240 L. B. SLOBODKIN 



Daphnia pvikx, an herbivorous crustacean and Hydra oligactis, a carnivorous 

 coelenterate. 



Maximum ecological efficiency in laboratory Daphnia pulex populations 

 is essentially identical with ecological efficiency estimates in natural aquatic 

 situations. This identity permits the tentative conclusion that the range of 

 ecological efficiencies in nature is relatively narrow. Further laboratory and 

 field work will be required to test this conclusion. 



It is possible to define and evaluate a concept of efficiency which takes into 

 account the effect of predation on standing crop size. This has been called 

 'population efficiency'. 



Population efficiency can be defined as a function of death distributions, 

 individual growth efficiency, individual caloric contents, and population 

 size; and their interaction with predation. It may be evaluated for an entire 

 population if energy consumption, yield, and the effect on standing crop 

 are known. 



It can also be evaluated in terms of the population efficiency that would 

 be associated with a predation programme that selectively removed some 

 particular kind of animal as yield. This evaluation can be made from data 

 on energy consumption, yield, distribution of the yield among the possible 

 kinds of yield and standing crop in calories, for several independent popula- 

 tions. 



When more caloric standing crop data become available it may be possible 

 to solve for population efficiency in field situations. 



ACKNOWLEDGEMENT 



Financial support for these studies has been provided by the Faculty Research 

 Fund (Project 993) and Michigan Memorial Phoenix Project (127) of The 

 University of Michigan, the Rockefeller Foundation (GaBMR 5661) and 

 the National Science Foundation (G-4396, G-9058). 



My colleagues at The University of Michigan have patiently supplied 

 criticisms and suggestions. Special thanks are due Dr Richman and 

 Dr Armstrong. 



REFEPJENCES 



Armstrong, J. T. (i960). The Dynamics of Daphnia pulex Populations and of Dugesia tigrina 

 Populations as Modified by Immigration. Ph.D. Thesis, Department of Zoology, The Uni- 

 versity of Michigan, Ann Arbor, 93 pp. 



Hyman, L, H. (1959a). Coelenterata, in Edmonson, W. T. (Ed.) Ward and Whipple's Freshwater 

 Biology. 2nd ed. New York; 313-22. 



Hyman, L. H. (i959fc). Tricladida, in Edmonson, W. T. (Ed.) Ward and Whipple's Freshwater 

 Biology, 2nd ed. New York; 326-34. 



Lindeman, R. L. (1942). The trophic-dynamic aspect of ecology. Ecology, 23, 399-418. 



Nicholson, A. J. (1954). An outline of the dynamics of animal populations. Aust. J. Zool, 2, 

 9-65. 



