Bacteria like rhizobium provide nitrogen to plants, other type of microbes help in decomosition, humans and animals eat plants some other carnivores animals depend on herbivores/omnivores animals for food. if there were only herbivores animals the plant cover would be less if only carnivores animals were there they would not have any food to eat and if there were no microbes the decomposition of some things was not possible and the provision of nitrogen wouldn't have been there.
 It is an interlinked, complicated but balanced system in nature. We can't separate out the wildlife alone whereas it is a part. When we start breaking the system or else the cycle, the nature reacts to it and modify itself which we can't face even. Finally the destroyer we our self will get destroyed....