Published since 1946
Genetic Study Confirms Growth of Yellowstone Grizzly Bear Population
A recent paper by the Interagency Grizzly Bear Study Team (study team) and collaborators documents that, in spite of a century of isolation from other bears and moderately low genetic diversity, grizzly bears in the Greater Yellowstone Ecosystem (GYE) are not losing genetic diversity or suffering from inbreeding. In addition, the study provides independent evidence that the population has increased substantially since the mid-1970s and is approaching the point where the number of breeding individuals is sufficient to assure long-term genetic viability. These results could play a key role in the U.S. Fish and Wildlife Service's (FWS) decision on the future status of this population. A proposed rule once again delisting the GYE population could be released in early 2016.
"Effective population size" (Ne) describes the number of individuals within a population that contributes genes to future generations and is a critical aspect of population dynamics. Ne is generally smaller than the total number of animals in a population because not all individuals breed. When isolated, populations with a larger Ne lose genetic diversity at a lower rate, which enhances their ability to adapt to their environment. As a general rule, geneticists agree that an Ne greater than 500 is needed to maintain genetic diversity and avoid the effects of inbreeding.
As important as Ne is, it can rarely be determined in wildlife populations due to the difficulty in identifying the "boundaries" of a breeding population and the large volumes of genetic material needed to perform the analysis. Mark Haroldson, a member of the study team, recognized the potential to calculate Ne for the GYE population because it has been isolated for nearly a century from other grizzly bears and because the study team had access to genetic samples from over 700 bears in historic collections and bears captured for research or management over the past four decades. Mark and Dr. Frank van Manen, leader of the IGBST, teamed up with several geneticists to calculate Ne for grizzly bears in the GYE using multiple methods. Results of their analysis were published October 28th in the journal Molecular Ecology and presented to the Yellowstone Ecosystem Subcommittee (YES) of the Interagency Grizzly Bear Committee in Jackson Hole, WY on November 4th.
Dr. van Manen explained to the YES that due to the large number of genetic samples and the timeframe over which they were collected, the study team was able to calculate Ne near the time the GYE grizzly population reached its nadir in the 1970s and again in 2007. Results showed that Ne in this population increased from approximately 100 in the early 1980s to approximately 450 by 2007. The analysis also revealed that genetic diversity was stable across the past four decades and that the rate of inbreeding from 1985 to 2010 was only 0.2%.
Results of this study have important implications both for conservation of the GYE population and for the potential delisting of grizzly bears in this ecosystem. The Conservation Strategy drafted in 2005 for the GYE identified the potential loss of genetic diversity over time as an issue and suggested that moving a few bears into the GYE from other ecosystems could be used to address such a decline. With an estimated Ne of 450 in 2007 and indications that the population has continued to grow slowly since then, loss of genetic diversity is now much less of a concern. The fact that genetic diversity has been stable since the 1980s further reduces the chances that importing genes into the population by moving bears from another ecosystem to the GYE will be necessary. Dr. Chris Servheen, FWS Grizzly Bear Recovery Coordinator further commented at the YES meeting that natural movement of genes into the GYE is likely in the foreseeable future. This is because the gap between occupied portions of the GYE and the Northern Continental Divide Ecosystem is now less than maximum dispersal distance of male grizzly bears in the northern Rockies. Ongoing genetic monitoring of the GYE population will enable the study team to determine if the trend in Ne or genetic diversity changes, or if new genes enter the population naturally.
The more than four-fold increase in Ne also provides independent confirmation that estimates of population growth based on counts of independent females with cubs of the year are accurate. Those estimates increased from 207 in 1984 to 714 in 2015. This counters criticism from some opponents of delisting who challenged the FWS' conclusion that the population had grown to a level where removing the protections of the Endangered Species Act (ESA) and returning management to the states of Idaho, Montana and Wyoming is warranted.
Richard Hannan, Deputy Regional Director for the FWS in Portland, informed the YES that the FWS is currently holding discussions with the state agencies in the GYE regarding the details of post-delisting management. If an acceptable regulatory framework can be defined among the parties, Mr. Hannan said the FWS could move forward with drafting a proposed rule to once again delist the GYE grizzly population. This population was initially delisted in 2007, but a federal court overturned the FWS' decision in 2009, placing these bears back under protection of the ESA. Mr. Hannan said a new proposed rule could be released in early 2016 and that the public comment and peer review process would likely take about a year before a final rule could be published. (cs)