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).

The ecosystem that I would introduce a new species to would involve an aquatic environment. This creature that I envision would bear a resemblance to Zooplankton, characterized by its small size and remarkable reproductive speed. However, I would not bestow immortality upon this creature, as that would not add any enjoyment to its presence. Instead, I would grant it the ability to reproduce in vast quantities. This way, even if one is eliminated, there would still be countless others remaining. The main negative impact that my species would inflict would involve the release of toxic substances into the water, resulting in the destruction of other forms of life. Additionally, any creatures that consume substances from the polluted area would be poisoned, ultimately leading to the collapse of the ecosystem. Furthermore, this species would possess the remarkable ability to adapt to any environment, except for land, as it is reliant on water for survival.