PROBLEMS IN POPULATION INPUT-OUTPUT DYNAMICS 195 



represent the other extreme: the animal hves in a relatively stable environ- 

 ment, has a low biotic potential, and reproduces late in life. The sexually- 

 mature individuals may live for many years after the first mating season. 



Deer, beaver and cattle populations represent complex exploited systems. 

 Several age-groups are liable to exploitation, growth can occur after repro- 

 ductive maturity is attained, and the environment can vary from being very 

 favourable to very unfavourable. For example, all other factors being equal, 

 optimum level of harvest of deer (by hunters) would probably have to be 

 lower following a winter of heavy snow cover than a winter of light snow 

 cover. 



Competition as well as weather will be important for wheat and trees, and 

 fairly frequent rotation of grazing herds from one pasture to another is 

 necessary to maximize rate of grass growth. 



ELEMENTS COMMON TO ALL PROBLEMS OF 

 EXPLOITED BIOLOGICAL POPULATIONS 

 All the forgoing exploited populations present problems which have certain 

 features in common. 



A. Output is to be made as high as possible, with the following qualifications. 



B. The abihty of the productive system to keep producing at a maximum 

 possible rate must never be impaired by making the yield too high. That is, 

 what we want to determine is the highest possible sustained yield the popula- 

 tion can produce. 



C. Certain restrictions must be imposed on the technique for obtaining 

 maximum yield. For example, it may be economically infeasible to thiri out 

 trees or marine fish prior to recruitment age, in order to minimize intra- 

 specific competition. Also, a harvesting technique which would deplete, say, 

 the phosphorus available in an oHgotrophic environment would be grossly 

 shortsighted. 



D. Productive systems invariably have some kind of built-in rhythmicity. 

 This will be tied to seasonal climate cycles, and hence cycles of growth, 

 reproduction, and the harvesting season. 



E. There is always a delay between the time when man manipulates the 

 system for his gain and the time wJien results of the gain have become 

 significant. Study of the delay period should be an integral part of study of 

 the productive system. 



F. The efficiency of conversion into biomass of solar energy cascading 

 through, and matter cycling through a community is different for different 

 species, and for different age-groups within each species. Even within a 

 trophic level, different species have different efficiency rates of biomass 

 production. 



14 



