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.  

reasoning from several particular examples to establish a general principle