The Holocene: 11,000 to 200 years ago
This section covers the history of North American grizzly bears before the arrival of Europeans and after the dramatic changes that accompanied the end of the last Ice Age--roughly between 11,000 and 200 years ago. I cover the Pleistocene-Holocene transition in a separate section on Early prehistory. Much of what I present in this section is organized around some peculiarities in the Holocene distribution of grizzly bears that I interpret as highlighting factors governing the distribution and numbers of grizzly bears during the last 10,000 years. I speculate that these key drivers were: broad-scale patterns in dominant vegetation; scramble competition for food with American black bears; scramble competition and predation by First Peoples; availability and abundance of bison; and, of course, climate. These factors probably operated synergistically either to the considerable detriment or benefit of grizzlies, with resulting effects on where grizzly bear populations could persist. I cover the effects of black bears, bison, and First Peoples in greater detail in separate section. Here I focus on the Holocene distribution of grizzly bears in North America and relations of that distribution to vegetation and, to a lesser extent, climate.
So...what is it about the Holocene distribution of grizzly bears that seems so odd?
Estimating Densities, Distributions, and Total Population Sizes of Extirpated Grizzly Bears in the Contiguous United States
Of relevance to this section, you can download a technical report by clicking on the highlighted link or the image at right that contains all of the density estimates for grizzly bear populations in North America that I've been able to find, and then have used here. The report contains an estimate of grizzly bear population sizes for the contiguous U.S. that builds on estimates of contemporary grizzly bear densities.
The map above left depicts (in green) what was probably the main distribution of grizzly bears in North America during much of the Holocene, especially after the huge changes caused by melt of the last Pleistocene ice sheets had played out between 10,000 and 9,000 (10k and 9k) years ago. By that time the major climates and vegetation zones that we see today had been pretty well established (see Early prehistory). The map above right provides a little more detail on the distribution of grizzly bears in the contiguous United States and the evidence that I used to construct it. Each of the dots within the light green area denote a specific observation of either grizzly bear remains or grizzly bears in the flesh. The red dots record observations made by Europeans explorers and settlers which I gleaned from journals or various state-level compendia. The white dots (which are smaller within the delineated range of grizzlies in the lower 48) denote an archeological find that dates between 10,000 and 200 years ago (see Early Prehistory as well, where I address the anomalous Utz site). The Canadian portion of grizzly bear range is based largely on a map in an authoritative report authored by Philip McLoughlin; this map is my basis for delineating a blob of grizzly bear range in far-northern Labrador, in the Ungava region.
I should note, again, that these maps cover only the last (roughly) 10,000 years. There is conclusive evidence that grizzly bears occurred in eastern North America between roughly 16k and 10k years ago. I provide more information on these eastern grizzlies and the probable cause of their extirpation in Early prehistory. Perhaps needless to say, my speculations about these eastern extirpations are closely related to the thesis I present here.
Before delving too deeply into answering the questions that I pose immediately above, it is worth observing that the margins of grizzly bear range were spatially correlated with the distributions of several key bear foods (see Ancestral diet). These relationships are summarized in the three panels above which show the relationship between likelihood that bears were present in a given area (on the vertical or y-axis) and the regional abundance of a particular food (on the horizontal or x-axis). Edges of grizzly bear range were strongly associated with the presence of oak-dominated vegetation, which was a source of acorns. This relationship clearly delineated the southern and western margins of bear range. Secondary to this oak-related affiliation, grizzlies also seem to have been associated with abundant pinyon pines (a source of pine seeds) and bison (a source of meat). Like oaks, pinyon pines typified the southern margin of grizzly bear range, whereas bison typified the eastern edge. For much more on bison see the section on the Bison factor.
The medium-scale relations between grizzly bear foods and grizzly bear distributions summarized above right still leave my large-scale central question unanswered: Why were grizzlies largely absent from eastern North America during the last 10,000 years? The map immediately above, to the left, begins to provide an answer. The red lines in this map delineate distributions of brown and grizzly bears in Eurasia and North America, respectively. Note that both types of bears belong to the same species (Ursus arctos), and that grizzlies in Alaska and northern Canada are quite closely related to ancestral brown bears in Siberia (see Evolutionary biogeography). The areas of dark and medium green denote boreal forests on both continents. The areas in yellow denote grasslands (or steppe, as they are often called in Asia). Boreal forests are dominated by conifers, typically spruce, fir, and pine, with the addition of larch in Asia, plus enclaves of deciduous trees such as aspen and birch. You will notice that Asia includes areas colored two shades of green that are not represented in North America--the darkest and lightest green. These colors correspond with the distribution of stone pines, which are a collection of pine species belonging to a group called Cembrae. Dark green denotes areas of boreal forest that support stone pines. From the perspective of bears, these pines are important because they produce large fatty seeds--much like pinyon seeds--that the bears heavily consume. No other boreal conifer produces seeds that are as large or fatty.
