Each and every plant and animal is essential for the survival of the bio-system. Changes in the biodiversity of a place change its bio-system.
The tiger is a carnivore and kills other animals for its survival. However, if it becomes extinct, the deer and cow population will thrive and increase in number. With such a large number of herbivores, they will graze on the green cover and in due course, there will be no plant cover. Without the roots of the plant to hold the soil in place, the place becomes prone to soil erosion.