Over a period of time, many habitats change with respect to the types of plants and animals that live there. This change is known as succession. Succession occurs because plants and animals cause a change in the environment in which they live. The first weeds and grasses that appear on a bare field, for example, change the environment by shielding the soil from the direct sunlight. As these plants spread, the ground surface becomes cooler and moisture than it was originally. Thus, the environment at the ground surface has been changed. The new surface conditions favor the sprouting of shrubs. As shrubs grow, they kill the grasses by preventing light from reaching them and also build up the soil in the area. In addition, they attract animals that also enhance the soil. Pine seedlings soon take hold and as they grow, they in turn shade out the shrubs. They are not able to shade out oak and hickory seedlings; however, they have found the forest floor suitable. These seedlings grow into that eventually shade out the pines.
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