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16 Ecosystems out of Balance Investigation • 1–2 C L a S S S E S S I O N S

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16 Ecosystems out of Balance Investigation • 1–2 C L a S S S E S S I O N S 16 Ecosystems Out of Balance investigation • 1–2 c l a s s s e s s i o n s Overview For each student In this activity, students interpret population graphs as they Student Sheet 16.1, “Cod-dominated Ecosystem Graph” investigate four fisheries that are altering surrounding eco- Student Sheet 16.2, “Tiger-shark-dominated Ecosystem Graph” systems. Students advise the fisheries on the consequences of Student Sheet 16.3, “Orca-dominated Ecosystem Graph” their fishing and how to make their fishery more sustainable. Student Sheet 16.4, “Caspian-seal-dominated Ecosystem Graph” Key COntent Scoring Guide: ANALYZING DATA (AD) (optional) 1. Relationships between species in an ecosystem are Scoring Guide: UNDERSTANDING CONCEPTS (UC) interdependent. (optional) *Not supplied in kit 2. Changes in the population of a species within an ecosys- Masters for Scoring Guides are in Teacher Resources IV: tem affect other species in that ecosystem. Assessment. 3. Removing a species from an ecosystem can have unin- tended consequences. teaChing suMMary 4. Some fisheries have significantly damaged associated ecosystems. Getting Started 5. Changes in the population dynamics of an ecosystem can • Review the population graphs for an ecosystem with a be a factor in an invasive species becoming established sustainable fishery. within that ecosystem. Doing the Activity 6. Sustainable management of ecosystem services depends • Students formulate hypotheses on the impact of fishing on continual input from scientists and other experts. on four ecosystems. • (AD ASSESSMENT) Students analyze how sustainable fish- Key PrOCess sKills ery practices might alter the effects seen in the fisheries’ 1. Students graph and analyze data. ecosystems. 2. Students interpret data. Follow-up 3. Students identify and describe trends in data. • (UC ASSESSMENT) (AD ASSESSMENT) The class discusses why 4. Students make accurate interpretations, inferences, and sustainable practices are not currently implemented in conclusions from text. all fisheries. Materials and advanCe PreParatiOn For the teacher transparency 16.1, “Pacific Halibut Population” transparency 16.2, “Pacific Halibut Food Web” transparency 16.3, “Pacific-Halibut-dominated Ecosystem Graph” transparency of Scoring Guide: ANALYZING DATA (AD) transparency of Scoring Guide: UNDERSTANDING CONCEPTS (UC) 208 ecosysteMs Out Of balanCe • Activity 16 baCKgrOund infOrMatiOn ecosystems can compensate to a degree if disrupted, but that Scientists have documented many instances of the effect of too much disruption throws the ecosystem out of balance, ecosystem disruption due to species removal. Some ecosys- causing a cascade effect. This can occur due to the loss of one tems have a keystone species, which if removed, critically species, or the reduction of several species. disrupts the balance for the ecosystem. One of the best- Fisheries have been found to have a major impact on many known examples of this is the sea otter and the kelp forests. ecosystems, although the effects are often not immediately Sea otters were hunted so much in the 18th and 19th centu- apparent. For example, if an apex predator, such as a shark, ries that by early in the 20th century they were near extinc- were heavily fished, the species it had preyed on would tion. At this point scientists documented that the numbers of increase due to lack of pressure from predation. As that spe- sea urchins, once prey for the otters, increased dramatically, cies increases, its prey will decrease, and so on. In some consuming kelp forests and leaving rocky barrens devoid of instances the removal of a species (due to overfishing or the vast array of species that had resided there previously. other factors) has led to shifts in ecosystems whereby species This is referred to as a cascade effect, where the removal of are consuming less desirable prey. Stellar sea lions off Alaska, one species causes a number of associated species to be for example, have switched from eating herring to eating affected as a result. The cascade effect is not limited to pollock, which have a lower nutritional value. In other cases removal of a species, but can also occur with the introduc- overfishing has led to complete ecosystem collapse, such as tion of an invasive species or as the result of an ecological off the coast of Namibia in Africa where overfishing of most disturbance. In the case of the sea otter, when they were no fish in the ecosystem is believed to have led to massive jelly- longer hunted, the populations eventually began to recover, fish blooms due to the cascade effects of overfishing. and the kelp forests and associated ecosystems slowly returned as well. It should be noted, however, that in most It is also important to note that ecosystems are in a constant instances the interactions are much more complex. state