"The number of aborigines is rapidly decreasing. In my whole ride, with the exception of some boys brought up by Englishmen, I saw only one other party. This decrease, no doubt, must be partly owing to the introduction of European diseases (even the milder ones of which, such as the measles, prove very destructive), and to the gradual extinction of the wild animals.... Besides these several evident causes of destruction, there appears to be some more mysterious agency generally at work. Wherever the European has trod, death seems to pursue the aboriginal." Charles Darwin, Voyage of the Beagle.
This morning I was in an amazing mood. Climbed high to get a great view of the Wyoming Valley, saw great birds, had a great cigar. Then you come down into the valley, through the poverty, the litter, the homelessness. Kinda depressing. How is it that a small city with a university and a college feel so unlike such a city. Step up leadership.
Sidetracked. If we assume human populations that are non-agrarian are stable (not implying they are "in harmony with nature" - a term I despise) then it would not take much to have negative population growth. Thus, something like measles can have disastrous consequences.