Interagency Grizzly Bear Study Team

Unique Females with Cubs-of-the-Year  

Adult females are the most important demographic segment of the grizzly bear populations because they are the reproductive engine.  Adult females with cubs-of-the-year (cubs) are the most easily and reliably recognized cohort of grizzly bear populations.  Consequently females with cubs are a major focus of IGBST’s monitoring program in the Greater Yellowstone Ecosystem (GYE).  Tracking trend for this segment of the population is generally representative of the trend for entire population.  Thus, if we observe growth in numbers of female with cubs, we can be reasonably sure the entire population is growing.  Efforts by IGBST to document the abundance and distribution of females with cubs began in 1973 and have improved over time by implementing standardized protocols and new statistical procedures.  During the past 10 years (2006-2015), IGBST has estimated an average of 56 unique females with cubs annually. When combined with other data, these estimates serve as the basis for estimating total population size.  Estimates of total population size are used by IGBST to evaluate annual mortality and to assess whether mortality is sustainable.  Sustainable mortality establishes a limit on the number of grizzly deaths that can occur given population objectives.  Information from these studies is crucial to evaluate trends in the grizzly bear population and assess the long-term health of the population.

Radio-Collaring Bears

IGBST began radiocollaring grizzly bears in the Greater Yellowstone Ecosystem (GYE) in 1975.  Since then, we have radio-monitored over 830 individuals for varying durations, typically for 2 to 3 years.  Over 100 individuals have been monitored during more than 5 different years.  Our monitoring program changed in 1986 based on directions from the Interagency Grizzly Bear Committee’s Population Task Force.  They recommended we maintain and monitor a minimum of 25 radio-collared adult females annually.  We also use telemetry to monitor a representative sample of adult males.  Data collected from radio-marked bears provide information necessary for tracking key population parameters.  By observing radio-collared bears, we document age of first reproduction, average litter size, cub and yearling survival, how often a female produces a litter, and causes of mortality.  These data allow us to estimate survival among different sex and age classes of bears.  Collectively, this is referred to as “known-fate monitoring”.  In conjunction with other estimates (i.e., number of females with cubs, annual mortality), this information is used estimate population size and evaluate sustainable mortality.  Location information obtained from collared bears also provides reliable data that helps resource managers focus their activities toward landscape issues that impact grizzly bears in the GYE.

All research animals are handled by following the specific requirements of USGS Animal Care and Use policies.

Whitebark Pine Monitoring

Whitebark pine seeds are a high-calorie food resource available to grizzly bears during late summer and fall.  Because whitebark pine is a masting species, cone production is variable, with good crops every 2 to 3 years.  Whitebark pine seeds are high in fats and proteins and, when available, allow grizzly bears to build up fat reserves during fall in preparation for hibernation.  Grizzly bears harvest these cones by raiding seed caches of red squirrels (middens).  Since 1980, we have annually monitored whitebark pine cone production throughout the Greater Yellowstone Ecosystem (GYE) using surveys of established transects. Our data show that mature, cone-producing whitebark pine trees in the GYE have experienced substantial mortality, primarily due to a mountain pine beetle outbreak that started in the early 2000s, but also from white pine blister rust, and fire.  IGBST research shows that grizzly bears are adapting to these changes by shifting their diets (see Food Synthesis Report).  However, we will continue to monitor this food resource because bears still make extensive use of it in years of good cone production.  Click here for IGBST Whitebark Pine Cone Production Annual Summaries.

Direct Links:

IGBST 2015 Annual Report Summary

IGBST Annual Reports

IGBST Complete Publications 1974 - 2016

IGBST Boundary Maps

IGBST Boundary Layers

Grizzly Bear Mortality Database

IGBST Grizzly Bear Food Synthesis Report

IGBST Whitebark Pine Cone Production Annual Summaries

Reports:

Response of Yellowstone grizzly bears to changes in food resources: a synthesis

IGBST Demographic Workshop Report