Teacher Background Information
NEW! Webinar for K-12 educators: “Sea Otters and People.” Overview of Alaska sea otter ecology and recent research related to human-sea otter conflicts in Kachemak Bay and Southeast Alaska. Presenters: Angela Doroff, Science Director of the Kachemak Bay Research Reserve, and Sunny Rice, Alaska Sea Grant Marine Advisory Agent, Petersburg. Teaching tips on use of the information in teaching Alaska Seas and Watershed units at various grade levels are included. 3/4/2016.
Teacher Background on Ecosystems
An ecosystem is made up of plants, animals (including humans), microbes, and physical environmental features that interact with one another. Ecosystems are dynamic and interconnected, both by the physical environment (e.g., currents transporting larvae from one part of the ecosystem to another) and by biological interactions (e.g., kelps or creating habitat or predators consuming prey).
Ecosystems come in many sizes, often with small ecosystems nesting inside of larger ones. For example, a kelp forest represents a small habitat ecosystem nested within a large ecosystem connected by the Alaska Coastal Current. Large ecosystems contain multiple habitats such as sandy beaches, rocky beaches, kelp forests, or pelagic habitat.
(The Beaufort Sea, the Eastern Bering Sea, the Chukchi Sea, and the Gulf of Alaska are recognized as Large Marine Ecosystems among 10 within the U.S. Exclusive Economic Zone.)
Teacher's Background Information on the Story
The story begins with biologist James Estes being surprised by the results of his sea otter survey of an area in the Aleutian Islands that he has been studying for 15 years. The numbers of otters he sees are far less than in previous surveys. What has happened to the missing otters is a mystery.
Dr. Estes was surprised by the obvious decline in numbers because he had previously studied the sea otters when populations were expanding. Sea otters disappeared from much of their range from the Aleutians to California as a result of the fur trade during the late 18th and the 19th centuries. Several small remnant populations remained, however, in the Aleutians. These populations reproduced rapidly and eventually recolonized all of the available habitat around the islands.
Past Studies of the Ecosystem:
In 1977, scientists Jim Estes and John Palmisano completed an ecosystem study in the Aleutian Islands that demonstrated how ecosystems change over time, and the influence that one "key" or "keystone" species can have in the overall structure of the ecosystem. They compared waters around two islands—Amchitka Island, with abundant sea otters, and Shemya Island, which the otters had not yet reached. In addition to the absence of otters, another obvious difference between the two islands was the absence of kelp beds around Shemya and their presence around Amchitka. The otters were often seen wrapped up in kelp, resting or sleeping. But what else did otters have to do with kelp beds?
The scientists compared the sea otter population with the density and biomass of the kelp beds, and also measured the density of a variety of marine invertebrates inside the kelp bed habitat. They also set out fish traps in the kelp beds. Their data demonstrated that around Amchitka, a high density of sea otters correlated with a high density of kelp beds, low numbers of small sea urchins, and high numbers of kelp greenling, which was the most abundant fish and one that otters preyed on. Conversely, around Shemya, the absence of otters was correlated with sparse kelp beds, high numbers of large and small sea urchins, and low numbers of kelp greenling. This led to the concept of the sea otter as a keystone species whose removal greatly changed the dynamics of the ecosystem, and the roles and influence of other species. In the absence of otters, large numbers of sea urchins could graze the kelp beds and actually topple them by gnawing on their base, turning them into "urchin barrens" of low diversity. With otters present, the kelp beds flourished as the "forests" that provided food and shelter for a diversity of marine invertebrates and fish.
The study of Amchitka and Shemya was repeated in 1986-87 and again in 2000. Otters had still not reached Shemya and the correlations of otters, urchins, and kelp greenling were the same as in 1977. In 2000, Shemya and Amchitka had similar characteristic of few otters—abundant, small urchins and sparse kelp beds.
Part 1 of the Story
In 1992, Jim Estes finds low numbers of otters at Amchitka, where he has studied the otters and kelp forest ecosystem for 15 years. The otter population had been high all through the 1970s and 1980s. His report of the data to the U.S. Fish and Wildlife Service leads to more surveys in 1992 and 2000 of the entire Aleutian Islands.
