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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 37. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. How did New York City decide to correct its problem with poor water quality?
  2. Why are hawks and owls so much less common than the rodents they eat?
  3. How is agriculture in the Midwest related to the Dead Zone in the Gulf of Mexico?
  4. If energy can neither be made nor destroyed, why can’t it be recycled in an ecosystem?
  5. What two factors best predict the level of net primary productivity for any region?
  6. In what way does the cycling of phosphorus differ from all the other major nutrients?
  7. What pollutant is responsible for most acid rain?
  8. How successful were the scientists that built Biosphere II in designing an artificial ecosystem?

A Guide to the Reading

When exploring the content in Chapter 37 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.


An ecosystem consists of a community of organisms along with the physical environment in which those organisms live. Two primary activities characterize an ecosystem, the unidirectional flow of energy and the recycling of nutrients. Ecosystems often have no physical boundaries, and vary in size from areas as small as mud puddles to entire oceans. With no specific size requirement and no discernable boundaries, it might seem impossible to identify an ecosystem. Rather than using physical features, ecologists delineate ecosystems using functional criteria. All the organisms that utilize the energy and nutrients from a single set of producers belong to the ecosystem. Organisms obtaining their energy and nutrients elsewhere belong to a different ecosystem. Of course, such a simple explanation is rarely suited to the complexities of nature. Many organisms are highly mobile (the Arctic tern, for example, flies 22,000 miles annually) and frequently move between ecosystems.

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

  • Section 37.1, How Ecosystems Function: An Overview
  • Figure 37.1, How Ecosystems Work

Energy Capture in Ecosystems

All living organisms require energy. Consumers obtain energy by digesting the complex chemical compounds they obtain from their food. Producers, however, collect their energy from non-living sources, primarily the sun. Producers use solar energy, carbon dioxide, and water (Chapter 8)  to photosynthesize the energy-rich compounds that will eventually nourish their bodies, primary consumers directly, and all other consumers indirectly. Because photosynthesis is the essential first step in ecosystem function, ecologists find it useful to characterize different ecosystems by the amount of photosynthesis that takes place within them. Total photosynthesis, however, would be a misleading measurement, since a portion of the captured energy is used by the producer. Subtracting this quantity from total photosynthesis gives a value called net primary productivity (NPP). It is the standard measure of ecosystem productivity, and has been used worldwide to compare one ecosystem with another. Patterns of NPP are predictably variable. For example, since the intensity of sunlight varies with the latitudes, you might predict high NPP near the equator, with declines as latitude increases. Of course, photosynthesis also requires water. You learned in Chapter 33 that the Earth’s atmospheric convection cells produce complex patterns of rainfall. Areas receiving ample sunlight and abundant rain, like the equator, do experience high NPP. Areas with abundant sunlight and little rain have a low NPP. The NPP in aquatic biomes is less predictable.  Aquatic organisms are able to use much lower light intensities than terrestrial plants since light penetrates the water column poorly. Productivity in aquatic biomes is often more dependent on the availability of dissolved nutrients. Rivers and streams typically deliver these, creating highly productive wetlands such as estuaries and marshes where the nutrients begin to accumulate. Human activity can significantly increase or decrease NPP. Nutrient enrichment from agricultural runoff or sewage outflows can produce almost unimaginable increases in productivity in aquatic ecosystems. Growth can be so abundant that the water becomes opaque from the organisms it contains. Recall that exponential growth seldom lasts for long. These organisms die rapidly, followed by bacterial decomposition that can entirely deplete the dissolved oxygen within the area. Oxygen depletion has now become an apparently permanent feature of the Gulf of Mexico. Such massive disruptions to ecosystem function are almost always detrimental.

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

  • Section 37.2, Energy Capture in Ecosystems
  • Figure 37.5, The Dead Zone

Nutrient Cycles

Ecologists use the term nutrient differently than most people. To an ecologist, nutrients are the various atoms and molecules needed to build the body of an organism. Nutrients may or may not provide energy. Thus, the most important nutrients tend to be simple chemicals like CO2, NO2-, and K+. Producers obtain their nutrients from the soil, water, or air. Consumers must eat either producers or other consumers. The transfer of a nutrient between organisms and the physical environment is termed a nutrient cycle. Nutrients may be difficult for producers to obtain, and shortages often limit the productivity of an ecosystem. The time required for a nutrient to cycle between a producer, the physical environment, and another producer depends on whether the nutrient cycles in an atmospheric cycle or a sedimentary cycle. In atmospheric cycling, the nutrient is a gas. It moves freely as the atmosphere circulates and tends to be available to almost all ecosystems. Nutrients that cycle in the atmosphere are rarely limiting. In the sedimentary cycle, soluble nutrients are transported by river to the sea, where they eventually become incorporated into marine sediments and become sedimentary rock. Over millions of years, these sediments may be uplifted, exposed to weathering, and eroded, allowing the nutrients to reenter the biosphere. Because this time interval is so long, these nutrients have the potential to become limiting factors. Phosphorus is the only major nutrient with a sedimentary cycle. Human activity commonly disrupts nutrient cycling, leading to surplus nutrients in some ecosystems and depletions in others. One of the earliest studies into nutrient cycling demonstrated that forest clear-cutting significantly accelerates the rate of nitrogen loss from forest soils. The practice of wastewater treatment often adds phosphorus to lakes and rivers, contributing to eutrophication and a deterioration of water quality.

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

  • Section 37.4, Nutrient Cycles
  • Figure 37.9, The Sulfur Cycle
  • Figure 37.10, The Phosphorus Cycle
  • Figure 37.11, Altering Nutrient Cycles in a Forest Ecosystem

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:


  • Chapter 37 – Section 35.2, Exploitation

Energy Capture in Ecosystems

  • Chapter 8 – Section 8.2, Photosynthesis: Capturing Energy from Sunlight

Nutrient Cycles

  • Chapter 31 – Biology on the Job, ‘Growing Plants without Soil
  • Chapter 34 – in Section 34.3, Growth is limited by essential resources and other environmental factors
  • Chapter 38 – Section 38.3, Changes in the Global Nitrogen Cycle
  • Chapter 38 – Section 38.4, Changes in the Global Carbon Cycle

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