The Holocene encompasses everything that happened between roughly 11-12,000 years ago and the present, although some call the current unfolding epoch the Anthropocene, in reference to the dominant effect of humans on the Earth's environment. The Holocene has been much warmer and less variable, climate-wise, than the late Pleistocene, with (until recently) peak warmth and drought occurring during a period roughly 9,000 to 5,000 years ago called the Altithermal. When thinking about the Holocene, much of my focus has been on explaining the anomalies of grizzly bear distribution in North America that reveal much about the ecology and environments of millennia past. More to the point, why didn't grizzlies occupy eastern deciduous and boreal forests, at least in any numbers for any length of time? By contrast, Eurasian brown bears (the same species as grizzlies) are (or were) abundant and widespread in the deciduous forests of Europe and throughout the boreal forests of Asia.

 

I hypothesize that the distribution of grizzlies in North America was curtailed by the combined and synergistic effects of problematic physical environments, competition with resident American black bears, and competition with and predation by burgeoning numbers of humans--First Peoples. Boreal forests in North America are less rich than those of Asia because our forests lack the fat-rich seeds of stone pines, which are abundant in northern Asia. Moreover, Asian boreal forests lack black bears, unlike in North America where grizzlies competed with black bears (and humans) for shared foods, but at a severe competitive disadvantage especially when it comes to a scramble for the richest patches. Because of their smaller size, smaller home ranges, and higher reproductive potential, black bears can exist at much higher densities than grizzlies. They can also more efficiently use less rich foods and food patches. Add some mortality meted out by people hunting bears for meat, and the balance was and is very likely definitively tipped against grizzlies in boreal forests.

 

Farther south, the Great Plains very likely posed their own problems for grizzlies. Rich foraging environments on these grassland spanning the mid-continent north to south are concentrated along rivers that run west to east. Competition and other problematic interactions with humans and black bears were probably heightened by this funneling effect, very likely, again to the detriment of grizzlies moving east. The only food that grizzlies probably dominated was carrion from bison--because humans were disinterested, and because grizzlies could physically displace black bears. As a result, I speculate that the fate of grizzlies on the Great Plains, which included potential colonizers of the deciduous forests farther east, hinged about the distribution and abundance of bison. Given that bison populations waxed and waned with wet and dry periods and with the progressive spread of less-nutritious C4 grasses from south to north, it is easy to imagine grizzly bear populations waxing and waning in synchrony. In fact, there is evidence that grizzlies may have been entirely absent from most of the Great Plains during periods of peak drought.

 

Taken together, these various lines of evidence suggest that North American grizzly bears were being affected by people for millennia prior to the arrival of Europeans. Resident black bears were likely having their effect as well, at odds with blithe assumptions on the part of most people that grizzlies are at the top of the ursid heap. And bison, which we now find with grizzlies only in and near Yellowstone Park, were in the past, unlike the present, a critically important food for grizzly bears in a significant part of their continental range. I elaborate on all of these theses in sections related to each titled The human factor, The black bear factor, and The bison factor.