paraphrase:
The new species that I would introduce into an ecosystem would be aquatic. My creature would resemble Zooplankton, tiny and incredibly fast to reproduce. I would not make my aminal immortal because that would not be amusing. I would give it the ability to reproduce in industrial quantities so that when you kill one, you still have trillions of millions more to kill. The prominent harm my species would cause would be releasing toxic chemicals into the water, destroying other living organisms, and poisoning any other creatures that eat anything from the polluted area, causing the collapse of the ecosystem. The species would have the ability to adapt to any environment in order to survive (except land because it needs water).