Chapter 28

Summary		Questions		Media Resources
28.1 Chemicals cycle within ecosystems.
•  Fully 98% of the water on earth cycles through the atmosphere. In the United States, 96% of the fresh water is groundwater. •  About 10% of the roughly 700 billion metric tons of free carbon dioxide in the atmosphere is fixed each year through photosynthesis. About as much carbon exists in living organisms at any one time as is present in the atmosphere. •  Carbon, nitrogen, and oxygen have gaseous or liquid reservoirs, as does water. All of the other nutrients, such as phosphorus, are contained in solid mineral reservoirs. •  Phosphorus is a key component of many biological molecules; it weathers out of soils and is transported to the world's oceans.		1.  What are the primary reservoirs for the chemicals in biogeochemical cycles? Are more of the life-sustaining chemicals found in these reservoirs or in the earth's living organisms? 2.  What is denitrification? Which organisms carry it out? 3.  How is the phosphorus cycle different from the water, carbon, nitrogen, and oxygen cycles? What are the natural sources for phosphorus? 4.  What effect does deforestation have on the water cycle and overall fertility of the land?		•  Carbon Cycle •  Introduction •  Nutrient Cycle •  Concept Quiz •  Water Cycle •  Groundwater •  Water Quality •  Fossil Fuels •  Carbon Cycle •  Water Cycle •  Carbon Cycle
28.2 Ecosystems are structured by who eats whom.
•  Plants convert about 1 to 5% of the light energy that falls on their leaves to food energy. Producers, the herbivores that eat them, and the carnivores that eat the herbivores constitute three trophic levels. •  At each level, only about 10% of the energy available in the food is fixed in the body of the consumer. For this reason, food chains are always relatively short.		5.  How might an increase in the number of predators affect lower levels of a food chain. How might an increase in nutrients affect upper levels?		•  Nitrogen Cycle •  Nutrient Cycle •  Nitrogen Cycle •  Nitrogen Cycle
28.3 Energy flows through ecosystems.
•  The primary productivity of a community is a measure of the biomass photosynthesis produces within it. •  As energy passes through the trophic levels of an ecosystem, much is lost at each step. Ecological pyramids reflect this energy loss.		6.  What is the difference between primary productivity, gross primary productivity, and net primary productivity? 7.  Which type of diet, carnivorous or herbivorous, provides more food value to any given living organism?		•  Energy Flow •  Trophic Levels •  Path of Energy Through Food Webs •  Trophic Levels within a Food Chain
28.4 Biodiversity promotes ecosystem stability.
•  Increasing the number of species in a community seems to promote ecosystem productivity. Controversial experiments suggest that communities with increased species richness are more stable and less vulnerable to disturbance.		8.  Why might rain forests have high levels of species diversity? 9.  Why do distant islands tend to have fewer species than nearer islands of the same size? Why do different-sized islands tend to differ in species number?		•  Exponential Population Growth   •  Food Web in Cayoga Lake •  Island Biogeography—Equilibrium Model

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