Some Animals Are More Equal than Others: Trophic Cascades and Keystone Species Student Handout 7. Before the 1960s, most

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Some Animals Are More Equal Than Others Trophic Cascades And Keystone Species Student Handout 7 Before The 1960s Most 1 (49.33 KiB) Viewed 69 times

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Some Animals Are More Equal than Others: Trophic Cascades and Keystone Species Student Handout 7. Before the 1960s, most ecologists thought that the number of producers in an ecosystem was the only variable that limits the number of herbivores. The idea was that every level was regulated by the amount of food from the trophic level below it. a. How did the green world hypothesis differ from this "bottom-up"* view? b. Imagine a simple food chain: Grass-> Grasshoppers-> Mice. If snakes that eat mice are added to the ecosystem, how would you redraw the food chain to represent this change? c. After the snakes are added, would you expect the amount of grass to increase or decrease? Explain your reasoning. Otter No. (% max. count) gme 0.25m % lose 24hr e No, per 0.25 m *** BRUAR 20 1972 Sea otter abundance -Amchika L N. Adak L Kagalaska I L. Kiskal Sea urchin biomass Grazing Intensity Total kalp density 1985 1989 1993 Figure 1. Since 1972, Dr. Jim Estes had been studying a food chain of kelpurchins->sea otters, and then in the early 1990s orcas began eating the sea otters. The data collected by Dr. Estes are shown. Panel A shows sea otter abundance around four different islands from 1972 to 1997. Panel B shows the amount of sea urchins (sea urchin biomass) in 1987 and 1997. Panel C shows the amount of kelp that sea urchins ate over a 24-hour period (grazing intensity) in 1991 and 1997. Panel D shows the number of kelp plants within a specific area (density of kelp) in 1987 and 1997. The thickness of the arrows illustrates the strength of the effect one species has on the species below it in the food web.
Some Animals Are More Equal than Others: Trophic Cascades and Keystone Species Refer to Figure 1 for questions 8 through 11 below. 8. In 1997, which species is the apex predator in the food chain? a. Killer whales b. Sea otters c. Sea urchins d. Kelp 9. Which of the following statements describes the data in Figure 1?, a. An increase in sea urchin biomass is associated with more intense grazing. b. An increase in sea urchin biomass is associated with greater kelp density. c. Predation of sea otters by killer whales is associated with greater kelp density. Sea otter abundance was relatively stable from 1972 to 1997. d. Student Handout 10. Complete the blanks in the following sentence from the multiple-choice selections below. Figure 1 illustrates that when orcas started eating sea otters, the sea otter population [blank 1], the urchin population [blank 21, and the kelp population [blank 3].. a. decreased, decreased, decreased b. decreased, increased, decreased c. Increased, decreased, increased d. increased, increased, increased 11. The arrows on the left and right sides of Figure 1 show the effects of one species on the species that are on lower trophic levels. Thicker arrows indicate a large effect and thin arrows a smaller effect. The arrows on the left show a system in which there are a lot of sea otters. The arrows on the right show a system in which there are few otters. Explain why the down-pointing arrows on the left side of the figure look different from the arrows on the right side of the figure. In the 1990s, ecologists Deborah Letourneau and Lee Dyer studied a tropical forest shrub called the piper plant and the various species of insects that live on and near the shrub. A species of ant uses the piper plant as a home by hollowing out some of its branches and building colonies inside the hollow branch cores. The ants do not eat the plant's leaves. Instead, the leaves are consumed mostly by caterpillars. When the ants encounter caterpillars or caterpillar eggs on the plant's leaves, they either eat them or kick them off. Letourneau and Dyer added beetles that eat ants. Figure 2 shows the results of one of Letourneau and Dyer's experiments in which they compared the leaf area of piper plants in control plots to that of experimental plots to which they had added beetles that eat ants.