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Yellowstone and the State of Grizzlies

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Recent research by the Interagency Grizzly Bear Study Team provides new insight into grizzly population dynamics and the hazards that influence bear mortality within the Greater Yellowstone Ecosystem, which is one of the largest strongholds for grizzly populations in the United States. Chuck Schwartz, USGS biologist and lead of the study team, discusses these findings.




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Marisa Lubeck: Welcome. Thanks for tuning in to this USGS CoreCast. I'm Marisa Lubeck. The mountainous expanse of the greater Yellowstone ecosystem is known for the native wildlife of its national parks and picturesque private lands. Grizzly bears live in this area of the Northern Rockies making it one of the largest strongholds for grizzly populations in the United States. Recent research by the Interagency Grizzly Bear Study Team provides new insight into grizzly population dynamics and the hazards that influence bear mortality within this ecosystem.

I'm here with Chuck Schwartz, USGS biologist and lead of the study team to discuss these findings. Thanks for speaking with us, Chuck.

Chuck Schwartz: You're welcome.


Marisa Lubeck: What is the current status of the grizzlies in terms of an endangered or threatened species? How are they doing with population numbers and what does the status mean for the greater Yellowstone ecosystem?

Chuck Schwartz: Grizzly bears in the Continental United States are listed as threatened under the Endangered Species Act. They were listed in 1975 and remain listed today. Grizzly bears in the greater Yellowstone ecosystem were actually delisted in 2007 but they were put back on the list following a decision from a district court judge.

Bears in Yellowstone represent one of six populations in the United States that still exist today. They've been eliminated from about 98% of their historic range and they only occur in smaller pockets in a few remaining mountainous areas of the Western United States.

In Yellowstone, there's probably somewhere around 600 grizzly bears and the population since it was listed has been growing at a rate of about 4% to 6% per year from about 1975 through today. They now reoccupy habitats in the ecosystem where they were basically eliminated in the late 1800s and early 1900s.


Marisa Lubeck: Previously grizzly studies have suggested that human activity can affect bear populations in certain ways. What new findings about grizzly hazards have come from your recent research and how much of a role do people play?

Chuck Schwartz: One of the things that we know for grizzly bears in general, and this is pretty consistent across all of North America where grizzly bears or brown bears live, is that if you're a bear and you're no longer dependent upon your mother for care so excluding cubs and yearlings, there's about a 90% chance that your cause of death will be directly associated with humans. Bears die in accidents on roads. They're killed in defense of life or property when they get into trouble with people, agencies going and remove them if they're in conflict for livestock killing or damaging property.


So what really drive bear demographics is humans. What our study show and they repeat some of the previous work is that roads in developed areas, for example, impact grizzly bears. Essentially, what we did was we took radio-collared bears; we looked at where those bears lived and whether they lived or died during our period of study. We took the bears' home range and essentially what we showed was what we call human disturbance variables strongly influence whether a bear is going to live or die.

So bears that live in very secure areas that have little or no road density or very few developed sites in the form of camp grounds or lodges or facilities that attractants for bears, bears that live in areas that don’t have these human developments typically have very, very high rates of survival. As the amount of human development increases, it becomes more hazardous and they tend to die at higher rates.


Some of the new findings that we have shown is that ungulate hunting also is hazardous to bears. We know for example that an important food of grizzly bears is ungulate meat so in one way when a hunter kills an elk and cleans that elk, bears use that as a food source so that brings bears into close proximity with hunters and on occasion, they do get into trouble; they are killed in defense of life.

Rural development in the form of ex-urban or outside of urban areas also increases the hazards for bears and so when we have large ranches, for example, that have been maintained as open space for agriculture but the human activity on that land is very low, when those lands are developed into ranch divided into 40-acre parcels that each get a house, it also becomes much more hazardous for grizzly bears to survive in areas like that.


There's a saying that biologists have that a fed bear is a dead bear and so these kinds of developments just increase the probability. But as far as natural hazards are concerned, cubs and yearlings which are dependent upon their mother for care face a different suite of mortality agents. What we typically see is if you're a young bear and you're going to die, you're more than likely going to die from natural causes. There's starvation or predation.

In the instance of starvation, in some years when we have poor food production, these little cubs don’t get enough nourishment and they starve to death.

The other thing that we see is more predation. In the world of bears, cubs and yearlings are vulnerable to being preyed upon by large adult males.


Marisa Lubeck: You mentioned that you used years of bear tracking information from collared bears to come to your conclusions. Can you tell me more about how this information was acquired?

Chuck Schwartz: The grizzly bear study team is responsible for monitoring the grizzly bear population in the greater Yellowstone ecosystem. One of the ways that we do our monitoring is we capture and radio-collar a sample of bears so we trap every year and we try to catch a sample of random bears from the population. We then put radio collars on those individuals and we track them through time. That's what allows us to get a very clear understanding of how bears use the environment, what factors influence rates of reproduction and survival.

So we can look at a female in the fall and determine whether she's weaned her offspring and then determine the following year if she's produced a new litter of cubs, how large that litter might be. That's basically how we track the demographics of the Yellowstone bear.


Marisa Lubeck: Can such bear mortality data from the past contribute to insight on future grizzly populations? Can it be used for predictions?

Chuck Schwartz: We can't really predict but we can project what the future might look like based on information from the past. As long as systems don’t change dramatically, those projections provide some insight and guidance to managers on what they might need to do or consider for maintaining a healthy bear population in the future.


Marisa Lubeck: How do grizzly population dynamics in the somewhat remote greater Yellowstone ecosystem affect people and places on a national level?

Chuck Schwartz: The Yellowstone ecosystem is one of the last remaining intact temperate systems in the Continental US and as such, it has a large amount of wild lands where we still have healthy populations of large carnivores, including grizzly bears, black bears, gray wolves, cougars.

One of the things that we know about bears is that bears are very popular animals. There are millions of people that come to this system every year and one of the reasons they come is with the hope of getting to see bears and wolves in a natural state. So maintaining these large systems like that in an intact fashion I think is important to people in our country.

Marisa Lubeck: Thanks for taking the time to speak with us today, Chuck.

Chuck Schwartz: Thank you.

Marisa Lubeck: This podcast is a product of the US Geological Survey, Department of the Interior. I'm Marisa Lubeck. Thanks for tuning in.


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