Feibleman published his paper in 1951 and in it he developed his 12 laws of integrative levels. Some who had read or had heard of the paper deliberated on what comes next after organisms. He concluded they have contiguous volume. Species don't have contiguous volume. Populations don't have contiguous volume.
Ecosystems have contiguous volume.
This must have been something of a relief, because if you tried to apply those 12 laws of integrative levels to species or populations, they broke down. But once the volumetric criterion had been identified, then those 12 laws applied to ecosystems also. Out of Arnold Schultz's and J. Stan Rowe's papers have come some ideas that are important to us at The Land Institute. We had already moved to the ecosystem level of organization in thinking about agriculture and culture. That journey began with a paper which appeared in Science in 1972.