Grots productivify 



nd efRcionciej (1/^ 



Trophic level 



Radiation 

 Producers 

 Primary consumers 

 Secondary consumers 

 Tertiary consumers 



Cedar Bog Lake, Minnesota Lalte Mendota, Wisconsin Minnesota pond 



Lindeman (1942) Lindeman (1942) Dineen (1953) 



g-cal/cm'/yr Efficiency g-cal/cm'/yr Efficiency g-cal/cm»/yr Efficiency 



118.872 

 111.3 

 14.8 

 3.1 



0.10 

 13.3 

 22.3 



118,872 

 480 

 41.6 

 2.3 

 0.3 



0.40% 

 8.7 

 5.5 

 13.0 



118,872 

 49.9 

 9.2 

 3.4 



0.04 

 18.4 

 36.9 



the onset of winter tliese organisms die, and during 

 the following season tlieir dead bodies are worked 

 over by the transformers so that the nutrients and 

 surplus energy that they contain recirculate through 

 the ecosystem until all energy is dissipated. Measure- 

 ments of COo exchange between plants and animals 

 in western Lake I'.rie indicate that photosynthesis of 

 the producers and respiration of the total aquatic 

 community are approximately equal (Verduin 

 1956a). In a coral reef community in the Pacific 

 Ocean, the balance in the ecosystem was reached at 

 a level of energy exchange between producers and 

 consumers of approximately 96 Kcal/m-/day (Odum 

 and Odum 1955). 



Such an equilibrium of energy exchanges is found 

 only in some clima.x communities. It is more charac- 

 teristic of serai communities for the total net pro- 

 duction of all trophic levels to be greater than can 

 be utilized during the course of the year. On the 

 death and decomposition of these organisms, more 

 organic matter is added to the substratum than can 

 recirculate through the ecosystem. This increases the 

 fertility of the soil. In marsh and bog areas this sur- 

 plus accumulation of organic matter may be consider- 

 able. Fi.xation of nitrogen commonly exceeds de- 

 nitrification in serai communities ; there is absorption 

 of minerals from underlying rock ; and in ponds and 

 lakes there is an influx of nutrients from the sur- 

 rounding drainage basin. All these processes increase 

 the carrying capacity of the habitat so that progres- 

 sively larger standing crops can occur. 



As fertility increases, changes also occur in the 

 species composition of the community. Species that 

 formerly were unable to occur because of low energy 

 resources or other intolerable situations now find 

 conditions favorable. Their invasion forces those 

 resident species that cannot withstand competition to 

 disappear. Oligotrophic lakes become eutrophic, and 

 then marsh. Bog mats change to bog forests and 

 eventually to the climax (Lindeman 1942). Sandy 

 and rock habitats accumulate greater fertility with 

 each succeeding stage. These changes persist until 

 the climax stage is reached, where respiratory loss of 

 energy balances the energy gain of the producers. 



YIELD 



In such applied fields of ecology as wildlife 

 management, forestry, animal industry, and agricul- 

 ture, the objective is to harvest the available net pro- 

 duction for human benefit rather than to let it ac- 

 cumulate in the natural habitat or be used by other 

 organisms. Crops of game animals, timber, or food 

 are removed periodically to give a yield. Man is an 

 animal consumer and removes the net production that 

 would ordinarily be taken by the herbivore or carni- 

 vore plus whatever additional growth, b, occurs. If 

 man takes his yield as a primary consumer he will 

 obtain more energy in food per unit area than if this 

 yield is obtained at higher trophic levels (Fig. 13-8d). 

 Since excretory and respiratory losses and non- 

 predatory deaths bring an accumulative dissipation 

 of energy at progressively higher trophic levels, the 

 potential yield of game or food animals decreases the 

 higher the position of the species in the trophic level 

 of the community. The fewer the links present in the 

 food chain, the greater the yield of game or fish. 

 More plankton-eating cisco or herbivorous carp can 

 be harvested in a given area than fish-eating bass, 

 more muskrats than mink, and so on. In human eco- 

 nomics, a land will support larger populations of 

 people if they are content with eating rice, wheat, or 

 corn than if they require the extravagance of beef, 

 pork, and lamb. 



Yield should never exceed net production, lest 

 with reduction of the standing crop the productive 

 potential become exhausted. On the other hand, hu- 

 man economics make it desirable to harvest the maxi- 

 mum yield that the ecosystem can supply without 

 jeopardizing continued production year after year. 

 The determination of maximum sustained yield or 

 optimum yield in harmony with the productivity and 

 maintenance of a steady state in the ecosystem is one 

 of the most vital and complicated problems in applied 

 ecology (Russell 1931). Maintenance of maximum 

 productivity is also of importance to the organisms 

 themselves as it permits attaining of large populations 

 in each species. 



There is a point in the growth curve of all popu- 



Exchanges, productivity, and yield 



207 



