There are two different types of long-term variability for an ecosystem. One-way; progressive environmental changes such as pollution.  Also, cyclical; such as anoxia and flushing of a basin. One-way changes cause differences in the dominant species and changes in the rate of production and accumulation of tests.
Cyclic changes from anoxic environments kills off the fauna to oxygenation.

    As a result, other species who to not require sufficient oxygen have a chance to prosper while the more dependent (once dominant) fauna is gradually decreasing.