Ecological Background 2

The Imperative for Large Connected Populations


All of the information here is germane to developing goals for viable populations of grizzly bears. Population viability is, in theory, determined by a number of factors, most commonly: demographic variability, environmental variability, genetic variability (e.g., inbreeding, drift, and mutation), and catastrophes. The results of minimum viable population (MVP) and population viability analysis (PVA) simulations depend in large measure on the risk benchmarks, especially specified probability of persistence, time frame for this reckoning, and the kinds of factors that are considered.  At one extreme, Shaffer and Sampson (1983) estimated that only 50-90 adult bears were needed to achieve a 95% probability of persistence over a 100 year period, but only considering demographic variation, which is the least consequential of all. Weilgus (2002) similarly estimated that roughly 250 adult bears were needed to achieve a 95% probability of not falling below 100 (a pseudo-extinction threshold) over 20 years, but only considering demographic and environmental variation.  

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At the other extreme, when Reed et al. (2003) and Traill et al. (2007) considered all four sources of variation they estimated that between 4200 and 5800 adult animals were needed to achieve a 99% probability of persistence over 40 generations which, for bears, translates into >500 years, a duration in accord with the recommendations of Boyce et al. (2001). Reed et al. (2003) based their estimate on PVAs for 102 species which they parameterized themselves, whereas Traill et al. (2007) based their estimate on a meta-analysis of PVAs done by other scientists for 212 different species.


Looking strictly at risks of inbreeding depression and loss of evolutionary potential, geneticists have long adhered to a rule of thumb of 50/500 effective population sizes for conservation, where the effective population (Ne) corresponds to the number of animals that contribute to the genetic material of succeeding generations. Ne is commonly estimated as being 1/10 of total population size (N), although this ratio for grizzly bears has been estimated to range from roughly 1/10  to 3/10.  An Ne of 50 is thought to be enough to guard against inbreeding depression whereas an Ne of 500 is thought to be enough to balance genetic losses due to genetic drift with increases due to mutation.  Concern about inbreeding depression is legitimized by observed declines in fecundity of inbred captive brown bears.  Given the ratio of Ne to N, effective population sizes of 50 and 500 translate into total population sizes of 155-500 and 1,550-5,000 for grizzly bears. These rules of thumb still hold despite being developed over 30 years ago.


The known fates of isolated brown and grizzly bear populations are also instructive. Isolated bear ranges with the potential to support >200-500 bears all experienced population increases when aggressive hunting and other control measures were stopped, with evidence of resilience greatest of all for ranges with the apparent potential of supporting >2,000 bears.  By contrast, bear populations in Eurasia and North America that have been constrained by growth of human populations and related infrastructures, and which have been reduced to <100 adult bears, have all failed to recover or have even continued to decline toward extirpation.  The two grizzly bear ranges in North America with the apparent potential of supporting >300-600 bears – the Selway-Bitterroot and North Cascades – have not exhibited robust population growth simply because a sufficient inoculum of bears has been lacking.

Put together, this evidence from modeling, theory, and case studies is a basis for deriving two sets of population goals to direct recovery of grizzly bear populations. At a minimum, recovery efforts should strive for populations of 250-500 adult bears to provide for reasonable prospects of genetic conservation and population survival in the face of normal demographic and environmental variation for periods of 20-100 years. However, recovery efforts to promote long-term (i.e., indefinite) persistence should strive for populations of >2,000-5,000 bears. Populations of this size would guard against catastrophes and provide for evolutionary potential. In the case of populations smaller than 100 adult bears, recovery should, if possible, be premised on connection with other larger populations and, if not, involve concerted efforts to improve habitat conditions in areas large enough to potentially support at least 250 bears.


The Importance of Human Lethality

Lethal humans continue to be the greatest threat to grizzly bears because roughly 80% of all adult bears that die are killed by people.  The rate at which humans kill grizzlies can be usefully understood as a function of how often bears encounter people (i.e., frequency of contact) and the likelihood, given an encounter, that the bear will be killed (i.e., lethality of encounter).  This distinction is important because people’s lethality to bears dictates in some measure the protection from human contact that is needed to insure the persistence of grizzly bear populations. In areas where people are intolerant, armed, and otherwise motivated to kill bears, and where the environs used by people are attractive to bears because of natural or anthropogenic foods, enormous areas typified by rugged terrain and little or no road access will be needed to conserve grizzly bears—by sequestering them from people.  By contrast, if people are tolerant, unarmed, active in less attractive habitats, employing preventative husbandry, and assiduously managing anthropogenic attractants, there may be much greater opportunity for people and grizzly bears to coexist.


