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  • David Mattson

Grizzly Twister and Other Games that Scientists Play

by David Mattson

Those of you who were born after 1970 probably don’t remember a game made by Milton Bradley called Twister that in 1966 exploded onto the national stage and into America’s living rooms and dens. Its popularity was considerably enhanced when Johnny Carson leaned scandalously over Eva Gabor’s partially clad buttocks during a live demonstration of the game on the Tonight Show. The game plays out on a large mat sporting multi-colored dots laid on the floor. The players are the figurative game pieces, contorting themselves as they place hands and feet on dots according to the dictates of a spinning dial. The winner is the one who stays upright, the loser the one who tips over in a twisted knot of limbs.

This game often comes to mind as I listen to or read quotes from Yellowstone’s grizzly bear researchers and managers, although at one level their mantra is pretty straight-forward. We’ve got ever more grizzly bears in ever more places being driven farther out onto the edges of the ecosystem by saturation of carrying capacity in the center. Moreover, carrying capacity is a static parameter creating a context within which the ever-adaptable omnivorous grizzlies are readily switching diets to fully compensate for changes in comparative availabilities of foods, drawing on a repertoire of more than 200 roughly equivalent foods.

At one level this official rhetoric can be understood simply as a brew of misrepresentation, disingenuousness, and partisan selectivity. As a literal matter of fact, carrying capacity is not static; the quality and quantity of Yellowstone’s grizzly bear foods vary by orders of magnitude; continent-wide, substantial differences in foods drive similar orders-of-magnitude differences in grizzly bear densities; nor are grizzly bears Latin taxonomists. Regarding this last point, the putative laundry list of >200 bear foods in Yellowstone was concocted by researchers based on phylogenetic differences among foods codified in Latin names. The problem is that such distinctions don’t matter to bears, whereas the many-fewer functional differences in nutritional quality and architecture of foods do.

These counter-factuals are perhaps so obvious as to beg the question of what motivates researchers and managers who spout the official rhetoric—and who should know better. Without belaboring the point, none of the explanations I’ve been able to come up with so far are very flattering.

Claims about Population Size and Trend

But my focus here is on claims of ever more bears in ever more places being driven to ecosystem extremities by progressive saturation of carrying capacity.

Perhaps the first point worth reiterating is that carrying capacity varies—potentially by a substantial amount—and has likely declined in the ecosystem core with losses of cutthroat trout, whitebark pine, and elk. All three of these foods had been high-quality dietary mainstays for Yellowstone’s grizzly bears, but are all now much diminished—cutthroat trout to the point of functional extinction as a bear food. Thus, even if true, saturation of carrying capacity has more likely occurred because of a lowering of this figurative ceiling rather than because of burgeoning bear numbers. And any changes in habitat have almost certainly been compounded by the reorientation of bears to alternative high-quality foods disproportionately concentrated on the ecosystem periphery; e.g., livestock and army cutworm moths. The data for increased consumption of moths and terrestrial meat are unambiguous, as is the sky-rocketing increase in human-bear conflicts over livestock.

But, more to the point of this piece, I find the numerous potentially contradictory statements made by agency spokespeople about population size and trend to be especially intriguing. For simplicity’s sake, its’ worth looking at a few illustrative examples from Frank van Manen, head of the Interagency Grizzly Bear Study Team (IGBST), and the man charged with overseeing grizzly bear research in the Yellowstone Ecosystem.

  • He has said “The population has been at a pretty stable level since the early 2000s,” in the same article as “…there’s no doubt that we are now at a point that we have the largest population size.”

  • He has likewise said that “grizzly numbers outside the monitoring area aren’t counted” (the “monitoring area” being the Demographic Monitoring Area [DMA] that defines the bounds within which bears are counted), while invoking bears outside the monitoring area to claim “the largest number of bears ever.”

  • During a recent meeting in April 2019, he was seen pointing with agitation to a graph showing that grizzly bear mortalities had mounted exponentially during recent years, while saying that “mortalities were not unusual last year, and exceeded by death rates during 2008.”

