Rethinking Eastern Grasslands: A Deep Time Perspective

By Michael R. Hall

For eastern grassland enthusiasts, the questions and mysteries kept piling up. Ponder the background behind some of the answered riddles:

As far east as Ohio, prairie remnants can still be found, although 99% of this habitat has been lost to agriculture and development in historical times. The problem is that east of the Mississippi River the climate favors forests. In most instances, the prairies will disappear or degrade rather rapidly without almost annual burning. Yet, frequent burning tends to destroy the very insect populations, such as butterflies and beetles that the prairie plants depend upon for pollination. Such burning also takes a toll on certain plants (e.g., mosses and lichens) and other animals, and temporarily disrupts the microbiological life in the soil. In addition, although there is abundant evidence that Indians frequently burned grasslands and some forest areas to improve hunting and to get rid of ticks, this does not explain how prairies persisted in the east prior to such human intervention. Natural fires do not occur annually on eastern prairies.

There were other mysteries as well. Why is this region the main natural habitat of the federally-endangered Indiana Bat (Mysotis sodalis), a bat which recent studies have shown prefers open-canopy forests and savannas, when the climate virtually always results at present in closed-canopy forests? Their populations have been declining ever since they were first discovered as a separate species in 1935. Likewise, why are Osage Orange, Honey Locust, and Kentucky Coffee trees only found in limited areas in the wild in bottomlands when they grow perfectly well upland and were once, according to fossil records, far more abundant and widespread until the Holocene (i.e., the last 10,000 years)?

The late eminent botanist, E. Lucy Braun, after careful examination of both plant species and geology, concluded that the unusual mid-grass prairie remnants in southwestern Ohio (i.e., Adams County) were pre-Illinoian in age (over 600,000 years old).[1] The last warm interglacial interval that swept tall-grass prairies into the east from the Great Plains (the so-called Prairie Peninsula) occurred 6-8,000 years ago, and did not extend as far south as Adams County, Ohio. Examination of species from the Sangamonian Interglacial (120,000 years ago) suggests the wide-spread presence of large grazers and, therefore, grasslands in the east. The Sangamonian is now thought by paleontologists to have been a warmer interglacial than the more recent one which followed the Wisconsinan. For example, hippos occupied London, England and jaguars and tapirs occurred as far north as the southern Great Lakes states during the Sangamonian.[2]

It is apparent that prairies and grasslands had swept eastward during at least several separate interglacial intervals over the last million years or more. But how did these prairies and grasslands survive when the climate changed periodically to favor forests?  The only plausible answer appears to be grazing and browsing by megafauna. However, when we look at all the extant megafauna that have ever existed in the Great Lakes region, none existed in pre-Illinoian times that would be suitable candidates for the task.

Bison did not enter North America until about 400,000 years ago, about the same time as elk (Cervus elaphus).[3] Likewise, tundra muskox (Ovibos moschatus) and the extinct woodland muskox (Bootherium bombifrons) are large grazers, but didn’t arrive until the Illinoian.[4] The tundra muskox likes colder conditions than the climate that favors prairies. The caribou (Rangifer genus) has been around for most of the Pleistocene (for at least 1.6 million years) and has been found as far south as Virginia and Tennessee, but it prefers tundra and woodland habitat.[5] Black bear (Ursus americanus) and white-tailed deer (Odocoileus virginianus) were certainly around in pre-Illinoian times, but no one would seriously argue that these forest and woodland-edge species could keep the woody plants from taking over a grassland when the climate favored forests.[6] The grizzly bear (Ursus arctos) and the modern moose (Alces alces) are even more recent arrivals from Eurasia than the bison and elk (i.e., from the Wisconsinan).[7] Pronghorn fossils (Antilocapridae family) have never been found in the Great Lakes region.[8]

Clearly modern explanations for grassland habitat viability and long-term ecosystem health in the east are inadequate to explain pre-Illinoian prairies. Something important is missing. To find the answers, it is necessary to explore conditions of grassland ecosystems in the east prior to the appearance of extant megafauna from Illinoian glacial times and thereafter, and of humans some 15,000 years ago. How did these systems evolve? How were they maintained?

To look for the ecological origins of modern prairies and grasslands, one can begin with the Blancan Age, which commenced about 3.5 million years ago and ended with the start of the Pleistocene, some 1.9 million years ago. Rhinos had already become extinct in North America around the beginning of this time (over 3 million years ago), but the Great Plains were vibrant with horses, camels, mastodons, pronghorns and giant ground sloths.[9] Camels existed solely in North America until 4 million years ago and, in Blancan times, there were camels as tall as 12 feet at the shoulders with long necks.[10] They could browse almost as high up as modern giraffes. Likewise, some ground sloths grew up to 11 feet long with an additional reach of several feet with its long forearms when it stood, tripod style, on its hind feet and tail.[11]

Horses also originated in North America and spread to several other continents. In Blancan times, a one-toed American zebra (Equus simplicidens) shared the plains with the last survivors of the three-toed gazelle-horses.[12] Experts now agree that the American zebra was closely related to the living African species called Grevy’s zebra (Equus grevyi).[13] The Grevy’s zebra lives in smaller groups than the more common plains zebra of Africa, and prefers more arid conditions.

