Introduction

National parks along Alaska’s coastlines are some of the most remote, undeveloped, and wild places on the planet. It is here, in Katmai National Park and Preserve (Figure 1), that we can see the natural world and how it changes over time and across ecosystems. We can ask questions about how many animals live here, what they eat, and how the populations are changing. Two species, brown bears and sea otters, are used as vital signs to help us monitor the overall health of the park. If we think of vital signs just like we do for our bodies—for example, a strong, regular heartbeat is one indication of good health—we can use bears and sea otters as vital signs to check on the health of Katmai. You can think of the number of animals in the park like the park’s heartbeat—when the park’s heartbeat is strong, the park has just the right number of bears and sea otters, but if the number of bears or sea otters decreases, it might be a sign that the park is not well. So, how many brown bears and sea otters can live at Katmai? In ecology, this concept is called carrying capacity, which means the number of individuals of a species an ecosystem can support. This article will describe Katmai’s carrying capacity for both brown bears and sea otters, how and why it has changed over time, and how these changes might continue to occur [1]. All this information helps us to understand the overall health of the ecosystems in Katmai.

Bears and Sea Otters

Surprisingly, the habitats of bears and sea otters overlap more than you might think. Bears eat some of the same foods sea otters eat, like clams! But bears and sea otters also use different spaces: bears are primarily on land (terrestrial), and sea otters are primarily at sea (marine). Studying them both can tell us about each habitat as well as how the habitats are linked, so we get a better sense of the health of the park’s different ecosystems.

Brown bears are the largest terrestrial mammals in North America, with some bears weighing more than 1,000 pounds! After more than 300 years of over-hunting and habitat loss, the range of brown bears in North America has decreased drastically, but Alaska still has healthy brown bear populations. Bears are especially healthy in Katmai because so many types of food are available. Bears catch salmon in the summer and fall, and they eat washed-up whale carcasses when they find them, to fatten up. All that fat helps the bears prepare for hibernation as winter approaches. They also dig up clams and catch fish on the tidal flats and eat grasses in tidal meadows [2]. The amounts of these foods that are available to bears helps determine the carrying capacity of their environment.

Sea otters can weigh up to 100 pounds and live near the coast in the North Pacific Ocean. In the 1800s, sea otters were hunted for their fur and went extinct along the Katmai coast by the early 1900s. Without sea otters to eat them, some of the otters’ favorite foods, like clams, crabs, and urchins, increased in size and number [3]. Once sea otters were protected, they returned to Katmai and had lots of food to eat. At first, all the extra food helped the sea otter population to grow. But once they ate most of the extra food, the number of sea otters went down a bit and now appears to have settled at a level that their food supply can support. That is carrying capacity at work!

Things we Learned

Over the past 20 years, the number of bears counted along the Katmai coast has decreased by about 66%. The bears’ diets also changed over this same time, from about two-thirds salmon and one-third plants in the 1990s to one-quarter salmon and three-quarters plants in recent years. What happened? One change is that salmon used to be more plentiful. Between 1978 and 2006, nearly a million salmon returned every year to spawn along the Katmai coast. Over the 10 years that followed, only about half as many salmon returned. In 2018, scientists counted more sedges, a favorite coastal plant that bears like to eat, than were counted in 2008. Lower numbers of salmon likely decreased the carrying capacity for bears along the Katmai coast, even though there were more sedges to eat. Why? Well, salmon have more calories than sedges, and so it takes more sedges than salmon to fatten up a bear. But individual bears remain healthy, there are just fewer of them being seen (Figure 2) [4].

Sea otters came back to the Katmai coast around 1950. Their population size peaked in 2012 at 8,600 when there was a lot of extra food, but today there are about 6,000 sea otters (a 30% decline). In addition to counting sea otters, scientists also measure how much food the otters eat. In 2006, because food was still abundant and easy to find, the average sea otter could eat lots of food in a short time, but this plentiful food did not last. From 2006–2019, we measured how many clams there were, and found that the number of clams went down following the peak in the sea otter population! Fewer clams meant the sea otters had to spend more time looking for food, and this eventually led to a reduced carrying capacity for sea otters along the Katmai coast [5] (Figure 2).

