. Animal Ecology. Animal ecology. 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 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 h
. Animal Ecology. Animal ecology. 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 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 Annual heat budget of water Annual heat budget of bottom Evaporation Reflection Conduction, convection, radiation Gros
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