Why animals in nature cannot be divided into useful and harmful?

In an ecosystem, all living organisms are closely intertwined. Each species has its own function. If one species destroys another, then this provides natural selection and regulation of the number of animals. For humans, useful or harmful animals can be identified. For any particular species, you can do the same. But if we take the biosystem as a whole, then it is impossible to single out harmful and useful ones, since they are all important or neutral, that is, when they disappear, there will be no consequences for other species and the biosystem as a whole.