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Most temperate grasslands have long since succumbed to the cow and the plow. Only a few retain the unmodified vegetation of the plains, prairies, steppes, velds, or pampas. But whether they bear, native vegetation or support the highly selected grasses we know as grains, these grasslands supply the bulk of human food.

Cattle and sheep graze on their grasses; cattle, hogs, and chickens fatten on com grown on former grasslands; and most of our cereals, especially wheat, are produced on soils that once supported native grasses.

Biology of Grasses

All the grasses belong to one large family and share distinctive characteristics that fit them to their environment and make them of particular use to humans. Many people are surprised to learn that grasses are flowering plants. Open grasslands are windy, and it is the wind that pollinates grass flowers.

Among these wind-pollinated plants, natural selection has favoured those that have conserved resources by producing small, petal-less flowers. Therefore, grasses and some other wind- pollinated species lack the showy petals that other plants use to attract
ovarian tissue of the fruit fuses with the single seed inside in such a way that they become one continuous structure.

Adapted to Withstand Grazing. As grasslands evolved, their abundant thin leaves offered a bonanza to herbivorous animals. As a result, there evolved a large group of grass-eating mammals, including the ancestors of today’s cattle, sheep, and horses.

The selective pressures of these grazers promoted evolution of traits that resist grazing damage. For one thing, grasses store nutrients in their roots. As a result, loss of stems does not substantially reduce their reserves.

Another factor is the presence of silica, the substance we know as glass, in grass cell walls. Of course, grazers have evolved excellent grinding teeth. But grasses take their toll. Tooth wear is so severe that may old herbivores eventually starve to death.

The growth pattern of grasses has several distinctive characteristics that many also adapt them to withstand grazing. Grass leaves originate singly at prominent nodes along the stem. The lower part of each leaf forms a sheath that surrounds the stem for considerable distance before bending sharply outward as a flattened blade.

The leaves retain a meristematic growth region at the base of the sheath and another at the base of the blade. If the blade is cropped back, these meristems resume growth and lengthen the leaf to compensate for lost photosynthetic tissue. Thus grass leaves grow from the base rather than from the tip. You can see this growth pattern in any lawn.

Until time of flower, the grass stem remains short, and the leaves grow upward beyond the tip of the stem. Thus the stem tip is protected from damage as long as possible. Only when it is time to reproduce does the stem elongate between several nodes and the tip stretch skyward. Here the flowers develop where they are exposed to the winds that pollinate them.

Binding the Soil. Grasses bear abundant roots. Most are adventitious roots, which arise not from the primary root formed by the embryo but from the lower nodes of the stem. The sod-forming perennial grasses have extensive stems lying just on top of the soil or in upper soil layer; these serve both to hold the soil and to spread the plant.

From rhizomes (underground stems) arise adventitious roots and occasional branches that penetrate to the surface, giving rise to what appear to be new plants. Quack grass, a garden pest across the nrothem half of the United States, often reproduces through rhizomes. Plowing or cultivating disrupts and scatters the quack grass rhizomes. Each piece with an intact node may form a new plant. Often grasses have stolons, stems that lie on the ground and root at intervals. Each rooting node may give rise to aerial branches.

Rhizomes and fibrous roots constitute over half the mass of a grass plant. Together they form a network penetrating throughout the soil and binding it into a nearly inseparable plant-soil complex known as sod. Neither wind nor water can erode a healthy sod.

Growing roots and rhizomes break the soil repeatedly and contribute to the characteristic crumbly texture of grassland soils. Many of the roots of perennials die each year and are replaced by new roots.

Decay of dead roots supplies humus and leaves species that aerate the soil. Decaying roots return minerals to the soil, where they are immediately reclaimed by other roots. This process is part of the rich and efficient nutrient cycle of grasslands.

Dead stems and leaves provide an extensive ground cover in most grassland. Unmowed grasses accumulate as much as 10,000 kg of humus per hectare (about 9000 lb/acre) each year. It takes three of four years for litter components to decompose.

Hence the litter forms a deep layer that shades the ground and lessens evaporation due to the wind. The litter also holds rainwater and aids its penetration into the soil. Because the water soaks in, there is no surface runoff and therefore little erosion. Except in the moistest grasslands, the rainwater selom soaks deep enough to join the groundwater. Instead, the extensive root system picks up most rainfall while it lies in the upper layer of the soil.

Then the xylem carries the water immediately to the leaves, where it is transpired back into the atmosphere. Because little water percolates down far into grassland soils, minerals are not leached away; instead, they remain near the roots. Retention of minerals contributes to the richness of grassland soils. This and the crumbly texture produced by the roots make grassland soils excellent for agriculture.

Grassland Communities

The factors that limit forests and permit grasslands to develop are hard to determine. Some biologists maintain that grasslands develop only where there is insufficient rainfall to support trees that could shade out the grasses.

