UV-B radiation degrades DNA which can induce many forms of cancer. The major constituents of living organisms are made up of carbon, oxygen, hydrogen, nitrogen, phosphorous and sulfur. Oxygen is readily usable in its elemental form but carbon, nitrogen, phosphorous and sulfur must be fixed for biological utilisation. These substances cycle between living and non-living reservoirs and these cycles are referred to as biogeochemical cycles. Part of global change research is to identify the major reservoirs of each of these elements and determine the rates at which they are transported or converted between these reservoirs.
Man alters the carbon cycle through fossil fuel emissions and forest clearing.
The nitrogen cycle has historically involved the uptake of nitrogen by bacteria which makes it available to plants. Thus it is kept in the organic reservoir until death and decomposition return nitrogen to the atmosphere. Through chemical processes man now fixes nitrogen in order to create fertilizer. Additional nitrogen is added as a combustion product and is a major ingredient in photochemical smog. Land-Use Change Habitat Fragmentation Fragmentation of native habitat is one of the biggest causes of extinctions.