In 2016, the northwestern corner of Yellowstone National Park was truly a hotspot for females with cubs-of-the-year (COY). During 2016, my study identified 3 sow's with COY inside Yellowstone National Park. The Interagency Grizzly Bear Study Team (IGBST) recorded 4 sow's with COY inside Yellowstone National Park.
While my study focuses on demographics and monitoring inside the park, the IGBST focuses on grizzly monitoring in the entire ecosystem. Outside northwest Yellowstone is where a large number of females with COY were located during 2016.
Pictured is a grizzly sow seen on Swan Lake Flats during June 2016. This same bear pictured started with 2 COY, and days later was only observed with 1 remaining COY.
Dr. Frank van Manen, the supervisory research wildlife biologist and head of the IGBST provided the following information regarding the documentation of mortalities for annual reporting purposes:
"On the mortality list we document known and probable mortalities from all causes for dead bears; "known" refers to mortalities "in hand" and "probable" mortalities are instances with strong evidence that a bear died even though there is no carcass in hand.
For independent age bears (>=2 years of age) we estimate total mortality from all causes and report that annually. This includes an estimate of unknown/unreported mortalities using a technique that employs an estimate of reported rate from radio-collared, independent-aged bears. We typically do not instrument dependent offspring (cubs and yearlings) unless they are involved in a management action. Thus we cannot obtain an estimate of unknown/unreported mortality for dependent young. This is why we only evaluate and report human-caused losses for dependent aged bears.
For losses of dependent young from radio-marked females, we do document and include on the annual mortality list when they lose cubs-of-the-year. Cubs-of-the-year (COY) separated from their mothers have a very low probability of survival. We list them as probably mortalities because we rarely get a carcass in hand, but are reasonable sure they will not survive (however, we should not that some COY do survive and recruit in to the population: our genetic data supports the fact that some COY of killed females have survived).
We also have data from both marked yearlings and DNA evidence shows that about half of the yearling that are separated from their mother survive and recruit into the population. Thus for the mortality list we do not assume yearlings died when they are no longer with the mother. Recall that we evaluate only human-caused losses for cubs and yearlings. However, when we derive our estimates of COY and yearling survival, we do include as dead both COY and Yearlings that are no longer with their radio-marked mothers, even though some survive. For this reason, IGBST estimates of COY and Yearling survival rates are likely conservative because they likely overestimate mortalites and thus provide a lower survival rate.
If we were to make any changes we would likely stop including probable COY losses from radio-marked females on the list. After all, we are not able to observe losses of offspring from unmarked females so, if anything, we are somewhat inconsistent in reporting probable cub losses from radio-marked females."
In my study, we speculate the missing COY is a "suspected" mortality (by definition not considered probable). However the mechanism and specific cause of disappearance or death remain unknown, and may never be revealed.
That is the wild of Yellowstone.
Tyler Brasington is a native born and raised Pennsylvanian, yet proud current Wisconsin resident. He graduated from the University of Wisconsin-Whitewater with a B.S. in Environmental Science. Currently, Tyler is pursuing his masters in Natural Resources with the University of Wisconsin-Stevens Point. He has worked in Yellowstone National Park under the guidance and supervision of Dr. George Clokey and Dr. Jim Halfpenny.
Disclaimer: The information and views expressed on this page do not necessarily represent the views of the Department of Interior, US Geological Survey, National Park Service or the United States Government.
The Greater Yellowstone Grizzly Project
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