“The interrelation and interdependence of the species are naturally more obvious and more intimate within the common habitat hat elsewhere. Furthermore, as correlations have multiplied and competition has decreased, in consequence of mutual adaptations of the competing species, the habitat and habitants have tended to assume the character of a more or less completely closed system.” -Robert Ezra Park on Human Ecology. American Journal of Sociology, Vol. 42, No.1, Jul 1936.

What does he mean by a ‘closed system’? I only really understand this in terms of physics. Is it at all possible for a community to be a completely closed system? and in what way?