Ecological Inertia

Except for the earliest plant pioneers in newly deglaciated land, all later arrivals, if they were to success, had to take possession of ground already vegetated. They had to get started in gaps in the existing vegetation. But gaps would have been uncommon, and such gaps as there were would have received far more seeds from whatever plants happened to be abundant in the neighborhood than from the few, isolated specimens forming the vanguard of an invading species. This would have been true even if the climate were slowly becoming less suitable for the established vegetation and more suitable for the invaders. Hence the lag in vegetation change; once a plant species is abundant in a particular area, it can usually hang on there for a long time in spite of the climate's gradually becoming less suitable for it. Many plants have an astonishing ability to persist in unfavorable habitats. The old adage "possession is nine points of the law" sums the matter up.

From After the Ice Age: The Return of Life to Glaciated North America. By E. C. Pielou (University of Chicago, 1991) p. 99

This incumbency effect is very pronounced in all kinds of places in ecology, and even in society. If you are already there you don't have to be as efficient or as effective in order to compete, because you have the advantage of already being there. It is not just in politics either, what else explains the continued existence of General Motors?