What once was an open pasture on a long abandoned farm has changed over time, without the direct influence of humans. after the cattle had been removed, shrubs and bushes could be seen dotting the grass-covered hillsides. a few years later, small pine trees and then larger deciduous trees appeared. now, there are so many trees and shaded regions it is difficult to tell that this was once a field?

The mentioned case represents secondary succession.  

Secondary succession is one of the two kinds of ecological succession of plant life. Secondary succession refers to a procedure initiated by an incident, like harvesting, forest fire, hurricane, and others, which minimizes an already settled ecosystem to a lesser population of species.  

Thus, it can be said that the secondary succession takes place in preexisting soil on the other hand primary succession takes place in a place devoid of soil.  

Secondary succession is the ecological succession, which takes place after the initial succession has been disrupted and certain plant and animal species do prevail. It is usually faster in comparison to primary succession.  

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