TRENDS IN CONFLICTS, MORTALITIES, AND POPULATIONS
The Government Art of Spin
The public has been swamped with misinformation about the growth of the Yellowstone grizzly bear population during the last decade, with some agency officials claiming a doubling of the population in the last handful of years. Using a 3-year running average of the most reliable estimates of females with cubs of the year (based on the Mark-Resight method), Interagency Grizzly Bear Study Team data shows that the population little if at all since around 2002, and almost certainly declined since 2014. A marked stalling of growth occurred in 2007, at the end of period when most of the whitebark pine forests died in the Yellowstone ecosystem.
In contrast, grizzly bear mortalities have increased dramatically since roughly 2000, far in excess of anything that can be explained by changes in population size. The most important trend is the substantial increase in mortality that followed hard on the heels of when we lost most whitebark pine in the ecosystem. Grizzly bear conflicts with livestock producers and hunters have surged dramatically since the loss of whitebark pine, which his consistent with a turn by many bears to eating more meat in compensation for loss of pine seeds and cutthroat trout. The increase in hunter-related mortalities has occurred despite a substantial decline in hunters afield. This belies claims on the part of agency personnel that hunters are “behaving better” than in the past.
Livestock and Hunter-Related Mortalities 1990-2015
The graphs at left shows trends in hunter and livestock-related conflicts and mortalities in the Greater Yellowstone Ecosystem, expressed as 3 year averages, up to Oct. 20, 2015. The top graph shows hunter-caused mortalities, with number of hunters afield in grey. The bottom graph shows livestock-related conflicts (pink) and mortalities (maroon). The yellow vertical bar denotes the period of maximum loss of whitebark pine from mountain pine beetles. Bottom line: grizzly bear conflicts with hunters and livestock producers over meat have surged dramatically since loss of whitebark pine, which is consistent with a turn by many bears to eating more meat in compensation for loss of pine seeds...and cutthroat trout. Note that the increase in hunter-caused mortalities has occurred despite substantial declines in numbers of hunters afield. This belies claims by agency spokespeople that hunters are "behaving better" than in the past. Data were compiled by Dr. David Mattson from annual reports of the Interagency Grizzly Bear Study Team
Population Growth 1999-2014
The public has been swamped with misinformation about growth of the Yellowstone grizzly bear population during the last decade, with some agency representatives claiming a doubling of the population during the last handful of years. The graph above shows a 3-year running average of the most reliable estimates (medians and the lower quantile of the estimate) for numbers of females with cubs-of-the-year in the population. Given a 3-year interval between litters, a three year average captures a baseline for numbers of reproductive females, which is relevant because estimates of this cohort undergird all estimates of total population size for the population (the details are beyond what can be treated here).
Bottom line: The population has not increased since around 2002 and has probably declined since 2007, which benchmarks the end of the interval during which we lost most of the cone-producing whitebark pine trees in the Yellowstone ecosystem. The red line is fitted to the data and suggests that we have been losing around 2 females with COY per year, net, since 2007. The trend line is statistically significant (P = 0.027, r2 = 0.54, for those who pay attention to such things).
Total Known & Probable Mortality 1998-2016
Trends in total mortality are consistent with estimated trends in population size. The graph immediately above shows a 3-year running average of total known and probable bear deaths in the Yellowstone ecosytem (see immediately above for the rationale behind use of a 3-year running average). The dominate trend since roughly 2000 has been a dramatic upsurge, far in excess of anything that can be explained by changes in population size. In fact, the population has been static since roughly 2002 (see above). The most important trend is the substantial increase that has followed hard on the heals of when we lost most whitebark pine in the ecosystem. 2014 wasn't bad insofar as mortality was concerned, at least compared to the years before. But we have seen record numbers of dead bears during 2015 and 2016, with 2017 shaping up to be bad as well.