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. A single road bisecting a habitat patch cuts the habitat in half effectively reducing the individual population sizes in half.
Different creatures are variably sensitive to fragmentation.
Birds can access different patches separated by small divisions but small mammals, being less mobile, can be seriously affected. The edge of an area provides a buffer between two different habitat types. As habitat is fragmented not only is the land divided but by the act of division part of the habitat is converted to buffer or edge which may support a different flora and fauna from the original habitat. Invasions by Exotic Species Exotic invasions are the second largest cause of extinctions. Exotics out compete both animals and plants.