Up or down? New research shows Yellowstone grizzly bears have an uncertain past and future
Since Yellowstone grizzly bears were first listed on the Endangered Species Act back in 1973, the US Fish and Wildlife Service has monitored the population and concluded that the number of bears has steadily increased from 200 to now around 600 bears. It is this positive, upward trajectory that has led the Service to propose removing the Yellowstone population from the endangered species list as recently as 2007. However, new research indicates that this perceived population growth may actually be the product of flawed methodology rather than an actual reflection of population recovery.
A new study by Dan Doak and Kerry Cutler shows that between the years of 1983-2010 the Service greatly increased their “observer effort” – that is, they counted more bears over the years not because there necessarily were more bears, but because they simply spent more time looking for them. Additionally, changes to the landscape including the loss of two major food sources for grizzly bears, cutthroat trout and more recently whitebark pine, have caused bears to forage for food in more open areas such as rocky hillsides where there are moth sites. This shift in where the bears were found led to increased “sightability”- meaning that the Service saw more bears, again, not necessarily because there were more bears, but because they were in locations where the bears were easier to see. Finally, the Service has been relying on two fundamentally flawed assumptions about grizzly bears to calculate their population estimates: 1. that bears categorically live to the age of 30 and 2. that they have a constant reproductive rate for that entire time. In actuality bears, like almost all organisms, “senesce,” or age, meaning that some don’t live past the age of 20 and their reproductive rate decreases with age.
When all of these factors are accounted for, the new study shows that the Yellowstone population may have grown very little, if at all, in the past decades and that it most certainly has not been increasing at the rates touted by the Service. The authors conclude “that we actually know very little about the past trends of this population, and hence about their likely future fate, especially with rapid declines in multiple food resources and increases in opportunities for human conflicts.”
Even with all of the previous errors that led the Service to estimate an inflated growth rate, the Service’s own numbers have shown a recent slowing in the population growth leading the Service to conclude that the population has met its “carrying capacity”, or the maximum number of bears that the ecosystem can support. NRDC has argued that rather than leveling off, the population is likely in a decline due to the bears’ response to the loss of key food sources like whitebark pine. The authors of this new study agree stating, “the flattening … estimates in the last decade, even as search effort has continued to increase, are consistent with a population that is now, in fact, declining.”
This new research calls into question not only the current status of the Yellowstone grizzly bear population, but the Service’s ability to accurately assess the population. If we do not even know whether the population has increased much at all over the years, we cannot know that the management practices in place are effective. In the meantime, the Service has been gearing up for another push to delist this population – including participating in promotional videos touting the successful recovery of the grizzly bear. It’s time for the Service to take a big step back and reevaluate everything they thought they knew about the Yellowstone grizzly population. While Endangered Species Act protections have most certainly contributed to the continued existence of grizzly bears in the area, this new research indicates that this is no time to celebrate their recovery. It’s time to double down on efforts to ensure their eventual and actual recovery.
Photo credit: FWS