During the last glaciation, a band of ice-free steppe extended from northern Europe to the western edge of Greenland. Known to paleoecologists as Berengia, this virtually treeless expanse not only formed a 500 mile wide land bridge between two great northern continents but sustained an extraordinary migration corridor above the ice sheets, much as the thin belt of the Sahel skirts across Africa below the Sahara today.
The fossil record shows that woolly mammoth, horse and steppe bison predominated in this ecosystem: 50% of all fossil bones from the unglaciated regions of northern Yukon and Alaska in this period are from bison species. In comparison, carnivores represent just 1-2% of fossil bones from that time: lions, scimitar cats, short faced-bears, as well as wolves, wolverines, and Asiatic Brown bear, the last of which crossed the land bridge at about the same time as the ancestors of the mammoth hunting Clovis point people arrived in North America.
The arid steppe ecosystem of Berengia sustained and responded to the movements of large herbivores, but Woolly Mammoth appears to have played a critical role in keeping down shrubby vegetation and allowing palatable grasses to proliferate. An alternate name for the grassland vegetation of Berengia is the Mammoth Steppe. Scimitar cats fed almost entirely on mammoth, while its carrion sustained a host of biota, from the omnivorous short-faced bear to the huge teratorns and New World vultures, of which the California condor is a modern relict. Mammoth have been described as Berengia's keystone species during this last, great ice age.
The keystone species hypothesis, first proposed by University of Washington professor Robert T. Paine in 1969, is now well ingrained in conservation biology. As the architectural keystone supports an entire arch, so a keystone species sustains and upholds a functioning ecosystem through its activities. Like the single keystone supporting the cathedral vault, the effect of such species is disproportionate to its numerical abundance. They may dramatically modify habitat, like the beaver or prairie dog, or by their activities sustain complex ecological synergies , like migrating salmon and Grizzly Bear. In ecosystems as in architecture, removing the keystone causes collapse.The American Chestnut is sometimes described as a keystone species for the forests of the eastern United States, and the full impact of its loss has yet to be measured in our woodlands today.
Some combination of climate change destroying habitat and human predation brought down the Woolly Mammoth. Its loss would have removed one of the most significant natural disturbance mechanisms keeping the willows, alders and other shrubby vegetation of the Mammoth Steppe in check. While a moister, warmer climate may have played its part, such periods between prior glaciations did not bring on the massive extinction event of the late Pleistocene. Mammoths survived significant fluctuations in climate from their arrival in North America 1.7 million years ago to their final extinction. Tim Flannery writes in his extraordinary ecological history of North America, The Eternal Frontier, that the loss of the woolly mammoth would have had significant, ecosystem-wide impacts:
"(T)he extinction of the mammoth led to the destruction of the Mammoth Steppe...a ten-ton mammoth would have consumed 100 and 120 kilograms of vegetation (much of it possibly woody) each day, dumping out waste in huge bowel movements at almost hourly intervals. The effect of such large herbivores is profound in regions, such as Alaska, where cold or drought reduces levels of primary productivity.
What happened when the mammoth finally vanished from Alaska? Doubtless much course vegetation which otherwise would have been eaten remained untouched. Whatever lesser megafauna remained, such as caribou, would not have been able to consume this course herbage because only the largest herbivores can profit from such poor-quality food. This vegetation would have eventually built up into insulating mats that inhibit nutrient flow and support the inedible vegetation that occupies Alaska today. (Flannery 2001: 208-209)"
With climate change accelerating the process, the ecology of mammoth steppe of Berengia would have rapidly proved inhospitable for many of the Pleistocene herbivores and the carnivores that stalked them. If excessive predation by human hunters was a critical factor in the elimination of the mammoth as a keystone species, then it is hard to avoid drawing parallels to the role of our species in the current, global extinction event playing out in a time of enhanced and accelerated climate change.
Some ecologists have recently made the controversial argument that North America's impoverished megafauna would benefit from the reintroduction of species like the lion and elephant with recently extinct evolutionary antecedents here. While this suggestion is hard to take seriously at face value, the fact remains that here in the Eastern woodlands we are missing our top carnivores (cougar, gray wolf), our signature mast bearing tree (American chestnut), and one of our major seed dispersers (passenger pigeon). Conceivably, the boreal dwelling mastodons of the late Pleistocene that ranged the eastern half of the United States played a role similar to the mammoths of Berengia in maintaining some of the early successional habitats of this region that require periodic disturbance to retard succession and are now quite rare. Conservation land managers turn to fire, mechanical and chemical means to manage some of these threatened natural communities.
What other keystones, known and still unexamined, sustain our present ecology? Can we, who disproportionately modify habitat in comparison to all other biota, keep from kicking down ecological keystones and modify our behaviors to support rather than degrade ecosystem function?