May I suggest that this is a bit oversimplified?

When you cite a "purpose," to what ends or for what reason? To simply say that it has a purpose, ascribes motivation or intention. when really, the only purpose they serve is to survive. And yes, survival includes making oneself useful to the ecosystem to facilitate longevity, however, I don't know that one can say for certain that the way a particular species fits into the ecosystem is a purpose. After all, species can become extinct if their survival strategies do not fit a changing ecosystem.

Further more, how can one say how an ecosystem should be! Sure, we have the state it was in as we developed within it, but we cannot say for certain that it is the natural world. From what we can tell, the ecosystem was very different indeed in the Paleozoic era. How can one say that wasn't the original or purposeful ecosystem?