Although mastodons (Zygodont family) and stegomastodons (Bunodont family) had been in North America for a total of about 15-17 million years, representatives of the true elephants, the mammoths, wouldn’t arrive until 2 million years ago.[14] The last stegomastodon (Stegomastodon mirificus) is recorded from North America sometime around 1.6 million years ago.[15]  The last America mastodon (Mammut americanum) survived until only about 10,000 years ago, and is now believed by some paleontologists to be the eastern seed disperser for many large seeded plants such as Osage Orange, Honey Locust and Kentucky Coffee trees.[16] Its browsing activities are also thought to have kept eastern forests more open in places, which might help explain the Indiana bat population in the east.  Although the mastodon is said to have preferred spruce and pine forests as well as wetlands, its wide distribution throughout the continental United States and northern Mexico suggests it was not picky about its browse.

            In Blancan times we also have a North American hyena (Chasmaporthetes ossifragus), although it was built more for speed in open areas than modern hyenas. In addition, we have an American cheetah (Miracinonyx studeri) found in an area that was thought to have forest–savanna at that time in the Great Plains.[17] This cheetah likely hunted pronghorns and deer. There was a scavenger-built dog, the bone-eating dog (Borophagus diversidens) with powerful jaws and short legs; and a saber-toothed cat, the western dirktooth (Megantereon hesperus).[18]

During the Pleistocene (1.9 million years to 10,000 years ago) we have some new and additional cast members of the megafauna on the American grasslands. Enter the mammoths, 95% of whose diet is grasses; the American lion (Panthera leo atrox); the short-faced bear (Arctodus simus) who was twice the weight of modern brown bears and more of a true long-legged carnivore; the flat-headed peccary (Platygonus compressus), a grassland species of peccary; a later cheetah species (Miracinonyx trumani); a later saber-toothed cat (Smilodon fatalis); etc.[19]  In fact, cheetahs are possibly the one genus of the cat family now believed to have evolved originally in North America.[20]

            If the above descriptions appear more than coincidentally to resemble the Serengeti of Africa, we should remember that Africa was already connected to the Eurasian continents at this time which, in turn, were periodically connected to North America via the Bering land bridge during several glacial ice advances. Prairies and savannas and other grasslands around the world during the Pleistocene were the home of more and larger megafauna than any other habitat. By the Wisconsinan (100,000 years ago), the megafauna diversity on the Great Plains surpassed anywhere else in the world, including the modern Serengeti.[21] North America lost over 70% of all its megafauna just 8,000 to 12,000 years ago. Our native grasslands have been impoverished habitats ever since. They are like rivers without fish.

The arrival of ancient bison and elk around 400,000 years ago and the survival of one species of pronghorn (Antilocapra americana) undoubtedly helped preserve the grasslands of the Great Plains after the demise of the other megafauna. A drier climate and, in some cases, dissolved minerals forming a hard pan a short distance below the soil surface kept out the woody plants there, but what about the eastern grasslands?

Unfortunately, there is almost no record of fossils in the east from the Blancan Age outside of Florida. In fact, there are few fossils at all until the warmer interval called the Sangamonian Interglacial (around 120,000 years ago) in the Great Lakes region.  Although in this region there are a few fossils of peccaries, camels, and horses from Sangamonian times, their limited presence does not seem to provide an adequate explanation for grassland persistence. Likewise, giant ground sloths, like the jefferson ground sloth (Megalonyx jeffersonii), and the giant land tortoise (Hesperotestudo crassiscutata) have too slow a metabolism to keep grasslands clear.

The most probable answer in the fossil record of the Great Lakes area all point to one keystone group of species: the elephants. More specifically, we are referring to the mammoths and the mastodons. There are more mammoth and mastodon fossils (mostly teeth) from more counties from the southern half of the lower peninsula of Michigan and from all of Ohio than from any other region of the country.[22] Indeed, there are more fossils of elephants in these two states than of all other Pleistocene megafauna combined. This then was the center of North America elephant country. Woolly mammoths (Mammuthus primigenius) ate the grasses in the colder tundra or steppe-like areas during glacial advances, while Jefferson mammoths (Mammuthus jeffersonii) ate the grasses in the warmer prairie areas after glacial retreats. The mastodons were major browsers in this ecosystem that also helped keep out the woody vegetation and this, in turn, also encouraged prairie and savanna conditions.

What was it about this area that was so good for elephants? Elephant experts say that, while virtually all mammals need salt and water, elephants need these items in such large amounts that it is considered a limiting factor for their populations.[23] Their diet is high in potassium which gradually leads to a sodium imbalance which must be periodically corrected.[24] Ohio, southern Michigan, and northern Kentucky are especially rich in large salt deposits at the surface (i.e., salt licks) and, unlike some areas with large salt deposits out west, there is plenty of fresh water here as well.[25]

There is also abundant clay. Elephants are known to walk up to 200 miles to get salt and mineral rich clay. That puts all of Ohio and southern Michigan in easy range of such substances for elephants (and other mammals). Elephants also have enormous appetites, and the greater rainfall in the eastern United States allowed their grazing and foraging areas to recover more rapidly. It is well known that you can support more grazing animals in the east than in the more arid west on a given amount of acreage.