The 1977 study is described to help students understand how scientists collect data and study ecosystems in Investigation 2.
The data about the declining sea otter populations in the Aleutian Islands trigger a major scientific effort to determine the cause of the decline and changes in the ecosystem related to requirements of the Endangered Species Act. In Investigation 3, the students review the evidence for six hypotheses, the killer whale predation hypothesis being the only likely one.
The Killer Whale Predation Hypothesis
The only way to prove this hypothesis would be to observe it happening or find a dead predator whose last meals could be determined from its stomach contents. Coincidentally, the first killer whale attack on a sea otter was observed and reported in 1991 and nine more were reported in the early1990s. Very few dead killer whales were ever found, although in 2005 the stomach of a dead killer whale in Prince William Sound had remains of four otters along with many other marine mammals, salmon, and even some rocks. But these observations were not sufficient data to reach a conclusion that killer whales were the cause of the sea otter decline. Several biologists who had studied killer whales in Alaska over long periods were skeptical, since they had never observed predation and rarely observed attacks. The estimated number of killer whales within 100 miles of the Aleutian Islands at that time was 4000. Of those, 90% were estimated to be in resident groups that stayed in fairly predictable areas and ate only fish, and the remaining 10% were individual whales that traveled widely, joined transient groups, and fed on marine mammals. The scientists reasoned that sea otters would make a very small and very furry meal for a large killer whale, and hardly worth the trouble of catching it. The University of Washington’s Jim Paine declared otters to be like "dental floss" to the large toothed whales.
A study of radio-collared otters provided an opportunity to test the hypothesis. In 1993, Jim Estes put radio collars on otters in two different groups that lived in different types of areas, and flew surveys over five years to count the number of animals in each group. One group stayed in Clam Lagoon, which had a narrow mouth and was inaccessible to killer whales. The other group ranged over Kuluk Bay, which was open and accessible to the whales. Over five years, the reproductive rates were the same—the same percentage of pups were being born and surviving—but the loss of animals from the area accessible to killer whales was much higher than in the protected lagoon. The study continued and the patterns held, but in1998 the otters left Clam Lagoon possibly to find more food.
Another type of evidence is related to the mathematical model for how killer whale predation could have reduced the sea otter population at the observed rate. Jim Estes worked with colleagues Terrie Williams and others to figure out the math. They asked: How many sea otters would an adult killer whale have to eat to get all the calories it needed in a single day? In a year? Using information on the caloric needs of captive animals for the killer whale predator needs, and using "caloric bomb" equipment, which could reduce a whole sea otter into its constituent parts and translate its chemical composition into calories that each otter prey could supply, they calculated that a group of five killer whales (four females and one male) feeding exclusively on sea otters could have been responsible for the observed loss of 40,000 sea otters over six years.
The Theory of a Trophic Cascade
A more general theory emerged from Estes and several other scientists, based on thinking about the larger oceanic ecosystem of which the killer whale, otters, and kelp bed communities were a part. Industrial whaling in the 1800s and early 1900s had killed thousands of large whales. Had they been a major target of the killer whales? Had the whalers disposed of the remains of the whales and provided the killer whales with a source as scavengers? In the absence of the whales and scavenging opportunities, had the killer whales switched to feed on smaller marine mammals like sea lions and seals? Scientists had documented a major decline in sea lions and seals in western and southwestern Alaska. Were the sea otters the next on the list as the killer whales ate their way down the food chain? If so, it would be a larger scale example of the effect of removing species from the food web and triggering a cascade of trophic effects related to who ate whom (additional information 1). Other scientists point out the lack of data to support this hypothesis and the impossibility of testing the hypothesis about relationships that happened in the past (additional information 2).
Has the mystery been solved? Status of the research in 2007
Alaska’s marine wildlife populations are often difficult to study, particularly those in more remote areas like the Aleutian Islands where the cost of research is high and extreme weather conditions are common. The "sea otter story" was chosen for this unit because of its focus on changing ecological relationships and the clear science story it provided. The data collected by scientists demonstrated clear trends.