So some obvious patterns: Ursus arctos is distributed throughout the boreal forests of Eurasia, but absent from most of the boreal forest in North America. That seems odd. Ursus arctos is (or was) also distributed throughout all of Europe, which in ancestral times was covered mostly by deciduous forests similar to those of eastern North America. Yet Ursus arctos was largely absent from deciduous forests on our continent. That also seems odd. Finally, the eastern edge of grizzly bear distribution clearly aligns with the eastern part of the Great Plains--our extensive mid-continental grasslands--which are oriented north-south. Notice how (with the exception of the Russian steppes) brown bears in Eurasia occupied extensive chunks of grassland. Two key points about these Eurasian grasslands: first, that they are oriented east-west and, second, the grasslands most extensively occupied by brown bears are all high-elevation, most dramatically so in the case of the grasslands on the Tibetan Plateau.
Some implications? There is or was something about the North American Great Plains and boreal forests that was inhospitable to grizzlies--in contrast to the boreal forests and high-elevation grasslands of Eurasia. Even if the Great Plains were a barrier to eastward-travelling grizzly bears, that still doesn't fully explain the apparent dearth of grizzlies in the East. Grizzlies could have very likely established themselves in eastern forests even if only a few colonists made it there. There must have been something else about North America's deciduous forests that made them less hospitable than the deciduous forests of Europe.
My hypothesis in brief
My hypothesis? North America's boreal forests were more inhospitable than those of Eurasia because they lacked stone pines and additionally hosted a major competitor--our American black bear. By contrast, Asia's boreal forests are rich with stone pines (note the correspondence between the southern boundary of brown bears and the southern boundary of stone pines in the map above) and lacked a competing ursid. Asiatic black bears are all confined to the southern part of Asia. North America's grasslands posed a barrier to grizzlies because productive habitats were concentrated along eastward flowing rivers that funneled grizzlies into direct contact with people and black bears--both attracted to the same rich conditions. North America's deciduous forests were less hospitable than those of Europe because of high resident densities of forest-adapted black bears and (in later millennia) highly-adaptable people, both of which vigorously competed with grizzlies for food. Probably as an historical artifact, Asiatic black bears never made it to Holocene Europe. On top of this, I hypothesize that the added increments of mortality meted out by people tipped the scales against grizzlies wherever grizzlies were at a competitive disadvantage when it came to food, which was probably especially the case in North America's boreal forests. I elaborate on and substantiate critical elements of this hypothesis in the sections on Human and Black bear effects.
A final note about climate, bison, and the Great Plains
Highly variable conditions in the Great Plains during the Holocene probably played a role in limiting grizzlies to the western part of mid-continental North America. Bison were almost certainly a key food of the grizzlies that roamed the Great Plains (see the Bison factor). But archeologists have amply documented that bison populations varied substantially in size throughout the Holocene, most dramatically so on the more arid southern and central Plains. In fact, bison were apparently entirely absent from the southern Plains during extended periods of drought (see Bison). No doubt other bear foods were in short supply during extended droughts as well. In fact, I suspect that grizzlies were locally extirpated from significant chunks of the Great Plains when extended droughts hit, which would have pushed the colonizing front of grizzly bears even farther west away from the eastern deciduous forests.
Which brings me to the graphs immediately above. The figure to the left represents a consensus view of broad-scale climate changes during the Holocene of west-central North America. The blue line represents temperature indexed by deuterium isotope concentrations derived from analysis of ice cores drilled in the Greenland Icecap. You can see the dramatic increase in temperature that marked the end of the last Ice Age along with two brief periods of cooling at roughly 9.2 and 8.2k years ago. Two more recent climate anomalies have also been delineated: the Medieval Climate Anomaly, or Warm Period, and the Little Ice Age, which are both mere blips when compared to the major swings of the past. The vertical orange bands denote periods of extended drought, with periods of greater consensus marked by darker hues. One major pattern to note is the period of peak warmth and drought that occurred between 9,000 and 5,200 years ago--a period often called the Altithermal. This was a particularly tough period for anything trying to make a living in drier regions of the American West, including the Great Plains.
In the graph at upper right I've taken the bars denoting extended periods of Holocene drought and overlain them on some data showing levels of carnivore scavenging at human-related kills of bison on the Great Plains. These data were compiled by Chrissina Burke. Bison herds probably declined substantially during both droughty epochs, so the displayed levels of scavenger activity probably hold for substantially fewer sites at these times. The red line indicates the extent to which the bison bones that were available were modified by mammalian scavengers of all sorts. The gray line represents the total number of sites per century at which carnivore activity was detected. Bottom line: Overall levels of carnivore activity were at exceedingly low levels during the Altithermal. More to the point, these trends are consistent with the notion that grizzlies might have been absent from especially the southern and central Great Plains for extended periods of time. It is especially relevant that Chrissina noted in her dissertation that grizzly bears seemed to have been absent for the period denoted by the red bar at the top of the upper right graph.
