The following text is from a biology journal. For years, the 'enemy release hypothesis' has been used to explain the success of invasive species. It suggests that when a species is introduced to a new region, it leaves behind its natural predators and parasites, allowing its population to grow ______. However, recent studies of the European green crab in North America have found that while it has fewer predators, it still faces significant competition from native species, which can limit its expansion as effectively as its original enemies.