Published in "Department of Scientific Affairs, Pan American Union [General Secretariat for the Organization of American States]"
On p. [1]: Presented at a short course in the Administration of National Parks and Equivalent Reserves, University of Michigan, May 12, 1966
Accompanied by a previous English version : To be presented at a short course in the Administration of National Parks and Equivalent Reserves, University of Michigan, May 13, 1965
Index file states this to be his earliest formulation of ecosystems approach (in Undated list).