The black bear factor

 

Under my introduction to the Holocene, I invoke American black bears as a major factor in explaining where grizzly bears were able to establish and persist in North America. American black bears are self-evidently another ursid. More to the point, they are another large-bodied omnivore with much the same life strategy as grizzlies. In fact, one could argue that the niche of grizzly bears and American black bears overlap substantially, largely as an artifact of their relative isolation from each during most of their history as separate species--American black bears in North America and brown bears (i.e., grizzlies) in Asia. On the other hand, an off-hand response to the idea that black bears could negatively affect grizzlies might be based simply on the comparative size and ferocity of grizzlies versus black bears--grizzly bears are both the larger and more aggressive of the two. If grizzlies are so much larger and more intimidating, why wouldn't they be consistently calling the shots?

 

In what follows I elaborate on why American black bears might have posed such a competitive challenge to grizzlies in North America. I emphasize not only the potential extent of dietary and spatial overlap, but also the much greater fecundity and considerably higher densities of black bears in many areas where they coexisted with grizzlies--to the potential detriment of the grizzlies when it came to a scramble for food. Moreover, as I elaborate below, black bears are better able to subsist on food in trees (i.e., they can climb) or in sparse patches because of their smaller body size, which likely put them at an even greater advantage over grizzlies when both were trying to exploit nuts and berries from trees or less dense shared resources. That said, grizzlies likely had an increasing advantage over black bears wherever foods were in the form of concentrated meat (for example, bison or spawning salmon) or underground in the form of roots, rodents, or squirrels. Grizzlies are built to dig whereas black bears are built to climb (see Front limbs). Grizzlies can also typically dominate black bears when it comes to head-to-head contests over a concentrated patch of food.

The productivity advantage

Under most conditions black bears can quite simply out-produce grizzlies when it comes to pumping out young and sustaining high population densities. The bar graphs above elaborate on this theme, with the orange bars corresponding to averages for interior populations of North American grizzly bears and the brown bars the same for black bear populations within or near existing or former grizzly bear range. These brown bars are presented in pairs, with the leftmost one presenting values for black bear populations in areas that still support grizzlies, and the one to the right, values for black bear populations that occur in an area of former grizzly bear range. Again, all of this is for populations in western interior North America--meaning that coastal populations of bears subsisting on spawning salmon are not included.

 

So, to the data. At far left you can see average body sizes. Grizzlies tend to be twice as large as black bears in both current and former black bear range. This simple difference in size undergirds pretty much everything else (see the section on Implications of body size). For one, it partly drives the huge difference in home range sizes between grizzlies and black bears, shown by the next set of bars to the right. Above and beyond this, though, grizzly bears just tend to use more space. It seems to be part of their life strategy which is built around creating and exploiting as many foraging options as possible, especially in more variable environments. More space entails more opportunities. The next two sets of bar graphs pertain to reproductive potential. Put succinctly, black bears produce substantially more cubs per year mostly because of a shorter interval between litters of cubs. Moreover, partly because of smaller home ranges and partly because of smaller size, black bears seem to be able to exist at much higher densities than grizzlies (at least interior grizzlies) can. The difference is as much as 5-10 fold. That's a really big difference when you think about the number of animals roaming a landscape, and the buffer and bet-hedging that such large numbers provide at a population level. Finally, when you put density together with a higher per-bear reproductive rate to calculate number of cubs produced per year within any given 100 square km, the difference between black and grizzly bears is even greater (the set of bars farthest to the right).

 

The implications? If there is a richer patch out there of shared food that is characteristically dispersed rather than highly concentrated (think most berries), odds are that a black bear will find it first. Once high-graded, a grizzly that happens upon it later likely won't be able to benefit, whereas the opposite will probably not be true if a black bear is following in the footsteps of a grizzly (see my point immediately below). Moreover, if there is a predator out there that kills both black and grizzly bears (think humans), the grizzly bear population will be less able to sustain the predation compared to black bears. So...an environment with a lot of shared vegetal foods, such as berries, and with humans that mete out even a modicum of mortality to bears will probably yield bad outcomes for the grizzly bear population--perhaps even local extirpations.

