for Fruit-Producing Species
The Earth is rapidly warming, with escalating temperatures greater at progressively higher latitudes. Projections specific to the northern US Rocky Mountains, roughly half-way between the equator and North Pole, show dramatic increases in temperature during the next 50-100 years, sufficient to swamp the more uncertain but modest projected increases in precipitation to yield intensified fire regimes and mounting chances of sustained widespread drought. As with so many other bear foods, this sort of warmer drier world will affect the future abundance of fruit-producing species that currently sustain grizzly bears.
A veritable cottage industry has arisen among scientists devoted to projecting the future abundance and distribution of various plant and animals species. Over time, the models used for such projections have become increasingly sophisticated and robust, despite often being more complex. Kirsten Ironside, currently an ecologist with the US Geological Survey, produced a series of such projections for fruit-producing species in the northern US Rocky Mountains using standardized climate change scenarios and models that incorporate, not only climate, but also factors related to substrate and topography. All of the species that Kirsten modeled are currently major sources of food for bears in this this part of the world.
The maps on this page depict the results of Kirsten's projections at the benchmark future year of 2050. In each map, yellow denotes areas where odds are greater than 50% that the species will disappear, and green areas where the species has a greater than 50% chance of persisting or colonizing. I've also included illustrations of important pollinators for each species primarily to emphasize the fact that future fruit-production will depend as much on the presence of pollinators as it will on the simple presence of a fruit-producing plant. This factor is highlighted by the fact that many species of bees have already experienced significant declines, for reasons only imperfectly understood, and that these very bees are important pollinators of species in the Rose and Heath families, including serviceberry (Amelanchier) and huckleberry (Vaccinium).
Although complex models are always intrinsically uncertain, the results at left are stark, and of such a magnitude to suggest a certain robustness to the vagaries of model building. Serviceberry is projected to virtually disappear from the northern US Rocky Mountains (top) and survive only as fragmented remnants at higher elevations. Chokecherry (Prunus) is also projected to nearly disappear from this region (middle), although survive, even thrive, in areas farther east on the Great Plains. The future is projected to be less bleak for buffaloberry (Shepherdia; bottom), although this species will also likely decrease in abundance, especially at lower elevations and in areas farther west. Overall, projections are for there to be much less fruit available to bears from these three currently-important species as soon as 35 or so years hence.
Parenthetically, and consistent with numerous other projections, future distributions of these berry species will not simply be a result of plants moving upslope in elevation. Some species will exhibit such a pattern, most will not. Some will fare better to the east, to the west, or to the south or north, all depending on climatic constraints, and the vagaries of how climates will reconfigure spatially.
Kirsten's projections for huckleberry are a special case. She produced two substantively different models for this singularly important species, one based largely on climatic factors, and a second emphasizing the effects of substrate. Both models showed a projected net decline in huckleberry, but the magnitude and distribution of losses were otherwise quite different.
The top map at right shows results for the climate-heavy model, the bottom map, for the substrate-heavy model. The first model shows major losses of huckleberry especially to the south and east, with near extirpation projected for the Yellowstone ecosystem in an area where fruit-production on this species is already scant. So the result makes sense. By contrast, when huckleberry is figuratively pinned in place by affinities for certain substrates, losses are projected to be minimal and concentrated at the lowest elevation. Of the two models, I give more credence to the climate-heavy model (top map) given that it comports better with what I know of current productivity and vigor of this species in different ecoregions.
Projections for Serviceberry, Chokecherry, & Buffaloberry
Projections for Huckleberry
The Aggravating Effects of Fire
Recent research has converged to suggest that projected changes in fire regimes in the northern Rockies will likely aggravate rather than alleviate the projected effects of a warmer drier climate, as such, on fruit-producing species. Research specific to the Yellowstone ecosystem projects increasingly widespread and frequent fires sufficient to reduce forest cover by upwards of 40-60% simply because, barring Douglas-fir, trees will not be able to reproduce and recruit at a requisite hectic pace. Even though species such as buffaloberry and huckleberry are fire-adapted, and most prolific, fruit-wise, at around 25-years post-fire, the projected frequency and extent of wildfires is such that even these species will not be able to keep pace with fire-caused mortality, especially in the drier Yellowstone and East Front ecosystems.
Even in wetter west-side ecosystems changes will probably not advantage fruit-producing species. Some recent work published by Bob Keane of the US Forest Service highlights what might happen in such an environment. The figure at left immediately above features key results of his projections that integrate effects of climate change, species autecology, and fire out to around 250 years. The total extent of forests declines by around 20% at the same time that lodgepole pine and spruce and fir-dominated forests decline, and Douglas-fir dominated forests increase from roughly 32% to 64% of total forest cover. Average intervals between fires will correspondingly decline from around 160 years to roughly 50 years in a much larger portion of the ecoregion.
Taken together, these results portray a future in which there is less fruit available for grizzly bears to eat in the northern US Rocky Mountains. In some places, losses could be catastrophic, especially for serviceberry and chokecherry; potentially also including huckleberry in places such as Yellowstone; but less likely for buffaloberry.
Put another way, there is little basis for optimism about future fruit abundance. This bottom line stands in stark contrast to blithe assertions by apologists for the status quo. An example of this rhetoric comes from the Yellowstone ecosystem during the last decade, voiced by biologists and managers invested in denying the consequences for grizzly bears of functional extirpations of cutthroat trout and whitebark pine in this ecosystem. These agency spokespeople have contended that grizzlies will compensate by eating more fruit, specifically fruit born by lower elevation species such as serviceberry and chokecherry. In the absence of critical analysis, they simply assume that lower-elevation species will migrate up in elevation in response to climate warming. The results presented here contradict this simple-minded invocation and are, moreover, consistent with what many other researchers have found in projecting future distributions of plant and animal species; that shifts will be idiosyncratic with respect to elevation, latitude, and longitude; and that losses will more often exceed gains.