DISCUSSION 301 



in this question. Consider Windermere perch and pike: the production of 

 pike is about one-tenth that of perch, this agreeing with Slobodkin's bracket. 

 But some young perch go to feed larger perch instead of pike, and occa- 

 sionally young pike are found in perch stomachs. 



L. B. Slobodkin: When energy enters an organism, potential energy 

 is assimilated. This can be left as potential energy or converted into heat. 

 Only the former can be re-used either as food, or as a corpse decomposed 

 by other organisms. In DapJmia if no animals are removed from the popula- 

 tion, about 5 per cent of the algal input is ultimately obtained as dead 

 Daphnia, the rest being liberated as heat. I, as a predator can increase this to 

 10-12 per cent by removing animals. 

 The efficiency of the population is then : 



Potential energy obtained from culture 

 Potential energy consumed in food 



This is not the same as the total energy output of the Daphnia population. 



All the organisms I have studied are aquatic. The zooplankton itself is 

 consuming phytoplankton, and is being consumed by fish. In such systems 

 there are four or five trophic levels. The top is assumed to have zero effi- 

 ciency, which is incorrect as the decomposing bacteria do something. Other 

 steps, such as the conversion of zooplankton to fish, range from 5-15 per 

 cent in efficiency. 



D. Jenkins : I am not happy about the implications of the concept that 

 if an animal Hves longer its numbers are less likely to fluctuate. It must 

 follow that species with a short generation time should all show marked 

 fluctuations. Is this true ? Is not the efficiency of adaptation to environment 

 not also highly relevant; If a climatic accident is considered, this is likely to 

 be equally as lethal to an animal with a life expectation of fifty years as it is 

 to a one-year species. It is the capacity to withstand environmental extremes 

 that is vital. 



M. E. Solomon: Most organisms have at least one exceptionally 

 vulnerable stage in the life cycle. In insects, the whole population may go 

 through this stage more or less synchronously, and thus be more vulnerable 

 than a population of, say a fish, of which only one-fifth the total number 

 are exposed to the adverse conditions at a time. 



REFERENCES 



LoTKA, A. J. (1925). Elements of Physical Biology. Baltimore (reprinted as Elements of Mathematical 



Biology, by Dover Publications, New York), p. no. 

 SuRTEES, G. (1958), Laboratory studies on the survival of the eggs ofAedes aegypti under adverse 



conditions. W. Afr. med.j., 7, 52-53. 



