A natural forest creates climate. These ecosystems evolved over a very, very long period of time and through many geologic and climatic changes. That is what happens when a forest is clearcut. When rivers are polluted by chemicals and erosion, oxygen is depleted from the water and living things die. When land is overgrazed or the nutrients in the soil used up from improper farming practices, the soil cannot renew itself and plants and crops cannot grow.
Crop rotation is one way to help maintain soil fertility.
When natural vegetation is removed from land to clear it for construction projects, mining operations or farmland, the plants and trees are no longer there to absorb rainwater and protect the soil from wind. Erosion occurs and the soil is washed away or soil particles are blown away in the winds. When land is cleared of vegetation in watershed areas that drain into rivers, lakes, streams, estuaries, etc., soil erodes into the water and the silt smothers the fish and plant life. When pollutants drain into these waters, the toxins kill aquatic life and the pollutants can be carried through the water far distances. When pollution destroys an area, like the devastating Exxon Valdez oil spill and the Chernobyl nuclear accident.