Recent studies show that ecosystems, even when undisturbed by people, are not static but change over time.  Studying the patterns of the changes in ecosystems  in response to natural or human disturbance is an important tool for learning how to restore lands damaged by people.  This section should foster your understanding of the concepts covered in the book specific to how an ecosystem redevelops following a disturbance through a process known as ecological succession, the important role that disturbances play in the persistence of ecosystems, and the different effects on the land of physical forces and biological processes.

A CLOSER LOOK  - Reforestation of Mount Saint Helens. 

   Mount Saint Helens, a long-quiet volcanic mountain in Washington State, was a pleasantly wooded, peaceful area until early in the morning of May 18, 1980.  Forests of Douglas-fir and western hemlock grew on the slopes below an elevation of 1200 m (4000 ft); mountain hemlock and firs grew above.  Wildlife was abundant.  On that May morning, life on the mountain was subject to a rapid series of devastating events when the volcano violently erupted.  A combination of an avalanche of debris, superheated steam and water, mudflows, volcanic ash and lava damaged 61,000 ha (151,000 acres) of land; 21,000 ha (52,000 acres) of forests were completely flattened, and much of the soil was lost.

   The landscape was devastated and seemed devoid of life; 1600 elk were killed and fish almost disappeared from the streams.  But to the surprise of almost everyone, including ecologists, life returned rapidly.  Fireweed and other plants common in clear-cut forest areas appeared, spring flowers such as trillium grew, and trees sprouted in areas protected from the blast and heat.  By 1984, 600 elk were living in the area, and the streams were populated by 10 times as many steelhead trout as there had been before the eruption.  Life was returning, and the process of ecological succession--the establishment of an ecosystem and its biological community--had begun on the mountain.