An Ecosystem S Disturbance By A Pollutant Term paper

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"An Ecosystem's Disturbance by a Pollutant

Paul Cordova

L. Lehr

December 11, 1995

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 assumes that only certain

species that come early in the succession are capable of colonizing the site. In

contrast the other two models both assume that any individual of any species

that happens to arrive at the site is capable of colonizing it, although all

models accept that certain species will tend to appear first because of their

colonizing abilities. All models also suppose that the first colonist will so

modify the site that it becomes unsuitable for those species that normally occur

early in the succession. The three hypotheses then suggest three different ways

in which other species will appear. Model I suggests that early occupants modify

the environment so that it becomes more suitable for species that come later in

the succession. Model II (the "tolerance" model) suggests that the sequence in

which species appear depends solely on their speeds of dispersal and growth.

Model III (inhibition) - the species already present makes the environment less

suitable for subsequent recruitment of later species. All these hypothesis do

not rely on the idea of a community as a sugra-organism but on succession as a

sugra-organism but on succession as a process that relies on two factors:

1) the probabilities that propagules of different species will be

present and

2) the ability of these propagules to survive,

develop and reproduce.

Now to look at the whole picture, we ask ourselves: "How...

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