We must stack people vertically, not horizontally, and green cities are the only healthy way to encourage that. At the drop of a currency. Until we were overrun by the aforementioned parachutists. Cities, on the other hand, are far from sustainable, relying on vast external supports utterly. If eco-city could crack the nut of simply supporting itself within its boundaries, it would be a major contribution.
Wildlife can get around the periferies.
Having said this, I have to begrudge that city boundaries may be quite extensive beyond the human-settled portions in order to meet human needs. It is on those edges that considerations for the needs of other species are critical. I was impressed recently on a visit to the National Museum's exhibit of Vermeer with a map of Delph in the 15th Century. It showed the city's agricultural support system, both inside and outside the walls and moats, very clearly. Nature's whole needs are better met with ecovillages than ecocities.