How many ancient civilizations downfalls were due to ecological issues?

Answer 1

Some have argued that most of them ultimately had some element of ecology as an issue. This includes that Anasazi in the American Southwest, the Mayans, the Persians, the Roman Empire as well as others.

Answer 2

That depends. Many historians feel that extreme periods od cold and drought, combined with a rapdily expanding population drove the Huns, Mongols and other Steppe peoples to periodically "flee" west in search of more accomodating land. Some also feel that the Vikings started out that way as well.

Answer 3

Zero that we know of, but don't tell Algore...