Introducing evasive species to a food web can damage the ecosystem as the existing life depends on each other. Having a new organism come in and take extra food harms the organisms that eat that food, and in turn if there is nothing left from (lets say the carnivores), the producers will have very little nutrients to absorb from the soil. Once the producers die, the herbivores will die because they have no food. Then the chain of death will continue till there's nothing left.