Drivers of Change

What else might have changed the carrying capacity for brown bears and sea otters? Currently, there are no cities on the Katmai coast, but human activities still affect the carrying capacity of Katmai for both bears and sea otters. For example, because of whaling in the 1900s, there are now fewer whales in the ocean [6]. That means fewer whales wash ashore for bears to eat. Since whale carcasses have lots of blubber and provide lots of energy for bears, the loss of whales reduces the carrying capacity of the coast for bears.

Although Katmai is a remote wilderness, both the ocean and the land will still feel the effects of a changing climate. As the planet warms, glaciers are melting and the rivers that salmon need are changing. As the ocean warms, the whole food web may be impacted. There may be less plankton (or different plankton) to feed the small fish that salmon eat. Less plankton also affects the clams and mussels that sea otters eat. A changing climate will probably shift the carrying capacity of Katmai for brown bears and sea otters again, as well as for the plants and animals that call Katmai home.

We have also seen a change in the number of people that visit Katmai. Before 1990, for example, few people visited the coast in Hallo Bay, but lately more than 3,000 visitors come to the bay each year by plane or boat. They come to watch brown bears and to see sea otters. Since hunting is not allowed in the park, bears have become less fearful of humans, but they are wary, and large numbers of visitors might affect where bears go. If too many people are watching the bears at their favorite places, the bears may not stay to eat as much as they would if people were not there. In this case, the carrying capacity may change—not because less food is available but because more people are visiting (Figure 2).

Conclusion

We expect some changes in an ecosystem to have a larger effect than others on the carrying capacity of that region for a given species. For example, in Alaska’s coastal habitats, salmon may be the most important food for brown bears. We expect larger numbers of salmon to increase the number of bears and fewer salmon to decrease the number of bears. In contrast, we expect decreasing numbers of clams to have a small effect on the region’s carrying capacity for bears because clams make up a small part of the bears’ diet. But fewer clams could have a large effect on the sea otter carrying capacity because otters like to eat lots of clams. In another example, we expect that fewer whale carcasses would tend to have a negative effect on bears, but little effect on sea otters because sea otters do not eat whale carcasses. One thing that has not changed is the size and general health of the bears. This presents a riddle. We would expect that, as food becomes less abundant or nutritious, the bears’ physical condition and size should diminish. However, we think that bears may adjust their abundance and stay healthy as opposed to keeping the same numbers of individuals with potential declines in overall health. This example is an indication of how animals may adjust their numbers in response to changes in carrying capacity.

As scientists, we will continue to study brown bears, sea otters, and other species at Katmai to see how they interact and how their populations change over time (Figure 3). Both bears and sea otters live in dynamic environments. Environmental changes can be natural or caused by humans. The ability of these species to adapt to change allows them to stay healthy as individuals and as populations. Because of the important role these mammals play by interacting with many other species, bears and sea otters are good indicators of ecosystem health. Thus, they serve as effective vital signs for “taking the pulse” of Katmai.

Funding

This work was supported by the National Park Service Foundation, the National Park Service and Gulf Watch Alaska through the Exxon Valdez Oil Spill Trustee Council. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.

Glossary

Vital Signs: ↑ Signals that can be used to gauge the health of an individual (a pulse rate or temperature), a population (abundance) or an ecosystem (number of species present).

Ecology: ↑ The study of how different organisms interact with each and their environment.

Carrying Capacity: ↑ The ability of a habitat to support a given number of a particular species. Carrying capacity can change over time as the habitat changes.

Terrestrial: ↑ Describes an organism or object that resides on land.

Marine: ↑ Describes an organism or object that resides in salt water (oceans).

Sedges: ↑ A grass-like plant that grows in wet ground, like tidal meadows.

Food Web: ↑ All the linkages in an ecosystem that relate to food. A food web links consumers and producers together.

Plankton: ↑ Organisms that cannot swim against a current of water. Most plankton are microscopic and can be plant-like or animal-like.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank Jim Pfeiffenberger for exceptional efforts in the creation of outreach and education materials for this project. He also provided his media expertise throughout the Changing Tides project in general and the photo collage (Figure 3) specifically for this paper. We also thank Lucas Westcott for his review prior to submission. We thank the staff of Katmai National Park and Preserve and we thank USGS, SWAN, and NPS staff that have helped collect data over the years.