What factors keep an ecosystem going?

Time, interface, &sequence; What ever the composition of the system, be it static (balanced) or dynamic (subject to change) it exists because of the dimensions that govern its composition. Time = a physical measurement of the expenditure of energy. Interface= each component reaching for dominance, verses all other components seeking to achieve the same result, thus achieving a compromise, or balance. Sequence= each event measured in milliseconds, or nanoseconds, or what ever increment of stability is achieved, until balance succeeds.

A) early horses is your answer