The potential for coexistence between grizzly bears and benign humans is substantial. The most dramatic evidence for the gains that can be derived by reducing human lethality come from the history of grizzly bear extirpations in the western US and from the differences in overlap between brown bears and people in Eurasia and grizzly bears and people in North America. As noted above, the Europeans that settled North America were remarkably lethal to grizzly bears, leading to the rapid extirpations between 1850 and 1950.  However, the trend toward extirpation was dramatically halted and reversed with protections afforded by the ESA, which contributed to major reductions in human lethality.  Of similar import, the overlap between brown bear range and areas of substantial human impacts (i.e., the “human footprint”) are much more substantial in Eurasia than in North America (Figure 2).  This difference in overlap is consistent with a more extended coevolution of occupancy between brown bears and people in Eurasia during which Eurasians were, and perhaps continue to be, less lethal. These lines of evidence support the proposition that grizzly bears could live in more places in the western US than they exist in now, and that the current distribution is largely an artifact of highly lethal people during an earlier century.

Figure 2. Overlap of brown and grizzly bear ranges with areas impacted by humans (the “human footprint”). Dark green is bear range in remote areas and light green is bear range overlapping with areas subject to moderate to high human impacts. Dark and light burgundy are areas without brown or grizzly bears and subject to high to moderate human impacts, respectively (Sanderson et al. 2002).

More concretely, the means by which people can better coexist with grizzly bears have been worked out, and with demonstrable reductions in conflict and bear mortality. Of foremost importance is the sanitation of human residences and facilities to make them less attractive to bears. Careful management of human-bear interactions, especially in National Parks, allows bears to be habituated to and consistently much closer to people, but without harmful consequences. Deterrents such as bear pepper spray have been shown to be a viable alternative to firearms for protection during close encounters with bears. Finally, management of agricultural attractants such as carrion, sheep, and cow calves configures conflicts in ways that create ample opportunities for improvement.

However, there are definite limits to the extent to which grizzly bears and humans can comingle—to how frequently they can come into contact with each other. Some degree of intractable conflict follows from the fact that grizzly bears are large carnivores that pose a threat to human safety and to things that we value such as domesticated animals and agricultural crops.  Because of that, grizzly bears will always depend on areas with limited human activity and access—on wilderness areas—for their conservation. This unavoidable reality requires that restrictions on human access and activity be an integral part of grizzly bear management, manifest in efficacies of regulating of backcountry human travel  and densities of open roads.  Numerous studies have shown that mortality risk for grizzly bears is dramatically higher near roads or, more generally, in areas with greater road access. The important point here, though, is that the extent of restrictions on human activity and access will necessarily be determined in part by the lethality of involved people, with the possibility that access restrictions might perversely increase human lethality. Conservation of grizzly bear populations will always require management of both human lethality and habitat security, with the need for security—for remoteness—greater under circumstances where people are armed and intent on harming bears.


The Ways That Grizzly Bears Integrate Ecosystems


Grizzly bears sit at the center of a uniquely dense web of ecological relations. Figure 3 depicts a simplified version of such a web for Yellowstone grizzly bears, with substantial energy flows from roots, foliage, fruits, seeds, insects, rodents, and ungulates to bears. Grizzlies are intelligent omnivores that interact with ecosystems not only on the basis of inter-generational knowledge, but also on the basis of new knowledge rapidly gained from close monitoring of their world. The upshot is that grizzly bears are un-paralleled among vertebrates in the extent to which they integrate and amplify signals emanating from multiple elements of all trophic levels.


Moreover, grizzly bears have been shown to substantially affect ecosystems. Grizzlies accelerate geomorphic processes, enrich soils, enhance biodiversity, regulate prey populations, and transport nutrients from marine to terrestrial systems. A large body of research has established the key role that grizzly bears play in enriching upland environments through extraction of salmon from spawning streams and the redeposition of salmon biomass in the form of carcasses and bear feces. Bear excavations of roots and rodents have been shown to increase the diversity of plant communities and elevate soil N levels. Under certain conditions, grizzly bear predation on calves can also regulate or even limit boreal moose populations,  and has the potential to do the same with interior elk populations. Bears can be the dominant predator of elk calves, and recent research has shown that calf survival plays a larger role than previously thought in governing growth of elk populations.