of flux. Scientists once believed that a “healthy” eco- system exhibited only minor fluctuations unless there was Initial theories approached the cascade effect as either some kind of disturbance. They have now found that many “top down” (when the apex predator[s] of a food chain was populations regularly undergo large fluctuations, and not removed) or “bottom up” (when the producers for an necessarily in response to major disturbances such as vol- ecosystem were removed). As scientific understanding of eco- canic eruptions or floods. system interactions has deepened, it has become clear that 209 sCienCe and glObal issues/biOlOgy • ecolOgy getting started 1 Start by asking students, What do you think population levels would look like if you graphed them for an 1 6 Ecosystems Out of Balance ecosystem that had no pressure from fisheries? RGANISMS WITHIN an ecosystem interact at 1 Oall levels, depending on each other for their Students’ answers should point out survival, either directly for food or shelter, or indi- that while population levels of the rectly through the carbon cycle. As you saw in the previous activity, both native and introduced species species in the ecosystem might fluc- have a signifi cant effect on the ecosystem around them. The entire ecosystem is a complex network tuate a bit, they should remain fairly that can be drastically altered if the population of one constant. Then ask students, based (or more) species goes into a decline or a surge. In this activity you will take on the role of a fi sheries on what they have studied, What do biologist and determine how to make several fi sheries you expect to happen to the graph if more sustainable. You are a fi sheries biologist who has, in the past, an important species within the eco- 2 advised the Pacifi c halibut (Hippoglossus stenolepis) system were overfished? fi shery, which is known for its successful sustainable practices. The Pacifi c halibut fi shery is carefully moni- tored, and each year you and other fi sheries biologists They should point out that the levels make a new set of recommendations on where and how many fi sh can be caught. The limits are adopted by of all populations might change, the entire fi shery. Several other fi sheries have asked for either going up if they were a prey your expert advice. These fi sheries would like you to examine population data and other information for the species, relative to the organism species they fi sh and the species’ ecosystems and, if they being fished, or down if they were a are not sustainable, to advise on how to make them more sustainable. A fisherman weighs the Pacific halibut he caught. predator species. Ask the students, If the fishery were sustainable, would Challenge you expect the same results? � How does information about relationships among organisms help to determine the 3 sustainability of a species and an ecosystem? Students should answer that they would not expect to see as much of an impact, if any. If necessary, remind students that a fishery would be considered sustainable if it allows the ecosystem to remain in 129 a relatively stable state. Project Transparency 16.1, “Pacific Halibut Population.” Discuss the graph, noting the labels on Students should answer yes because the population levels the axes and what is happening to population levels. Next, for all organisms stay fairly steady with slight fluctuations project Transparency 16.2, “Pacific Halibut Food Chain.” relative to each other. Be sure to clear up any misconcep- Review the food chain with the students and have them sug- tions about the slight fluctuations in the population levels. gest the roles of the organisms in the food chain. Now Students should understand that population levels will project Transparency 16.3, “Pacific-halibut-dominated Eco- normally not be a flat line, and some fluctuation is normal. system.” Have students identify which line correlates with Ask the students, What do you notice about the population each organism, and what the different axes of the graph levels for the different organisms? show. Ask the students, Do you think this is a stable, healthy ecosystem? What evidence supports your thinking? 210 ecosysteMs Out Of balanCe • Activity 16 Students should point out that the brittle stars and Pacific halibut SCIENCE & GLOBAL ISSUES/BIOLOGY • ECOLOGY follow the same pattern, and that Pacific halibut are large flatfish that live on the ocean floor in the sablefish do the opposite. Then coastal waters off the western United States and Canada. review the food chain with the stu- dents. Remind them that an upward arrow indicates that the organism above gets energy from (eats) the organism below. Have the students examine the graph at two specific points in time, one from the begin- MATERIALS ning of the graph and one from the FOR EACH STUDENT Student Sheet 16.1, “Cod-dominated Ecosyypstem Graph” end. Ask the students to describe Student Sheet 16.2,,g “Tiger-shark-dominated Ecosystem Graph p” what the data show at each point.
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