The thriftiness advantage

One reason black bears may be able to sustain such high densities is because, when compared to grizzlies, they more efficiently gain weight and store up body fat when feeding on berries and vegetation. This difference is largely a function in differences in body size between the two species. As evidence of this phenomenon, the graph at upper left shows the average rate of weight gain (expressed in both absolute and relative terms) for bears of different sizes feeding on characteristic assemblages of berries. Like so much of what we know about the nutrition of bears, these relationships are the result of research by Charlie Robbins and his graduate students, in this case Christy Welch (see Nutrition for a lot more information). The absolute and relative rates of gain increase to a peak and then decline with body size. I've given the body mass associated with each peak--somewhere between 40 and 70 kg (ie., 90-150 lbs). I've also shown, in gray, the range of average body weights for black and grizzly bear females from interior regions. The point is pretty obvious. Black bears are of the optimal size to put on weight when eating berries. Grizzly bears are not. This same holds for bears eating the foliage of vegetation.

 

To give these phenomena a little more spatial focus, there are extensive areas where the diets of both black and grizzly bears consist mostly of berries and vegetation foliage, and where overlap of diet between the two species is nearly complete. Mostly notably this holds for wet interior regions along the US-Canada border and for relatively limited parts of the Canadian boreal forest occupied by both species. I've illustrated this graphically with the green bars in the figure above right. The height of each bar corresponds to the degree of overlap between diets of black and grizzly bears in a given study area. A value of 1.0 indicates that diets are identical, a value of 0.0 that they have no similarity. All of the areas identified with the tallest darkest green bars occur along the US-Canada border in areas where there are abundant berries and comparatively little meat.

 

To put this in context, if you go just about anywhere other than the mid-continental boreal forests or the US-Canada borderlands, the diets of black and grizzly bears diverge considerably. More specifically, where meat is available, grizzlies tend to eat more of it. Where roots and ground-dwelling mammals are available, grizzlies tend to eat more of them as well. Such divergences are illustrated not only by the lesser overlap of black and grizzly bear diets in Banff National Park and the Yellowstone region, but also by the two sets of paired bars farthest to the right. The height of these bars corresponds to the amount of energy obtained from meat in areas where berries dominate the diet and in areas where meat is a major component, which accounts for most of the remainder. The golden bars are averages for grizzly bears; the gray bars are averages for black bears. In other words, where berries dominate the diet, ingestion of meat does not differ much between black and grizzly bears, and diet overlap is nearly complete. In other areas where meat from either spawning salmon or large-bodied herbivores is available, grizzlies eat twice as much meat as black bears do--and a lot of it.

 

The reasons for the disparity in meat consumption between grizzlies and black bears, at least where meat is relatively abundant, go back to body size and behavior. Here the larger size and greater aggressiveness of grizzlies work to their advantage. Meat in meat-rich areas often comes in concentrated packages, either within the restricted confines of spawning streams, or built into the bodies of large herbivores such as bison, moose, elk, and even caribou. Deer, interestingly enough, are rarely a major source of meat for grizzlies. The point being: concentrated proteinaceous foods such as these are more easily dominated by grizzly bears compared to more dispersed resources such as berry patches. Hence, grizzlies are more likely to benefit from--and disproportionately consume--meat from concentrated sources. In terms of biogeography, grizzly bears predictably fare better when confronted with competition from black bears in places where they can dominate food sources, which are often rich in protein. Such places include coastal areas with spawning salmon and areas like the Great Plains, which (at least historically) supported vast herds of bison.

The biogeography of it all

 

At this point my argument turns spatial. Where is the geospatial nexus of competition between brown and black bears, and what is the geospatial evidence in support of my thesis? The map above as well as a closely-related map shown on the introductory page to this section (the Holocene) depict some relevant patterns. Here I show the overlap of brown and black bears (grizzlies are the same species as brown bears). The overlap is shown as a grayish-green, which is an extension of the grayish shading that delineates the sole distribution of black bears in both Asia and North America. The Asiatic black bear is Ursus thibetanus, the American black bear is Ursus americanus. Regardless of how closely related they are (see Evolutionary relations), both species are smaller than brown bears, and, unlike brown bears, both climb trees to forage on nuts and seeds, with are most abundant in the canopies of temperate deciduous trees.

