Nutritional ecosystem, natural

The structure and function of these taste systems will be discussed in the context of a natural nutritional ecosystem, i.e. one in which man is not a disruptive element. Human taste systems are assumed to have developed to function in this natural system and to have changed little as a result of the cultural dietary changes that have < ccurr d in the last 10- 20,000 years. 

Figure 1. Flow diagram of the natural nutritional ecosystem of the human (simplified)...

Figure 2. Diagram of some of the major inputs and outputs of typical carnivores in a natural nutritional ecosystem...

The major man-induced change in the chemistry of his nutritional ecosystem is the production of flavor compounds by cooking foods whereas microbial production of flavor compounds is seminatural, many heat produced compounds are not found in nature in any quantity. Although it has been speculated that food is cooked for hygienic purposes or to increase the nutritional value of foods eaten (certain starches are rendered edible by heating), most foods man presently eats can be eaten raw with little or no loss in nutritional value. With heating, in fact,... 

The results of the traditional acute single-species toxicity tests conducted in the laboratory cannot be used alone to predict effects on natural populations, communities, and ecosystems. The cultural species in laboratory tests are different from those in most ecosystems. Conditions such as the size of the test species, its life stage, and nutritional state can have an effect on toxicity. Furthermore, the experimental conditions in laboratory tests cannot duplicate the complex interacting physical and chemical conditions of ecosystems, such as seasonal changes in water temperature, dissolved oxygen, and suspended solids. In addition to these environmental modifying factors, aquatic life is usually exposed simultaneously to numerous potential toxicants (mixtures). Although the toxicities of binary and ternary mixtures have been evaluated for some chemicals in laboratory toxicity tests, the resultant information has predictive limitations. 

In agricultural ecosystems, the P cycle is more open than in undisturbed ecosystems, because some losses occur by the removal of agricultural products and in some situations by surface runoff and erosion. Applications of fertiliser are needed to compensate for these losses, as well as to raise the level of production above native levels. Where grazing animals are present, unevenly distributed returns of P through dung and, to a small extent, urine also occur. Consequently, in managed ecosystems the natural P cycle is modified by these additional factors, so that the importance of P mineralisation for plant nutrition is more difficult to assess. This is why... 

Insects as herbivores occupy a paramount position in nature. On a comparative biomass scale, the total biomass of insects in tropical forests is considered to be seven times that of vertebrates (Holden, 1989). Yet even with a high potential to severely impact plant species to satisfy nutritional needs, they are estimated to consume only 10 percent of the natural ecosystem and roughly 13 percent of the cultivated plants. Clearly, plants must possess inherent defensive mechanisms which provide resistance to insect predation. The naturally occurring chemical constituents in plants are recognized to be a primary factor in the defense of plants against insect attack. 

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