Download The Physical World of Grasslands

The Physical World of Grasslands
A sea of grass stretches as far as the eye can see in every direction. No mountain
interrupts it, few trees break the surface. It is an empty place, beneath a vast, cloudless
sky-and a restless place. The grass bends and sways ceaselessly in the constant wind.
Herds of zebras graze peacefully on the plains. In the distance giraffes pluck leaves from
the small thorn bushes, and lions rest amid tall grasses. In North America this grassland
is known as the prairie, in Africa it is known as the savanna, but both are rich with life.
Formation of Soils
As plants begin to grow and animals feed on them, organic matter is mixed with the mineral particles near
the soil’s surface. Plant roots and animals tunnel through the soil, mixing it and allowing air and water to
circulate. In time, distinct layers or “horizons” develop. Grassland soils have a thick surface horizon.
Some are chestnut brown in color, others black, because they contain a large amount of organic matter.
Soil consists of tiny particles of rock mixed with plant and animal wastes in varying stages of
decomposition. Its formation begins when rock near the surface of the Earth is heated by the Sun and
expands, then cools and contracts. This cracks the rock. Water seeps into the cracks, expands as it freezes
in winter, widening the cracks, then melts in spring and flows away. Flakes and particles of rock fall away,
are rolled against one another, and are slowly worn down into fragments the size of sand grains or smaller.
This process is known as weathering.
Climate
As stated above, there are two distinct grasslands that dominate the world- temperate and tropical
grasslands. They have many similarities as well as some differences, climate being one difference.
Temperate grasslands have climates associated with continental air masses. They are fairly dry, with hot
summers and cold winters. In winter cold, dense air produces high surface atmospheric pressure over
North America and central Asia. This brings long spells of cold, clear, settles weather. In summer the
pressure falls as the air is heated by contact with the ground, but the air is dry, and droughts are common.
The North American prairies suffer extreme weather more often than most grassland regions. Violent
storms bring blizzards in winter (a snowstorm with winds of at least 35 mph and a temperature no higher
than 20 F). There are also more tornadoes here than anywhere else in the world. Storms are a
consequence of geography. In the spring and summer cold air from mountains and from the north often
meet warm moist air coming from the south. The warm, moist air meets the cold air and, because it is less
dense, rises over it. Its water vapor condenses to form huge storm clouds that extend almost 50,000 feet or
more. If a strong wind is blowing the clouds will begin to rotate and produce violent storms. Hail,
torrential rain, thunder and lightning will often appear within minutes. The spinning air may then emerge
as a funnel below the base forming a tornado. Inside the tornado the wind speed can reach 300 mph.
QuickTime™ and a
decompressor
are needed to see this picture.
To the left is one example of how a
typical tornado may form. Warm moist
air shoots upward meeting colder, dryer
air forming a mesocyclone. Only 30% of
mesocyclones actually produce a
tornado, but about 95% produce severe
weather.
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Tropical grasslands appear on or near the equator. It is only at the equator that there are no seasons.
Tropical seasons are either dry or wet but never cold. In the hottest months it will reach 90 degrees
everyday but will rain up to 60 inches during the wet season. The wet season can last between six to nine
months and can often include monsoons, which can bring heavy rains for days at a time. During the rainy
season the grasses and other plants grow vigorously. Then, as the rains become lighter and less frequent,
and the upper layers of soil dry out, the vegetation wilts and turns brown. The dead plant material becomes
very dry and highly flammable. A fire can quickly spread across the savanna clearing away all life above
ground. The organic material remaining after the fire in the form of ash (nitrogen, potassium, phosphorus,
sodium etc) will wash into the soil at the next rain becoming available to future plants.
The Natural World of Grasslands
Drought-resistant trees and shrubs break up the monotony of the flat landscape, in places
growing quite close together, but the most abundant plants are grasses. Tall and tough,
they wave gracefully in the wind, food for grazing animals.
Plants are divided into two large groupings- angiosperms which produce true flowers and their seeds are
enclosed in an ovary, and gymnosperms which have seeds not enclosed in an ovary and do not produce
flowers. These would be represented by pines, firs, and cone-bearing trees. We can further divide these
two groups into monocots and dicots. Monocots have seeds that produce only one leaf upon germination.
Monocots also produce leaves with parallel veins to transport food and water and they have fibrous roots
which spread to the sides and not very deep. This root system allows them to soak up surface water quickly
when it rains. Grasses are monocots. Dicot seeds produce two leaves upon germination. Leaves have
veins that branch in several directions to reach all corners and their root system is a single taproot, which
burrows deep into the soil allowing it to survive when water is scarce.
A plant that germinates from seed, grows, flowers, sets seed, and then dies don all within one year is called
an annual. Some grasses are annuals, and fresh plants grow from seed every year. Other grass species are
perennials. These also grow from seed and produce seeds, but they do not die down at the end of each
growing season, most savanna and prairie grasses are perennials.
Trampling and grazing are not the most serious problem that grasslands must overcome. They must
