Background/Question/Methods An oft-cited hypothesis in invasion ecology predicts that positive interactions involving non-native species cause systems to become more easily invaded and modified through potentially self-reinforcing feedbacks, resulting in an accumulation of non-native species and their effects—a process known as invasional meltdown. Invasional meltdown is a continuum of the effects of biotic facilitation, the most basic of which is a single invader being positively affected by another invader such that its establishment, population expansion, spread, or impact is enhanced. The concept has been oversimplified in the scientific literature and media, and the term has been invoked to describe any form of positive interaction between non-native species. Here, we review and extend the theory behind this concept.
Results/Conclusions Although invasional meltdown has rarely been studied beyond single pairwise facilitations, it can arise through multiple distinct mechanisms that lead to i) increasing invasion rates, ii) cumulative, sometimes synergistic, impacts, and iii) rapid biotic homogenization. These mechanisms include, for example, co-evolved species assembly, cascading disturbances, and keystone invaders that act as superfacilitators. Furthermore, native species can play key roles in invasional meltdown. We propose a mechanism that involves cascading interactions among native and non-native species: facilitations and synergies among invaders can occur indirectly across multispecies chains whose intermediate links may include natives. This model offers a unique explanation for conflicting relationships between native and non-native species richness (the so-called “invasion paradox”).