Why are native species are important to an ecosystem?
Because they are already adapted to live in this specific ecosystem and they have a well established role in the food web and the recycling of materials.
In any ecosystem energy coming from the sun has to be transferred from one organism to another to drive this ecosystem. And for the organisms to make new cells for growth or renewal, matter from the surrounding should be used, and hence this matter should be returned back to the environment after the death of this organism, to be used by other ones. This is important as matter in not added to the ecosystem, no new of it will be generated, so it has to be recycled. And each organism in the ecosystem has its role in both, transferring energy and recycling material.