Chapter 20 | Populations, Communities, and Ecosystems

  1. Figure 20.12 How might turnover in tropical lakes differ from turnover in lakes that exist in temperate regions?
    The illustration shows a cross-section of a lake in four different seasons. In winter, the surface of the lake is frozen with a temperature of 0 degrees upper case C. The temperature at the bottom of the lake is 4 degrees upper case C, and the temperature just beneath the surface is 2 degrees C. During the spring turnover, the surface ice melts and warms to 4 degrees C. At this temperature, the surface water is denser than the 2 degree C water beneath; therefore, it sinks. In summertime, the surface of the lake is 21 degrees C, and the temperature decreases with depth, to 4 degrees C at the bottom. During the fall turnover, the warm surface water cools to about 10 degrees C; thus, it becomes denser and sinks.
    Figure 20.12 The spring and fall turnovers are important processes in freshwater lakes that act to move the nutrients and oxygen at the bottom of deep lakes to the top. Turnover occurs because water has a maximum density at 4 °C. Surface water temperature changes as the seasons progress, and denser water sinks.
  2. Figure 20.15b If the major food source of the seals declines due to pollution or overfishing, which of the following would likely occur?
    1. The carrying capacity of seals would decrease, as would the seal population.
    2. The carrying capacity of seals would decrease, but the seal population would remain the same.
    3. The number of seal deaths would increase but the number of births would also increase, so the population size would remain the same.
    4. The carrying capacity of seals would remain the same, but the population of seals would decrease.
      Graph (a) plots amount of yeast versus time of growth in hours. The curve rises steeply, and then plateaus at the carrying capacity. Data points tightly follow the curve. Graph (b) plots the number of harbor seals versus time in years. Again, the curve rises steeply then plateaus at the carrying capacity, but this time there is much more scatter in the data. A micrograph of yeast cells, which are oval in shape, and a photo of a harbor seal are shown.
      Figure 20.15 (a) Yeast grown in ideal conditions in a test tube show a classical S-shaped logistic growth curve, whereas (b) a natural population of seals shows real-world fluctuation.
  3. Figure 20.18 Age structure diagrams for rapidly growing, slow growing and stable populations are shown in stages 1 through 3. What type of population change do you think stage 4 represents?
    For the four different age structure diagrams shown, the base represents birth and the apex occurs around age 70. The age structure diagram for stage 1, rapid growth, is shaped like a deflated triangle that starts out wide at the base and rapidly decreases to a narrow apex, indicating that the number of individuals decreases rapidly with age. The age structure diagram for stage 2, slow growth, is triangular in shape, indicating that the number of individuals decreases steadily with age. The age structure diagram for stage 3, stable growth, is rounded at the top, indicating that the number of individuals per age group decreases gradually at first, then increases for the older portion of the population. The final age structure diagram, stage 4, widens from the base to middle age, and then narrows to a rounded top. The population type indicated by this diagram is not given, as this is part of the Visual Connection question.
    Figure 20.18 Typical age structure diagrams are shown. The rapid growth diagram narrows to a point, indicating that the number of individuals decreases rapidly with age. In the slow growth model, the number of individuals decreases steadily with age. Stable population diagrams are rounded on the top, showing that the number of individuals per age group decreases gradually, and then increases for the older part of the population.
  4. Figure 20.42 Why do you think the value for gross productivity of the primary producers is the same as the value for total heat and respiration (20,810 kcal/m2/yr)?
    Flow chart shows that the ecosystem absorbs 1,700,00 calories per meter squared per year of sunlight. Primary producers have a gross productivity of 20,810 calories per meter squared per year. 13,187 calories per meter squared per year is lost to respiration and heat, so the net productivity of primary producers is 7,623 calories per meter squared per year. 4,250 calories per meter squared per year is passed on to decomposers, and the remaining 3,373 calories per meter squared per year is passed on to primary consumers. Thus, the gross productivity of primary consumers is 3,373 calories per meter squared per year. 2,270 calories per meter squared per year is lost to heat and respiration, resulting in a net productivity for primary consumers of 1,103 calories per meter squared per year. 720 calories per meter squared per year is lost to decomposers, and 383 calories per meter squared per year becomes the gross productivity of secondary consumers. 272 calories per meter squared per year is lost to heat and respiration, so the net productivity for secondary consumers is 111 calories per meter squared per year. 90 calories per meter squared per year is lost to decomposers, and the remaining 21 calories per meter squared per year becomes the gross productivity of tertiary consumers. Sixteen calories per meter squared per year is lost to respiration and heat, so the net productivity of tertiary consumers is 5 calories per meter squared per year. All this energy is lost to decomposers. In total, decomposers use 5,065 calories per meter squared per year of energy, and 20,810 calories per meter squared per year is lost to respiration and heat.
    Figure 20.42 This conceptual model shows the flow of energy through a spring ecosystem in Silver Springs, Florida. Notice that the energy decreases with each increase in trophic level.

