Well, there are a few explanations to this questions that could be right, but let's go to the obvious one.
What can happen sometimes is that, when a new species is imported into a new ecosystem, it can cause some problems. For example, a plant is imported from Japan to Brazil and it is planted in the soil of a forest. This plant can either adapt to the environment or die because it's missing nutrients from Japanese soil. If this plant adapts, it can happen to in a way that this plant will take place of plants that usually were growing right there, taking their resources to grow up more and more, spreading fastly too. That is a process that can happen, it's a possibility, but, there's a very big chance of this happening in reality.
We can either mention the example of an animal, it's easier to understand, if we put a new animal into a new environment, it can go wild because this animal can start eating other animals, and that can cause a serious problem for their population, since it's going to start growing faster and faster, and once this happens, it'll will reach a limit line, and that line is the line of competition for food.
