Clements’s prairie-inspired theory of “community succession” made a stir in the intellectual capitals of the world. And it was there, in London, England, that his intuitions were eventually brought to a new stage of development. The concept of the “ecosystem”—the idea that the Earth operates as a series of self-organized complexes in which all components (both living and nonliving) are linked—was first advanced by British ecologist Arthur Tansley in 1935, in a self-conscious attempt to marry the study of nature with advances in math and systems theory. Suddenly, the young-and-still-somewhat-woolly-headed science of ecology, which had heretofore made do with hazy notions of “relationships” and “community,” could gird itself in the vocabulary of the physical sciences. Instead of a bewildering clutter of sensations, the living world had become a “complex, adaptive, open, nonlinear system,” complete with “feedbacks” and thermodynamic “fluxes.”
Yet for all this new exactitude (which has served the science well), the idea of the ecosystem has proven reassuringly resistant to precise definition. As Tansley himself acknowledged, “the systems we isolate mentally are not only included as parts of larger ones, but they also overlap, interlock and interact with one another.” The task of charting these overlapping and fluid realities— of acknowledging the differences between particular localities without denying their interconnectedness—remains a major preoccupation of ecologists. It is a challenge that, over the last thirty years, has inspired a continent-wide spree of “ecological land classification” and mapping. At its heart, this is an attempt to see beyond the human impacts of the last few centuries and uncover the enduring components of the environment (climate, soils, landforms, vegetation, and so on) that make one part of the continent biologically different from the next. Although we cannot go back in time and view the wild prairie in full bloom, we can attempt to identify and assess the factors that, over the long term, made them what they are or, at least, what they once were.
Ecological mapping began in the 1960s and 1970s as a relatively straightforward attempt to examine the relationship between natural vegetation and climate. Nowadays, however, with satellites to assist with mapping and computers to do the grunt work, the possibilities have spiraled. Instead of being limited to single variables, researchers can now consider the interplay among dozens of different ecological components all at once, factoring in everything from climate to geology, hydrology, physiography, soil chemistry, vegetation, species diversity, and where relevant, human effects on the environment. Yet even at this high pitch of sophistication, scientists have not been able to pin the living world neatly to the wall, sure that they have mapped its subtleties once and for all.
Rough fescue
Over the last decade, the ecological regions of the Great Plains have been mapped in different ways by different agencies, whether in broad strokes as part of continentwide research or more minutely, state by state and province by province. One result of this effort is a set of maps created under the joint authorship of the World Wildlife Fund Canada and its counterpart in the United States, the wwf–U.S. Seen through the lens of these organizations, the Great Plains grasslands come into focus as a mosaic of fifteen eco-regions. The Aspen Parklands ecoregion, for example, lies across the midriff of the Prairie provinces (Alberta, Saskatchewan, and Manitoba) and provides an interface between the boreal forest and the open plains. Rising in the north as a closed poplar woodland with occasional stands of spruce, it gradually unfolds into a rolling grassland dotted with aspen groves and dominated by various spear-grasses, wheatgrasses, and most notably, fescues.
Because of the predominance of fescues in the Aspen Parklands, the region is sometimes known as a fescue grassland. The same term is also applied, for the same reason, to the community of plants found in the Foothills Grasslands. Located on the undulating slopes at the foot of the Rocky Mountains, this ecoregion is dominated by rough fescue, together with lesser quantities of June grass, speargrass, wheatgrass, and various flowers and shrubs.
East of the foothills and south of the Aspen Parklands lie two ecoregions with subtly different characteristics. The Northern Mixed Grasslands takes its name both from its northerly location and from its characteristic mixed— which is to say, mixed-height—cover of grasses. Here, the sparse, tufted vegetation of the foothills gives way to a groundcover of ankle-high grasses, notably blue grama, intermixed with an overarching canopy of knee-high stands, including various spear- and wheatgrasses. As this mixed grassland flows southward, the canopy of taller, midheight grasses gradually thins out, and the overall height of the vegetation diminishes. This transition from mixed-to short-grass prairie continues to the south in the Northwestern Short/Mixed Grasslands ecoregion.
Subtle variations in soil, precipitation, drainage, and growing season determine which species of plants thrive in different regions of the Great Plains. The mixed grasslands of southern Saskatchewan, for example, are dominated by needle-and-thread grass and spangled with wild flowers like ascending milk vetch.
The trend to shorter grasses culminates in the Southern Short Grasslands of the High Plains. Here, in an area once memorialized by Coronado as a land of “very small plants,” the vegetation is dominated by a ground-hugging mat of grama and buffalo grass. Yet here, too, there are subtle shifts. For if the grasslands diminish in height from north to south, they shoot up from west to east as they escape from the rain shadow of the Rockies. This trend is reflected in the transition from the short-grass prairies of the west, with their carpet of stunted plants, to the multilayered, knee-high vegetation of the Nebraska Sand Hills and the Southern Mixed Grasslands.
On the eastern flank of the plains lies the tall-grass-prairie region, so named for the luxuriant stands of big bluestem and Indian grass that grow—or, at least, once grew—there. “The grass is so very high that a man is lost amongst it,” reported explorer Pierre François-Xavier de Chevalier as he crossed southern Wisconsin in 1761. Bright with brown-eyed susans and other flowers, these magnificent prairies extended from the Northern and Central Tall Grasslands south through the Flint Hills to the Blackland Prairies of east-central Texas. Many of the same species of grasses are also found, as an understory, in the juniper breaks of the Edwards Plateau Savannas and in the hickory-and-oak woodlands of the Cross Timbers Forest and the Southern Prairie-and-Oak Transition.
And finally, right out in the middle of everything, stand the lonely, displaced islands of ponderosa pine, white spruce, and paper birch that make up the Black Hills coniferous forest.
The ecological interactions that find expression in these varying landscapes have been at work for thousands of years. Even today, characteristics such as average temperatures, precipitation, length of growing season, and drainage patterns provide the physical framework or, one could say, the loom on which the fabric of the Great Plains ecosystem is woven. Yet for all their continuing importance, these long-term physical features are no longer the only powers in the land. Other interests have taken over; other hands are pulling threads. Those hands, of course, are human.
Over the last two hundred years, human beings have hit the prairies with the force of a major geological crisis, triggering not only extinctions and extirpations—of plains wolves, plains grizzlies, plains elk, plains bighorn sheep, free-ranging plains bison—but also dramatic shifts in the vegetation. Taken as a whole, the Great Plains grasslands now rank as one of the most extensively altered ecosystems on Earth. There is scarcely a patch of ground where we have not left our footprints. The southernmost Short Grasslands, for instance, are as sun-baked and arid a country as you could ever expect to see, yet even there an estimated 29 percent of the ecoregion has been brought under cultivation. The surviving native prairie in the region is now devoted to livestock or converted to ranchettes, on the advancing front of urbanization.