It’s very important to distinguish between theories, testable hypotheses, and conclusions while teaching this unit. In the case of sea otter declines, if killer whale predation is considered the answer to the mystery, other factors that may have contributed to declines in several marine mammal populations in southwestern and western Alaska might no longer be considered or addressed. Other factors to consider could include changes in ocean conditions related to climate change or overfishing. The mathematical calculations about the number of killer whales that could have been responsible for the observed decline paint a plausible scenario, but they do not prove that this actually occurred. Killer whale predation is instead considered a theory based on a "weight of evidence" approach. Scientists and wildlife managers are continuing to study other factors that may also be responsible and important to understand in order to devise ways to avoid contributing to further declines or to help the sea otter populations recover (additional information 3).
What about Other Areas of Alaska and Other Places in the Range of Sea Otters?
Following survey efforts, the sea otter decline first detected in the Aleutians has been determined to be widespread through the entire Aleutian Islands chain and the waters of the Alaska Peninsula and Kodiak. This southwestern population was declared a Threatened Species in 2005. In 2005, the U.S. Fish and Wildlife Service trained volunteers in Kachemak Bay to report all dead sea otters found washed up on beaches and to send carcasses or skulls to them for study. So many were found that an "unusual mortality event" was declared in 2006. The major causes of death have been disease and boat strikes (DeVaughn 2006).
Sea otter populations are stable or increasing in other areas of Alaska. Although sea otters prefer to eat sea urchins when they are available, they eat a wide variety of other animals, including chitons, mussels, clams, crabs, fish, and octopus. Their habitat is not limited to kelp beds. The sea otter-kelp-urchin relationship was first studied and documented in California as well as the Aleutians. The large kelps that form the "forest" habitat require rocky bottoms for attachment of their holdfasts. Sea otters are also found in large numbers in areas with "soft" bottoms that are a combination of mud, sand, and smaller rocks. Here, they feed primarily on clams and mussels.
No otters were present east of Prince William Sound or in Southeast Alaska in the early 1900s until they were reintroduced in Southeast Alaska. Scientists have been studying their expansion into Glacier Bay and are finding similar relationships among the otters, kelp stands, and urchins along with use of other habitats and a variety of food sources.
The sea otter was one of the many species harmed by the Exxon Valdez oil spill in 1989. Nearly 1000 otters were found dead in the immediate aftermath of the spill, and scientists estimate that as many as 3900 died. An otter's fur, when oiled, loses its insulating properties; most perished from hypothermia or oil toxicity. Rehabilitation was a difficult process that involved capture, cleaning, and a period of recovery in captivity. Although 300 oiled otters were caught shortly after the spill and were taken for rehabilitation, and 200 were released into the wild, their survival was low. In 2007, sea otters remained on the list of species that had not recovered from the oil spill. Urchin populations did not eliminate the kelp stands in Prince William Sound following the reduction in otter numbers.
Sea otters had also disappeared from California after the fur trade, but were reintroduced by animals transplanted from Alaska populations. They have recently been declining. The major cause of death that has been documented is a disease that is spread from cats through the dumping of cat litter that finds its way into the marine ecosystem.
Killer whale predation as the cause of the sea otter decline, and the trophic cascade in the ocean ecosystem remain theories. Because they depend on finding evidence of events that occurred in the past, those theories will likely never be proved or disproved. In the case of the Southwest Alaska sea otter population, which was declared Threatened under the Endangered Species Act in 1972, scientists are continuing to look for evidence of other causes of the decline that can be addressed to aid in the recovery of the sea otter populations. Killer whale predation remains the best hypothesis.
Scientists predicted that declining otter numbers would result in a shift from thriving kelp beds to urchin barrens. In 2000, another study of kelp beds, urchins, and kelp greenling was conducted at four islands. The conclusion of the study stated: "The abundances of urchins, kelp and greenling were unchanged at islands where otters were initially rare but had shifted to the characteristic pattern of otter-free systems at islands where otters were initially abundant."
