Class Session 20>

I. Land Ho!
As we humans are a terrestrial based species, land is a pretty important resource. The area of the earth’s surface is about 196,940,000 square miles. Of this amount, about 71% is water and 29% is land. To determine the amount of land surface on the earth, take the earth's total area -- 196,940,000 square miles - and multiply it by 29%. Twenty nine percent of 196,940,000 equals 57,112,600 and so there are some 57,112,600 square miles (14.9 billion hectares) of land surface on earth.

Not all land is the same. Because of the either underlying geologic forces, or processes occurring on the surface, such as weathering and erosion, the earth’s land surface is divided into different landforms and landscapes. Landform and landscape types include mountains, valleys, deserts, plateau, plains, peninsula, mesa, lowland, isthmus, hill, highland, delta, cliff, cape, canyon, and basin. The study of land types is known as geomorphology.

Land also differs a great deal in terms of its photosynthetic productivity or vegetation. Of the 57,112,600 square miles of land surface, 78% has some amount of vegetation on it either natural vegetation such as forests or grasslands, or agriculture. Some is semi-arid while some is sub-polar. Land which has enough heat, moisture and sunlight and which is not too steep is called arable and only a portion of all land, about 30%, or 40% of all vegetated land is arable. Twenty-two percent of the earth’s land surface is barren, either from the cold, dryness or slope. Areas of natural vegetation are known as biomes. A biome is a large ecosystem where plants, animals, insects, and people live in a certain type of climate. Major biome types include tundra, coniferous forests, deciduous forest, desert, grasslands, mountains, and rainforests.

We humans use land for a wide variety of activities. These include agricultural production, livestock grazing, forestry, open space, urbanization, transportation, recreation, and resource extraction. Farming is the biggest human land use, accounting for about 12% of the earth’s land surface. Livestock grazing is the second most common human use of land.

The way in which we use land, however, can sometimes have deleterious effects on the natural environment. One common problem is soil erosion. Soil erosion occurs when particles of soil are transported (carried) by either water, wind, or ice. Soil erosion happens every time it rains or there is a windy day. Erosion from the movement of ice is rarer and occurs where continental and alpine glaciers are found. Whereas certain types of air and water pollution are relatively recent phenomenon, soil erosion is nothing new. Societies have been farming on the delta lands built up at the mouths of rivers for centuries. It's also important to remember that erosion is a natural process.

The issue is not that soil erosion is occurring but the amount of erosion that is occurring as a result of human activity. In the last 100 years, the rate of erosion has increased substantially; even more so in the last fifty years as the human activities that disturb soil and accelerate the rate of erosion have increased significantly. All of these activities disturb the soil and can accelerate the "natural" rate of erosion:

* inadequate soil conservation techniques on cropland * increased use of irrigation * grazing of cattle
* the construction of buildings * the building of highways * activities associated with mining

All of these activities that result in erosion, agriculture probably is the biggest single contributor. Soil erosion is measured in tons of soil lost per acre. About 6 inches of topsoil is usually cultivated in modern agriculture. These 6 inches weighs approximately 1,000 tons per acre. If 15 tons per acre per year is eroded, about .10 inch of soil is lost each year and the whole plow layer in 60 - 70 years. It is thought that only 1-5 tons of soil can be lost each year without affecting productivity.

It is estimated that 4 billion tons of topsoil is eroded by water each year in the U.S., 3/4 of it from farmland. Wind erosion may erode another billion tons. Of the 3 billion tons of topsoil lost on farmland in the U.S., the national average for erosion losses is 4-5 tons per acre per year, just on the outer edge of affecting productivity. Most of the soil lost from farms in the U.S., however, comes from a small number of farms. Some 43% of the topsoil lost annually in the U.S. comes from 6% of the farms. The exact amount that erodes off of any specific farm varies greatly depending on the amount of rain a place receives or how much irrigation is used, what types of soil conservation practices are in place, if any, and the slope of the land.

Soil erosion decreases the productive capacity of the soil. Soil is composed of minute fragments of rock, organic material, water and air. Most of the organic matter is in the top layers of soil and this makes sense since most of life occurs at the surface. As this topsoil erodes, the organic material is carried off as well, leaving soil with low organic content behind.

Soil erosion also affects soil structure. Soil structure affects the permeability of the soil. Permeability allows for storage of water in soil root zone as well as passage of water to groundwater. When topsoil has been carried off, the remaining soil may not absorb water as well, hence more runoff may occur, making less water available to the crop.

