Secondary invasion re-redefined: The distinction between invader-facilitated and invader-contingent invasions as subclasses of secondary invasion

In their recent article in Ecology and Evolution, O’Loughlin and Green (2017) set out to (1) redefine the term secondary invasion as the condition "when invader success is contingent on other invaders altering the properties of recipient ecosystems" and (2) propose a framework for accounting for this phenomenon in invasion ecology. We applaud the second objective. However, redefining the term secondary invasion in the very narrow manner they propose is problematic and unfounded. Here, we establish that secondary invasion represents a broad range of phenomena that encompasses invader-facilitated invasions of which the condition described by O’Loughlin and Green, invader-contingent invasions, is a special case. We further demonstrate how recognizing these distinctions and applying these definitions in this manner expands the applicability of the framework they propose for thinking about facilitation in invasion ecology.