The theory about an ecosystem trophic cascade initiated by industrial-scale whaling in the 1900s was published in the scientific literature in1998. James Estes and several other biologists published a paper (Williams et al. 2004) that set out the hypothesis that an ecological chain reaction dating to industrial-scale hunting of whales in the North Pacific has driven the widespread decline of Alaska seals, sea lions and otters that had puzzled scientists for decades. The killing of whales caused a collapse in the food chain. As a half-million whales were wiped out by Japanese and Russian whaling fleets after World War II, killer whales that once preyed on the larger "great" whales had to look for other food to eat. So, the scientists theorized, some of the killer whales turned to seals instead. But before whaling, seals were never as numerous as whales. And it takes lots of seals to equal the calories in a single great whale. It wasn't long before most of the seals were eaten and the killer whales—also known as orcas—turned their attention to Steller sea lions. Then, when those grew rare enough, they went after otters. "If our hypothesis is correct, either wholly or in significant part, commercial whaling in the North Pacific Ocean set off one of the longest and most complex ecological chain reactions ever described," says the paper being published online in the Proceedings of the National Academy of Sciences (McClure 2003).
The historical trophic cascade theory has been subject to debate among marine mammal biologists. Marine mammal researchers Craig Matkin and Doug DeMaster have pointed out that there is no evidence that killer whales ate the large whales historically and then switched to sea otters. They point out the small amount of nutrition, and large amount of hair, that a single sea otter meal would provide a killer whale. DeMaster also points out that rarely is one factor responsible. As described in the 2002 U.S. Fish and Wildlife Service report, the evidence of recent killer whale predation on otters is based on a small number of dead killer whales and sea otters found on land and a small number of observations of attacks. Craig Matkin, in his many hours observing killer whales, has seen attacks, but never the consumption of the otter by the whale. His recent study (2005-2006) of humpback whale migration through Unimak Pass in the Aleutians, however, provides evidence that transient killer whales concentrate there in large numbers and are likely targeting the large whales as prey. (web refs) (web ref - NPR interview)
In April 2002, the U.S. Fish and Wildlife convened a panel of expert scientists to review all of the existing scientific information. The report of that meeting said that "although there are questions about the overall magnitude and geographic extent of the decline, the trend is clear: the sea otter population has undergone a dramatic population decline throughout most of southwest Alaska in the past 10-15 years. Within this area, the most intensively studied region is also the most remote. Research during the 1990s at several islands in the western and central Aleutian Islands indicated that the decline is the result of increased adult mortality. There was no evidence that starvation, disease, contaminants, human harvest, or commercial fishing activities were responsible for the decline. A significant increase in observed attacks by killer whales, a stable sea otter population in the protected waters of Clam Lagoon on Adak Island, and relatively few beach-cast carcasses suggested that predation by killer whales may be the cause of the decline in the Aleutian Islands."
It is likely that we will never be able to identify the cause of the current sea otter decline with certainty. The current theory of killer whale predation is based on a "weight of evidence" approach. Evidence that supports this theory includes:
- A significant increase in the rate of observed killer whale attacks on sea otters;
- Stable sea otter numbers in a protected area (Clam Lagoon on Adak) as compared to exposed areas; and
- Lack of evidence for other causes (body condition, food availability, scarcity of carcasses) (U.S. Fish and Wildlife Service 2003).
DeVaughn, M. 2006. Why are the sea otters dying at Kachemak Bay? Anchorage Daily News, Anchorage, Alaska, September 27, 2006.
McClure, R. 2003. Researchers say whaling altered the food chain. Seattle Post-Intelligencer, Seattle, Washington, September 25, 2003.
U.S. Fish and Wildlife Service. 2003. Southwest Alaska sea otter decline workshop: Summary report. U.S. Fish and Wildlife Service, Anchorage, Alaska. 17 pp.
Williams, T.M., J.A. Estes, D.F. Doak, and A.M. Springer. 2004. Killer appetites: Assessing the role of predators in ecological communities. Ecology 85(12):3373-3383.