A widely cited hypothesis in ecology is that species-rich communities are less vulnerable to invasion than species-poor ones, owing to competition for limiting resources (the "biotic resistance" model). However, evidence for biotic resistance in aquatic ecosystems is equivocal. Contrary to the view that communities become more resistant to invasion as they accumulate species, the rate of invasion has increased over the past century in areas that have received frequent shipping traffic. Furthermore, introduced species may facilitate, rather than compete with, one another. A review of invasions in the Great Lakes indicates that direct positive (mutualistic and commensal) interactions among introduced species are more common than purely negative (competitive and amensal) interactions. In addition, many exploitative (e.g., predator–prey) interactions appear to be strongly asymmetric in benefiting one invading species at a negligible cost to another. These observations, combined with an increasing invasion rate in the Great Lakes, tentatively support the Simberloff – Von Holle "invasional meltdown" model. The model posits that ecosystems become more easily invaded as the cumulative number of species introductions increases, and that facilitative interactions can exacerbate the impact of invaders. It provides a theoretical argument for substantially reducing the rate of species introductions to the Great Lakes.