gate on the right). The product is considered useful because it serves as an input to a productive process. Waste material also is created by the initial process, but because it doesn't serve as an input to a subsequent productive process, it is considered to be useless (or in other words, a waste), and therefore, it is a by-product. While this is logical, a paradox arises when it is realized that the same amount of energy, from the source to the left and from the feedback out of the storage, is required to make both the intended, useful product and the unintended, waste by-product. From the perspective of the energy theory of value (H.T. Odum, 1996; see Chapter 8), both the useful product and the waste by-product can be thought to be equally valuable because they both required the same amount of energy to produce. Kangas (1983a) applied this theory to strip-mined landscapes in central Florida. He proposed that the spoil mounds formed by mining had value in proportion to mining energy inputs and that they should not be leveled for reclamation. Plant communities colonized the mounds and, with sufficient time, succession could produce forests with structure comparable to natural forests. Ultimately, however, the argument was not convincing enough to change mine reclamation policy. Obviously, the energy theory contrasts dramatically with the utility-based value system of market economics, where the balance of supply and demand determines value or price. In human society, which is driven by market economics, wastes occur when no demand exists for their use. In fact, these wastes actually have a negative value because there are costs associated with their disposal.
In natural ecosystems, wastes seldom arise because some population or process evolves to use by-products for a productive purpose. For example, in the basic P-R model of the ecosystem (see Chapter 1), primary productivity (P) produces oxygen as an apparent by-product that is used as an input by respiration (R) and respiration produces carbon dioxide as an apparent by-product that is used as an input by primary productivity. There is no waste in this P-R system because by-products are utilized. The energy theory of value represents this evolutionary perspective of the natural ecosystem by assigning equal values to all outflows from a process. Industrial ecology is striving to achieve this goal in human economics, wherein all waste byproducts are used as resources (Allen and Behmanesh, 1994). In a sense, the loss of value that occurs when by-products are wasted, such as in landfilling, is the cost to human society for the evolution of closed material loops characteristic of natural ecosystems.
Of course, some wastes are being recycled but markets are not widely available (Aquino, 1995; Lund, 2001). Many creative uses have been found for certain wastes, often outside of market transactions (Piburn, 1972), such as the use of waste tires for artificial reefs (see Chapter 5), composting examples described earlier in this chapter, and the recycling of oceangoing vessels by "ship breakers" (Langewiesche, 2000). An extreme case occurs when trash is used as art (Greenfield, 1986), which reveals a surprising aesthetic value of waste. Finding productive uses for waste byproducts is a goal of ecological engineering, and examples are described throughout the text.
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