     

     

  5. Figure 20.44 Pyramids depicting the number of organisms or biomass may be inverted, upright, or even diamond-shaped. Energy pyramids, however, are always upright. Why?
    Section A, biomass, indicated by dry mass g slash m squared. On the left is a pyramid diagram of dry biomass in grams per meter squared in Silver Springs, Florida. The biomass of plants is 809. The biomass of primary consumers, including herbivorous insects and snails is 37. The biomass of secondary consumer fishes is 11, and the biomass of tertiary consumer fishes is 5. Primary, secondary and tertiary decomposers have a combined biomass of 5. On the right is a pyramid diagram of dry biomass in grams per meter squared in the English Channel. The biomass is 4 phytoplankton and 21 zooplankto. Section B, number of individuals per 0.1 hectare. On the left is a pyramid diagram of the number of individuals per 0.1 hectare in a summer grassland. There are 1,500,000 grass plants, 200,000 herbivorous insects, 90,000 predatory insects, and 1 bird On the right is a pyramid diagram of organisms per 0.1 hectare in a temperate forest. There are 200 trees, 150,000 herbivorous insects, 120,000 predatory insects, and 5 birds. Section C, energy, k cal slash m squared slash year. In Silver Springs Florida, the energy of plants is 20,810. The energy of primary consumers, including insects and snails, is 3,368. The energy of primary consumer fishes is 383, and the energy of secondary consumer fishes is 21. The energy of decomposers, including fungi and bacteria, is 5,060.
    Figure 20.44 Ecological pyramids depict the (a) biomass, (b) number of organisms, and (c) energy in each trophic level.
  6. Figure 20.51 Which of the following statements about the nitrogen cycle is false?
    1. Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH4+).
    2. Denitrification by bacteria converts nitrates (NO3−) to nitrogen gas (N2).
    3. Nitrification by bacteria converts nitrates (NO3−) to nitrates (NO2−)
    4. Nitrogen fixing bacteria convert nitrogen gas (N2) into organic compounds.
      This illustration shows the nitrogen cycle. Nitrogen gas from the atmosphere is fixed into organic nitrogen by nitrogen-fixing bacteria. This organic nitrogen enters terrestrial food webs, and it leaves the food webs as nitrogenous wastes in the soil. Ammonification of this nitrogenous waste by bacteria and fungi in the soil converts the organic nitrogen to ammonium ion, or upper N upper H 4 plus. Ammonium is converted to nitrite, or upper N upper O 2 minus, then to nitrate, or upper N upper O 3 minus by nitrifying bacteria. Denitrifying bacteria convert the nitrate back into nitrogen gas, which re-enters the atmosphere. Nitrogen from runoff and fertilizers enters the ocean, where it enters marine food webs. Some organic nitrogen falls to the ocean floor as sediment. Other organic nitrogen in the ocean is converted to nitrite and nitrate ions, which is then converted to nitrogen gas in a process analogous to the one that occurs on land.
      Figure 20.51 Nitrogen enters the living world from the atmosphere via nitrogen-fixing bacteria. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need. (credit: modification of work by John M. Evans and Howard Perlman, USGS)

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