Disturbance (ecology) - Disturbance As Part of A Cycle

Disturbance As Part of A Cycle

Often, when a disturbance occurs naturally, it provides conditions that favor the success of different species over pre-disturbance organisms. This can be attributed to physical changes in the abiotic conditions of an ecosystem in combination with reduced levels of competition. Because of this, a disturbance can change an ecosystem for significantly longer than the period over which the immediate effects persist. However, in the absence of further disturbance, many ecosystems will trend back toward pre-disturbance conditions. Such alteration, accompanied by changes in the abundance of different species over time, is called ecological succession. Succession often leads to conditions that will once again predispose an ecosystem to disturbance.

Pine forests in the western North America provide a good example of such a cycle involving insect outbreaks. The mountain pine beetle (Dendroctonus ponderosae) play an important role in limiting pine trees like lodgepole pine in forests of western North America. In 2004 the beetles affected more than 90,000 square kilometres. The beetles exist in endemic and epidemic phases. During epidemic phases swarms of beetles kill large numbers of old pines. This mortality creates openings in the forest for new vegetation. Spruce, fir, and younger pines, which are unaffected by the beetles, thrive in canopy openings. Eventually pines grow into the canopy and replace those lost. Younger pines are often able to ward off beetle attacks but, as they grow older, pines become less vigorous and more susceptible to infestation. This cycle of death and re-growth creates a temporal mosaic of pines in the forest. Similar cycles occur in association with other disturbances such as fire and windthrow.