1. Ch. 37.14 - 37.23: Ecosystem Structure and Dynamics

2. 37.14 Energy flow and chemical cycling
In an ecosystem,
energy flow moves through the components of an ecosystem and
Chemical/Matter cycling is the transfer of materials within the ecosystem.
Student Misconceptions and Concerns
 Without an understanding of basic physics and the inefficiency of aerobic metabolism, students might not understand how chemical energy in food is lost as heat. Consider expanding upon the explanations given in the book.
Teaching Tips
 The heat generated as a by-product of metabolism, which is quite evident during strenuous exercise, is much like the heat produced by a running automobile engine. In both circumstances, heat is a by-product of the fuel-burning process.
 Energy flow through an ecosystem is analogous to the flow of fuel through a car or electricity through a vacuum cleaner. These systems will not work without a steady input. NASA, however, must rely upon some closed systems for its spacecrafts. Students might enjoy investigating the recycling of gases and fluids in these systems. 2

3. 37.15 Primary production sets the energy budget
The amount of solar energy converted to chemical energy (in organic compounds) by an ecosystem’s producers for a given area and during a given time period is called primary production.
Ecologists call the amount, or mass, of living organic material in an ecosystem the biomass.
Active Lecture Tips
 Challenge students to work in small groups or pairs to explain why the areas of greatest primary production are near the equator. (Answer: Primary production is a consequence of photosynthesis. Regions near the equator receive the highest levels of solar input.) 3

4. Different ecosystems vary in their primary production and contribution to the total production of the biosphere.
Open ocean
Estuary
Algal beds and coral reefs
Desert and semidesert scrub
Tundra
Temperate grassland
Cultivated land
Boreal forest (taiga)
Figure Net primary production of various ecosystems
Savanna
Temperate deciduous forest
Tropical rain forest
500
1,000
1,500
2,000
2,500
Average net primary production (g/m2/yr)
Data from R.H. Whittaker, Communities and Ecosystems, second edition, New York: Macmillan (1975). 4

5. 37.16 Energy supply limits the length of food chains
A caterpillar represents a primary consumer.
Of the organic compounds a caterpillar ingests, about
50% is eliminated in feces,
35% is used in cellular respiration, and
15% is converted to caterpillar biomass.
Active Lecture Tips
 Ask your students to work in pairs or small groups to explain why food chains and food webs typically have only three to five levels. This question, which is seldom considered by students, is addressed directly in this section of the chapter. It can spark a good opening discussion before a lecture on food chains and food webs. 5

6. 37.16 Energy supply limits the length of food chains
A pyramid of production illustrates the cumulative loss of energy with each transfer in a food chain.
Only about 10% of the energy stored at each trophic level is available to the next level.
Active Lecture Tips
 Ask your students to work in pairs or small groups to explain why food chains and food webs typically have only three to five levels. This question, which is seldom considered by students, is addressed directly in this section of the chapter. It can spark a good opening discussion before a lecture on food chains and food webs. 6

7. 37.16 Energy supply limits the length of food chains
The amount of energy available to top-level consumers is small compared with that available to lower-level consumers.
Only a tiny fraction of the energy stored by photosynthesis flows through a food chain all the way to a tertiary consumer.
This explains why top-level consumers such as lions and hawks require so much geographic territory.
Active Lecture Tips
 Ask your students to work in pairs or small groups to explain why food chains and food webs typically have only three to five levels. This question, which is seldom considered by students, is addressed directly in this section of the chapter. It can spark a good opening discussion before a lecture on food chains and food webs. 7

8. 37.17 CONNECTION: A pyramid of production explains the ecological cost of meat
When humans eat
grain or fruit, we are primary consumers,
beef or other meat from herbivores, we are secondary consumers, and
fish like trout or salmon, we are tertiary or quaternary consumers.
Student Misconceptions and Concerns
 The environmental impact of eating farm animals is little appreciated by most students who otherwise may be concerned about global climate change and the conservation of natural ecosystems. This chapter section helps explain the basis of the increased costs associated with a diet that includes meat.
Teaching Tips
 Some students might be interested in eating more proteins and fewer carbohydrates because of popular diet plans. But do high-protein diets always require the consumption of more meat? The many sources of plant proteins might be surprising to students. Some high-protein vegetarian options are described by the Vegetarian Society at its website, 8

9. Biogeochemical cycles include
37.18 Chemicals are cycled between organic matter and abiotic reservoirs
In addition to energy from the sun, life also depends on the recycling of chemicals.
Biogeochemical cycles include
biotic components,
abiotic components, and
abiotic reservoirs, where chemicals accumulate or are stockpiled outside of living organisms.
Student Misconceptions and Concerns
 Students are unlikely to have any prior knowledge of biogeochemical cycles. Although some transfers between the biotic and abiotic components of ecosystems, such as the use of fertilizer on plants, may be known to them, the broader fact that the biosphere is a self-cycling system is not appreciated by most students. Before you lecture, consider asking your students to explain how carbon, phosphorus, and nitrogen cycle through the atmosphere. Pre-testing your students on their knowledge can confirm both what they understand and what they may need explained to them in more detail.
Teaching Tips
 As you discuss the importance of the biogeochemical cycles, consider explaining the basic label information provided on a container of plant fertilizer. Consider an example that might be used on houseplants, and therefore more likely to be familiar to students. Typically, plant fertilizers contain various forms of nitrogen and phosphorus, which are essential chemicals for growth. 9

