All process rates are in physics described as a gradient times specific conductivity or inverse resistance times the area over which the process is distributed; compare for instance with Fick's laws of diffusion and Ohm's law.

Consequently, the size of an organism determines the rate of exchange between an organism and its environment. The dependence of a biological variable X on body mass, W, is typically characterized by an allometric scaling law of the form X = Xo Wb, where b is the scaling exponent and Xo is a constant that however is different for different variables and may be different for different kinds of organisms. Loss of heat by an organism for instance is proportional to the surface area of the organism and the temperature difference, according to the law of heat transfer. Many rate parameters for plants and animals are therefore highly related to the size, which implies that it is possible to get very good first estimates for many ecological parameters based only upon the size. Naturally, the parameters are also dependent on several other characteristic features of the species, for instance, the shape, but their influence is often minor compared with the size. It is possible, however, to take these variations into account by the use of a form factor (= surface/volume). This form factor may vary considerably among species.

The conclusion ofthese considerations is that there are many ecological parameters and properties that are related to the size ofthe organisms, and that such relationships are based upon fundamental thermodynamics.

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