Factors that Mediate Ecological Competition
Within an ecosystem that is relatively isolated from outside influences, competitors undergo coevolution with each other, eventually ending up with either competitive exclusion (the loser goes extinct in that area) or some sort of stable setup of niche segregation (could be symmetrical, could be asymmetrical). However, many ecosystems have regular introductions of species from outside the region. When invasion occurs, an inferior species can maintain itself within the ecosystem even though it can't compete with the resident dominant species; evolution therefore occurs over broader, allopatric conditions (multiple geographically-separated ecosystem areas).
Competition is also mediated by other types of ecosystem interactions, especially predation. If the superior competitor is preferentially eaten by a predator, its population gets reduced regularly, allowing the inferior competitors a chance to access the resources. Ecological disturbances are another way for an ecosystem to clean its slate of competitive conditions and allow the various species to start over.
Finally, the conditions that make one species superior and another inferior are also subject to change. In ecological succession, different groups of species take turns being the best possible fit in the ecosystem, then get outcompeted by the next group. Likewise, cyclical changes like seasons will favor different species at different times, allowing all of them to coexist within the ecosystem.