Ecotoxicology

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Ecotoxicology

Ecotoxicology is a field of science that studies the effects of toxic substances on ecosystems. It analyzes environmental damage from pollution and predicts the consequences of proposed human actions in both the short and long term. With more than 100,000 chemicals in commercial use and thousands more being introduced each year, the scale of the task is daunting. Ecotoxicologists have a variety of methods with which they measure the impact of harmful substances on people, plants, and animals. Toxicity tests measure the response of biological systems to a substance to determine if it is toxic. A test could study, for example, how well fish live, grow, and reproduce in various concentrations of industrial effluent . Another could evaluate the point at which metal contaminants in soil damage plants' ability to convert sunlight into food. Still another could measure how various concentrations of pesticides in agricultural runoff affect sediment and nutrient absorption in wetlands . Analyses of chemical fate (i.e., where a pollutant goes once it is released into the environment ) can be combined with toxicity-test information to predict environmental response to pollution. Because toxicity is the interaction between a living system and a substance, only living plants, animals, and systems can be used in these experiments. There is no other way to measure toxicity.

Another tool used in ecotoxicology is the field survey. It describes ecological conditions in both healthy and damaged natural systems, including pollution levels. Surveys often focus on the number and variety of plants and animals supported by the ecosystem , but they can also characterize other valued attributes such as crop yield, commercial fishing , timber harvest, or aesthetics. Information from a number of field surveys can be combined for an overview of the relationship between pollution levels and the ecological condition.

A logical question might be: why not merely measure the concentration of a toxic substance and predict what will happen from that? The answer is because chemical analysis alone cannot predict environmental consequences in most cases. Unfortunately, interactions between toxicants and the components of ecosystems are not clear; in addition, ecotoxicologists have not yet developed simulation models that would allow them to make predictions based on chemical concentration alone. For example:

An ecosystem's response to toxic materials is greatly influenced by environmental conditions. The concentration of zinc that will kill bluegill sunfish in Virginia's soft waters will not kill them in the much harder waters of Texas. Most of the relationships between environmental conditions and toxicity have not been established.
Most pollution is a complex mixture of chemicals, not just a single substance. In addition, some chemicals are more harmful when combined with other toxicants.
Some chemicals are toxic at concentrations and levels too small to be measured.
An organism's response to toxic materials can be influenced by other organisms in the community. For example, a fish exposed to pollution may be unable to escape from its predators.

Ecotoxicologists use all three kinds of information: field surveys, chemical analyses, and toxicity tests. Field surveys prove that some important characteristic of the ecosystem has been damaged, chemical measurements confirm the presence of a toxicant, and toxicity tests link a particular toxicant to a particular type of damage.

The scope of environmental protection has broadened considerably over the years, and the types of ecotoxicological information required have also changed. Toxicity testing began as an interest in the effects of various substances on human health. Gradually this concern extended to the other organisms that were most obviously important to humans—domestic animals and crop plants—and finally spread to other organisms that are less apparent or universal in their importance. Hunters are interested in deer, fishers in fish, bird watchers in eagles or pelicans, and beachcombers in loggerhead turtles. Keeping these organisms healthy requires studying the effects of pollution on them. In addition, the toxicant must not eliminate or taint the plants and/or animals upon which they feed nor can it destroy their habitat . Indirect effects of toxicants, which can also be devastating, are difficult to predict. A chemical that is not toxic to an organism, but instead destroys the grasses in which it lays eggs or hides from predators, will be indirectly responsible for the death of that organism. Protecting all the species that people value, along with their food and habitat, is a small step toward universal protection. An ambitious goal is to prevent the loss of any existing species, regardless of its appeal or known value to human society.

Because each one of the millions of species on this planet cannot be tested before a chemical is used, ecotoxicologists tested a few "representative" species to characterize toxicity. If a pollutant could be found in rivers, testing might be done on an alga, an insect that eats algae, a fish that eats insects, and a fish that eats fish. Other representative species are chosen by their habitat: on the bottom of rivers, midstream, or in the soil. Regardless of the sampling scheme, however, thousands of organisms that will be affected by a pollutant will not be tested.

Some prediction must be made about their response, nonetheless. Statistical techniques can predict the response of organisms in general from information on a few randomly selected organisms. Another approach tests the well-being of higher levels of biological organization. Since natural communities and ecosystems are composed of a large number of interacting species, the response of the whole reflects the responses of its many constituents. The health of a large and complex ecosystem cannot be measured in the same way as the health of a single species, however. Different attributes are important. For example, examining a single species like cattle or trout might require measuring respiration , reproduction, behavior, growth, or tissue damage. The condition of an ecosystem, on the other hand, might be determined by measuring production, nutrient spiralling, or colonization. Since people depend on ecosystems for food production, waste processing, and biodiversity , keeping them healthy is important.

[John Cairns, Jr. ]