There is a veritable cottage industry of scientific scholarship devoted to terminology that expresses the special ways that grizzly bears and other species can affect or reflect ecosystems. According to this body of work, species can be foundational, keystones, umbrellas, flagships, indicators, engineers, strong interactors, and sources of trophic cascades.  Yet there is much contention about the utility, veracity of evidence, and qualifying criteria for these schemas.  Clearly, all are abstractions, yet abstractions that in some way denote species playing key roles in ecosystem processes that generate outcomes of value or interest to humans. And there is no doubt that grizzly bears potentially qualify for many of these categories if for no other reason than their charisma and centrality to a dense web of ecosystem relations, as well as the extent to which they receive, amplify, integrate, and return ecological signals.

Figure 3. A simplified representation of energy flows from vegetation to herbivores to carnivores in the Yellowstone ecosystem. Flows to grizzly and black bears are shown by blue arrows.


The implications of this ecological specialness are clear for grizzly bear conservation. First and foremost, the unique role of grizzlies in ecosystems and the related services that they provide to humans need to be recognized and considered in conservation planning.  Second, ecologically functional and otherwise healthy grizzly bear populations need to be a part of conservation goals.  Finally, grizzlies can provide extraordinary amounts of information about the states and rates of ecosystems, but only if managers are monitoring bear behaviors and ecological relations, and learning from what they observe.     

Implications for Recovery of Grizzly Bears in the Contiguous U.S.


The implications of the above background for recovery of grizzly bears in the contiguous U.S. are straight-forward. Recovery efforts should be focused on connecting all of the existing populations with each other to create a contiguous single evolutionarily robust population of >2,000-5,000 bears.  This is feasible if the Yellowstone population were to be connected with a restored population in central Idaho, in turn connected with the Cabinet-Yaak and Selkirk populations, in turn connected with the Northern Continental Divide population, partly through adjacent Canada. The potential of these combined populations would be roughly 2,200 bears.  The prospects for connecting the North Cascades to this mega-population are more uncertain, but potential linkages have been identified by Singleton et al. (2004). In the near term, efforts should be focused on restoring grizzlies to central Idaho, restoring habitat linkages of this pivotal area with the Cabinet-Yaak, Selkirk, and Yellowstone populations, recovering grizzly bears in the North Cascades and, at a minimum, maintaining current levels of protection for the Northern Continental Divide and Yellowstone populations.

The creation of a large contiguous grizzly bear population in the northern US Rocky Mountains, potentially connected to the North Cascades through Canada, would provide for long-term genetic resilience and demographic and genetic rescue of locally imperiled population segments by more robust segments elsewhere, especially if variations in habitat conditions or human lethality are not highly correlated among regions. Such a large population would also provide a buffer against uniform declines in habitat productivity and grizzly bear densities driven by climate warming and drying. Moreover, creation of such a population is feasible given evidence (above) that grizzly and brown bears can abide and survive densities of people higher than is currently the case, as long as the people are not lethal to bears and as long as there are effective programs in place to manage anthropogenic attractants and grizzly bear-related hazards to people.  Managers have also demonstrated the wherewithal to increase the permeability of the major highways that currently create fracture zones  by using strategically located over-and underpasses.  Creation of a large contiguous population of grizzly bears in the northwest U.S. is certainly possible. Current conceptions of recovery are bound more by history and lack of imagination than by a realistic appraisal of biophysical constraints and management options.

Finally, grizzly bears are capable of living in diverse habitats eating diverse foods, as long as mortality does not exceed reproduction. Moreover, the diversity of behaviors and diets engendered by diverse environments comprise an important facet of overall biodiversity. Much of this behavioral diversity has been lost, or nearly lost, for grizzly bears during the last 150 years,  but without any recognition in government recovery planning. Restoration of behavioral biodiversity is an important recovery goal that transcends mere tallies of grizzly bear population sizes.  In practice, restoration of behavioral biodiversity will require identification of habitat biophysically suitable for grizzly bears throughout their former range, followed by efforts to restore bears to a diversity of areas deemed capable of supporting them. There are several areas in former grizzly bear range—in Colorado, the Southwest, Utah, and California—that contain diverse and unique habitats and bear foods, and that, because of size and security, necessitate consideration for restoration of grizzly bear populations if the true mandate of the ESA is to be met.