Successful invasions and failed biocontrol: the role of antagonistic species interactions
Understanding the successes and failures of nonnative species remains challenging. In recent decades, researchers have developed the enemy release hypothesis and other antagonist hypotheses, which posit that nonnative species either fail or succeed in a novel range because of the presence or absence of antagonists. The premise of classical biological control of invasive species is that top-down control works. We identify twelve existing hypotheses that address the roles that antagonists from many trophic levels play during plant and insect invasions in natural environments. We outline a unifying framework of antagonist hypotheses to simplify the relatedness among the hypotheses, incorporate the role of top-down and bottom-up influences on nonnative species, and encourage expansion of experimental assessments of antagonist hypotheses to include belowground and fourth trophic level antagonists. A mechanistic understanding of antagonists and their impacts on nonnative species is critical in a changing world.