On the other hand, there is both historical and experimental evidence that many grassland edges, such as those between the American prairies and the eastern deciduous forest, were maintained by fire. Where forests and grasslands meet, burning usually favours grasses over trees.

Variations in climate and accidents of grazing have resulted in complex and dynamically balanced grassland communities. Contrary to popular belief, the American prairies were never a uniform sea of grass ranging from the deciduous forests of the East westward to the Rocky Mountains.

Mixed in with the grasses there were other herbs, especially members of the aster family and legumes, such as the lupines. Nitrogen fixation by symbiotic bacteria in the nodules of the legume roots contributes to the fertility of the soil and the vigour of the community.

The roots of various species extended to different levels and were best developed in specific regions of the soil. Concentration of roots of one species at a particular level reduces competition between species. But because each level is populated by roots of some type, resources are fully exploited.

The grassland landscape is even more varied along the moist borders of streams, river, marshes, lakes, and ponds. Where inland waterway pass through grasslands, groves of trees and shrubs thrive, along with other organisms ordinarily associated with woodland communities.

Grassland Animals. A large and diverse animal community lives in every grassland. The deep soil that covers rocks and the absence of woody vegetation limit above ground shelter, except where grasslands merge with woodland or along rivers and streams. Thus most small animals depend on the soil for protection. Numerous little mammals burrow beneath the sod.

Prairie dogs, rodents related to squirrels, are symbolic of the short grasslands or Great Plains of North America. Stockmen exterminated most of the prairie dogs, believing that these animals competed with livestock for forage. Undoubtedly the selective grazing of prairie dogs on grasses favoured the growth of other plants.

Pronghom antelopes, which were once almost as numerous as bison and are now few in numbers, relied heavily on the broad-leaved vegetation of prairie dog “towns”.

Elimination of prairie dogs also resulted in near extinction of one of its predators, the black-footed ferret. This handsome, weasel like animal hunted in the prairie dog burrows. Because vast numbers of prairie dogs are necessary to support even a tiny breeding population of ferrets, this species is now extremely rare-or perhaps extinct except for a few in captivity.

Prairie dog burrows and those of gophers, pocket mice, kangaroo rats, and ground squirrels provide shelter for other animals, including grass-hopper-mice and snakes.

At least one bird, the burrowing owl, relies on old rodent burrows for nesting sites. So do cottontail rabbits. Beetels and camel crickets also use burrows; the dung, fungi, and hoarded vegetation afford these insects a ready food supply.

Ants are abundant in grassland soils; some species build huge mounds surrounded by a zone stripped clean of all visible life except for the ants themselves. Animals that dig underground benefit the grasses, because they aerate the soil and mix in humus from the surface. Even those that are underground transients contribute fecal material where it is directly available to the roots.

Some animals, such as grasshoppers, are found only aboveground. Nevertheless, they depend on the soil for shelter. The female grasshopper uses the tip of her abdomen to penetrate deep into the soil and bury her eggs. The adults perish in the rigour of winter, but the species continues because the developing embryos of the next generation lie safely protected in the soil.

A number of medium-sized animals that live in the surface vegetation face danger from predators. Rapid movement through thick grasses is very difficult, and vision is limited; consequently, it is easy for predators to stalk their prey.

Escape must be fast and sure; otherwise it comes too late. Consider how grasshoppers, jackrabbits, and jumping mice meet this challenge. Huge aerial leaps permit such animals to clear the top of the vegetation, get an unimpeded view for a moment, and then drop some distance away without leaving a trail.

No Hiding Place. In wide-open habitats large animals find little or no shelter. Under these circumstances “predator control” becomes as group activity, and the animals aggregate together.

The pronghorns, bison, and muskoxen of North America, the kangaroos of Australia, the saga antelopes, wild horses, and asses of the steppes of Russia, the gnu and zebra and even the ostriches of Africa show herd instincts. In a group there are many eyes to watch for predators, and alarm spreads rapidly.

Frightened pronghorns raise their tails and display a large white patch on their rumps. The flash of white rumps and white flag like tail on a running pronghorn starts every other pronghorn in sight on its way! Mixed herds, such as are common in Africa, cooperate in watching for danger. Ostriches are taller than most mammals they herd with and are usually the first to sound alarm.

Large grassland animals are exceedingly fleet; they have long length that covers the ground quickly. The faster runners in the world inhabit grasslands.

The American pronghorn, our faster native animal, can do 60 miles an hour. At this rate the pronghorns leave a solitary predator or a pack of wolves so far behind that no amount of strategy or cunning can lead to another encounter until the pronghorns have had time to feed and recuperate from the original confrontation. Of course, natural selection has also promoted evolution of fast running grassland predators.

In Africa, the cheetah has been clocked at 65 miles an hour. It is known to accelerate from a stand-still to 45 miles an hour in a few seconds.

