Search for the missing sea otters Objectives: Based on a real study of sea otter declines Apply your knowledge of basic concepts of population and community ecology Interpret data from graphs and tables Understand indirect effects in biological communities 1
Part 1 Questions What could be the cause of decreasing otter numbers? List as many causes as you can think of (at least 4) and whether they are density dependent or independent. Briefly describe how you would test one of your ideas. Background Sea otters are members of the kelp forest community 2
Part 1 What could be the cause of decreasing otter numbers? Around 1991, Dr. James Estes and his colleagues at the University of California, Santa Cruz, noticed that the otter populations they had been studying for over 20 years were beginning to shrink. Sea otter populations inhabiting several of the Aleutian Islands had declined as much as 90 percent in fewer than 10 years (Figure 1). What could cause such a sharp drop in sea otter numbers in this island chain of Alaska? Read the following excerpt from an article published in the New York Times and see if you can determine where all of the missing otters had gone. Could the otters simply have migrated from one part of the region to another? To find out, the researchers analyzed populations over a 500-mile-long stretch of the Aleutians from Kiska to Seguam. By 1993 otter numbers in that whole stretch had been cut by half. Here the geographical scope of the research effort became critical; a smaller region would not have been large enough to reveal the decline. In 1997, they found that the population decline had worsened, to about 90 percent. That told us for sure it was a very large-scale decline, but we were still trying to understand the cause, Dr. Estes said. The researchers ruled out reproductive failure. Their studies enabled them to keep track of how often otters gave birth and how many young survived, and this revealed that reproduction was continuing to re-supply the population. With other possible causes eliminated, mortality had to be the explanation. In the past, they had seen temporary declines in otter populations because of starvation, pollution or infectious disease. In all those cases, Dr. Estes said, we find lots of bodies. They get weak and tired and come ashore to die. This time not a single dead otter was found a clue, he said, that something really weird was going on. Part 1 Based on this new information, what is the most likely cause of the otter decline? 3
Part 1 Based on this new information, what is the most likely cause of the otter decline? What predator could be causing the large decrease in otter numbers? Killer whale (Orca) 4
Part 2: more information Dr. Estes and his group hypothesized that increased predation by killer whales was the cause of the sea otter decline. This was an unusual idea, since killer whales and sea otters had been observed together in Alaska for decades with no obvious interactions occurring between them. The first time a killer whale was observed attacking a sea otter was in 1991. Nine more attacks were observed in the next seven years and it was these attacks that finally led Dr. Estes and his colleagues to propose their hypothesis. Your task: Describe an experiment that would allow you to test the hypothesis that increased predation by killer whales was the cause of the sea otter decline. Estes and his colleagues estimated the impact of killer whales on sea otter populations by comparing trends in population size and survival rates of individually marked otters between two adjacent locations on Adak Island Clam Lagoon and Kuluk Bay. Kuluk Bay is on an open coast, so sea otters there are exposed to killer whales. In Clam Lagoon, the entrance from the open sea is too narrow and shallow for killer whales to get in. Based on Figures 3 and 4, what can you conclude about the effects of killer whales on sea otter populations? Figure 3. Changes in sea otter population size over time at Clam Lagoon Figure 4. Survival rates of sea otters individually marked in 1995 with flipper tags and radio transmitters a and Kuluk Bay, Adak Island, Alaska. Redrawn from Estes et al., 1998. Clam Lagoon and Kuluk Bay, Adak Island, Alaska. Redrawn from Estes et al., 1998. 5
Part 3 Why are killer whales eating sea otters now? All the evidence collected by Estes and his coworkers points to killer whales as the cause of the decline in sea otters: the increase in observed killer whale attacks on sea otters, the differences in sea otter survival and population trends at the two locations on Adak Island and an energetics study. Yet prior to the 1990s, killer whales and sea otters had co-existed peacefully for decades. What has caused the killer whales to start eating sea otters at such an alarming rate? Understanding the community Draw a food web using all of the following species: kelp, plankton, sea urchins, herring, ocean perch, pollock, humans, sea otters, orcas, seals and sea lions. Draw arrows to show where each one gets its energy. 6
Understanding the community Using your food web, describe two hypotheses explaining why orcas may be eating otters now. Part 3 Why are killer whales eating sea otters now? Some plausible hypotheses: 1. The species composition of Bering Sea fish has changed dramatically in recent decades. Oily fish species, such as ocean perch (a rockfish) and herring, have declined sharply while the abundance of pollock, a less oily fish, has increased. The causes of these changes in the fish community are not clear. Several plausible hypotheses have been proposed, including overexploitation of perch and herring by humans through the 1960s and the warming of the northern Pacific in the 1970s, which may have made the temperature unsuitable for some species. Human hunting of plankton-feeding baleen whales may even indirectly play a role. Since pollock are also plankton feeders, a reduction in whales may have allowed pollock to proliferate. 2. The decline in oily fish like herring and ocean perch probably was a cause of the subsequent collapse of Stellar sea lion and harbor seal populations in the North Pacific starting in the late 1970s. Although pollock were increasingly available as an alternate food and are eaten by sea lions and seals, they are a less nutritious food and may not provide sufficient nutrition, especially for the growing juveniles. Whatever the cause, North Pacific harbor seal and sea lion populations had declined by as much as 80 percent by 1992. 3. Seals and sea lions are the major food of some killer whale groups. Faced with a shortage of their normal prey, some whales may have switched to the next available thing: sea otters. 7
Interspecific relationships within the community Now make a list of all the species in your food web and put an up or down arrow next to the name to show whether you predict that populations are increasing or decreasing. What effect is the sea otter decline having on the rest of the kelp community? Based on what you know about sea otter biology, what do you think the relationship was between sea urchin and kelp abundances in areas with and without sea otters. Hint: make a list of all the species in your food web and put an up or down arrow next to the name to show whether you predict that populations are increasing or decreasing. 8
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