ecology 1. a plant capable of invading bare sites, e.g., newly exposed soil, and persisting there or colonizing them until supplanted by successional species —note pioneers often invade in large numbers and over considerable areas 2. by extension, any new arrival in the early stages of succession, with particular reference to certain species whose presence appears to promote the establishment of more exacting species —see ecolysis, primary host selection, ruderalThis definition last updated 07/24/2008