Replenishment is at current population levels. ( Births - Deaths =0 ) No other considerations accounted for.
Optimally sustainable leads on from the quality of life. I've read articles that indicate Aus could support 150 million people but the standard of living would be much much lower than today.
Environmentally sustainable may reduce numbers below 5 mill ( Think Aboriginals were about 1.5 mill pre 1770)
