Essay/Term paper: "an ecosystem's disturbance by a pollutant

Essay, term paper, research paper:  Society

"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 do we predict the
response of a community from a pollutant?" Should we look at one population at a
time, or in some holistic approach. Moriarty suggests that some of the currently
favored approaches rest ont he assumption, often implicit rather than explicit,
that communities are sugra-organisms. He (Moriarty) suggests that two topics
that should be discussed when dealing with the idea of community response: 1)
indicator species
2) biological or environmental health may be
misleading.

The term indicator species, which is used in the classification of communities
(p. 62) is also used in ecotoxicology, with a variety of meanings. At times it
indicates the idea that knowledge of one species within a community will
indicate the well-being or biological health of the whole community. Moriarty
suggests that this seems a reasonable proposition if one accepts the traditional
view of community as sugra-organism, but suggests that it is in fact misleading.
He (Moriarty) adds that there is no fundamental reason from community structure
to suppose that any particular species within the community will give a better
measure of impact from pollutants than will another. Pollutants will affect
populations of particular species, and which species are first affected will
depend on the relative degrees of exposure and susceptibility and these are
functions much more of the particular pollutant and of the individual species
than of the community. An indicator species can only be used to assess the
impact of pollution on a community if quite a lot is known about both the
pollution and the community (Moriarty, 69). Concerning the idea of the concept
of biological or environmental health being misleading: one may properly refer
to the health of a community. A community can change "markedly" if affected by a
pollutant, but it will just become a different community that is neither more
nor less "healthy" just different (Moriarty, 69). It may be a less desirable
community, for economic, social, scientific or aesthetic reasons, but that is
quite a different matter. Effects of pollution may be described as a
retrogression - a decrease in diversity, productivity, biomass and structural
complexity. Moriarty argues that while there may be the appearance of a
retrogression process it should not be taken as a generality. In conclusion, on
the effect and response of an organism from a pollutant, the most appropriate
emphasis is on populations. The effect of pollutants on populations within a
community can be complex and apart from reduction or elimination of populations
- resurgence, population increase or introduction of rarer species, sublethal
effects and genetic changes may all be part of the changes that occur.

Another very important characteristic of populations that we cannot overlook is
their emetic composition. Much of the variation between individuals is inherited
from their parents. It is common knowledge that relatively few offspring of any
species survive to reproduce. Charles Darwin (biologist, 1859) formed the idea
of natural selection: the idea that some individuals will have a higher
probability of survival than others, and on average such individuals will then
leave more descendants than other less well adapted individuals. We will use
Darwin's, Mendel's and Watson and Crick's and other information to investigate
our concern - the role of pollutants in natural selection. It has been shown
many times that pollutants can exert powerful selective forces, and we need
therefore to understand something of the mechanisms of inheritance and how
natural selection acts on populations.

For the purpose of this assignment I will outline/review all the general
findings of important works that proved significant in understanding the
concepts of genetics. A good place to start would be with an outline of some of
Mendel's results obtained when breeding peas (Pisum sativum). "A" indicates the
dominate gene for yellow seed, "a" the recessive gene for green seed.

However, genes do not always fall into this simple dominant/recessive pattern.
Some may be incompletely dominant in the heterozygote, showing a transition
stage between the phenotypes of the homozygous dominant and recessive conditions.
Later workers also found that there are often more than two alternative forms
alleles) of a gene. One such worker was Avery (1944) who showed that the genetic
material in a bacterium consists of the nucleic acid DNA (deoxyribonucleic acid),
and in 1953 Watson and Crick first suggested the three-dimensional structure of
DNA from which has developed all the subsequent work on the genetic code. The
essential feature of this code is that: genes are arranged along chromosomes,
which in essence may be regarded as giant molecules of DNA. The DNA molecule
consists of two intertwined helical chains of many nucleotides, with ten
nucleotides in both chains for each complete turn of the helix (Watson, 1965).

