COMPONENT RELATIONSHIPS 



Figure IX-6 — PLANT-MOUSE-WEASEL CHAIN 



SUNLIGHT 



47.1 x 10* 



46.5x10" -* 



GRASSLAND 

 Grass Production 



Mouse Consumption 



Mouse Production 

 >,170 - — 1,350 Import 



Mouse Population Increases 1,569 



WEASELS 

 Weasel consumption 



Weasel Population Increase 117 



Decomposer Respiration 



Decomposer Production 



The diagram illustrates an actual energy-flow budget for a plant-meadow mouse- 

 weasel food chain in an old-field habitat. Numerical values are in kilocalories per 

 hectare. About 1 percent of the incoming solar energy is converted into plant tissue. 

 Most of energy represented by this plant tissue is accounted for by respiration and 

 decomposition. Of the remaining energy, the meadow mice consume only 2 percent. 

 The weasels, in turn, utilize 30 percent of the available mouse biomass. Of the 

 energy consumed in each stage of the food chain, the plants use 15 percent in 

 respiration, the mice 68 percent, and the weasels 93 percent. This supports the 

 suggestion that successive stages in food chains exhibit an increased utilization of 

 the energy taken up. However, in this particular food chain, so little of the energy 

 entering the system was eventually utilized in the conversion of weasel flesh that it 

 would have been impossible for the habitat to support a secondary carnivore preying 

 upon the weasels. Because of this tapering off of available energy in a food chain, 

 food chains rarely exceed five steps and commonly have less. 



depends upon the other. The present 

 composition of the atmosphere is the 

 direct result of life on the surface, 

 and life itself depends on the par- 

 ticular character of the atmosphere. 

 Ozone in the stratosphere, which 

 screens the surface from the actinic 

 ultraviolet rays of the sun, is a direct 

 photochemical product of the oxygen 

 that comes from plants. Carbon di- 

 oxide and water vapor absorb and 

 emit infrared radiation, thereby di- 

 rectly affecting the heat balance of 

 the earth, but these chemical con- 

 stituents interact intimately with the 

 green photosynthesizing surface. The 

 atmosphere has a narrow semi- 

 transparent spectral window that al- 

 lows sunlight to flow to the earth's 

 surface and some radiant heat to flow 

 to space. 



It is this delicately balanced, unique 

 system of life and atmosphere, in 

 cooperation with the oceans of the 

 world, which is the life-support sys- 

 tem for man. Yet man persists in 

 dirtying the atmospheric window and 

 tampering with the energy flow, gas 

 exchange, and life-support system 

 itself. 



Energy Relations of Plants 



Energy exchange for plants is by 

 processes of radiation, convection, 

 transpiration, and photosynthesis. We 

 now have excellent theoretical, math- 

 ematical models to describe how a 

 particular plant leaf is coupled to the 

 climate surrounding it by means of 

 energy exchange. The plant leaf will 

 assume a particular temperature and 

 a particular transpiration rate (the 

 two dependent variables) as a func- 

 tion of the total amount of radiation 

 absorbed by the leaf, air temperature, 

 wind speed, and relative humidity of 

 the air (the four independent vari- 

 ables). The plant's dependent vari- 

 ables are coupled to the environ- 

 mental independent variables by the 

 absorptivity of the leaf to radiation, 

 the size, shape, and structure of the 

 leaf, and the internal resistance of 

 the stomates to diffusion of water 



287 



