Why does Nash use the term "island" when describing his vision of a civilization in the future? What does it mean to live on an island?

This question seems to be incomplete. However, there´s enough information to find the right answer.

Nash uses the idea of Island because his proposal involves "clustering on a planetary scale" to reduce human environmental impact. The idea of an island represents the turn against "integrating our civilization into nature". Instead, boundaries are established around humans in a reduced amount of islands, in reference to technological clusters that hold a reduced number of people.  

Explanation:

In his Presentation to the Simon Fraser Institute for the Humanities, Island Civilization: A Vision for Human Occupancy of Earth in the Fourth Millennium (2008), Roderick R. Nash proposes Island Civilization as an alternative for the future of the relationship between mankind and nature.