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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 33. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What is the relationship between the zebra mussel and cargo ship ballast water?
  2. What is the estimated annual impact on the U.S. economy attributable to introduced species?
  3. Why are the Earth’s equatorial regions so much warmer than its polar regions?
  4. Why are most deserts located near 30 degrees latitude?
  5. What best explains why water temperatures on the Pacific coast of the U.S. are so much cooler than on the Atlantic coast?
  6. What two climatic factors most strongly influence the development of a terrestrial biome?
  7. Terrestrial biomes are named after the dominant vegetation of the region; how are aquatic biomes named?
  8. What two factors most strongly influence the development of an aquatic biome?

A Guide to the Reading

When exploring the content in Chapter 33 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

The Importance of Ecology

Chapter 33 begins with the question “Why Is Ecology Important?” Although a complete answer is beyond our current understanding, it has become increasingly evident that people are dependent on the continuous operation of the biosphere’s natural systems. Unfortunately, human activity is changing the biosphere in many ways, some of which may be impossible to correct. Introduced species clearly illustrate this concern. Whether introduced intentionally or accidentally, plant and animal species from other regions may expand uncontrollably, displace native species, and reduce the usefulness or dependability of natural systems. Current estimates for economic damage related to introduced species are near $120 billion annually for the United States alone. Once established, removing an introduced species becomes all but impossible. Yet banning all future introductions may not be in our best interests either. More than 98 percent of the food currently produced in the Unites States comes from introduced crops. Plant species with potentially valuable uses are routinely discovered and, despite our best efforts, accidental introductions will continue.  A more complete understanding of ecology can help us understand the consequences of these and other environmentally damaging events (such as the dispersal of toxic chemicals), direct an appropriate response to current problems, and perhaps prevent future ones from occurring entirely.

For more information on this concept, be sure to focus on:

  • Section 33.1, Why Is Ecology Important?

Interactions with the Environment

Throughout the remainder of this unit you will be repeatedly exposed to the idea that the interactions between living organisms and the environment are reciprocal. Biologists have known   for hundreds of years that the physical environment influences the appearance and distribution of plants and animals. Even without a formal education, almost everyone you know can explain why polar bears don’t live at the equator. What’s become increasingly clear is the influence living organisms have on the physical environment. This is illustrated by the example of nitrogen availability given in your text. Nitrogen is used by plants in only two molecular forms. When these forms are reduced in the soil, plant growth slows or ceases entirely. Since different plants often have differing nitrogen needs, any particular plant may become rare or more common   when nitrogen declines. Thus the levels of soil nitrogen strongly influence the pattern of plant growth within any particular area. Some bacteria convert atmospheric nitrogen into the forms that support plant growth. The presence of these bacteria will elevate nitrogen levels in the soil, providing some plants with a competitive advantage over others.  Two physical localities, identical in all respects, but differing in soil bacteria, may look entirely different.

For more information on this concept, be sure to focus on:

  • Section 33.2, Interactions with the Environment

Climate Has a Large Effect on the Biosphere

The Earth’s surface is not uniformly heated. Sunlight strikes the equator and the Earth’s tropical regions directly, but strikes the poles and higher latitudes at a slanted angle. Although these angles change somewhat with the seasons, the tropics experience the least variation, providing a relatively warm, stable climate throughout the year for the organisms living there. Polar regions cycle through relatively short, mild summers to the bitter cold of winter. Such extremes over the course of a year make life in the polar regions much more challenging than in the tropics. Differences in the temperature of the Earth’s surface heat or cool the adjacent atmosphere. In tropical regions the moist air warms and rises, but expands and cools when doing so. Because cool air has a lower capacity to hold moisture, rain will eventually fall. Although now cooler than the air below it the loss of moisture has also reduced its weight and the air is unable to sink through the moist air rising from the surface beneath it. The air is first displaced to the north and south, eventually sinking at about 30o latitude. As it descends the air is compressed, causing it to warm and gain moisture from the Earth’s surface as it moves back toward the equator. By the time it reaches the equator the air is warm and moist and the cycle is repeated. This cyclical circulation of air is termed a convection cell. Two convection cells exist in each hemisphere. A polar cell, with air descending at the pole and flowing to about 60o latitude, where it rises, and the tropical cell just described. The horizontal surface component of each cell produces the winds that most directly influence the biosphere. These winds are relatively consistent in the tropics and polar regions, but somewhat variable in the temperate regions between 30o and 60o latitude. The winds, however, do not move directly north or directly south as the orientation of the convention cells would suggest. Influenced by the rotation of the Earth, they curve to the right in the northern hemisphere and to the left in the southern. In the northern hemisphere the surface winds blowing south toward the equator turn to the right, appearing to come from the east; they are called easterlies. Surface winds traveling poleward are also turned to the right, appear to come from the west, and are called westerlies.

For more information on this concept, be sure to focus on:

  • In Section 33.3, Incoming solar radiation shapes climate
  • In Section 33.3, Global movements of air and water shape climate
  • Figure 33.3, Earth Has Four Giant Convection Cells
  • Figure 33.4, Global Patterns of Air Circulation


The biosphere can be divided into several terrestrial and aquatic life zones termed biomes. Seven terrestrial and eight aquatic biomes are commonly recognized. Terrestrial biomes are named for the dominant forms of vegetation within each region. Examples include forests and grasslands.  Rainfall and temperature are the primary factors that influence biome development. When rainfall is abundant a forest can form. Recall that rainfall is high at the equator and at 60o latitude. Forests exist in both locations, but the specific form of forest, tropical rain forest or boreal forest, for example, is determined primarily by average temperature. Because water is relatively thermally stable, aquatic biomes are less influenced by temperature than terrestrial biomes. Thus the annual temperature range for most aquatic biomes is small. Climate, however, still significantly influences most aquatic biomes. As was true of air, the temperature of water directly affects its density. Vertical mixing can occur in lakes and the ocean when surface waters cool and sink. In lakes, such mixing may move oxygen-rich surface water to the bottom and displace nutrient-rich water toward the surface. Marine aquatic biomes may be affected by temperature in additional ways. High temperatures may evaporate water from a marine biome, concentrating salt and increasing salinity while rainfall may dilute saltwater and lower salinity. While the situations described above tend to be localized, physical conditions may sometimes alter a significant portion of an entire ocean basin. Consider the El Niño events that develop periodically off the western coast of South America. Unusually warm ocean water floats over the top of cooler nutrient-rich water, preventing it from reaching the surface. Without the replenishment of nutrients the abundance of marine life declines spectacularly.

For more information on this concept, be sure to focus on:

  • In Section 33.4, The biosphere can be divided into biomes
  • In Section 33.5, Aquatic biomes are influenced by terrestrial biomes and climate
  • In Section 33.5, Aquatic biomes are also influenced by human activity
  • Figure 33.12, El Nino Events

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

The Importance of Ecology

  • Chapter 34 - Section 34.6, Human Population Growth: Surpassing the Limits?
  • Interlude G, The Current Human Impact is Not Sustainable

Interactions with the Environment

  • Chapter 36, Recovery from Disturbances

Climate Has a Large Effect on the Biosphere

  • Chapter 36 – in Section 36.2, Communities change as climate changes


  • Chapter 37 – in Section 37.2, The rate of energy capture varies across the globe

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