3 ./f 



'quaternary consumers {C4) 



TERTIARY CONSUMERS (Cj) 



TRANSFORMERS (T) PRODUCERS (P) 



FIG. 14-2 Energy flow through an ecosystem the trophic levels 

 of which are in balance with each other; symbols are explained 

 in text. The weights of the arrows are intended to suggest the 

 relative proportions of energy flow in the various directions, 

 but this is schematic because the proportions vary widely in 

 different ecosystems. 



Certain of the food taken in, I, is indigestible or 

 may simply be undigested, or if digested and ab- 

 sorbed is not completely metabolized in the tissues, 

 so that it is eliminated in feces and excreta. This is 

 designated excretory energy, E. Assimilated energy 

 is the energy of the food actually absorbed and uti- 

 lized (/ — E). These terms may also be applied to 

 the producer level when / represents the total solar 

 radiation reaching the plant, and E is that portion of 

 the radiation not used. 



Aside from being killed by predators, organisms 

 die from a multitude of other factors such as disease, 

 extreme weather, starvation, combat, old age, and 

 accident. In order for the populations of the different 

 trophic levels to be maintained at a more or less con- 

 stant level, organisms that die non-predatory deaths, 

 D, must be replaced by the reproduction and growth 

 of new individuals. 



For energy to be transferred from one trophic 

 level to a higher one, organisms must be killed and 



eaten by predators of the higher trophic levels. When 

 predators consume their prey completely, as do fish 

 feeding on plankton, there is no wastage, but with 

 many predators, the prey killed is so large it must 

 be eaten piecemeal, and much is not used. Predatory 

 kill must therefore be separated into the energy con- 

 sumed by the higher trophic level (/a+i) and the 

 energy wasted, W. 



Energy lost from a trophic level through excreta, 

 non-predatory deaths, and wastage from kills is used 

 by saprovores or transformers, T. This allows for 

 the decomposition and conversion of nitrogen and 

 other compounds into nutrients suitable for reabsorp- 

 tion by plants. Energy from the transformer level 

 recirculates into higher trophic levels when detritus 

 and the transformer organisms, bacteria, fungi, pro- 

 tozoans, and the like, are consumed, as indicated in 

 Fig. 14-2. 



When populations of different trophic levels are 

 in balance, the total net production of each trophic 

 level, after losses from e.xcreta, non-predatory deaths, 

 and wastage from kills have been subtracted, is con- 

 sumed by predators of the higher trophic levels. In 

 unbalanced populations, predatory consumption may 

 not equal the available net production, so that the 

 population of that trophic level increases. If the 

 predatory kill exceeds the available net production, 

 the population decreases. A change in the biomass, 

 b, of a population may, therefore, be either plus or 

 minus. An entire ecosystem is in balance when the 

 total exchange of oxygen and carbon dioxide between 

 consumers and producers is equal. 



Increase in biomass comes with the growth of in- 

 dividuals. When an individual organism grows, it 

 increases in size and weight by adding organic mat- 

 ter. When reproduction takes place there is an in- 

 crease in number of individuals, but not necessarily 

 an increase in biomass, which takes place only if the 

 offspring increase in size. Since individuals of most 

 species have limits of growth increase, reproduction 

 increases the potential productivity of a community 

 by adding to the number of individuals capable of 

 growth. 



MEASUREMENT OF PRODUCTIVITY 



Productivity may be measured during any 

 reasonable period of time. Because of essential met- 

 abolic differences between day and nighttime, how- 

 ever, the 24-hour day is the smallest practicable unit. 

 Similarly, because of seasonal changes in the environ- 

 ment and in community populations, the measure- 

 ment of annual production is probably most useful. 

 Production is commonly expressed in terms of indi- 

 viduals, biomass, or preferably calories per unit area 

 per unit time. The basic problem is that of analysis 



202 Ecological processes and dynamics 



