1-no predators,so uncontrolled population growth
2-is a predator that is stronger than the apex predator of the area,or is in a predator free environment so has little to no problem spreading
3-is a toxic animal that breeds quickly,so when eaten can kill what ate it,there for ruining the ecosystem as the predators die from the toxic prey they eat.which can lead to method 1 if not dealt with.
