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 direct sunlight. As these plants spread, the ground becomes cooler and more moist 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 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, that have found the forest floor suitable.  These seedlings grew into large trees that eventually shade out the pines. The best title of this passage is   _  .
The Importance of Weeds and Grasses The Success of Oak and Hickory How Environmental Habitats Change Animal and Plant Habitats