Cave Bear 
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!
