Freedman defines a pollutant as "the occurrence of toxic substances or energy in a larger quality then the ecological communities or particular species can tolerate without suffering measurable detriment" (Freeman, 562). Although the effects of a pollutant on an organism vary depending on the dose and duration (how long administered). The impact can be one of sub lethality-to-lethality, all dependent upon the factors involved. These factors need to be looked at when determining an ecosystem\'s disturbance by a pollutant.
Some of the most frequent pollutants in our ecosystem include: gases such as sulphur dioxide, elements such as mercury and arsenic, and even pollution by nutrients, which is referred to as eutrophication. Each of these pollutants pose a different effect on the ecosystem at different doses. This varied effect is what is referred to as dose and duration. The amount of the pollutant administered over what period of time greatly affects the impact that the pollutant will have on an ecosystem and population.
Pollutants can affect both a population and an ecosystem. A pollutant on a population level can be either non-target or target. Target effects are those that can kill off the entire population. Non-target effects are those that effects a significant number of individuals and spreads over to other individuals, such is the case when crop dusters spread herbicides, insecticides. Next we look at population damage by a pollutant, which in turn has a detrimental effect on the ecosystem in several ways. First, by the killing of an entire population by a pollutant, it offsets the food chain and potentially kills off other species that depended on that organism for food. Such is the case when a keystone species is killed. If predators were the dominant species high on the food chain, the organisms that the predator keep to a minimum could massively over produce creating a disturbance in the delicate balance of carrying capacity in the ecosystem. Along with this imbalance another potential problem in an ecosystem is the possibility of the pollutant accumulating in the (lipophilic) fat cells. As the pollutant makes it way through the food chain it increases with the increasing body mass of the organism. These potential problems are referred to as bioconcentration and biomagnificaiton, respectively. Both of these problems being a great concern of humans because of their location on the food chain. These are only a few of the impacts that a pollutant can have on a population and ecosystem.
Another factor to consider is the carrying capacity when evaluating the effects of a pollutant on an ecosystem. A carrying capacity curve describes the number of individuals that a specific ecosystem can sustain. Factors involved include available resources (food, water, etc.), other members of the species of reproductive age and abiotic factors such as climate, terrain are all determinants of carrying capacity.
If a pollutant is introduced into an ecosystem , it can affect the carrying capacity curve of several organisms (Chiras, 127). This effect on the curve is caused by the killing off of the intolerant and allowing more room for both the resistant strain and new organisms. In some cases the pollutant will create unsuitable habitats causing migration.
Another important part of the idea of a carrying capacity is the Verholst (logistic) equation: The actual growth rate is equal to the potential growth rate multiplied by the carrying capacity level. Three major characteristics exist for this equation. First, that the rate of growth is density dependent, the larger the population, the slower it will grow. Secondly, the population growth is not limited and will reach a stable maximum. Lastly, the speed at which a population approaches its maximum value is solely determined by the rate of increase (r). In a population with a stable age structure this would be the birth rate minus the death rate, but this is almost impossible. If a pollutant then the growth rate of an organism can be seriously affects any of the variables in this equation affected which can in turn affect the entire ecosystem (Freeman, 122).
Now using the approach of classical toxicology we study the poisoning effects of chemicals on individual animals resulting in lethal or sublethal effects. Effects on individuals may range from rapid death (lethal) through sublethal effects to no effects at all. The most obvious effect