In a balanced ecosystem, the predator and prey relationships are in balance as well. This means that the healthy rates for the ecosystem to be highly functional is mantained because there's the adequate amount of individuals in each species. When you introduce a non-native species, this species will have to feed and live somewhere, but as the ecosystem was in balance, this species will have to compete for resources and create an unbalance. The species can even feed so much in another species that the prey will be stinct from the area, for example. Non-native species can be introduced and don't have any predators, so they will reproduce until it takes so much space that it creates an unbalance in the ecosystem as well.