Undoubtedly an important reason why invasive species do not always invade habitats, the preemption hypothesis suggests that invasives can be prevented from invading because of the presence of other species already on a site. The plant colonization window is closed, because other species are physically occupying the space. Though often overlooked, preemption is logically one of the most important forms of competition in plants. If a species is already occupying a space, it is difficult for an invasive species (or any other) to eject that species from the space.
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