Grasslands

Environmental Science

Chapters 8

Grassland Biome

• A grassland ecosystem is an area that receives more rainfall than a desert, but not enough to support the trees of a forest.

• These usually exist between deserts and forest biomes

• It is believed that at one point in the Earth’s history that nearly half of all land was covered in grassland.

• Grassland is home to a wide diversity of organisms

Grassland

• Grasslands occur naturally on every continent except Antarctica.

• Natural grasslands primarily occur in regions that receive between 250 and 900 mm (9.8 and 35 in) of rain per year, as compared with deserts, which receive less than 250 mm (9.8 in) and tropical rainforests, which receive more than 2,000 mm (79 in).

• Deserts and grasslands are usually seperated by a dry area of land that can support a few grasses called a desert-grassland boundary

Desert-Grassland Boundary

As the name implies

• The most common biotic factor in a grassland ecosystem is, go figure, grasses

• However small shrub/sage brush can also be present

• Generally speaking woody trees are very limited and usually do not get very tall

• Grasslands usually support large numbers of both small and large herbivores

Grasslands Need To Burn

• Aside from low rainfall, one of the main reason that grasslands resist forestation is annual or semi annual fires

• Most grasslands experience hot, dry summers that cause many of the grasses to dry out

• When a storm does occur, often lightning strikes cause fires that spread rapidly

• These fires burn off the dead grass tops and most any tree that tries to grow but does not harm the roots of the grasses

• The ashes from the fire add nutrients and soil depth

Grassland = Cropland• One issue that grasslands have faced is that the

rich soil that supports the thick grasses also supports a different type of grass…

• Corn… which is actually a modified “seeding” grass, grows exceptionally well in areas where grassland exist

• For this reason, most grasslands have been destroyed by the farming industry

• This has caused major problems for the animals that need grasslands to survive because once the corn is harvested, it leaves behind little habitat

Grasslands Endangered?

• Over the past few decades, we as a society has realized the importance of grassland habitats as not only habitats for organisms, but also as barrier for keeping harmful fertilizers and pesticides out of waterways

• For this reason, the United States have joined other countries, world wide, in trying to restore farmland into grassland

• This increases an ecosystems diversity and helps increase ecosystem stability

Anthropogenic (human controlled) Grasslands

• Some areas have helped farmers to use grassland as a crop source

• These farmers allow their farmland to convert back to a healthy grassland ecosystem – Sometimes they even get money from the

government (subsidized) or receive tax breaks for allowing their farmland to revert to native grasses

• Then, instead of burning the grasses or allowing nature to allow it to burn (lightning) they harvest it for food for their livestock (cows)

Grasslands Biomes

• In order to better understand grasslands, ecologists have separated them into three types of grassland biomes.

– Steppe

– Prairie

– Savanna

Steppe Biome (Grassland)

• Steppes are similar to deserts in that they receive a very small amount of rain

– Only about 50cm of rainfall per year

• Steppes are usually dominated by “bunch grasses”

– Bunch grasses are hardy grasses that grow in tight groups and usually have deeper root systems

– Sometimes, these are called semiarid deserts because of their low diversity and rainfall

Steppe

Prairie Biome (Grassland)

• The most common grassland in the united states is considered prairie

• Prairies are characterized by rolling hills, plains, and sod forming grasses

• Prairies receive about 50-75cm of rainfall each year

– Occasionally, prairies will have a particularly wet season in which it can even exceed 150cm of rainfall

• Most of this is usually in the form of thunderstorms

Grasses = Rich Soil• Sod-Forming grasses are grasses that form a

dense mat of roots and soil – These mats of soil and root are so tightly woven

together that if cut into a square, they can be removed as if they were a single piece

– In colonial times, sod was used a renewable building product, especially on house roofs

• As the roots of grasses die, they form a layer of rich organic matter called humus– Humus holds high levels of nutrients and moisture

and makes for the rich soil found in the prairies

Bad Farming and Grassland• Farmers always need to be conscious of the

delicate processes of grassland communities

• If plants are removed, their roots are not able to hold the soil down to prevent wind erosion

• If this occurs for to many years in a row, the layer of rich top soil is lost and the grassland behaves similar to desert land

• Bad farming practices in the 1930s created just such a problem during one of the worst environmental disasters of all time called the dust bowl.

Savanna Biomes (Grassland)

• Savannas are tropical or subtropical grasslands ranging from dry scrubland to wet, open forests

• Savannas are classified as areas that have climates that alternate between heavy rainfalls and long dry droughts

• Savannas can see as much as 150cm of rain per year, but most of it falls during the brief rainy season

• These extremes make for a difficult living environment for organisms and high adaptations

Savannah Plants• Most grasses and shrubs in the Savannah are

characterized by extremely fast growing when water is available, and able to become dormant when water is not available

• Many plants use Runners which are horizontally growing stems that allow for rapid spreading

• Many of the plants can repair damaged parts (from fires or grazing animals) and usually have ways to defend themselves like thorns and toxins

Surviving the Competition• The warm temperatures and heavy rainfall of the

savanna allows for large amounts of animals to live in these areas

• Remarkably, the savannah is home not only to a lot of organisms, but it is also home to a large variety of different species

• This is because many of these organisms have adapted to fill specific niches

• One way this occurred is by a processes called a vertical feeding pattern each organisms feeding on vegetation at a different height