In a pond there are fish, turtles, algae, frogs and other living organisms that live in the pond. all the organisms that live in the pond are called the

Freshwater ecosystems

It is useful for historians to write narratives so that history can be passed on from their point of view.  think about a story being told in third person, then think about it again being told in first person, by someone who actually experienced the event.

Your answer would be 625,000,000.