In addition, the most successful known predator of baby mammoths (except for man) did not exist in the northeast quadrant of the United States: the scimitar-toothed cat (Homotherium serum).[26] A distant cousin, the saber-toothed cat (Smilodon fatalis) did occur in this region (or at least in southern Indiana) but it is not known to have been very successful at separating baby mammoths or mastodons from their protective mothers. To accomplish this it is believed that the scimitar-toothed cat likely hunted in pairs, unusual behavior outside of lions for large cats, with one adult drawing off or distracting the mother while the other slashed the infant’s neck or throat causing it to bleed to death.[27] 

What is the significance of all these elephants to eastern grassland habitats? Preserve managers in Africa have observed that rhinos and elephants are keystone species to the grassland and savanna biomes.[28] They keep out trees and shrubs and fertilize the grassland with copious amounts of dung (buried by armies of dung beetles). When elephants are eliminated, African preserve managers note that the area reverts to thorny shrubland and, with enough rainfall, to eventual forest.[29]  The mammoths became extinct only about 11,000 years ago in North America and the mastodons about 10,000 years ago. Eastern grasslands have been in trouble ever since, especially with the loss of other significant megaherbivores such as bison, horses, and camels, all of which were known to exist in the Great Lakes area.[30]

What are the implications for grassland restoration and preservation in the United States? First, while restoration of megafauna is important throughout North America, particularly for grasslands, such megafauna restoration is critical for grasslands east of the Mississippi River. Second, the prairies and other grasslands as far east as Ohio were once far more extensive than is commonly acknowledged. There is no other way to explain the large elephant populations in these areas. Jefferson mammoths and woolly mammoths are not forest elephants. Third, an eastern prairie with wild horses (such as Przewalski’s horse or Asian wild asses), camels (including bactrian camels and llamas), bison, and elk is a more natural ecosystem than one without them. Fourth, to support these large animals, prairie restorations must be on a much larger scale – thousands of acres. Finally, with the restoration of many large herbivores, fire management can be more infrequent, reducing ecosystem maintenance and species loss.

This approach will solve several other problems as well. In places such as Adams County, Ohio, several plant species such as false aloe (Agave virginica) are in decline because their present populations are said to be too widespread and fragmented to ensure long-term fertility. Even small grassland animals such as thirteen-lined ground squirrels, badgers, and a number of bird species need larger grassland areas to establish viable populations. Different grassland species of birds require different grassland heights, which disparate habitat needs cannot be met solely by a fire management approach. Large grazers are needed. Grassland species of birds are declining more rapidly in the east than any other habitat-dependent group of birds.

Alas, we cannot bring back the mastodons and mammoths, but the medium-sized and small mammals have not changed since the Pleistocene: i.e., badgers, bobcats, beavers, coyotes, weasels, rabbits, skunks, mice, moles, opossums, etc.,[31] Likewise, birds and reptiles and insects are pretty much the same but desperately need the restoration of their grassland habitats.  Experimentation with combinations of large mammal species that will work in different areas of the country or within a given state is needed, but we must quickly change our idea of what is a “natural” grassland so far as the megafauna are concerned. In most instances, we should simply try to recreate the ecological niches of such animals as best we can in our respective climates to approximate the only remaining intact grassland ecosystem in the world: the African Serengeti. We should give preference to suitable North American species but, where a given ecological niche no longer has an appropriate North America megafauna species, we should not hesitate to experiment with large species from other continents. Of course, care should be taken to prevent species from getting loose just in case a particular species’ net effect should turn out to be more negative than positive (e.g., European wild boar and feral pigs in the Great Smoky Mountains National Park).

For More Information:
Ka-ma-ma's November Discovery
Naming Ka-ma-ma
The Forest-Prairie Dance -- Plant Migrations
Calendar of Tours to Ka-ma-ma
Rethinking the Eastern Grasslands: Article by Michael Hall
 

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References:

A.       In Quest of Great Lakes Ice Age Vertebrates, by J. Alan Holman. ISBN 0-87013-591-0. Copyright, 2001.

B.       Before The Indians, by Bjorn Kurten. ISBN 0-231-06582-5. Copyright, 1988.

C.       The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms, by Connie Barlow. ISBN 0-465-00551-9. Copyright, 2000.

D.       Prehistoric America: A Journey through the Ice Age and Beyond, by Miles Barton, Nigel Bean, Stephen Dunleavy, Ian Gray, Adam White. ISBN 0-300-09819-7. Copyright, 2002.

E.        Fossils of Ohio, Bulletin 70, Ohio Division of Geological Survey, Department of Natural Resources. Copyright, 1996.

F.        “Glacial and Post-Glacial Plant Migrations Indicated by Relic Colonies of Southern Ohio,” by E. Lucy Braun, Ecology, Vol. IX, No. 3, (July, 1928).

G.       “Goodbye Eternal Frontier,” by Connie Barlow, Wild Earth 12(2): 16–22 (2002).

Footnotes: