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Opportunist plant species are plants typical of unstable or periodically extreme environments. One characteristic of such plants is the strong ability to spread widely and quickly. Such species are usually smaller than the typical species in an area, and live less long. Thus, initially they can colonize an area rapidly during a favorable period, though in the long term they may lose out to equilibrium species.
Here is a common example of such plant species in the Western U.S. In the upper Great Plains, there is the blue-bunch, a common and dominant plant species in the area. Cattle, however have nearly eliminated the plants. During the period of time when the blue-bunch disappears, another plant species, the leafy spurge has come to predominate the area. However, it only stays for a season like several months, and then the leafy spurge plants will be warded off by the dominant plants. The reason is that if the number of cattle is reduced, due to human activities or climate, to the level that the ordinary state of the environment can re-establish itself, the dominant plant species in the area will come to exist again. Once this is done, over time, the opportunist plant species will disappear.