These are organisms that are brought into a new environment, sometimes accidentally, sometimes on purpose; and have no natural enemies or controls.

The correct answer is - invasive species/organisms.

The invasive species, very often are very damaging to the new environment. The process of their introduction in a new environment can be:

- Natural, like with the Smilodon getting to South America once a land bridge opened between North and South America.

- On purpose (by humans), like the introduction of the coffee plant in South America.

- By accident (by humans), like with the introduction of the rats all around the world, that were dwelling on the ships of the explorers and migrants and managed to get to new territories.

All three examples have been damaging to the native species, in one way or another, with the Smilodon managing to destroy the Thylacosmilus, the biggest marsupial carnivore of South America, because of the coffee plant people have been destroying other plants to make space for it, and the rats have been out-competing lots of species around the world for food and also transmitted diseases to them which resulted in their demise.