Paul Cordova, L. Lehr, December 11, 1995. Freedman defines a pollutant as the occurrence of toxic substances or energy in a larger quantity than the ecological communities or particular species can tolerate without suffering measurable detriment” (Freeman, 562). The effects of a pollutant on an organism vary depending on the dose and duration of exposure. The impact can range from sublethality to lethality, depending on the factors involved. These factors need to be considered when determining the disturbance of an ecosystem 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 pollution by nutrients, which is referred to as eutrophication.
Each of these pollutants poses a different effect on the ecosystem at different doses. This varied effect 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 affect a significant number of individuals and spread to other individuals. This is the case when crop dusters spread herbicides and insecticides. Next, we look at population damage caused by pollutants, which have a detrimental effect on the ecosystem in several ways. First, the killing of an entire population by a pollutant offsets the food chain and potentially kills off other species that depended on that organism for food. This is particularly true when a keystone species is killed.
If predators were the dominant species high on the food chain, the organisms that the predator keeps to a minimum could massively overproduce, creating a disturbance in the delicate balance of carrying capacity in the ecosystem. Another potential problem in an ecosystem is the accumulation of pollutants in the (lipophilic) fat cells. As the pollutant makes its way through the food chain, it increases with the increasing body mass of the organism. These potential problems are referred to as bioconcentration and biomagnification, respectively.
Both of these problems are a great concern for 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 when evaluating the effects of a pollutant on an ecosystem is the carrying capacity. 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 and terrain. All of these are determinants of carrying capacity.
This curve is drawn below:
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 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, 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; and 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, the growth rate would be determined by the birth rate minus the death rate. However, this is almost impossible to achieve. If any of the variables in this equation are affected by a pollutant, the growth rate of an organism can be seriously impacted, which can, in turn, affect the entire ecosystem (Freeman, 122). Classical toxicology studies the poisoning effects of chemicals on individual animals, resulting in lethal or sublethal effects. Effects on individuals may range from rapid death (lethal) to sublethal effects or no effects at all. The most obvious effect of exposure to a pollutant is rapid death, and it is common practice to assess this type of toxicity by the LD50 (the lethal dose for 50%).
