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
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.