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A National Park Servicebiologist collecting data

from an immobilized bear.

Grizzly bears are an important component ofDenali National Park and Preserve. The legislationthat created the park (formerly Mt. McKinleyNational Park) established a game refuge for theanimals, so grizzly bears have not been huntedthere since 1917. The park now supports a natu-rally regulated grizzly bear population as an activecomponent of a large-mammal predator–preysystem that includes wolves, caribou, moose,and Dall’s sheep.

Grizzly bears are also a primary reason thatmany people visit Denali. A recent survey esti-mated that 90% of visitors travelling the park roadobserved at least one grizzly bear on their trip.Numerous studies have described the adverseeffects of humans and associated development ongrizzly bears. Harvest of grizzly bears outside thepark and concerns regarding the impacts of humanaccess within the park resulted in the need forobjective information on the status and trends ofthe grizzly bear population in Denali.

This study was initiated in 1991 to examine therole of grizzly bears as predators of caribou calves.The emphasis of the project was redirected early

on to describe the characteristics of a sample pop-ulation of grizzly bears in Denali National Park andto develop and test noninvasive techniques fordetermining the density of bears in the park. Thefocus has since shifted to long-term monitoring ofcub production and survival.

Study AreaOur study area lies along the north slope of the

central Alaska Range in Denali National Park andPreserve, from the east side of the Muldrow Gla-cier west to the Herron River. The 1750-square-kilometer (675-square-mile) study area includeselevations ranging from 600 to 2000 meters (2000to 6500 feet). The area includes important foraginghabitats, such as large, concentrated berry patcheson glacial moraines and hillsides, as well as winterdenning habitat. It also includes the principal calv-ing area for the Denali caribou herd. The climate isgenerally cool and wet during the summer, withtemperatures around 10–15°C (50–60°F). Freezingtemperatures and snow may occur during anymonth. Snow accumulation usually begins in Octo-ber and dissipates from lowlands and unshadedportions of foothills by mid- to late May.

MethodsThe study relies heavily on radiotelemetry to

acquire information about individual bears andtherefore requires catching animals to attach radiocollars. Bears in the study area are located forcapture using a small fixed-wing aircraft. Oncelocated, bears are darted from a helicopter usingan immobilizing drug delivered in a projectilesyringe fired from a syringe rifle. Darted bears aremonitored from the aircraft until they are immobi-lized, at which time the helicopter crew lands toprocess the bear.

Standard morphological measurements, such ashead and neck circumference and body length as

Grizzly Bear Population Ecology and Monitoringin Denali National Park and Preserve

This article was preparedby Patricia Owen, WildlifeBiologist, Denali National

Park and Preserve.

This document has been archived.

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well as weight, are taken to monitor growth andphysical condition. Bioelectrical impedance analy-sis (BIA) is used to determine percent body fat.BIA is the same method used to determine bodyfat in humans. It is a measure of the body’s resis-tance to the flow of a very weak electrical current.The resistance measurement is entered into a for-mula specific to grizzly bears to calculate percentbody fat. A small vestigial tooth is extracted fromindependent bears during their initial capture todetermine age. Teeth are sent to a laboratory,where they are sliced into thin sections andstained. The rings of cementum on the tooth canthen be counted under a microscope, much likethe rings on a tree, to determine age. Blood sam-ples are collected to assess disease exposure.

Since much of the study area is not easilyaccessible from the Denali Park road, bears aresubsequently located using a fixed-wing aircraft.The first radiotelemetry flights of the season beginin mid-April each year, continue through the sum-mer, and end in late October once all the bears aredetermined to be denning. Early-season flights

allow us to determine when bears emerge fromtheir dens in the spring so that we can get accu-rate counts of the number of cubs that eachfemale bear has produced. Flights throughout thesummer months enable us to gather informationon habitat use and cub mortality as well as mortal-ity of independent bears and family breakup. Inthe fall we determine den locations and denentrance dates and confirm the remaining num-bers of cubs.

ResultsSince the study is now focused on cub produc-

tion and survival, collars have been maintainedonly on female bears. Each year we examine theprevious year’s data to determine the number ofreproductive females (those six years old or older)and the number of “potential producers” (thosereproductive females that were available to breedthe previous year). Females are considered avail-able to breed the previous year if they did not pro-duce cubs that year or had cubs but lost them

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early enough to breed again. Starting at den emer-gence, we locate the “potential producers” todetermine the number with cubs and the numberof cubs produced. The number of females withcubs has varied widely over the years.

Though the number of collared female bears inthe study has varied, we are able to compare yearsby calculating reproductive rates. Reproductiverates compare the number of females with cubs tothe number of potential producers. Reproductiverates of zero in 1993 and 1997 and rates of lessthan 0.10 in 2000 and 2001 are yet to be fullyexplained. The lack of productivity in 1993 maybe explained by unusual weather patterns thatoccurred the previous year, when the areareceived heavy snowfalls in both mid-May andmid-September, resulting in an abbreviated sum-mer season. Female bears may not have had theopportunity to accumulate sufficient fat reservesto maintain themselves and cubs while denning

Female history, 1991–2002.

Reproductive rates, 1991–2002.

during the winter of 1992-93. Unfortunately, noobvious weather patterns could be identified toexplain zero to low productivity in 1997, 2000, or2001.

Accurate counts of the number of cubs pro-duced each year and the number of cubs still aliveat the time of den entrance have allowed us totrack cub mortality. Cub mortality was unusuallyhigh early in the study. Mortality rates calculatedfor each year of the study vary from 46 to 100%,with an average mortality rate over the course ofthe study of about 71%.

We attempted to determine the cause of deathin cubs by attaching small radio collars to six cubsin 1994. The collar was designed to enlarge as thecub grew and to fall off before denning. At thetime the collars fell off, all six cubs were still alive.Because a small unmarked bear carcass is difficultto locate, we have only been able to determine thecause of death in one cub in the 12 years of the

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Mortality of cubs of the year, 1991–2002.

Numbers of cubs, spring and fall, 1991–2002.

study. A necropsy of the carcass revealed that itwas killed in a rock slide.

DiscussionThough this study has been in progress for

some time, many questions remain about grizzlybears in Denali. From early survey work we havedetermined that the density of grizzly bears in thestudy area is about 27 independent bears per 1000square kilometers (386 square miles). It is difficult,however, to extrapolate this number into an esti-mate of grizzly bear density for the entire DenaliNational Park since much of the remainder of thepark contains habitat that may or may not supportgrizzly bears.

We have learned that grizzly bear cub mortalityis high and that productivity varies widely, but theunderlying reasons for these facts are still largelyunknown. Investigations into the role of female

physical condition in relation to cub productionand survival will continue. The availability ofabundant berry crops in the late summer and fallis likely key to the accumulation of adequate fatreserves in bears. Surveys to quantify berry cropsin the study area and measure fall body fat areneeded to test the relationship between femalebody condition and cub production and survival.Disease is probably not a significant factor affect-ing grizzly bear cub survival. Blood samplesshowed low prevalence when tested for wildlifediseases including infectious canine hepatitis,canine distemper, and leptospirosis.

Even with high cub mortality and variable pro-ductivity, the high density, high independent bearsurvival rates, and lack of human interference sug-gest that the Denali grizzly bear population islikely stable. Cub production and survival haslikely not been monitored long enough to includea pulse in recruitment.