Required Readings
Science in Action 7
pages 62-65
or
Science Focus 7
pages 56-59
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The types of living organisms found in a particular area changes over time. Some changes can be man-made such as fire, agriculture and logging of forest products, others may be caused by avalanches, volcanoes and floods and some are simply an orderly change of species in a community. Succession is a natural occurrence in the aging of an ecosystem.
A very young ecosystem is the bare rock of a mountain or a sand dune that forms along the edge of a large lake. At first little life survives in these areas. Lichens, mosses and algae begin to cling to the rocks, slowly breaking down the surface of the rock causing a small amount of soil to form. Falling into the cracks of the rocks, the soil provided a place for seeds of small plants and hardy bushes to grow and small insects to live. These species form pioneer communities which continue to break down the rock even further allowing more plants to grow.
Cedar and birch trees are common pioneer trees. Areas that contain these trees tend to be early in their cycle of succession. As leaves and dead plant material from the pioneer species fall to the ground, they help to form humus. Slowly more soil is built up on the rock, and worms, birds, mice and other small rodents populate the area. As the soil becomes deep, other trees such as Maple and Oak begin to grow, forming soil even faster as more plant matter falls to the forest floor.
Large animals make their home in the forest where the huge stands of decidious trees begin to darken the underbrush of the forest and keep the moisture in. This provides shelter and nutrients for many species, but the pioneering community can not compete with the larger trees for sunlight and slowly die out. This final stage is called the climax stage in the cycle that changes bare rock to a mature decidious forest. A maple forest is considered to be a climax This process of succession can take thousands of years.
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