February 2, 2019. I’m chaperoning some students today, and had too much to do yesterday to get a post done. This is a rerun from 2014.
[This is the first of a series about how Great Plains and High Plains species affect their physical environment. Sources will be available upon request.]
One of the conceits of popular environmentalism is that only humans cause major alterations to the physical and biological environment. A little research shows that this is far from the case, and while humans have indeed been major shapers of the world, non-human animals played, and still play, important roles in determining the lay of the land and what plants and other animals live on that land.
I’m going to start this series of posts with definitions of certain key terms and landscapes. Then I’ll look at the geology, before narrowing down to certain locations and individual species, notably beaver, bison, prairie dogs, domestic sheep, and domestic cattle on the High Plains. I’ll conclude with an overview of pre-1800 human activity in North America and how it greatly influenced a number of biota, most famously the grasslands of the Great Plains.
One of the terms we need to know is “keystone species.” Think of a Roman arch, how the components are held in place by a key stone. If you remove that stone, the arch loses its integrity and falls apart. A keystone species is one that has a dominant place in determining the composition of an ecosystem during the period of time that you are looking at that ecosystem. Bison and prairie dogs were both keystone species for the High Plains prior to the 1870s, their grazing habits and living conditions determining what other species did better and which fared more poorly. A keystone species is not necessarily the largest or best known animal in an area. Small rodents, if they form the prey base for the next level of animals and if they shape which plants dominate the area, can be a keystone species.
Another term I’ll use is “trophic level.” A trophic level is where on the food chain/ food web an organism lives. Plants are the first trophic level because they don’t eat anything else (pitcher plants and Venus flytraps don’t count). Grazers and other herbivores, of any size, form the second trophic level. That which eats the grazers is the third trophic level. You could argue that bacteria and carrion eaters might form a fourth trophic level, but most textbooks stop at three.
The High Plains refers to the area between the Rocky Mountains in the west and the 20″ isoheyt to the east. That means the 20″ annual average rainfall line, or where short grass steppe shifts to the mixed and tall grass prairies, where pyrophilic grasses become more common. Today, if wheat dominates, you are probably in the High Plains and corn (maize) is the tall grass (irrigation notwithstanding.) The High Plains are a semi-arid belt of short grass ecosystems that extends from the southern end of the Llano Estacado Plateau to the Canadian border. Some books limit the area to eastern Wyoming and Montana, and call the southern stretch the South Plains, but that starts to get very confusing when you look at a smaller scale, so for our purposes, High Plains it is. Why “high?” Because much of the area is over 3000′ in elevation.
Semi-arid refers to the average precipitation and evaporation patterns in the area. Precipitation is strongly seasonal, with most of the rain coming in April, May, and June, with a secondary peak in August-September. The southern part of the High Plains has a very high evaporation rate caused by the dominant southwesterly winds and large amounts of sunshine. The northern areas have less evaporation and more winter precipitation, although it varies from year to year. An average of 20″ of moisture can mean 30″ one year and 10″ the next, as residents of the region know well. Some years are more arid, some lusher. The variation can be striking even over a few miles, depending on where storms form and drop their rain load. I’ve seen pastures on the east side of town withering while two miles west it looks like someone dumped a load of green paint. Back in the 1800s, the managers of the big ranches had to try and explain to owners “back East” or in Britain how they could get a good rain at the ranch headquarters and still have animals dying of thirst.
Fire plays a role in the Great and High Plains. The Great Plains, while not a truly pyro-dependent landscape, were close. The tall grasses and the associated forbs that grow with them leave a lot of dead material behind over winter, and fires (along with grazing) helped return nutrients to the soil. Fire also keeps the trees and shrubs at bay. Otherwise the eastern woods expand into the prairies. The High Plains are less congenial to trees, in part because the constant wind sucks moisture out of large, fluttering leaves aster than you can say “gush.” However, fires still raced over the land on occasion, set by lightning or by Indians. When rain follows fire, the land turns a rich, lush green that has to be seen to be believed.
Short grasses dominate the High Plains. Knee-high or waist-high at most, buffalo grass, grama grasses such as side-oats, black, and hairy, spartina, and western wheat grass formed clumps and patches. Buffalo grass responds to heavy grazing by sending out side shoots, or tillers, that allow it to grow without attracting attention from hungry bison. It was a rule of thumb that bison and buffalo grass went together, and they did.
To the east, where rain falls more generously, the tall grasses loomed higher than a man’s head. Big and little bluestem, Indian grass, and dozens of other smaller kinds covered the ground. Forbs such as lead plant, compass plant, goosefoot amaranth, and others returned nitrogen to the soil, forming a complete prairie package.
Having laid the cover down over the High Plains, next time we’ll look at the land under the plant roots and animal feet.
