(1) A fierce predator is removed from the ecosystem is the answer.
This is because a fierce predator controls the amount of primary consumers and in a respectable amount of cases, even secondary, tertiary, etc. consumers. Without the controlling factor, it may become that the ecosystem becomes overpopulated. Overpopulation would quickly deplete the resources of the ecosystem, which would destabilize the mutual relationships that the ecosystem and animals have with each other.