Grassland herbivores rely on the group not only for detection of predators but also defense. When threatened, bison from protective head- outward circles. The young are secure in the center of the circle.

Herding is itself a defensive measure. Predators are confused by large numbers of running animals. Thus if the individual a predator is following loses itself within the herd, the predator stops in confusion. For this reason, most predators kill only aged, diseased, deformed, and very young animals that are unable to keep up with the herd. These same animals are especially vulnerable, because they are also weak.

Grazing and Overgrazing

Each grassland is a balanced ecosystem of producers and consumers. The native herbivores have evolved with the plants and are adapted to the vegetation and to one another. Likewise, grasslands are adapted to withstand grazing by certain animals. Unfortunately, domestic animals often have destructive effects on ecosystems in which they do not evolve.

The abuse of natural grasslands by domestic animals arises from two factors. First, the domesticated grazers, with few exceptions, are from other parts of the world. Hence local grasses are not adapted to circumvent their feeding patterns. Domestic sheep, in particular, can graze more closely than native North American herbivores can. Most grasses are vulnerable to extensive loss of stems and leaves.

Second, overgrazing by domestic animals is common. The usual population controls that limit wild grazers are nearly if not totally absent from populations of domestic animals.

Humans do their best to protect their animals from severe weather, predators, or temporary food shortage. When they slaughter animals for food, people carefully select surplus young adults, particularly excess mates. Very young animals, which have a period of rapid growth ahead of them, pregnant females, and females of good breeding age are seldom killed. In almost all cultures, people exploit their understanding of animal growth and reproduction to maximize the number of animals and obtain as much food as possible.

Unfortunately, most societies lack a similar understanding of the ability of grasslands to support animals for long period. Perhaps this difference in understanding animals compared with grasslands results from the fact that the life cycle of the animals is comparatively short. Each human has the opportunity to see many generations of cattle grow up and reproduce. In comparison, the effects of overgrazing may appear only over decades and sometimes require several human generations to become acute.

Damage from Overgrazing. Repeated close cropping of grass blades reduces the photosynthetic potential of the plant and hence the energy that can be stored in the roots to support the following seasons’ growth. In response to grazing, most grasses undertake a compensatory growth.

The leaves elongate at their meristems, and new branch stems arise near the ground. Both results from the loss of auxin-producing tissue at the tips of leaves and stems. Under excessive grazing some grasses continue to make new growth until their underground stores are exhausted.

Heavy grazing prevents the stems from growing out, blooming, and setting seed. Sometime the supply of new plants is cut off completely.

Close cropping of leaves and stems also inhibits normal growth of roots that must occur to replace old, dying roots, to ensure absorption of water and minerals, and to bind the soil against erosion. If all leaves are gone, the grass crown (base of the stem) lies exposed to physical damage from freezing or trampling.

Finally, overgrazing harms hams the soil directly. Reduced vegetation and surface litter increase evaporation and reduce water absorption.

Much of the rain simply runs off along the surface, eroding the soil. Overgrazing has accelerated erosion of hte deep gullies and arroyos of the arid Southwest. Heavy grazing also changes the soil texture. Continuous trampling compacts barren soil and thereby destroy spaces between the particles. This reduces ability of the soil to hold air and water.

Range Management. Except for the constant pressure for short- term economic gains, range management is quite simple. Grasslands must be permitted to set seed.

The manager can achieve this goal by dividing the land into plots and rotating the grazing that each plot remains ungrazed every few seasons until after the seed has ripened and dipersed. If overgrazing on the other plots is to be prevented, more land must be available for a herd of a viven size, or the total number of animals must be reduced. Another problem arises from the fact that grazing animals never use available forage uniformly. They always prefer some areas, especially those near water holes. Sometimes judicious placement of salt blocks can balance the distribution of grazing.

Natural Checks. If range use must be so carefully managed, how did the thundering herds of bison live in equilibrium with the native grasslands of North America? The answer is quite simple: there really were not many bison in relation to the amount of grassland.

The highest estimates of bison populations suggest that there was only one animal per 20 acres of range. Although the bison travelled in herds, raising clouds of dust with their hoofs and trampling the vegetation badly in places, most plants went many seasons without a single bison passing their way.

Some pond and stream margins did suffer repeated damage, and these probably supported successional vegetation. But in general the grasslands were lightly used. Natural checks limited the bison population. Although only grizzlies from the western mountains could have killed healthy adults, the young bison and ill ones were prey for wolves and perhaps coyotes.

Weather may have been another limiting factor. According to Indian tradition, a large herd of bison disappeared from Illinois during a terrible blizzard late in tine eighteenth century. Always some bison became trapped in river muds, and some surely died from disease or parasites. But the best animals usually survived, and numbers were in balance with the food supply. However, there can be little doubt that there were times when drought brought starvation to the bison and damage to the grasslands.