American bison are one of the few species of megafauna to survive the late Quaternary extinctions in North America, only to be hunted to near extinction in the 19th century (though they're no longer considered endangered today). It's impossible to know for certain how many bison there were before the great buffalo hunts of the 19th century; early European anecdotes describe landscapes darkened by the giant animals as far as the eye could see (Krech 1999), though Charles Mann points out in 1491 that this may have been due to a population explosion following the devastation of Native American communities by European diseases. Earnest Thompson Seton, the first to estimate the population densities based on carrying capacity in the early 20th century, suggested that there were "not less than sixty million" bison in North America in 1500 AD (Krech 1999); subsequent estimates have been lower, in part because researchers have learned that bison tend to be very unevenly distributed across the landscape through time (a factor that some have suggested is responsible for their survival into the Holocene). While the exact numbers of bison may be unknown (and disputed), what is less contentious is the role of bison as a keystone species of the North American tallgrass prairie, maintaining patches of different species as they ate, wallowed, and rubbed their way across the Plains (Knapp et al., 1999).
Bison are picky eaters; their diets are typically composed almost entirely of grass, and they usually avoid other common flowering prairie perennials, like ragweed and ironweed (Fahnestock & Knapp 1994). In addition to avoiding these “forbs” in their diets, the very presence of bison makes the prairie a more forb-friendly place. Forbs tend to be shorter than grasses in the tallgrass prairie, and also have shallower root systems. This means that forbs can’t outcompete grasses for light in the shadow of grasses, and aren’t as able to reach the moisture in prairie soils that sustains grasses through dry spells. In other words, by acting as ecological lawnmowers, bison basically keep the grasses in check, allowing forbs get a bigger slice of the resource pie. Understandably, the late Holocene near-extirpation of bison is thought to have significantly influenced the ecology of the modern Great Plains, and bison grazing is thought to be an important component of prairie restoration efforts.
Bison do more than just eat, however. Like many other large animals, bison roll around in the dust or mud in natural depressions on the landscape. As they roll and lay in these “wallows,” they compact the soil, enlarging the depression when they carry away the mud that sticks to their fur. It’s not entirely known why bison wallow (yep, it’s a verb, too). They may be seeking relief from insect bites or parasites, cooling off, grooming themselves during their moulting period, or even just having fun (MacMillan et al. 2000). Regardless, these patchy disturbances in the prairie landscape can persist for a century or more; some modern wallows have been found in places where bison have been absent for >120 years. It’s estimated that there may have been >100 million wallows over the 70,000 ha of the prairie prior to European settlement (McMillan et al 2011).
While the diversity of plants in wallows has been found to be lower than that of the surrounding prairie, those species found in the wallows are often different, particularly along the disturbed edges (Collins and Uno 1983). If you read the Little House books, you may remember that Laura’s little sister Grace wandered off and was later found in an old buffalo wallow full of violets in By the Shores of Silver Lake. One reason for the difference in composition is that bison compact the soils in the wallow as they roll around, and so wallows provide disturbed habitat where ragweed and other forbs are more readily able to out-compete deep-rooted grasses for water. Since wallows are formed in the spring and summer and tend not to drain very well, they hold more water (and for longer) than the surrounding undisturbed soils, which can provide good habitat for wetland species and is a source of water for prairie animals. At Konza Prairie, reintroduced bison both reactivated old wallows and made new ones, which were then used as breeding sites by frogs in the years when the climates were cool and wet enough that the depressions formed standing pools (Gerlanc & Kaufman 2003). Today, the absence of wallows has contributes to the prairie being a less diverse place than it was before European settlement, all else being equal.
Modern prairie remnants or restorations may be grazed by cattle, which don’t wallow, or may not support any megaherbivores at all. Bison wallow research, even as something of a niche field, points to the importance of non-trophic interactions between animals and plants; landscapes, it seems, experience much more of a megaherbivore than its digestive system. If the Holocene prairie was more like a patchwork quilt than a bedsheet in terms of landscape diversity, what are the ecological legacies of the loss of bison wallows? Do the frogs, birds, insects, and wetland plants of the modern prairie record a signature of the loss of these missing potholes of diversity?
Collins & Uno, 1983. The effect of early spring burning on vegetation in buffalo wallows. Bulletin of the Torrey Botanical Club 110: 474–481.
Fahnestock & Knapp, 1993. Water relations and growth of tallgrass prairie forbs in response to selective herbivory by bison. International Journal of Plant Science 154: 432-440.
Gerlanc & Kaufman 2003. Use of Bison Wallows by Anurans on Konza Prairie. The American Midland Naturalist 150(1):158-168.
Knapp et al., The Keystone Role of Bison in North American Tallgrass Prairie. Bioscience 49: 39-50.
Krech, Shepard, III, 1999. The Ecological Indiana: Myth & History, W. W. Norton & Company, New York, NY.
MacMillan et al., 2000. Wallowing Behavior of American Bison (Bos bison) in Tallgrass Prairie: an Examination of Alternate Explanations. The American Midland Naturalist 144(1):159-167.
MacMillan et al., 2011.Vegetation Responses to an Animal-generated Disturbance (Bison Wallows) in Tallgrass Prairie. The American Midland Naturalist 165(1):60-73.