10. CO2 in atmosphere Burning Photosynthesis Cellular respiration
Figure 37.19
CO2 in atmosphere 5
Burning 3
Photosynthesis
Cellular respiration 1
Plants, algae,
cyanobacteria
Higher-level
consumers 2
Wood
and fossil
fuels
Primary
consumers
Figure The carbon cycle
Decomposition
Wastes; death
Plant litter;
death 4
Decomposers
(soil microbes)
Detritus 10

11. abiotic reservoir:
CO2 in atmosphere
enter food chain:
photosynthesis = building carbon based molecules
recycle:
decaying organic material
return to abiotic:
respiration
combustion
37.19 The carbon cycle
Increased burning of wood and fossil fuels (coal and petroleum) is raising the level of CO2 in the atmosphere.
Student Misconceptions and Concerns
 Students are unlikely to have any prior knowledge of biogeochemical cycles. Although some transfers between the biotic and abiotic components of ecosystems, such as the use of fertilizer on plants, may be known to them, the broader fact that the biosphere is a self-cycling system is not appreciated by most students. Before you lecture, consider asking your students to explain how carbon, phosphorus, and nitrogen cycle through the atmosphere. Pre-testing your students on their knowledge can confirm both what they understand and what they may need explained to them in more detail.
Teaching Tips
 As rising atmospheric carbon dioxide levels affect global climate, carbon cycling has become an increasingly important issue. If your course will not cover Module 38.5, on global climate change, consider including a brief discussion of the topic here.
 As you discuss the importance of the biogeochemical cycles, consider explaining the basic label information provided on a container of plant fertilizer. Consider an example that might be used on houseplants, and therefore more likely to be familiar to students. Typically, plant fertilizers contain various forms of nitrogen and phosphorus, which are essential chemicals for growth. 11

12. 37.20 The Phosphorus Cycle abiotic reservoir: rocks, minerals, soil
enter food chain:
erosion releases soluble phosphate
uptake by plants
recycle:
decomposing bacteria & fungi
return to abiotic:
loss to ocean sediment
37.20 The Phosphorus Cycle
Student Misconceptions and Concerns
 Students are unlikely to have any prior knowledge of biogeochemical cycles. Although some transfers between the biotic and abiotic components of ecosystems, such as the use of fertilizer on plants, may be known to them, the broader fact that the biosphere is a self-cycling system is not appreciated by most students. Before you lecture, consider asking your students to explain how carbon, phosphorus, and nitrogen cycle through the atmosphere. Pre-testing your students on their knowledge can confirm both what they understand and what they may need explained to them in more detail.
Teaching Tips
 Discussing the movements of water through your local community can help students better understand the concept of biogeochemical cycling. You may want to ask students to consider all of the possible inputs of water into your community as well as the possible routes of exit.
 As noted in Module 37.20, phosphate contamination of aquatic systems typically leads to increased algal growth and potentially disastrous fish kills.
 As you discuss the importance of the biogeochemical cycles, consider explaining the basic label information provided on a container of plant fertilizer. Consider an example that might be used on houseplants, and therefore more likely to be familiar to students. Typically, plant fertilizers contain various forms of nitrogen and phosphorus, which are essential chemicals for growth. 12

13. 37.21 The Nitrogen Cycle abiotic reservoir: N in atmosphere
enter food chain:
nitrogen fixation by soil & aquatic bacteria
recycle:
decomposing & nitrifying bacteria
return to abiotic:
denitrifying bacteria
37.21 The Nitrogen Cycle
Student Misconceptions and Concerns
 Students are unlikely to have any prior knowledge of biogeochemical cycles. Although some transfers between the biotic and abiotic components of ecosystems, such as the use of fertilizer on plants, may be known to them, the broader fact that the biosphere is a self-cycling system is not appreciated by most students. Before you lecture, consider asking your students to explain how carbon, phosphorus, and nitrogen cycle through the atmosphere. Pre-testing your students on their knowledge can confirm both what they understand and what they may need explained to them in more detail.
Teaching Tips
 The nitrogen-fixing bacteria living in the roots of soybeans add nitrogen to the soil. Corn does not enjoy this same relationship with bacteria. However, by rotating corn and soybean crops, farmers can allow corn crops to use some of the nitrogen fixed by the soybean crop in the previous year. Such rotation has other benefits. Since corn is a monocot and soybeans are dicots, few pests attack both corn and soybeans. Thus, crop rotation also helps to control the pest populations that target each type of plant, reducing the need for other pest-fighting strategies. 13

14. 37.22 CONNECTION: A rapid inflow of nutrients degrades aquatic ecosystems
Over time, standing water ecosystems gradually accumulate excess nutrients and primary production increases in a process known as eutrophication.
depletes oxygen levels and
decreases species diversity.
Student Misconceptions and Concerns
 Students are unlikely to have any prior knowledge of biogeochemical cycles. Although some transfers between the biotic and abiotic components of ecosystems, such as the use of fertilizer on plants, may be known to them, the broader fact that the biosphere is a self-cycling system is not appreciated by most students. Before you lecture, consider asking your students to explain how carbon, phosphorus, and nitrogen cycle through the atmosphere. Pre-testing your students on their knowledge can confirm both what they understand and what they may need explained to them in more detail.
Teaching Tips
 The studies of the Hubbard Brook Experimental Forest (described at and in a previous edition of this Concepts textbook) provide an opportunity to explain how basic principles of scientific investigation are applied to ecological studies. Consider discussing the difficulties of conducting these broad experiments in other locations, where water cycling may not be so restricted and other biogeochemical cycles not as well defined. 14

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