survive through the dry season. Many achieve this by changing the shape of their leaves according to the
amount of moisture in the air. Most of their stomata – pores through which plants lose water- are located
on one side of the leaf. As the leaf dries out it will curl into a tube with the stomata inside. By then the
leaves and stems of the grasses- but not their roots – have died. The tangled mat of wilted vegetation has
little nutritional value for grazing animals.
Grasses
There are about 9,000 species of grasses. These grasses provide food for a wide variety of animals. Herds
of bison and antelopes graze, and below ground communities of smaller animals such as prairie dogs live in
networks of burrows and chambers. The animals help manage the grassland. Without them much of it
would become forest.
Some grasses in the prairie have extensive root systems. Some fibrous roots, laid end-to-end, measure 40
miles. By holding soil together, the roots trap nutrients and moisture, after a spell of dry weather, when
exposed soil feels very dry, the soil held in the mat of roots beneath grass is still moist. Even if the weather
is so dry that the stems and leaves die, the roots will survive several months longer, by which time there
will probably have been enough rain for the plant to revive. Because the leaves grow from ground level,
having their leaves eaten does not harm grasses. In fact they may benefit, because the removal of the older
parts of leaves encourages fresh growth. Grasses can also withstand trampling. This is because of the way
their vertical stems grow. If a stem is flattened, it grows faster on the lower side than on the upper side, and
this pushes it upright again.
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Grasses, herbs, and in some places scattered shrubs are the green plants of the grasslands. These are the
producers on which all the animals- the consumers-depend. The food they offer consists of leaves and
grass blades, roots, and seeds, and all of it is to be found at or very close to ground level. Although the
grass and other plant species vary from place to place, the type of vegetation is fairly uniform throughout
the prairies. Consequently, the habitat changes little over large areas. Animal species that are able to thrive
in the grassland will be able to do so in almost any part of it, and so, although there are far fewer species
than in forests, the grassland ecosystem contains animal species with a wide distribution.
Tall grass prairies, (pictured to the left) have soil that is
rich and fertile. The grasses and herbs support a diverse
population of herbivorous animals, including many insects
and small mammals as well as the bison and other large
herbivores.
Grasslands support few animal species, but they do support a variety of herbivorous species. In the prairie
these range from grasshoppers to bison. If two species living side by side depend on precisely the same
food, one of them will be more efficient than the other at exploiting the food resource. It may be better at
locating food, or at gathering it, or its digestive system may work better so that its body derives more
nutritional benefit from the food it eats. Members of that species will be healthier and produce more
offspring than members of the less efficient rival species. After several generations the less efficient
species will disappear from the habitat.
This is called the competitive exclusion principle. The principle states that two or more species cannot
exist together in the same environment if all those species depend on the same resource and use it in the
same way, because all but one of the species will be eliminated. According to this principle, many animal
species can feed side by side on grassland plants only if they eat different plants or different parts of the
same plant. They must not compete among themselves directly. What allows them to avoid competing is
the differences in their sizes and the dietary requirements that those differences impose.
Warm-blooded animals need more food than cold-blooded animals because they use a great deal of energy
maintaining a constant body temperature. Small animals have bodies with a larger surface area in relation
to their volume than big animals. They lose heat more quickly through the larger surface area, and so they
need a more nutritious diet to sustain them. All herbivores eat plant material, but they do not have a free
choice of what they eat. Insects can eat small plants or parts of plants. These do not need to be very
plentiful because insects are highly mobile, and insect populations increase and decrease rapidly according
to the availability of resources. Their food has to be easy to eat and very nutritious, however. Big
herbivores, which do not need highly nutritious food but do need plenty of it, choose plants that are very
abundant. They might also have difficulty locating tiny plants, so will eat bigger ones.
The grassland herbivores can be arranged in groups of those requiring food that is nutritious but not very
plentiful, plentiful but not very nutritious, and the medium-sized animals that need a diet of moderately
abundant, moderately nutritious food. Some plants are both nutritious and plentiful, and all the herbivores
will eat those.
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Plants of the Temperate Grasslands
Plants of the grasslands will flower all spring summer and fall including until the first snowfall. Those that
flower in the spring will vary from those that flower in the summer and fall, each filling a special niche of
the prairie.
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Downy phlox (Phlox pilosa) also known as “prairie phlox” is an herbaceous perennial which
grows to 2 feet in height, flowers in spring.
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Prairie buttercup (Ranunculus fascicularis) the first yellow flower in spring is native to the
prairie’s of North Dakota is an irritant to the stomachs of grazing animals. Their scientific name
means “a little frog” as they grow in areas abundant with frogs.

