A potential complication is that species thought of as native may in fact be the result of earlier, undocumented introductions, particularly in the cases of plants and invertebrate fauna. Furthermore, since nature never stands still, the "original" state of a specific habitat, and the "true" ecological equilibrium biologists seek to restore, are always subject to interpretation.

There have been some real success stories, like the reintroduction of some of North America's game species, the desert bighorn sheep and the wild turkey, to much of their original range after these areas have been reforested and limits have been placed on hunting. Many reintroductions are relatively recent and it is therefore too soon to judge their end results. However, failures have outnumbered successful translocations to date. Costs are high and the number of variables are enormous.