lulmoiulsoii l'»4(): Dafh)iia. Kiclimaii 1"58; fisli, 

 Ricker 1946. Gerking 1954; birds, Keiuleigli 1949; 

 mammals, Golley I960). Occasionally, the amount 

 of food re(|iiired by a species can be determined un- 

 der natural conditions. Heavers in Michigan cut 216 

 jioplar trees, averaging 5.23 cm in diameter, per 

 year per individual, in order to use the bark and 

 cambium as food. With v3750 such trees per hectare 

 (1500 per acre), one iiectare (Z.'^ acres) could suj)- 

 port a ]in\v of heavers for about 9 years (Bradt 

 1938). 



Mortality from non-predatory causes, D has 

 rarely been measured. Combined rates of mortality 

 from predatory and non-predatory causes may be 

 computed from the size of populations of diiTerent 

 age groups, or from recaptures of marked animals. 

 If one knows the population of predators in an area 

 and their average food requirements, non-predatory 

 deaths may be calculated. 



PRODUCTIVITY VALUES 



The photosynthetic efificiency of plants in 

 the use of solar radiation under natural conditions is 

 very low, in terms of calories. Only a small percent- 

 age of the available radiation is actually absorbed and 

 used. The growing season of plants is limited to only 

 a portion of the year. Much radiation is reflected 

 back from ground and water surfaces, or is absorbed 

 by the ground or water or other non-living material, 

 and later radiated back into the atmosphere. Some 

 wavelengths of the spectrum are relatively ineffective 

 in photosynthesis, and complete utilization of even 

 the effective portion of the spectrum may be limited 

 by temperature, available moisture, concentration of 

 carbon dioxide, and amount of chlorophyll present. 



The utilization of solar radiation received at Lake 

 Mendota, Wisconsin, has been apportioned as follows 

 (Juday 1940), last two figures corrected: 



Melting of winter ice 2.97c 



Annual heat budget of water 20.4 



Annual heat budget of bottom 1.7 



Evaporation 24.7 



Reflection 24.0 



Conduction, convection, radiation 25.9 



Gross biological intake 0.4 



The extent to which solar radiation is used de- 

 pends considerably on the luxuriance of the vegetation. 

 Deserts and short-grass plains have a gross primary 

 productivity in dry weight of less than 0.5 g/m-/day ; 

 ocean, fresh-water communities, forests, and prairie 

 from 0.5 to 5.0 ; coral reefs, estuaries, mineral springs, 

 semiaquatic communities, and evergreen forests may 

 produce up to 20 (Odum 1959). Miscellaneous data 

 on secondary productivity are given in Chapters 5-9, 



FIG. 14 3 Rate of c 



different times of da 



night mean t)iat 



removed (after Verduin 1957). 



rbon 



moval, photosynthesis, at 

 Erie. Negative values at 

 leing added rather than 



In both freshwater (Verduin 1957) and the sea 

 (Shimada 1958), maximum rates of photosynthesis 

 occur in the early morning. Monthly variations show 

 a relationship between photosynthesis and solar radi- 

 ation, although it is observed that maximum photo- 

 synthesis occurs in June rather than in July for rea- 

 sons that are not clear. Photosynthesis in the sea 

 increases proportionally with light intensity until 

 saturation intensity is attained (Ryther 1959). 



The consumer trophic levels derive energy from 

 transformers, T, as well as from green plants, P. The 

 number or biomass of bacteria, fungi, saprovores, and 

 detritus feeders depends on the amount of organic 

 matter that accumulates from non-predatory deaths, 

 D. excreta, £,and waste from predatory kills, IV. 

 The ratio between predatory consumption (/a+i) 

 and E, D, W doubtless varies in different ecosystems. 

 Adult lions in Africa kill, on an average, 20 kg 

 (44 lb) of food per day, although they require only 

 about half of this amount for existence. The unused 



600 

 500 



400 

 300 

 200 



100 



JFMAMJJASONDJ 

 MONTHS 



FIG. 14-4 Rate of carbon dioxide removal, photosynthesis, in 

 different months of the year in western Lake Erie correlated with 

 intensity of solar radiation (after Verduin 1956a). 



Exchanges, productivity, and yield 205 



