growth, decreased oxygen, and ultimately large fish
kills—not to mention very distasteful and foul smelling water. These activities
tend to decrease biodiversity in aquatic systems.
3. In mountainous regions of the western United States, large predators (e.g., wolves
in Yellowstone National Park), which were previously driven out of the area, are
being reintroduced. People living in these areas are concerned that lifestock may
be lost if there is not enough wild food for the predators. What type of data are
needed concerning food webs and ecological pyramids to ensure successful
reintroduction of these predators and minimal impact on livestock?
Explanation/Answer: You would need information on the amount of biomass
produced by primary producers and the proportion of that biomass eaten by
primary consumers, secondary consumers and so forth, because energy is lost at
each trophic level in the food chain. You would need to know the trophic level
that your predator would be on the food chain so you know if there is enough
energy in the ecosystem to support the proposed predator you plan to introduce.
Based on estimates of available energy at that trophic level, scientists can
determine appropriate numbers of animals to introduce to minimize impacts on
domestic livestock.
4. It is the year 2100 and world population has surpassed 13 billion people.
Advances in technology have made it possible to create an artificial atmosphere
on the moon that traps appropriate levels of oxygen and allows ample sunlight for
life to persist. You are a member of a team of ecologists assigned the task of
making the moon inhabitable. You realize that much more than a suitable
atmosphere is necessary for survival. Construct an ecosystem that will support
human and other life as we move people to the moon; justify your choices of
organisms used in this ecosystem.
Explanation/Answer: Your ecosystem must contain a large biomass of primary
producers (i.e., green plants) to support the other trophic levels. Depending on
the proposed diet of your new society, you would want some herbivores and
possibly some carnivores, based on the amount of biomass and energy at the
lower trophic levels. If the diet were proposed to be omnivorous, as we are on
Earth, then some herbivores and secondary consumers would be necessary. We
generally eat herbivores, but unless we wanted to control the entire populations of
herbivores ourselves, we should probably introduce predators as well (building on
what was covered in terms of population regulation in Ch. 33).
5. Most food webs in any particular ecosystem have four or five trophic levels at
most. Why?
Explanation/Answer: As illustrated by the ecological pyramid (Fig. 34.5), most
energy is lost as heat between each trophic level; only 10% of energy is passed on
up the food chain. Therefore, although there may be many primary producers that
produce high biomass, they utilize much of the sun’s energy. Then, through each
trophic level, 90% of energy is lost as heat. Thus, very few animals can be
supported beyond the third and fourth trophic levels because little energy is left.