I do remember hearing there was debate if we should introduce invasive species, let alone terraform, or try to preserve life the way it is. There is also concern about it going the other way around mar's life becoming invasive on Earth or infectious to humans.
Obviously neither of our planet's life has evolved to be very fit in surviving in the other's environment. But evolution often has "arms races" where organisms evolve highly optimized weapons and defenses against them. It's possible one planet has evolved something the other has no defenses against which would give it an advantage.
E.g. on Earth, the cane toad produces a poison that no Australian predators can survive, and so they all die and the toad overpopulates. Same with many invasive species, like plants that produce poison that kill all the other plants nearby, and take over fields and native ecosystems. Or predators introduced on islands that are much better than the native predators and kill all the native life.
Obviously neither of our planet's life has evolved to be very fit in surviving in the other's environment. But evolution often has "arms races" where organisms evolve highly optimized weapons and defenses against them. It's possible one planet has evolved something the other has no defenses against which would give it an advantage.
E.g. on Earth, the cane toad produces a poison that no Australian predators can survive, and so they all die and the toad overpopulates. Same with many invasive species, like plants that produce poison that kill all the other plants nearby, and take over fields and native ecosystems. Or predators introduced on islands that are much better than the native predators and kill all the native life.