Bison Air Support

When we think of the Great Plains and the prairies, we most likely envision a landscape of grasses rolling on to the horizon.  But the skies above those grasses are as much a part of the ecosystem as the land itself.  Upon the air currents circulating over the land, birds, insects, and bats fly searching for food, nesting materials, and escape routes from predators.  These same air currents disperse seeds over the plains and prairies, increasing plant diversity.  Though the aerial species may be above it all, they, like any air force, are dependent upon ground support.  Key to that support is the bison.

                In tall-grass prairies, lupine, a flowering plant, has diminished along with the demise of the bison.  Lupine flourishes with a mix of shade and sun.  Without the bison, the trees and woody shrubs of the oak savannah [1] overshadow the other plants, depriving the lupine of much needed sunlight,  resulting in a decline of lupine across the northern prairies due to loss of habitat.  Much of that loss resulted from converting the prairies into farmland.  But in the remaining prairie, tree and other woody plant cover has suppressed lupine growth.  In the past, when bison were present, they maintained a reduction of woody plant cover.  They kept woody shrubs and trees out of the area by rubbing their horns and bodies against the shrubs and trees, and by causing minor soil disturbance, the bison had a significant impact [2].

Wild Blue Lupine
An Oak Savana

                Among the lupine, flitter approximately 50 rare species of butterflies.  One in particular is the Karner Blue Butterfly [3], listed as endangered in 1992.  The Karner relies on lupine as both a food source and a place to lay their eggs.  The bison create and restore the open habitats of the oak savannah prairie, which the lupine and the butterflies desperately need.

The Karner Butterfly

                Re-introducing the bison to one area, the Sandhill State Wildlife Area in Wood County, Wisconsin, [4] bore out this relationship among the butterflies, the lupine and the bison.   Since the return of the bison, a significant recovery of both lupine and the Karner has occurred [5].

                Birds of the Great Plains and the prairies benefit from the bison as well.  While bison graze the wealth of perennial grasses, they are often surrounded by a small flock of birds, including brown-headed cow birds (also known as buffalo bird), starlings, and magpies.   The grazing stirs up insects out of the grasses, providing an easy feast for the surrounding birds.  In addition, the birds get free rides, hitching on the backs of the bison, consuming the insects found in the bison’s hair.  The bison amenities, though, not only offer meals.  During inclement weather, the backs of bison also offer protection against the elements. 

The Brown-Headed Cow Bird
Birds and Bison

                While grazing renders insect-hunting easier, bison-grazing creates heterogeneity of grasses, resulting in a range of forage heights  from very low (heavy grazing) to high grass (no grazing).  These conditions promote a variety of bird species from those who nest only in low vegetation to those who nest only in high vegetation and those species whose nesting preference is somewhere in between. In contrast, cattle graze uniformly in regard to vegetation height, leaving forage at a mid-height which reduces habitat for those avian species that require either low or high vegetation [6].

                Amid the grasses where the bison roam, shallow soil depressions can be found.  For various reasons bison roll in the soil, creating wallows.  Filling with water during storms, wallows provide habitat for a variety of insects, frogs, and birds.  For the birds, the water-filled wallows provide drinking water and a staple of insects.  Dry, the wallows offer habitat for such birds as the sharp-tailed grouse and the burrowing owls [7].

                Aerial support though, also involves providing materials for ground cover.  Bison hair and wool, as well as dung, [8] are used in the construction of nests.  Hair and wool provide insulating and water-repellant materials and more.  Nests lined with bison wool suffer less predation because the hair, serving as olfactory camouflage, masks odors of the nest site [9], warding off predators.

                A significant example of the importance of the bison to bird species would be the burrowing owl, which has been declining for many years due to the loss of habitat and the control programs of prairie dogs and ground squirrels.  Prairie dogs are just as essential to the Great Plains and prairies as the bison are.  A full discussion of the significance of prairie dogs is beyond the scope of this article, but in respect to the borrowing owl, the prairie dog burrows provide ideal habitat for the burrowing owl.  The bison’s contribution, especially in tall-grass and mixed-grass prairies, comes through their grazing.  The bison make the prairie dog towns possible.  Prairie dogs will not live in tall-grass since it is less nutritious and hides predators.  But bison grazing keeps the tall-grass short enough to promote prairie dog towns [10].  

