lations at which the species is making the greatest 

 use of the energy resources of the ecosystem and 

 growing most rapidly without the depressing efifects 

 of intra- and inter-specific strife, predation, and dis- 

 ease becoming excessive. This is the point of inflec- 

 tion between the accelerating and inhibiting phases 

 of the growth curve, the point at which the incre- 

 ment curve reaches its highest point. In most species 

 investigated, this occurs at a population size approxi- 

 mately half that of the asymptote. Theoretically if 

 the yield were so great that the population is kept 

 below this point, total production would be reduced 

 because of the small parent breeding stock that is left. 

 If the population were allowed to go beyond this 

 point of inflection, fewer offspring would be brought 

 to maturity because of increased competition and 

 other factors. It appears that the optimum yield 

 should be such as to maintain the population con- 

 tinuously at this level, and thus balance maximum 

 annual production (Hjort et al. 1933, Ketchum et al. 

 1949, Scott 1954). The problem of optimum yield 

 of animal and plant species for human use seems to 

 reduce, then, to determining the point of inflection 

 in the population growth curve of the species con- 

 cerned, keeping in mind that productive capacity 

 varies between different species and habitats. 



It may well be that balanced ecosystems have 

 evolved under natural conditions so that predation is 

 of such intensity as to maintain populations of prey 

 at this level of maximum productivity. In a small 

 pond in southern Michigan, in which no predatory 

 fish were present, the benthos biomass increased two- 

 or three-fold during the season to an upper asymp- 

 tote, after which there was no net productivity. In 

 another similar pond with fish present, the benthos 

 biomass eaten by fish never reached this asymptote, 

 and productivity was maintained continuously at such 

 a high rate that the production during the growing 

 season amounted to 17 times the standing crop 

 (Hayneand Ball 1956). 



Experimental work has not so far demonstrated 

 a relation between optimum sustained yield and the 

 point of inflection in the population growth curve. In 

 laboratory cultures of flour beetles, productivity in- 

 creased progressively with rates of exploitation that 

 brought the surviving population far below the point 

 of inflection (Watt 1955). With Daphnia pulicaria. 

 maximum sustained yield occurred over a period of 

 time when 90 per cent of newborn animals were re- 

 moved at regular intervals (Slobodkin and Richman 

 1956). In experimental populations of guppy fish, 

 the standing crop was reduced but the yield was 

 greatest when the tri-weekly exploitation removed 

 30-40 per cent of the individuals, and the population 

 mass was at one-third its asymptotic level. An ex- 

 ploitation rate of 75 per cent brought extinction of 

 the population (Silliman and Outsell 1957). 



The lack of agreement between experimental re- 

 suits and theory may be due in part to the fact that 

 the age distribution of the population after such 

 harvests is not the same as under conditions of nor- 

 mal population growth. In an attempt to keep a nat- 

 ural population of Norway rats in Baltimore at the 

 inflection point, it was found necessary to remove one 

 and a half times as many animals as expected from 

 an analysis of the growth curve. After a few months, 

 however, the populations collapsed, probably because 

 the average age of the females was reduced until they 

 were too young to breed (Davis and Christian 1958). 



Certainly much more study is required to deter- 

 mine practical means of estimating optimum yield, 

 to understand the factors involved, and to put proper 

 harvesting procedures into operation (Beverton and 

 Holt 1957, Ricker 1958). In North America, several 

 species have been exterminated through overuse ; the 

 passenger pigeon, for instance. On the other hand, 

 there is evidence that in some localities the yield an- 

 nually taken of fish, muskrats, and deer is not as 

 great as populations of these species could support. 



With organisms that have no specific adult size 

 but continue growth throughout life, such as fish, 

 yield should be calculated in terms of biomass. With 

 these animals there is the additional problem of de- 

 termining the minimum size limit of individuals which 

 would provide the greatest sustained yield in weight 

 for the population as a whole (Saila 1956, Ricker 

 1958). 



In undisturbed ecosystems, non-predatory deaths 

 and excreta at all trophic levels return both organic 

 and inorganic nutrients to the substratum in amounts 

 sufficient, when completely regenerated, to maintain 

 the standing crop, and input of solar energy replaces 

 respiratory losses. With the harvesting of plant and 

 animal crops by man, however, there is removal of 

 nitrogen, phosphorus, calcium, and many other min- 

 erals from the ecosystem that can be replaced only 

 very slowly by natural processes. Artificial fertiliza- 

 tion is ultimately necessary, therefore, when yields 

 are taken. Artificial fertilization is often also desir- 

 able in early stages of succession, when the natural 

 supply of nutrients in the soil or water is a limiting 

 factor. Addition of nitrates and phosphates to sterile 

 ponds usually results in a sudden bloom of phyto- 

 plankton. This bloom is later followed by increases 

 in animals at the consumer levels and greater yields 

 of fish. 



SUMMARY 



Energy, unlike nutrients, does not circulate 

 indefinitely through the ecosystem. It is continuously 

 dissipated to perform work and produce heat, and 

 hence must be continuously replaced. The chief 



208 Ecological processes and dynamics 



