Pesticides are an example of contaminants deliberately introduced into the environment that have played a significant role in the decline of species and the loss of habitat diversity. They interfere with and cause the breakdown of fundamental biogeochemical processes that support life support systems, including decomposition, mineral oxidation, nitrogen fixation and photosynthesis.
Pesticides are chemical compounds used to control plants and animals that are classified as pests. They are most widely used on crops, but they are also used in and around the home on insects, rodents, weeds, and plant diseases; in wood processing and preserving; in paint; in food storage; and in public health programmes. Insect and weed control are the two most common uses. Some pesticides are applied directly to plants or soil. Soon after application, they are dispersed into the environment, so that applications are often repeated. As the pesticides accumulate in the soil and wash into streams and rivers, they can affect fish and birds. Because of the wide spectrum of life forms which they attack, some call these pesticides biocides.
Because of the toxicity of pesticides in concentrated form and because of the frequency of exposure, the most serious human health effects are found among agricultural and production workers. Long-term and chronic health effects occur as the chemicals are ingested and inhaled. Bioaccumulation (the buildup of toxic materials in tissues) is evident in fish and birds as well as in humans.
Two different sets of problems are posed by the occurrence of persistent pesticides in the environment: (1) localized problems, tending to be acute, leading to recognizable effects with assignable origins, which can be dealt with if successfully sorted out and if the will and powers exist; and (2) more widespread problems, tending to be inferential with postulated effects, due to the universal presence of the materials in question. There is, for example, growing evidence from the amount of pesticide residues found in specimens of affected species as well as in the animals, fish, invertebrates, or plankton they feed upon, that there is a process of pesticide concentration in the food chains. As small amounts of persistent pesticides become more and more widely spread throughout the entire natural environment, they are absorbed by low forms of life. Where large numbers of these species serve as food for higher animals, some of the total pesticide remains in the eater. This can lead to a certain concentration of pesticides in a form of life at the end of the food chain. Eventually lethal doses may be reached for certain populations, or the species may be reduced because of adverse effects on reproduction or behaviour.
The importance of changes in the environment is that they may be irreversible: species may disappear altogether. This will cause ecological imbalance - certain forms of life whose numbers are controlled by the disappearing species will no longer be held in check. Thus an apparently harmless species may become a pest. Another result of reduction of species diversity may be the loss of the genetic possibilities that each organism disturbance of the selective pressures of the environment will reduce the chances of future development of desired plants and animals. Furthermore, as humans are at the top of food chains, they too tend to concentrate residues in their bodies with as yet unknown effects on them. While perhaps sufficient attention has been directed to acute toxicity problems, too little attention has been paid to the effect of long term ingestion by human beings of small amounts of these chemicals.
A serious dilemma is that the cheapest pesticides are the organochlorines, known to accumulate in human fat. The substitution of organophosphorus pesticides brings both higher cost and a need for more carefully trained operators, both of which pose problems.