To this point, my account of the balanced growth equation has centered on energy flow, but ecological fluxes involve more than the transfer of energy. When an animal eats or breathes, defecates or grows, a quantity of material with a definite chemical composition accompanies each energy flux. One could produce budgets for each compound and element in animal tissues and so repeat the foregoing chapters considering balanced growth equations for phosphorus (Peters and Rigler 1973), nitrogen (Nagy & Shoemaker 1975), nickel (Hall 1978), and so forth. These flows are the autecological expression of nutrient turnover, mineral utilization, and toxicant accumulation, areas of prime environmental concern that are not addressed by ecological energetics.

Whatever advantages such material budgets might offer, they have been far less extensively analyzed than energy budgets, and the treatment here will be correspondingly brief. The prime reason for this asymmetrical development is theoretical. Beginning with Lindeman's (1942) seminal paper and culminating in the International Biological Programme (1964–1974), ecologists hoped that patterns of energy flow would provide the basis for ecological generality. This theoretical attraction gave impetus to the development of techniques and traditions that emphasize energy transfers. Finally, the number of possible compounds that could be studied ecologically is so vast that biologists must hope that the basic proportionalities of energy flow will also apply to mass fluxes.