[Serereids]

uuh...this whole thing is based on a book called Catastrophe by David Keys, and he traces it to a volcano doing the volcanic winter bit in 535. Best candidate is Karakatoa but there are others.

The basic cause has some interesting data. (the tree rings come in here) The conclusions he draws are specualtive at best.

One problem is that the Western Roman Empire officially ended in 478, when the last Emperor was overthrown by Odoacer who sent the Imperial robes of Romulus Augustulus back to Constantinople and took the title King. Now Keys says Justinian was reviving the Empire at the time but the volcanic winter stopped it. Again, he makes an intesting case but many historians, for complicated reasons, look on the Justinian reconquests as the final nail in Rome's coffin, not a revival as much as a wake, an event which not only stopped a present revival but made any other impossible.

There are no end of theories as to why Rome ended and the mystery isn't why it did but why didn't it reinvent itself and go on? as it had several times before.

Not only that but afterwards why did world, (and this is the whole world, North and South Am too) populations decline at this time and not really recover for almost a millenium?

No one knows, theories abound. They range from Toynbee's Spenglerian view of civilisations as organic things to (literally) Diseases From Space by N.C. Wickramasinghe and Fred Hoyle (yes, THE Fred Hoyle, he may have won the Nobel Prize but he also wrote very good SF and had some theories which were a little out of his field, but make very interesting reading)