High rates of production in both natural and cultivated ecosystems occur when physical factors, such as water, nutrients, and climate, are favorable, and especially when energy subsidies (such as fertilizers) from outside the system enhance growth or rates of reproduction within the system. Secondary energy that supplements the sun and allows plants to store and pass on more photosynthate is called auxiliary energy flow or energy subsidy.
Such energy subsidies may also be the work of wind and rain in a forest, tidal energy in an estuary, or the fossil fuel, animal, or human work energy used in cultivating a crop. In evaluating the productivity of an ecosystem, one must consider the nature and magnitude not only of the energy drains resulting from climatic conditions, harvest, pollution, and other stresses that divert energy away from the production process but also of the energy subsidies that enhance it, by reducing the respiratory heat loss (the 'disorder pump-out') necessary to maintain the biological structure.
High productivity and high net gross productivity ratios in crops are maintained by large inputs of energy involved in cultivation, irrigation, fertilization, genetic selection, and pest control. The fuel used to power farm machinery is just as much an energy input as sunlight; for example, the energy subsidy input into agriculture in the United States increased tenfold between 1900 and the 1980s.
H. T. Odum was one of the first ecologists to state the vital relationships among energy input, selection, and agricultural productivity. He wrote the following: ''In a real way the energy for potatoes, beef, and plant produce of intense agriculture is coming in large part from the fossil fuels rather than from the sun. The food we eat is partly made of oil.''
Genetic selection has also increased crop yields. The ratio of grain-to-straw dry weight for wheat and rice, for example, has increased from 50% to almost 80% during the past century.
However, the gross productivity of cultivated ecosystems does not exceed that found in nature. We do, of course, increase productivity by supplying water and nutrients in areas where those are limiting (such as deserts and grasslands). Most of all, however, we increase net primary and net community production through energy subsidies that reduce both autotrophic and heterotrophic consumption and thereby increase the harvest.
A factor that under one set of environmental conditions or input level acts as a subsidy can under another set of environmental conditions or at a higher input level act as an energy drain or stress that reduces productivity; for example, flowing water systems tend to be more fertile than standing water systems, but not if the flow is too abrasive or irregular. The gentle ebb and flow of tides in a salt marsh, a mangrove estuary, or a coral reef contributes tremendously to the high productivity of these communities, but strong tides crashing against a northern rocky shore subjected to ice in winter and heat in summer can be a tremendous drain. Swamps and riverine forests subjected to regular flooding during the winter and the early spring dormant period have a much higher production rate than those flooded continually or for long periods in the growing season.
Finally, certain types of pollution, such as treated sewage, can act as a subsidy or as a stress depending on the rate and periodicity of their input. Treated sewage released into an ecosystem at a steady but moderate rate can increase productivity, but massive, irregular dumping can almost completely destroy the ecosystem as a biological entity. If the input level of stress is poisonous, the response will be negative at any input level. On the other hand, if the input involves usable energy or materials, productivity or other measures of performance may be enhanced. In summary, just about everything that civilization does has a mixed effect on the natural environment and on the quality of human life.
A discussion of biological regeneration is relevant because recycling has increasingly become a major goal for human societies. Therefore, it is appropriate to focus on the cycling of nutrients in the biologically active portion of the ecosystem. A microbial food web consisting of bacteria, fungi, and microorganisms that consume organic detritus is present in somewhat different forms in all soils and all natural waters. Both dissolved and particulate organic matter in soil and water are partially processed by bacteria, some attached to particles and some floating free in the water. The bacteria are eaten by protozoans, which excrete ammonium and phosphate, which in turn can be reused by plants. This food web is often termed the detritus pathway or detritus cycle. Measurements of turnover rates indicate that the nutrients that protozoans release during their lifetime is many times the amount of soluble nutrients released by microbial decomposition of their bodies after their death. These excretions include dissolved inorganic and organic compounds of phosphorus, nitrogen, and CO2, which are directly usable by producers without any further chemical breakdown by bacteria.
Human beings enter the recycling picture when they expend fuel energy to desalinate water from the sea, produce fertilizers, or recycle aluminum or other metals. Recycling work accomplished mechanically or physically can provide an energy subsidy for the system as a whole. In the design of disposal systems for human and industrial wastes, it is frequently profitable to provide an input of mechanical energy to pulverize organic matter and thus hasten its rate of decomposition. Without being disrupted or poisoned, natural recycling mechanisms can do most of the work of recycling water and nutrients. Industrial materials (such as heavy metals) involved in manufacturing are quite another matter; their recycling is costly in fuel and money, but there is little choice when supplies become limited or when the wastes endanger human health.
Cycling within ecosystems may be defined in terms of the proportion of incoming material that circulates from one compartment to another before exiting from the system. The recycled fraction is the sum of the amounts cycled through each compartment. The total system throughflow is defined as the sum of all inputs minus the change in storage within the system if it is negative or, alternatively, all outputs plus the change in storage if it is positive.
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