OVERVIEW

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ecosystem

wildlife

economic

policy

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STUDY TEAM

REFERENCES

Grizzly Bear Demography and Sources of Mortality

Demography 101

Demographics are used to characterize the structure of a population.  Demographic data includes data such as age class, sex, and fecundity (the number of offspring per individual), and survivorship.  Often times, information on mortality is included in demographic data sets.

What do we know about Yellowstone bears?

In the Greater Yellowstone Ecosystem, relatively little is known about population demography, though it has been studied profusely.  This is so for two primary reasons.  The first is that many bears prefer to remain in areas that are not regularly frequented by humans.  Most of the information about the demographics of Yellowstone bears is derived from habituated bears, as they are much more visible for study.  The second reason is because of assumptions made for analyses (Pease and Mattson 1999).  Though the data used by most researchers is virtually the same, conflicting conclusions arise due to variance in their assumptions.  In their paper on the demographic parameters and rates of mortality, Pease and Mattson (1999) made the following estimates: (1) the mean litter size for females at den emergence on April 15 of any given year is 2.4 cubs,  (2) the age at which grizzly bears are sexually mature is 4 years, and (3) the mean age of first reproduction is 5.6 years.  With these assumptions, they modeled a population that has changed little over the past 25 years.  Other researchers, using the same or similar data, conclude that the population is growing, while others conclude that the population has been reduced.

What causes bears to die?

For grizzly bears, human caused mortality accounts for nearly all mortality for individuals over the age of one (Pease and Mattson 1999.).  Of 174 radio marked grizzly bears between 1974 and 1996, humans killed 85 to 94%, with most of these being habituated bears (bears that have become accustomed to humans) (Meagher & Fowler 1989; Mattson et al. 1992).  Researchers have found that the habituation rates are highest in sub-adult males and females with young, as they spend the most amount of time in areas nearest to people, so as to avoid the risk of harm from adult males in the backcountry (Tracy 1977; Warner 1987; Dau 1989; Mattson et al. 1992; Fagen & Fagen 1994). 

Roads have been found to be particularly deadly for grizzly bears.  In one study conducted in Alberta, Canada, 75% of all bear mortalities occurred within 1.0 km of all-weather roads (Aune and Kasworm 1989).  The same seems to also be true of roads in central and southern Montana (Dood et al. 1986).  This is quite surprising, especially since other studies have shown that bears typically underused areas within 100 to 500 meters of roads, but in one study avoided areas as far away as 914 meters (Archibald et al. 1987; Mattson et al. 1987; McLellan & Shackleton 1988; Aune & Kasworm 1989; Kasworm & Manley 1990).  In Yellowstone, studies have shown that the use of available habitat within 400 to 2,000 meters of campsites or cabins was 40-67% less than expected (Elgmork 1983; Gunther 1990), demonstrating the fact that grizzly bears avoid humans if at all possible.  Reasons for vulnerability of bears near human frequented areas, such as roads, are numerous, ranging from an increased susceptibility to poaching to intolerance on behalf of humans and agency control of "hazardous" bears.

Where are bears most susceptible?

In the Greater Yellowstone Ecosystem, the mortality rates per unit area were greatest in sites near towns, and 5.8 to 11.0     times greater than the lowest rates of mortality in U.S. Forest Service roadless areas and U.S. Park Servicebackcountry, respectively (Mattson et al. 1992).  In fact, of radio-marked habituated bears were 3.1 times morelikely to be killed from 1975 to 1990, compared to their weary counterparts (Mattson et al. 1992).  These data areespecially true in years where whitebark pine crops are lowest.  Pease and Mattson (1999) found that although bears do not starve to death in years where the crops are lowest, their mortality rates are much higher.  This is dueto the fact that their behaviors are altered during these times, and bears that would normally remain in backcountryareas migrate into the front country in search of food.  The movement of bears seems tohave a spiraling effect, in that the bears that are not killed due to “conflicts” with humans are, rather, habituated, making them susceptible to conflicts later on (Meagher and Fower 1989).  In the Greater Yellowstone Ecosystem, approximately 33% of the total habitat available to grizzly bears is impacted substantially by humans (Mattson 1998), making this additional habituation a huge problem for the resident bear population.

Can’t we just coexist peacefully?

Habituation alone is not the only negative result of human use of available habitat.   Doak(1995) modeled the problem of habitat conversion in the Greater Yellowstone Ecosystem, and found that very small amounts of habitat degradation may lead to dramatic changes in population growth rates, depending upon the severity of degradation or its impacts upon mobility.  He found that additional conversion of 15% of an area from good habitat to bad could cause a 4% decline in what was otherwise a stable population.  This may be closely linked to the fact that the population of grizzly bears in Yellowstone is both low and relatively isolated, and as a result, the difference between a recovering population and a declining population amounts to the loss of one or two adult females per year (Knight and Eberhardt 1984, 1985).