In eco systems what can be or harmful depending on point of view

We can quote many examples for such things which are helpful or harmful depending on point of view or situation or amount etc.

a) Bacteria : they can be helpful for fermentation process used in manufacturing of many useful things (like beverages) and can be harmful as they cause rotting of many useful things

b) Beaver : it is one of the largest rodent on earth. It is harmful as it causes cutting of trees. It lives upon them.

However, it a natural dam builder. It has a tremendous power to swim or float in water.

It can create canals naturally.

But for living purpose it feeds upon trees and cause destruction of trees and thus forest.

tracks, burrows, eggshells, nests, tooth marks, or gastroliths