The Invasive Meltdown theory states that as species are introduced into an environment that environment has a certain threshold it can cope with and when that threshold is surpassed the environment can no longer resist further disturbances. Basically the prese3nce of invasive species can make it easier for other invasive species after a certain point. It can be summed up by saying that while each individual invasive is a threat they collectively cause enormous damage as well (1).
A contemporary example of this phenomenom is the zebra mussels. They get into lakes cause distrubances that contribute to the threshold of disturbance and make the lakes an easy target for other invasive species. this causes more disorder allowing for more species to invade and causes the environment to "meltdown" (1).
References:
(1) Ecology/Invasive species. (2011, March 6). Retrieved April 14, 2015, from http://en.wikibooks.org/wiki/Ecology/Invasive_species#Invasive_Meltdown_Theory