Prairie smoke (Geum triflorum) flowers from April through August across the entire high prairies
of North America.
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Perennial ryegrass (Lolium perenne) successful prairie grass, which germinates very quickly,
survives hot weather and is able to withstand heavy grazing. Prominent in preventing soil erosion
and leading cause of hay fever as pollen distributed by wind.
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Rattlesnake master (Eryngium yuccifolium) a member of the carrot family its large taproot allows
it to survive long dry seasons. Native Americans used this plant for its medicinal value as an antivenom against snake bites.

Silky aster (Aster sericeus) flowers in late summer and early fall, prefers cold harsh winters.
Aromatic plant that attracts animals to eat their seeds which then digest them and distribute them
elsewhere.
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Rough blazing star (Solidago riddellii) grows to 5 feet tall and blooms from late August until the
first snowfall. Produce tough woody stems and spread by producing “runners” which produce
new plants just a few feet away meaning they grow in small colonies.
Plants of the Tropical Grassland

Acacias (Acacia disambigua) also known as Wattles, grow to 35 feet and have distinctive flat tops.
Adapted to dry conditions, leaves covered with a thick, waxy coating. They are evergreens, but
will conserve water by producing leaves only when water is present and shedding leaves during
drought. To avoid being eaten they produce thorns or fruit, which is poisonous to many animals.
Some ants burrow holds in the large thorns and start colonies’ as they are attracted to the nectar.
The ants are aggressive when defending the tree, which keeps animals from grazing on the leaves.
The ants also destroy any insects that might be a parasite to the tree. The ant tree relationship
where both organisms benefit is an example of mutualism.

Porcupine grass (Miscanthus sinensis) also known as “needle grass,” forms in large clumps called
tussocks they are known for leaves, which are razor sharp. These tussocks of grass grow to about
8 feet in height and spread 4 feet wide. The edges easily cut the flesh of animals that attempt to
graze upon the plant. Also very drought resistant due to a deep and fast growing root system.

Sorghum (Sorghum vulgare) and Millet (Eleusine coracana) two fast growing grains that spread
quickly in tropical savannas. Provide nutrition to many grazers with their ability to produce high
protein seeds and leaf blades. Estimates indicate that these two grasses supply the sustainable diet
of most tropical grazers. Known for providing “mineral nutrients” they are members of the cereal
family.
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Tropical Herbivores
Herbivores of the tropics tend to be large migratory animals often traveling over a thousand miles in search
of fresh grass. The grasses, which produce large amounts of sugar since they grow in a climate with year
round sunshine. The high sugar content can easily support large animals in herds consisting of thousands
of individuals. Though these herbivores can grow quite large they cannot afford to be slow moving like
many grazers in temperate grasslands. On the savanna there is little cover so they seek safety in numbers.
Within these herds there is a surprisingly well-defined social structure.
In the savanna large numbers of carnivores will follow the herds and survival is often dependant on
outrunning the hunters. Many herbivores are members of the cattle family (Bovidae), which means they
have a four-chambered stomach and can digest grass and other tough plant material. Because of the
dangerous living conditions these herbivores developed a technique of “chewing cud.” When grazing they
are able to ingest large amounts of grass in a short amount of time without chewing. Later, when free from
predators they are able to force the grass back into their mouth where they can leisurely chew it and
swallow it to their waiting stomachs to be completely digested.
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Antelopes and Gazelles ( )
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Wildebeest (Connochaetes gnou)
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Zebra (Equus quagga) three species exist, all in Africa preferring drier grasses along the desert
edges. Very social animals tending to herd with other zebras antelopes and gazelles providing
protection from predators. Very organized, some will have their heads down to graze while the
others have their head up to watch for attackers. Zebra stripes are thought to blend with tall grass
as their main predator the lion is colorblind and will have difficulty seeing them.

Giraffe (Giraffa camelopardalis) known as an even-toed ungulate they survive by eating leaves
that are beyond the reach of most other animals providing an abundance of food. Males can stand
20 feet tall and run to 35 miles per hour for short bursts. They prefer Acacia trees for both food
and to hide from predators. Lions are the only threat to an adult but a giraffe can defend itself as
the kick from their hind legs can shatter a lion skull. Camel hearts weigh 22 pounds and double
the blood pressure of any other animals.

Elephant (Loxodonta africana) largest land animal weighing up to 13 tons and living for about 70
years. Trunk is a fusion of the nose and upper lip- African elephants have two finger-like
projections on the end of their trunk whereas Asian elephants have only one. Their trunk is
sensitive enough to pick up a blade of grass yet tough enough to tear a branch off a tree. Have
been known to walk 50 miles per day in search of food and consume 350 pounds per day. Very
social animals traveling in herds of 20 members generally. Tusks are their upper incisors and like
humans can be left or right dominant.

Rhinoceros (Diceros bicornis) known as an odd-toed ungulate, five species still exist. Have poor
eyesight but excellent hearing and sense of smell. Many lack teeth at the front of their mouth
using their lips to pull and eat grass. Most live to 60 years of age some species have one horn
while others have two. Horns are made of ketatin (think fingernails).
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