Is my compost heap aerobic or anaerobic?

It is often stated that heaps are either aerobic or anaerobic. Both types of bacteria co-exist in decaying matter. It is perhaps more relevant to think of a dominant population of one type of bacteria and the ability of the dominant population to 'switch' dependent on the oxygen concentration.

Aerobic bacteria dominate when oxygen is above 6% Anaerobic bacteria dominate when oxygen is below 6%.