Chapter 3 Review

chapter 3Review

Key Terms

Learning Objectives Revisited

• Explain the concept of ecosystem boundaries.

  Ecosystem boundaries distinguish one ecosystem from another. Although boundaries can be well-defined, often they are not. Boundaries are commonly defined either by topographic features, such as mountain ranges, or are subjectively set by administrative criteria rather than biological criteria.

  98

• Describe the processes of photosynthesis and respiration.

  Photosynthesis captures the energy of the Sun to convert CO₂ and water into carbohydrates. Respiration, whether aerobic or anaerobic, unlocks the chemical energy stored in the cells of organisms.

• Distinguish among the trophic levels that exist in food chains and food webs.

  The trophic levels consist of producers that convert solar energy into producer biomass through photosynthesis, primary consumers that eat the producers, secondary consumers that eat the primary consumers, and tertiary consumers that eat the secondary consumers. Omnivores eat individuals from more than one trophic group. Trophic groups that eat waste products and dead organisms are scavengers, detritivores, and decomposers.

• Quantify ecosystem productivity.

  Ecosystem productivity can be quantified by measuring the total amount of solar energy that producers capture, which is gross primary productivity, or by measuring the total amount of solar energy captured minus the amount of energy used for respiration, which is net primary productivity.

• Explain energy transfer efficiency and trophic pyramids.

  The energy present in one trophic level can be transferred to a higher trophic level and the efficiency of this transfer is generally about 10 percent. Because of this low energy transfer efficiency, the amount of energy present in each trophic level declines as we move to higher trophic levels. We can represent the energy in each trophic level as a rectangular block in a pyramid, with a size that is proportional to the energy found in the trophic level. Low ecological efficiency results in a large biomass of producers, but a much lower biomass of primary consumers and an even lower biomass of secondary consumers.

• Describe how water cycles within ecosystems.

  In the water cycle, water evaporates from water bodies and transpires from plants. The resulting water vapor cools and forms clouds, which ultimately drop water back to Earth in the form of precipitation. When the water falls onto the land, it can evaporate, be taken up by plants and transpired, percolate into the groundwater, or run off along the soil surface and ultimately return to lakes and oceans.

• Explain how carbon cycles within ecosystems.

  In the carbon cycle, producers take up CO₂ for photosynthesis and transfer the carbon to consumers and decomposers. Some of this carbon is converted back into CO₂ by respiration, while the rest is lost to sedimentation and burial. The extraction and combustion of fossil fuels, as well as the destruction of forests, returns CO₂ to the atmosphere.

• Describe how nitrogen cycles within ecosystems.

  In the nitrogen cycle, nitrogen fixation by organisms, lightning, or human activities converts nitrogen gas into ammonium (NH₄⁺) or nitrate (NO₃^–). Nitrification is a process that converts ammonium into nitrite (NO₂^–) and then into nitrate (NO₃^–). Once producers take up nitrogen as ammonia, ammonium, nitrite, or nitrate, they incorporate it into their tissues in a process called assimilation. Eventually, organisms die and their tissues decompose and are converted to ammonium in a process called mineralization. Finally, denitrification returns nitrogen to the atmosphere.

• Explain how phosphorus cycles within ecosystems.

  The phosphorus cycle involves a large pool of phosphorus in rock that is formed by the precipitation of phosphate onto the ocean floor. Geologic forces can lift these sediments and form mountains. The phosphorus in the mountains can be made available to producers either by weathering or by mining. Producers assimilate phosphorus from the soil or water and consumers assimilate it when they eat producers. The waste products and dead bodies of organisms experience mineralization, which returns phosphorus to the environment where it can be ultimately transferred back to the ocean.

• Discuss the movement of calcium, magnesium, potassium, and sulfur within ecosystems.

  Calcium, magnesium, and potassium are derived from rock and can be held by soils. Producers can assimilate these elements, and mineralization of waste products and dead organisms returns the elements back to the environment. Most sulfur exists in the form of rocks and is released through the process of weathering, which makes it available for plant assimilation. Some sulfur exists as a gas in the form of sulfur dioxide (SO₂), which can be produced by volcanic eruptions and the burning of fossil fuels. In the atmosphere, SO₂ is converted into sulfuric acid (H₂SO₄) when it mixes with water. The sulfuric acid can then be carried back to the ground when it rains or snows.

• Distinguish between ecosystem resistance and ecosystem resilience.

  The resistance of an ecosystem is a measure of how much a disturbance can affect its flows of energy and matter. In contrast, the resilience of an ecosystem is the rate at which an ecosystem returns to its original state after a disturbance has occurred.

  99

• Explain the insights gained from watershed studies.

  Because watersheds contain all of the land in a given landscape that drains into a particular water body, experimental manipulations such as logging allow scientists to determine how a disturbance to an ecosystem alters the flow of energy and matter.

• Explain the intermediate disturbance hypothesis.

  The intermediate disturbance hypothesis states that ecosystems experiencing intermediate levels of disturbance are more diverse than those with high or low disturbance levels.