Threshold and Nonthreshold Agents

Following the general terminology, we distinguish between threshold agents, that in a suitable concentration promote growth, and nonthreshold agents, that in all concentrations have a negative or no effect on the growth.

Threshold agents include various nutrients, such as phosphorus, nitrogen, silica, carbon, vitamins, and minerals (calcium, iron, zinc, etc.). When they are added or taken in excess, the organism or the ecosystem can be overstimulated, and the ecological balance may be damaged. Examples are the eutrophication of lakes, streams, and estuaries from fertilizer runoff or municipal wastewater. The threshold level and the type and extent of damage vary widely with different organisms and stresses. The thresholds for some pollutants may be quite high, while for others they may be as low as i ppm or even i ppb.

The threshold level is often closely related to the concentration found in nature under normal environmental conditions. This shows that the organisms have become adapted to the chemical composition of the environment.

Nonthreshold agents are a number of heavy metals, pesticides, and a number of toxic organic compounds. It is important to determine the noneffect level or noneffect concentration for these compounds to determine at which level or concentration a negative effect on organisms and ecosystems can be expected.

Figure 2 shows the different effects of threshold and nonthreshold agents. Threshold agents have often harmful effects when they are not present in sufficient quantities to support growth. The range where there

Figure 2 A indicates threshold agent; B indicates nonthreshold or gradual agent. To have a threshold agent it is sufficient that one of the two A-plots is valid. The two B-plots represent two different dose-response curves.

is no harmful effect corresponds to the range of concentrations as mentioned above. Shelford's law was formulated in 1913 to expand Liebig's law to include the maximal limit. Thus, Shelford's law states that a value below a critical minimum or a quantity in excess of the critical maximum would exclude organisms.

There is a strong interaction between the chemical composition of the environment and the organisms. It was already mentioned above that an adaptation of the chemical composition of organism to the chemical composition of the environment takes place. The biochemistry developed reflects the available elements and compounds. For instance, the role of magnesium and manganese ions and later zinc ions as cofactors for many enzymatic processes could be due to the presence of these ions in a suitable concentration, while other ions could have been used had they been present in an appropriate concentration. The use of calcium in skeletons can be explained more or less in this way.

See also-. Biomass; Ecological Stoichiometry: Overview; Environmental Tolerance; Limiting Factors and Liebig's Principle; Limits to Growth; Plant Growth Models; Principal Components Analysis; Reproductive Toxicity; Tolerance range; Tree Growth; Xenobiotic (Pesticides, PCB, Dioxins) Cycles.

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