Once a burrow becomes available, the burrowing owl brings together the burrow and bison dung into a unique arrangement.  Using bison dung to line the burrow, the burrowing owl ambushes dung beetles.  Attracted by the feces, dung beetles are an important food source for adults and developing nestlings [11] (Perhaps this is why owls are considered wise, because they know their shit).

Burrowing Owl

Even in death the bison provide for the aerial life of the plains.  Scavenger birds such as crows, ravens, magpies and turkey vultures feed on the carcasses. Typically such scavenger birds as ravens and magpies along with turkey vultures will show up first.  Initially, these birds feed on carrion flies, since they cannot break through the hide.  Later, perhaps after spotting the ravens and magpies, bald and golden eagles may join the feast. But even the eagles may have difficulty tearing open the hide and generally depend on other predators such as wolves to perform that task. Once opened, the eagles are able to rip apart the innards of the carcass into smaller chunks, creating serving sizes for smaller mammals [12].  Indirectly, the carcasses, as they decay, nourish the plant life which in turn offers habitat and food for birds and butterflies.

From flittering butterflies to soaring and burrowing birds, bison support a variety of invertebrate and avian species which populate the skies above the plains and prairies.  The decline of the bison and the conversion of the Great Plains and prairies into farms, ranches and concrete has devastated the indigenous avian, riparian and lepidopteran species. The avian species of the Great Plains have seen significant population decline [13], while the many butterfly species of the surrounding prairies also appear on the threatened or endangered species lists.  Fortunately, such efforts as those of the American Prairie Reserve in Montana and the Sandhill State Wildlife Area in Wisconsin are making strides in recovering many of those species.  But the questions will be:  Will these efforts be expanded? How will we act upon what we have discovered?  How quickly will we act?  Some time ago Joni Mitchell wrote a song called Big Yellow Taxi, which starts out:

They paved paradise
And put up a parking lot
With a pink hotel, a boutique
And a swinging hot spot
Don’t it always seem to go
That you don’t know what you’ve got
‘Till it’s gone

It would seem these words are still apropos in regard to the ecosystem of the Great Plains and the surrounding prairies.

____________

[1] An oak savannah is a plant community where the oak tree is a dominant component but the density of the oaks is so low it allows grasses and other vegetation to become the actual dominants of the community. Retrieved from https://oaksavannas.org/.

[2] Bison Bellows: Bison Bolster Endangered Blue Butterfly Recovery. https//www.nps.gov/articles/bison-bellows-8-4-16.htm

[3] Named by the novelist Vladimir Nabokov. 

[4] In 1962 12 bison were donated to the Sandhill State Wildlife Area.  The current herd consists of 15 bison on 260 acres.

[5] Bison Bellows: Bison Bolster Endangered Blue Butterfly Recovery

[6] Heidebrink, Scott. Bison Restoration Manager. American Prairie Reserve. Email to author dated 3/8/19.

[7] Retrieved from https://blog.nwf.org/2016/06/wildlife-that-depend-on-wild-bison/

[8] Coppedge, Bryan R. Patterns of Bison Hair Use in Nests of Tallgrass Prairie Birds.  The Prairie Naturalist. 41: December 2009. 110-115.

[9] Baily, James A. American Plains Bison: Rewilding an Icon. 2013. Sweetgrass Books. Helena, MT. 46. and Coppedge

[10] Lott, Dale F. American Bison: A Natural History. 2002. University of California Press. Berkley, CA. 128.

[11] Coppedge

[12] Lee, Barbara. Decomposition.  Montana Outdoors. 2014. 21-22. Retrieved from

[13] Brennan, L.  A., and  W.  P.  Kuvlesky. 2005. North American  grassland  birds:  an  unfolding  conservation

crisis?  Journal of Wildlife Management 69:1–13.