What once was an open pasture on a long abandoned farm has changed over time,

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 changes in this field, from grasses to shrubs to trees, represent 

(a) ecological succession 

(b) climax production 

(c) biological evolution 

(d) ecological adaptation