A

a. US farmlands were threatened by severe drought in the 1930s. This, exacerbated by the use of sub par farmland, resulted in massive erosion. When the U.S. economy went into the Great Depression in the late 1920s, many farmers tried to plant on submarginal acres or plant different crops, often not suited to the Great Plains, which would bring better prices. When the crops failed, the soil is exposed to the elements. High winds accompanying the drought resulted in the dust bowl.

Currently, the biggest threat to farmland habitats is urban sprawl. To alleviate this threat, building codes need to be carefully evaluated. Rather than constructing new subdivisions, old urban areas need to be revitalized. Arable land should be set aside and not used for building.

b. When natural landscapes are altered, erosion results. Removal of the native plant species exposes the soil to wind and rain. Improper irrigation also increases soil loss through water erosion. Desertification, or the formation of an almost impermeable upper layer, on the soil can result.

The four environmental spheres are the atmosphere, lithosphere, biosphere, and hydrosphere. The farming of natural landscapes disturbs the biosphere and the lithosphere. The lithosphere is the solid shell of inorganic materials at the surface of the Earth. It is composed of soil particles and the underlying rocks down to a depth of about 50 km. The biosphere is the thin shell of organic matter on the Earth's surface. It occupies the least volume of all of the spheres but it is the heart, or the chemical pump, of much of the flow of matter through nature. The atmosphere continues to operate in the same manner, resulting in wind, while the hydrosphere can cause erosion if drought occurs.