Ecological Studies
Developed and refined methods to estimate the amount of fat in the living bear: People can measure their body fat content by stepping on battery-powered bathroom scales. Bear researchers now have a similar capability to measure the body fat content of bears, although the bear does have to be anesthetized. This technique has been critical to understanding how a bear’s size, fatness, and cub production are related to its food resources. For example, we now know that leaner females produce cubs later and that those cubs grow more slowly and are less likely to survive than those produced by fatter females. Similarly, biologists in Yellowstone National Park have learned that “problem bears” that get too close to people or kill livestock are leaner than the rest of the bear population, which may explain why those bears are willing to take a chance and seek high-risk foods.
Developed and refined methods to quantify the diets of wild bears from hair or bone samples: Bears eat a wide variety of foods. However, rarely have we been able to determine the relative contribution of those foods to growth, reproduction, and the well-being of wild bears. A relatively new technique called stable isotope analyses does allow us to use a few hairs, a drop of blood, or a flake of bone to quantify which foods are being used by a particular bear. This technique can be used on both modern bears and on bears that died thousands of years ago. For example, we’ve learned that grizzly bears currently living in Yellowstone National Park have about the same diet as their ancestors had a thousand years ago. However, things have changed in other ecosystems. For example, salmon provided 58% of the nourishment for grizzly bears living 100 years ago in Idaho, Washington, and Oregon. This level of salmon consumption is about the same as that occurring today in many populations of Alaskan grizzly bears with access to abundant salmon. Unfortunately, no grizzly bear in Idaho, Washington, or Oregon currently consumes salmon.
Quantified the interaction between people, bears, and salmon in Alaska: Grizzly bears that eat a lot of salmon are 80% larger, produce 25% more cubs, and live in populations that are up to 50 times denser than grizzly bears that fatten primarily on fall berries. Thus, human activities that disrupt foraging by bears on salmon or other important seasonal food resources could seriously harm individual bears or their populations. Bear-viewing has become very popular in many areas of Alaska. We’ve learned that bear-viewing affects both the number of bears and the age and sex ratio of bears willing to visit salmon streams while people are present. Managers can reduce those impacts by minimizing the number of trips people make into viewing areas, standardizing the timing of those trips, confining people to well defined viewing areas, and providing significant blocks of time when people are not present
Quantified the importance of various foods to Yellowstone grizzly bears: Although grizzly bears are frequently identified as large carnivores, whitebark pine nuts are one of the more important foods for grizzly bears in Yellowstone National Park. During years of good pine cone production, two-thirds of the bears derive over half of their nourishment from pine nuts. Surprisingly, bears depend on red squirrels to harvest the cones and store them in their middens, where bears can efficiently excavate and consume the energy and protein-rich nuts. Fortunately, the squirrels also gather other cones that grizzly bears don’t use, so there are food resources left for the squirrels. Unfortunately, whitebark pines are being attacked and killed by white pine blister rust, a non-native fungus. Another important food for Yellowstone grizzly bears are cutthroat trout that migrate out of Yellowstone Lake each spring and summer to spawn in at least 59 tributary streams, much like spawning salmon in Alaska. However, illegal introductions of lake trout have led to a dramatic reduction in cutthroat trout. While a study conducted in the 1980s prior to the crash in the cutthroat trout population found that female bears ate more fish than did males and that 90% of their diet came from cutthroat trout, a later study by us in the late 1990s found that male grizzly bears were the only ones making extensive use of the remaining cutthroat trout and the entire population of bears consumed less than 10% of what had been eaten in the 1980s.
Grizzly bears assist in moving marine-derived nutrients into terrestrial plant communities: In large scale global nutrient cycles, nutrients from the land are leached into freshwater and ultimately flow via rivers into oceans. However, salmon that return from the oceans to spawn in streams and lakes are one way that nutrients can return from oceans to land. While dying salmon provide the nutrients necessary to nourish the next generation of salmon, bears also move the nutrients that salmon consumed in the ocean back onto the land. For example, each adult female grizzly bear on the Kenai Peninsula in Alaska eats about 3000 lbs of salmon per year. While bears retain the energy from the salmon as fat, much of the nitrogen, phosphorus, and other excess nutrients used by plants are excreted in urine and feces as bears walk through surrounding areas. Spruce trees growing along Alaskan streams with healthy salmon runs get almost 20% of their nitrogen from salmon, and over 80% of that nitrogen passed through a grizzly bear. Thus, grizzly bears are extremely important in moving nutrients from the ocean back to the land.
Assisting the public in understanding the relationships between food resources and the characteristics of bear populations: Most people do not understand the link between the types of foods that occur in an area and the number, size, and productivity of resident bears. For example, developers wanting to build homes on salmon streams and thereby prevent bears from accessing salmon have argued that the grizzly bears that currently live along those streams will learn to feed on berries and other dispersed, low quality foods that exist in the surrounding forests. After all, other bears are already feeding on those food resources. However, the very large size of salmon-feeding bears means they also have very large daily energy and nutrient requirements. These big bears will find it impossible to physically gather and process enough berries and vegetation on a daily basis to meet even their minimum requirements. While they can lose some excess fat, their large skeletons and muscles are not shrinkable to the size of a bear that can live on lower quality, dispersed foods. Because there is no physical solution to this imbalance, these bears are destined to become “problem bears” as they try to regain access to salmon or starve. Although the foods are different, the same problem can occur when long-term, open-pit garbage dumps are closed or abundant livestock carrion are removed as food resources for grizzly bears.