Which definition best describes the term succession a series of changes that cause a population to increase or decrease. an instant change caused by a disturbance. changes that only happen when species die. series of gradual changes that occur after a disturbance.

Succession in biology is the series of gradual changes that occurs after a disturbance. 
the disturbance may have caused all life to have been cleared off that region. this causes a pioneer species to come and inhabit the region and settle down. Along with the pioneer species, other species such as trees and other other organisms gradually establish themselves in the region. This paves the way for a stable ecosystem to be established.