Collapse

From http://www.therpgsite.com/showthread.php?38135-Apocalyptic-Revolution/page3

Cave Bear 

Another problem is that we seem to be assuming that nuclear war will be the most likely cause of our civilization's decline.
Epidemics, ecological collapse, and declining birth rate can do the job too.

Epidemics like the Black Death (with 50% death rate in Europe) seem to be recovered from very quickly - infrastructure remains in place, there are just fewer people using it. Ecological collapses have been significant historically, though not so much recently. I could imagine something like the recent issue with pesticides killing off all the honey bees (& other insects), or monoculture disease as with the Irish potato famine, as potentially leading to some sort of crisis. But there seems to be a lot of global excess capacity for food production currently (albeit concentrated in the USA & Canada) so I suspect this would be localised. 

I remember a decade or so ago when they were converting corn to ethanol because of a supposed coming fuel shortage - it resulted in an actual global food shortage because most of the planet now apparently lives off subsidised US food exports, sometimes disguised as 'aid'. I could imagine a disease, dust bowl or other event (Yellowstone eruption?) in North America reducing food yields dramatically, resulting in global starvation - mostly outside North America, since remaining food would be eaten there first.

Declining birth rates - likely did contribute to the collapse of the western Roman empire (& decline of the eastern RE). As seen there though, only a major issue if the declining population is replaced by dominant immigrants/conquerors who are unable/unwilling to maintain the more complex society of the declining population, resulting in a step-change collapse. Sans immigration, a society like Japan with declining birth rate & population but no change in ethny will not decline radically.

Also, looking at WRE case, initially there was only a total civilisational collapse in Britain, which had not been fully culturally Romanised. Once the Legions left, the native Britons were struggling to maintain Roman civilisation even before the Anglo-Saxon conquests. This resembles the situation in some areas undergoing European de-colonisation in the 20th century after WW2; the colonising civilisation had not truly taken root. Civilisational collapse in the Mediterranean region of the former WRE was only completed 300 years later with the Arab-Muslim conquests, which caused an ecological collapse in North Africa (goats ate the olive trees) and raiders/pirates destroyed sea trade and forced abandonment of coastal territories in southern Europe. See The Fall of Rome by Ward-Perkins for lots of data on this. The north African pottery records are fascinating!