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Pollutants and Food-Chains

Pollutants will, if present in sufficient quantity, have adverse effects on animals and plants. Many of these compounds do, of course, break down so rapidly that there is little risk of toxic concentrations. But compounds such as the organochlorine insecticides and poly-chlorinated biphenyls (PCBs) can persist long enough for living organisms to acquire considerable concentrations within their bodies. It is commonly believed that such persistent substances accumulate or concentrate as they pass along or up the food-chain. This idea of concentration developed from field observations on the occurrence of organochlorine insecticides in wildlife specimens, and most of our evidence still relates to these insecticides. There is, however, little good evidence that pollutants in general, or organochlorine insecticides in particular, do concentrate along the food-chain.

Animals are known to be selective in what they eat - one man's meat is another man's poison. The energy that animals obtain from their food is derived ultimately from the sun's radiant energy, some of which is utilized by green plants for photosynthesis. The herbivores are now usually thought to be the first link in the food-chain, or food-web. These may be preyed on by carnivores, which may themselves be eaten by other predators. It is then easy to assume that persistent compounds will accumulate along the food-chain. If 10 herbivores all have, and retain, equal amounts of a persistent substance, then the carnivore that eats these 10 herbivores will contain 10 times as much of this substance as did any one of the herbivores.

But this is not a real situation, and the confusion comes possibly from the unappreciated difference between persistence in the physical environment and persistence within organisms. Organochlorine insecticides last a long time in soils-given suitable conditions 50 percent may remain for well over 10 years. And elemental pollutants such as the heavy metals obviously persist indefinitely, either in the elemental form or in a variety of compounds. But we do not yet know of any pollutants that remain indefinitely within a living organism.

In general terms, there are two processes by which animals can remove foreign substances from their bodies: metabolism and excretion. Metabolism, or chemical change, can by itself remove some compounds from the body. It is sometimes a necessary preliminary too before a substance can be excreted. This is particularly true of compounds such as the organochlorine insecticides that are the residue left unaffected by metabolism and excretion.

Considerable studies have been made of the quantitative relationships between rate of intake and rate of loss of drugs, and we can probably use the same mathematical models for studies on the kinetics of pollutants. Such analyses have been made for organochlorine insecticides. It is appropriate to use the compartmental model - so called because the body is considered to be viewed as a series of compartments, each defined as a quantity of insecticide with uniform and distinguishable kinetics of metabolism and of transport to the other compartments. In biological terms these compartments usually correspond to specific tissues or organs. The data available are mostly for mammals, but the little information we have for other groups, principally birds and fish, suggests that they are similar. These quantitative relationships can usefully be summarized in four simple statements:

1. The concentration of insecticide in a tissue depends on the rate of intake: the higher the rate of intake, the higher the concentration in the body.

2. Eventually, if the rate of intake is constant, and continues for
long enough, the concentration of insecticide in a tissue reaches a plateau level. Sometimes, months after a plateau concentration has been reached with constant rates of insecticide intake, the tissue concentrations may change again, either up or down.

3. One can deduce from the above two relationships that the concentrations

In different tissues should be correlated. They are. In general terms, adipose (fat) tissue usually contains by far the highest concentrations. This is presumably related to the fat-soluble nature of the organochlorine insecticides.

4. The existence of a plateau level implies that the animal is excreting and/or metabolizing the absorbed insecticide at the same rate as it is being absorbed. In other words, the consent rations of insecticide in the various compartments are in a steady state. One would expect, therefore, that if the rate of exposure to insecticide decreases or ends, the tissue concentrations will drop. They do. Sometimes when exposure ends there is a simple exponential decline. But sometimes, in some tissues, the rate of decline is more rapid at first than later on. These results are believed to be consistent with a model in which insecticide can be excreted directly from the blood, whereas insecticide in other tissues has to pass into the blood first.

1. Skimming

Read through the passage very quickly. Do not try to understand every word; just try to get a general idea. Now choose a new title for the passage, from this list:

(a) In defense of insecticides

(b) The dangers from insecticides

(c) How organisms reject foreign substances

(d) The concentration of insecticides in animal tissue

(e) Concentration and elimination of pollutants

(f) Research into the dangers from chemicals

2. Understanding in detail

Choose the correct interpretation of the phrases quoted from the text

a) there is little risk of toxic concentration =

(i) no real danger of toxic concentration

(ii) some danger of toxic concentration

(iii) a good chance of toxic concentration

(b) there is little good evidence =

(i) some good evidence

(ii) a small amount of good evidence

(iii) not really any good evidence at all

(c) adipose tissue contains by far the highest concentration =

(i) concentrations in adipose tissue are much higher than in other tissue

(ii) concentrations in adipose tissue are nearly the highest of all tissues

(iii) concentrations in adipose tissue are a little higher than in other tissue

(d) the little evidence we have =

(i) some of the evidence we have

(ii) the small amount of evidence we have

(iii) part of the evidence we have

(e) it is common knowledge that animals are selective =

(i) it is obvious that animals are selective

(ii) everybody knows that animals are selective

(iii) most people think that animals are selective

(f) one man's meat is another man's poison =

(i) different animals like different kinds of food

(ii) some men can eat poison

(iii) some meat is poisonous to men

3. Find in the text the following words and expressions and translate them into Russian