 

So...a couple of interesting features. American black bears overlap extensively with grizzly bears in North America. Yet grizzly bears are absent from extensive parts of the eastern latitudes occupied solely by black bears--latitudes that should be hospitable to grizzlies if one considers the environment within which they evolved (see Evolutionary biogeography). The point being; grizzlies are (or were) absent from extensive areas in North America where we might have expected them to flourish. Not by accident, these areas coincide with the eastern deciduous and boreal forests of what was eventually the United States and Canada. By contrast, brown bears occupy (or occupied) all of boreal Eurasia along with all of temperate Europe. Moreover, overlap with the Asiatic black bear is minimal and largely confined to areas at the margin of brown bear range, coincident with the occurrence of deciduous forests. The Amur region of far eastern Russia is notable in this regard. Put another way, a potential competitor in the form of a black bear is wholly absent from the boreal and higher-latitude deciduous forests of Eurasia. In the absence of such a competitor, Eurasia's brown bears seem to have flourished.

 

Perhaps, unlike in North America, competition from brown bears occupying Asia's boreal forests confined Asiatic black bears to their southeastern Aisan stronghold. This seems to be a plausible explanation for distributional patterns in Asia's boreal forests. It could be that the added enrichment provided by stone pines (see the Holocene) gave brown bears an edge, along with the fact that brown bears were well-established throughout northern Asia during the entirety of the last Ice Age (see Evolutionary biogeography). So the Asiatic black bear would have been expanding into an area already dominated by brown bears. Perhaps only in areas covered by deciduous forest were Asiatic black bears able to effectively compete--the Amur being a potential prime example.

 

But what about the extensive temperate forests of Europe. Why were Asiatic black bears not dominant there, to the potential exclusion of brown bears? Probably because of an historical artifact. Black bears actually were common in Europe during the interglacials preceding the last Ice Age. But during the Last Glacial Maximum, deciduous forests were reduced to a mere fringe along the Mediterranean. The rest of Europe was non-forested. Black bears apparently could not survive. Experts such as Reginald Pocock, Dirk Erdbrink, and Bjorn Kurten have suggested that Asiatic black bears never made it back to Europe at the end of the last Ice Age because bridges of favorable habitat (i.e., productive deciduous forests) did not connect spreading deciduous forests to the far west with the Ice Age refugium of black bears in southeastern Asia. It is possible that if black bears had been able to survive in Europe, or get back there expeditiously, brown bears would have faced tough competition in the deciduous forests of Europe. As it turned, brown bears had a free hand. More to the point, in the absence of better-adapted black bears, brown bears can probably thrive in deciduous forests such as those of eastern North America. But black bears were well-established in eastern North America, and probably had been throughout the last Ice Age.

The graph at left provides evidence that Ursus arctos functions dietarily much like black bears where this species is able to establish itself in deciduous forests--as in the case of European brown bears. The numerous bars at left each correspond in height to the degree of similarity-or overlap--between the diet of a population of European brown bears and the diet of a population of black bears in temperate forests of eastern North America. A value of 1.0 corresponds with 100% similarity, a value of 0.0, no similarity at all.

 

The main result? 45% of the comparisons exhibit 70% or greater similarity; 19% exhibit 80% or more. The brown bears with diets most similar to those of black bears in North America include those living in the Balkans and Carpathians Mountains, which are typified by extensive beech forests.

 

My main point? Deciduous forests are not intrinsically hostile environments for brown (or grizzly) bears. This species does just fine in these kinds of habitats. But, apparently, only in the absence of better-adapted black bears. Hence the likelihood that competition from black bears contributed to keeping grizzlies out of the mid-latitudes of eastern North America, but probably in concert with the effects of people and the bottleneck introduced by conditions on the Great Plains (see the Holocene and the Human factor).

My final entry on this page pertains to yet another piece of evidence suggesting that American black bears can be strongly competitive with brown bears under the right set of circumstances.

 

It turns out that there is an interesting anomaly when it comes to the distributions of black and brown bears along the Pacific coast of Alaska and British Columbia. Some islands only support brown bears. Other islands only support black bears. And in most cases these islands are within swimming distance of one species or the other. Meaning, that it is not the physical inability to reach an island that keeps a species at bay, but rather something else. That something else is likely competition from the already-established species, whether brown or black bear.

 

This stasis is probably driven mostly by competition. But some features of dispersal probably contribute to excluding one species or the other. Most long-distance dispersers tend to be younger males. These bears tend to be smaller. In the case of an adolescent male brown bear showing up on a black bear-dominated island, the reception committee will likely be comprised of at least some adult black bears, which tend to be particularly large on these food-rich islands. At some point coming from an aggressive breed doesn't make up for being small. This has been amply demonstrated by observations of conflict between large black bears and small grizzlies at concentrated food sources. The large black bears typically win. All in all, this adds up to further evidence that once black bears are established in rich habitat, they can exclude colonizing grizzlies.