ABSTRACT
Human populations have brought about significant changes to the natural landscape with their activities; they have caused the extinction of many plant and animal species and major disturbances to the biosphere functions. The civilizations that flourished in various places and eras found themselves in greater or smaller competition with the natural environment. Biosphere is the planetary ecosystem, and it exhibits great complexity in relation to its structure and function. The cultivation of marginal lands, deforestation of areas with a marked relief or vulnerable soil, unsuitable irrigation of friable earth, over-grazing of the natural vegetation and use of fire were widely applied practices that had disastrous consequences. Terrestrial ecosystems can be divided, mainly on the basis of the prevailing vegetation type, in four basic categories: forest, grassland, desert and tundra.