  • Meanwhile the press is littered with his quotes variously asserting that the distribution of Yellowstone grizzlies has expanded by 11%, 34%, 38%, and even 300%, referencing vagarious temporal benchmarks.

  • He and the US Fish & Wildlife Service have also repeatedly claimed that a certain method used for estimating population size and trend (the so-called Chao2 method) is “the best available science,” while in near the same breath invoking other unspecified but presumably more deficient methods to claim that Chao2 under-estimates population size by “a lot” or “by 40 to 50 percent.”

You should be confused. I’m confused. Perhaps the intended outcome is for a confused public to yield to the confusion and surrender their trust and judgement to the presumed experts—the only winners left standing in the game of Grizzly Twister.

Nonetheless, there is an imperative to penetrate the veil of obfuscations and contradictions to help foster a truly informed rather than faith-based public conversation about how best to conserve our iconic Yellowstone grizzly bears. So, in service of that end, what follows is my best shot at penetrating the tangled official rhetoric, and elucidating what’s probably been happening with size, trend, and mortality of Yellowstone’s grizzly bears…

Imposition of a Political Boundary

A main source of confusion arises from the fact that the IGBST estimates population size and sex-specific mortality rates only for the Demographic Monitoring Area (DMA), and not for any areas outside. Estimates of population size are derived, at root, from estimated numbers of reproductive females within the DMA, which are then multiplied by various more-or-less static factors to account for other sex and age classes of bears.

Leaving intrinsic problems with methods aside for the moment, up until recently the DMA encompassed all of the sightings of reproductive females and nearly all of the documented grizzly bear mortalities. But during recent years an increasing fraction of both mortalities and sightings of reproductive females have occurred outside the DMA, leaving a presumably increasing but unknown fraction of the total population unaccounted for in estimates of population size and mortality rates that are limited to the DMA.

Importantly, estimated numbers of reproductive females have been more-or-less static within the DMA since roughly 2002, hence a population at a “pretty static level.” I’ve shown trends in numbers of reproductive females inside the DMA derived from two different methods in figure 1 below. Chao2 represents the presumed “best available science,” whereas MR refers to the less esteemed Mark-Resight method—the apparent (although unacknowledged) basis for claims that there may be 40-50% more bears in “the population” than indicated by the Chao2 method.

Outside the DMA, there are scant grounds for determining how many bears might be unaccounted for, although agency researchers and managers seem to be using aerial extent of the population's distribution outside the DMA as a surrogate--despite the fact that bear densities are certainly lower on the periphery and comprised of a greater proportion of males. Regardless, these unaccounted-for bears seem to leave agency spokespeople ample latitude for claims that we have “record numbers” of bears.

As a bottom line, we don’t, in fact, know total size of the population, or what the trend has been, simply because the bounds within which numbers of live bears are tracked were arbitrarily truncated at the DMA through a process that had more to do with partisan politics than bear biology. What we do know is that numbers of reproductive females within the DMA have been static for the last 15 plus years; that the DMA accounts for most—a super-majority—of bears in the ecosystem; but with the proviso that total population distribution is increasingly extending beyond the DMA.

A Compounding Problem

An additional and on-going source of confusion arises from the fact that researchers and managers have changed the fundamental basis for estimates of trend since 2002, but with each successive change leading to an artificial inflation of population estimates over what had come before. More specifically, trend was tracked on the basis of estimated MINIMUM numbers of bears up through 2006, accounting for the totality of the distribution. Beginning in 2007, trend was tracked on the basis of TOTAL population size, still accounting for all bears in all areas.

But with this change came the chronic problem of managers claiming a substantial increase in the population, but based almost solely on comparing the apples of a minimum estimate with the oranges of a total estimate. This conflation has been egregious and obvious enough to, again, beg the question of motives.

Nonetheless, a further change was introduced in 2012 when the IGBST started using a larger multiplier to account for numbers of males in the population. So, here again, we had a seeming instantaneous increase in estimated numbers of bears, but based largely on a change of method and the addition of a putative burgeoning of demographically extraneous males.