Considering science has only gotten robust enough to prove anything like that far more recently than any good examples of ecological collapse, I'd say this parameter is a little arbitrary.

The best example I can think of regarding ecological collapse is during and after the fall of the Western Roman Empire. Their climate decreased in temperature, which reduces crop yields, which weakened the empire and encouraged migration from northern Europe, which brought their collapse (plus like 12 other things lol).

In 535AD, during Justinian's reign in the east, the first black plague happened following a supermassive volcano that left the sky covered in ash blocking the sun. This was a massively ecologically damaging period of history and it caused the death of countless plant and animal life, along with the deaths of half the population of the Mediterranean.

It's not like people of this age were taking soil samples and references trends or whatever, but they certainly understood how things were going poorly.