L. Lehr
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 sublethality 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. This curve is drawn below:

# of individuals

Years

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 any of the
variables in this equation are affected by a pollutant then the growth rate of
an organism can be seriously 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 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% of test animals) values, scientist can judge the
relative toxicity of two chemicals. For example, a chemical with an LD50 of 200
milligrams per kilogram of body weight is half as toxic as one with an LD50 the
more toxic a chemical. Death is rarely instantaneous, and even cyanide takes at
least some tens of seconds to kill a human being. Death is alwaBAD BAD BAD BAD
BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD one set of conditions, often ill
defined, with one type of exposure, and with no indication of the influence of
other environmental variables.

Perkins (1979) suggests that a sublethal exposure kills at most only a small
proportion of a population, but the possibility that s sublethal exposure could
cause a small proportion of individuals to die from acute toxicity seems self
contradictory (Freedman, 126). For both the sake of this assignment and for
practical purposes, it would be incautious to suppose that a sublethal exposure
that affects individual organisms adversely is not close to that which will
affect the population. There is no good reason to suppose that there is a
constant relationship for different pollutants or different species, between the
dose needed to kill and that needed to impair an organism. Therefore, given the
difficulties of studying an ecosystem, the most effective way to predict
biological effects is likely to be by discerning the least exposure that
produces a deleterious response in individual organisms (Moriarty, 1960) and
then examining the extent to which different environmental conditions alter this
minimum exposure.

Further adding to the complexity several additional factors come into play with
the effect and response of an organism from a pollutant. One such factor is age.
Although we think of youngsters of all species as resilient creatures, young,
growing organisms are generally more susceptible to toxic chemicals than adults
(Chiras, 127). Health Status is determined by many factors, among them one's
nutrition, level of stress, and personal habits such as smoking. As a rule, the
poorer one's health, the more susceptible he or she is to a toxin (Freeman, 214).
Toxins may also interact with each other producing several different responses.
Some chemical substances for example, team up to produce an additive response
that is, an effect that is simply the sum of the individual responses. Others
may produce a synergistic response that is, a response stronger than the sum of
the two individual ones. A pollutant can also synergize for instance, sulphur
dioxide gas and particulates (minute airborne particles) inhaledtogether can
reduce air flow through the lungs' tiny passages. The combined response is much
greater than the sum of the individual responses.
Plants have three strategies in response to a disturbance - this was
suggested by Grimes. These strategies are:
C - selection - having high competitive ability
S - selection - having a high endurance for stress
R - selection - having a good ability to colonize disturbed areas.

Plant response to a disturbance was suggested by Connell and Slatyer (1977)
using models. Model I (the "facilitation" model ...