From the Conclusion: 'The study of secondary productivity in grassland ecosystems is potentially of use in both theoretical and applied ecology. Energy flow studies, far from being complete or obsolete, are really just beginining. We have surprisingly little information on the energetics of natural terrestrial ecosystems and populations. What few data we do have are crude and in many cases are based on debatable assumptions. We need studies in which respiratory energy loss, production , and ingestion are independently estimated. There should be simultaneous measurement of the net primary production and of the type of food ingested by the various consumer species. The failure to compute productiion directly from sound population and growth rate data and the lack of detailed information about net primary production are the biggest defects in most existing studies. It is hoped that data gathered during the International Biological Program will help to fill these large gaps in our knowledge. Until this is done, the ideas presented in this paper cannot be adequately tested and must remain highly speculative.'