Question

When can an ecosystem sometimes repair itself?

Answer 1

After natural pollution. 
Hope this helps! :)

Answer 2

Answer: after clear cutting of the forest

An ecosystem can be defined as biological community where living and non-living components interact. The clear cutting refers to the cutting of the trees and plants in the forests, for using the wood product, medicines obtained by these plants and trees. The forests which is being clear cut will be disturbed. The animals and birds species will migrate to different location in search of food and habitat. The clear cut forests can be restored as the soil is fertile enough to support the growth of new plants. The colonization of new plants will take place with the already present precursors like seeds, roots and partially trees. The animals and birds species will soon immigrate after colonization of tree and plants species. Therefore, the ecosystem will repair itself with the help of these precursors.