Releasing any species into an environment it's not adapted to, or native to is dangerous because the species lacks the necessary adaptations to survive in the environment.
In the example of the frog, releasing it into a grassland would be a bad idea because the frog needs not only a "moist" area to thrive but it needs places to hide and plenty of insects to feed off of. Also, for reproduction it would need a pond, or some kind of water source!
I hope this is helpful!