Soil erosion makes tilling the soil more difficult. Beneath the layer of topsoil is a layer known as subsoil. Subsoil is more difficult to plow and till than topsoil. Subsoil also tends to form rivulets and gulleys which may interfere with plowing as well.

A fourth impact from soil erosion is increased sedimentation in streams. Eventually ends up in streams, which feed rivers, which feed bigger rivers, which feed the ocean. This sedimentation changes flow of streams and rivers and may make them more prone to flooding. Sedimentation also increases the amount of soil particles suspended in streams, rivers, and estuaries and this increase in floating material can interfere with the photosynthetic activity of water plants and grasses. Lastly, sedimentation also causes siltation of dams, shortening their functional and economic lifecycle.

Erosion can also increases runoff of unwanted substances, such as farm chemicals, into waterways. Many different types of chemicals are used as fertilizers, pesticides, and herbicides. These chemicals bind themselves to soil particles and, as these particles get carried off into waterways, the attached chemicals go with them increasing the chemical loading of waterways.

Because there is no global soil survey or data bank, no one knows exactly how much soil is being eroded from the continents around the world. One estimate developed by the World Watch Institute in Washington D.C. calculates that about seven tenths of the world's topsoil is eroded annually, resulting in a 7% loss per decade.

The actual rate of erosion varies tremendously from country to country. The type and size of farms, the agricultural technologies and techniques utilized, and the intensity of cultivation all affect the actual rate of erosion.

Another unintended environmental impact which can result from agriculture is increasing the salt content of soils, known as salinization. The concentration of salt in water, known as salinity, varies. In areas where water contains large amounts of dissolved salts, irrigation used for agriculture can, over time, result in salinization of soils. The long term effects of salinization can include decreasing productivity of soils, reduction of vegetative cover, and increasing salinity of river water as irrigation canals often drain into nearby streams. The increasing salinity of waterways can adversely affect aquatic ecosystems as well as make drinking water less palatable or unusable.

It is estimated that about 7% of the world's cropland has been affected by salinization. Salinization is a problem in North Africa, the Middle East, Central Asia, India, Pakistan, Australia, Argentina, and Mexico. Closer to home, it has been estimated that as much as 25% of the cropland in the United States has been affected by increasing soil salinity.

Overusing marginal agricultural or grazing lands can lead to the gradual spreading of deserts, known as desertification. Desertification occurs in ecologically fragile areas with minimal or erratic rainfall. Over cultivation of marginal lands, overgrazing of range and deforestation for fuel wood can lead to desertification. The long term effects of desertification include degraded soil structure, increased erosion, and loss of vegetative cover.

It is difficult to estimates exactly the amount of land that has been affected or is at risk to desertification. The United Nations Environment Program (UNEP) estimates that about one third of the earth's surface is at risk to desertification. Most of this land is in Africa and Asia, but the western United States and northern Mexico has also been affected.

II. Dry lands and Wetlands

The earth’s surface is 71% water and 29% land. Well, not all the surface considered land is “dry” land. A small, but important, percentage are “wet”lands. Wetlands are special environments where some standing water, either salt or freshwater, is present for at least part of the year. Salt water wetlands are known as coastal wetlands and freshwater water as inland wetlands. Wetlands, however, have many different names including swamps, marsh, bogs, and mudflats. In all, wetlands cover about 6% of the earth's surface.

Wetlands are special environments. Per unit area they contain as much biological diversity as any habitat on earth. The presence of water leads to accelerated amounts of photosynthetic activity which yields more biomass and biological diversity. Wetlands account for about 25% of the earth's total net primary photosynthetic productivity. Many saltwater fish, crab, clam, and oyster species spawn in wetlands and wetlands provide habitat for waterfowl and many other animal and insect species. In addition, because of the great amount of photosynthetic and biotic activity that occurs in wetlands, they also serve as natural filters for waterborne pollutants.

A variety of human activities can result in wetlands degradation and loss. These include agriculture, aquaculture, housing, industry, and transportation. The primary mechanisms behind wetlands loss are dredging and filling operations which convert wetlands to land suitable for development. Other causes of wetlands loss include, diking and damming of rivers, stream channelization, and pollution.

It is difficulty to say exactly the amount if wetlands lost globally as a result of human activity. Some estimates gauge the amount of wetlands lost since 1900 to be as high as 50%. The United States has lost some 55% of its' original wetlands.