Deforestation

Overview

Agriculture: To support an expanding population, forested land must often be cleared to make room for agriculture. In third-world countries, this is usually used for crop growth.
Urban Development: To account for their booming populations, developing countries often need to expand their cities and urban areas. Of course, this requires the destruction of local ecosystems.
Harvesting: The trees themselves are often harvested for the purpose of constructing buildings or furniture, for use as firewood, or for the production of goods such as paper.

All ecosystems are intertwined; the removal of even one element can throw the entire system off-balance. Large-scale deforestation is no exception. The removal of trees leads to the disturbance of the species that depend on them. If these species cannot survive, the cycle continues. Species continue to leave the area, drastically decreasing local biodiversity.
Living trees use their roots to hold the soil in place; when these trees are removed, the soil loosens. With rain the soil and its nutrients are lost to local rivers and streams. This has two adverse effects:

In some areas (such as those bordering the Amazon) inefficient agricultural practices deplete the soil of its nutrients, leading to its eventual abandonment by both farmers and the forest. More land is then deforested to make up for the loss. Luckily, crop rotation has proven effective in controlling soil depletion, perhaps curbing deforested areas’ steady growth.