Biology: Life on Earth 7e
Thinking Through the Concepts
Chapter 42: Earth’s Diverse Ecosystems: Answer Key!
1. Explain how air currents contribute to the formation of the Tropics and the large deserts.
Answer: Warm air saturated with water vapor rises over the equator, and as the air cools, it rains over
the tropics. The cooler, dry air then flows north and south from the equator and sinks but is warmed by
heat radiated from Earth, so by the time it reaches Earth it is both warm and dry and influences the
formation of deserts at 20¡ north and south of the equator.
2. What are large, roughly circular ocean currents called? What effect do they have on climate, and where
is that effect strongest?
Answer: Ocean circulation patterns are called gyres. Gyres distribute warmth from the equator to
northern and southern coastal areas.
3. What are the four major requirements for life? Which two are most often limiting in terrestrial
ecosystems? In ocean ecosystems?
Answer: The four requirements for life are nutrients, energy, water, and appropriate temperatures. Water
and temperature are the most limiting in terrestrial ecosystems. Nutrients and energy are the most limiting
in aquatic ecosystems.
4. Explain why traveling up a mountain takes you through biomes similar to those you would encounter
traveling north for a long distance.
Answer: In both cases, temperature is limiting primary productivity.
5. Where are the nutrients of the tropical forest biome concentrated? Why is life in the tropical rain forest
concentrated high above the ground?
Answer: Nutrients are concentrated in the vegetation. Life is arboreal because of the limited vegetation
close to the ground.
6. Explain two undesirable effects of agriculture in the tropical rain forest biome.
Answer: Soils are left infertile and soil becomes bricklike.
7. List some adaptations of (a) desert plants and (b) desert animals to heat and drought.
Answer: (a) Plants have large, shallow root systems, leaves modified as spines to conserve water, quick
reproductive cycles, and thickened stems that store water; (b) animals live in borrows during the day and
are able to derive water exclusively from the food they eat.
8. What human activities damage deserts?
Answer: Desertification is the spread of deserts brought on primarily by overgrazing in areas bordering
deserts and in grasslands.
9. How are trees of the taiga adapted to a lack of water and a short growing season?
Answer: Plants have short, waxy needles and are evergreens.
10. How do deciduous and coniferous biomes differ?
Answer: In deciduous forests, trees shed their leaves when water is limiting, whereas the evergreens in
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Biology: Life on Earth 7e
Thinking Through the Concepts
coniferous forests retain their leaves the entire year. Coniferous biomes have a lower productivity and are
found in areas of lower temperature and rainfall.
11. What single environmental factor best explains why there is shortgrass prairie in Colorado, tallgrass
prairie in Illinois, and deciduous forest in Ohio?
Answer: Precipitation levels explain the differences among the three biomes.
12. Where are the world’s largest populations of large herbivores and carnivores located?
Answer: Temperate deciduous forests.
13. Where is life in the oceans most abundant, and why?
Answer: In the coastal waters where there is a steady flow of nutrients from the land.
14. Why is the diversity of life so high in coral reefs? What human impacts threaten them?
Answer: Coral reefs have a high productivity due to the optimal combinations of bottom depth,
temperature, and nutrients. Human development creates silt that clouds the water, reducing
photosynthetic rates of the algae necessary to support the abundant reef life.
15. Distinguish among the limnetic, littoral, and profundal zones of lakes in terms of their location and the
communities they support.
Answer: The littoral zone is near the shore and supports the most diverse community because there is
abundant light and nutrients. The limnetic zone is the open water below the surface, and there is sufficient
light to support photosynthesis by algae, which support a limited community of protozoa, small
crustaceans and fish. The profundal zone lies below the limnetic zone, reaching to the ocean floor, where
light is insufficient to support photosynthesis, and life is limited to detritus feeders and decomposers.
16. Distinguish between oligotrophic and eutrophic lakes. Describe (a) a natural scenario and (b) a
human-created scenario under which an oligotrophic lake might be converted to a eutrophic lake.
Answer: Oligotrophic lakes are low in nutrients and clear, due to low sediment and few microscopic
organisms. Eutrophic lakes are high in nutrients and support dense communities. (a) A natural
progression toward eutrophy involves the accumulation of sediment over time. (b) A human-created
scenario involves the addition of nutrient-rich pollutants into waterways.
17. Distinguish between the photic and aphotic zones. How do organisms in the photic zone obtain
nutrients? How are nutrients obtained in the aphotic zone?
Answer: The photic zone is the upper layer of ocean water, where light is strong enough to support
photosynthesis. The aphotic zone lies below the photic zone, where the only energy comes from the
excrement and bodies of organisms that sink or swim down. Organisms in the photic zone obtain
nutrients from those dissolved in the water that are provided by runoff from land or that are brought up
into the photic zone by upwelling. Organisms in the aphotic zone obtain nutrients from dead photic zone
organisms that sink into the aphotic zone.
18. What unusual primary producer forms the basis for hydrothermal vent communities?
Answer: Sulfur bacteria are the primary producers of vent communities.
19. On the basis of the location of the worst atmospheric ozone depletion, which biomes are likely to be
most affected by increased UV penetration?
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Biology: Life on Earth 7e
Thinking Through the Concepts
Answer: Temperate rain forests in the Southern Hemisphere, as well as open-ocean ecosystems.
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