Diagram to illustrate the double helix of DNA with the two polynucleotide chains
linked by complementary base-pairs (Adenine (A) with Thymine (T), and Guanine
(G) with Cytosive (C). Replication occurs when the two strands separate and both
act as templates on which new complementary strands are formed (Moriarty, 62).

Occasionally, something goes wrong with the replication process and one or more
genes may be altered, lost or gained. These changes, or mutations are usually
less favorable to the organism than the original gene, and are often
sufficiently unfavorable to be lethal. Nevertheless, mutations in the
reproductive cells are of crucial importance: these are in favorable, the source
of new genetic variation in subsequent generations.

This knowledge about gene structure and function modifies the Mendelian view of
inheritance.

Now, after the brief introduction and history of genetics it is time to consider
the relevance of ecological genetics to pollution. Most current problems of
pollution occur on a much shorter time-scale than that required for the
evolution of new species. The critical difference between evolutionary change
and that wrought by pollution is the speed: populations can disappear very
rapidly from pollution and if unchecked, we would have a very impoverished fauna
and flora (Moriarty, 81).

One very popular example of the effects of pollution on wildlife, and perhaps
the most striking evolutionary change over to be actually witnessed was the
occurrence of melanism in moths. This effect is commonly associated with
industrial development. White moths would rest on white lichen on trees and were
well-nigh visible on them. But with industrial pollution (between 1848 and 1990)
lichen turned a black color exposing and making the white moth (f. typica) prey
to birds. Birds posed a selective pressure against the white moths. Now black
moths were favored evolutionary. This is known as the heterozygous advantage, in
which a bank of recessive alleles becomes favored due to a change in the
environment. The biological significance of melanism was a matter for debate for
some decades, and although it is now generally accepted that melanism in (f.
typica) is associated with atmospheric pollution, some of the details are still
unclear. Although several points are worth emphasizing. Pollution in this
instance is not having a direct effect on the moth populations, nor indeed on
their predators, but an alteration to the habitat has altered greatly the
relative fitness of different genotypes. Melanism also illustrates the
difficulty of producing adequate proof, or disproof, of cause and effect when
pollutants are thought to be causing major biological effects. In conclusion,
with regards to genetics, it is important to appreciate that the effects of
pollutants can be modified by an organisms genetic constitution, and that
pollutants can alter a population's gene pool (Freeman, 128). The interactions
between pollutants and genes can be relevant both to understanding and to
predicting effects and are potentially of great value for monitoring (Moriarty,
102).

In summary, as stated throughout this school year in my 2375 Pollution class,
the effects of pollutants on populations are mediated via their effects, direct
or indirect on individuals and the likelihood of these effects depends on the
dose. Sublethal effects can be unravelled from knowledge of the mode of action.
Alternatively, emphasis in the study of sublethal effects can be placed on the
health of the individual organism. With both approaches, the effect of other
environmental variables needs to be given much more prominence than heretofore
and this could profitably be linked with studies on amounts of pollutant within
organisms (Moriarty, 176). It is from this basis that Moriarty states that we
have to consider how best to predict and to monitor the ecological effects of
potential pollutants.

In my opinion, I feel that (as does Moriarty) one should relate pollution to the
wider aspects of man's impact on his environment. We can, to a considerable
extent, control and mitigate our negative impacts upon this planet because as we
have learned from our past experiences, this planet does have a finite carrying
capacity for our own as well as for all other species.

References

Campbell, N.A. Biology (3rd ed) 1993. Benjamin/Cummings Publishing Company.

Chiras, Daniel D. Environmental Science: Action for a Sustainable Future. (4th
ed), 1994. The Benjamin/Cummings Publishing Company.

Freedman, Bill. Environmental Ecology: The Ecological Effects of pollution,
disturbances and other stresses / Bill Freedman (2nd ed.), 1995.

Moriarty, F. Ecotoxicology (2nd ed), 1993. Academic Press Limited.