Surprisingly, this seems to be not true. Moscow, a city of 10+ million people, has huge forests inside or adjacent to the city limits. People leave rubbish here and there, but unless forests are rezoned and actively developed as "recreation zones" or some such, they are doing okay. One can easily find more species of birds in a large Moscow park than in the whole of Baden-Wuerttemberg. The trick is not depleting the ecosystem to begin with.