Simplified ecosystems are subject to collapse. Diversity that allows an ecosystem to survive during times of stress, such as drought. Because of diversity, drought will not kill every part of an ecosystem, so there is something remaining from which to rebuild. A grossly simplified ecosystem may not be able to rebuild.
From a human viewpoint, simple ecosystems can be very productive (fields of corn, for example), but they are fragile and subject to collapse - for example, the Irish potato famine of 1845-1851 killed a million people.
Wildlife also contribute to habitat degradation and loss, particularly elephants in areas such as national parks in Southern Africa, where they destroy forests by knocking down trees and 'simplifying' the habitat and ecological processes.