In a new environment, apparently insignificant aliens can't just be assumed to follow a series of simple linear steps in harmlessly adapting to their new surrounds. New introductions, especially accidental ones, must always be expected to evoke echoes up and down the food chain, reflecting the mysterious, unpredictable relationships between all of the living inhabitants. Research, for example, into the predation by cane toads, of other frogs, is difficult to undertake in the wild, because just about everywhere that cane toads have taken up residence, the native frogs have gone. Cane toads typically eat every moving thing within sight that will fit into their mouth, including most other frogs. Furthermore, most potential predators quickly realise cane toads are both poisonous and nasty tasting.
Alien species tend to fit the food chain poorly both upwards and downwards.
In the absence of effective controls, populations may simply explode. On the evolutionary tree, toads sit a fair way away from cattle. Yet one consequence of bringing cattle into Australia in 1788, was that pastures soon degraded because the ever deepening layers of dung broke down altogether too slowly. The system was, predictably, out of control. This was because the indigenous dung beetles were not adapted to the alien dung.