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Many factors contribute to the diversity of life in an environment. The
availability of nutrients and sunlight, along with other factors that play a
pivotal role in determining what and how much life an area can sustain.
While studying the Second Law of Thermodynamics, it came to my attention
that the classical pyramid shape of the producer, C1, C2, C3, biomass pyramid
did little to take into account the amount of detrital input. I hypothesized
that the amount detrital input greatly effected the number of C1, C2, and C3
consumers and thus the overall biodiversity of an ecosystem. Further, if you
could find a test-bed where detrital input was the only real difference between
two similar ecosystems you would find that organisms of each ecosystem
would be adapted to the peculiar conditions. This adaptation would lead you
to find vast differences in the taxonomic groups associated with each
ecosystem.
With this in mind, I first set out to find two similar ecosystems were I
could test this hypothesis. Second, to sample, categorize and compare the
diversity of these ecosystems along taxonomic lines. Next, I planned to use
several of the widely accepted diversity indexes (Simpson’s Index, Shannon’s
Index the Chi-Square Test) to compare statistically, the diversity of my
ecosystems.
Scientific Law states that in order to test the effects of one factor in an
equation you must eliminate all other factors . In order to test the detrital base
as the limiting factor, all other limiting agents must be eliminated. In a field
experiment this is technically impossible; though it is possible to come close by
choosing two ecosystems that are very similar.
In order to keep this experiment as simple as possible the ecosystem
chosen had to be nearly self contained and small. The smaller and more
contained the ecosystem the less chance for outside input that could destroy
our results. Alazan and Bernaldo creek provided just the type of test-bed
needed for this experiment. Both are third order creeks in the same
geographic area that are subject to same weather and climate conditions, but
differ considerably in the amount of detritus available. (Fleet)
Procedure
Alazan creek is a third order stream that feeds into the Angelina River.
It is bordered by several species of indigenous trees that form a small gallery
of overhanging branches. This gallery consisted of (pine, oak, sweetgum trees)
and was limited to a range of about twenty five feet from the edge of the
stream. These gallery trees are surrounded by open cattle grazing fields
covered by short grasses and an occasional scrub brush. Alazan creek ranged
from ten to fifteen feet wide with a water depth of six inches to two feet. The
water was generally clear, and flowed at a brisk ten to twelve mile per hour
pace. The creek bottom was primarily sand with little or no mud. Turbitity was
low to moderately low and the creek had a high oxygen content. Detrital input
was low and limited to leaves from the gallery trees.
Bernaldo creek is a third order creek that similarly empties into the
Angelina River. Bernaldo creek differs substantially in that it is entirely
surrounded by typical east Texas piney woods. (The particular area that
samples were taken from appeared to be relatively low lying in comparison to
the surrounding woods.) It is likewise ten to fifteen feet wide but, is
considerably deeper at four to eight feet than Alazan creek. Bernaldo creek
flows at a much slower pace, approximately six to eight miles per hour. The
bottom of Bernaldo creek consists largely of mud, which gives the water a
darker color. Overall turbitity is high and overall oxygen content is low.
Human disturbance at both creeks was minimal. Although at Alazan
creek the surrounding area was used for grazing animals and at Bernaldo
creek the sight that specimen were actually taken from was a concrete
washout bridge. Both sights appeared to be in a flood plain, one that probably
becomes inundated on a monthly basis during the rainy season.
Weather conditions at the time of the sampling were typical of east
Texas in spring, therefore unusual conditions caused by atypical weather can
be eliminated. What it boils down to is, the only difference between the two
creeks was the amount of detrital material available and the conditions
predicated by this difference.
Starting the week of February 8, 1999 daily 1p.m. trips were made by
four lab groups to both Alazan and Bernaldo creeks. During these trips
observations were made on terrain, topography, climate, vegetation and
specimens were taken from several spots along each creek. The specimen
were taken by netting at various depths and locations. The nets used had a
pore size of approximately 2 millimeters on four sides and a canvas bottom
(see diagram 1) and were attached to poles 8 feet long. In order to take a
sample, a student placed the scoop nets open end up stream and allowed the
water and it’s contents to be strained. The nets were then quickly pulled from
the water and the samples collected were immediately taken to opened
garbage bags and sorted through. (see diagram 2) When any living creature
was found, it was placed in a collection jar (labeled for the particular creek it
was taken from) to be examined later. The collection jars contained an
organic die known as FAA. FAA is a combination of formalin, ethyl alcohol,
and Rose Bengal and tints most of the small “bugs” a red/pink color.
The following week each lab examined the specimen jars one by one
and separated the contents by taxonomic groups. Once each creek’s
specimens had been counted and categorized by class period, a list was
compiled for the weeks totals. This list was then used to test by comparison
the validity of our hypothesis. (for the complete list and breakdown see chart 1)
Results
The hypothesis I was attempting to prove had three parts. The first and
most general was the creek with the greater detrital base would have greater
biodiversity. This can be proven in several ways. The first is to simply count
the number of species present in each of the two creeks and compare the
results. This is called richness, which is the number of species/taxonomic
groups. In that case Alazan creek contained 13 species/ taxomic groups and
Bernaldo creek had 17. Therefore, Bernaldo creek which had the greater
detrital base had 4 more species than Alazan creek. A second part of counting
species is to determine the evenness of the the creeks. Evenness is the
measure of how evenly divided the individuals are among the taxonomic
groups. Bernaldo creek had
Next I used several of the accepted diversity indexes to statically prove
which creek had the greater diversity. Simpson’s Index is the number of times
it would take to pick two individuals of the same species/taxonomic group.
Simpson’s index is calculated by the equation:
D = {N(N-1)} / {En(n-1)}
Where: N=Total number of species/taxonomic
groups
n=Number of individuals of a species. (Cox)
In this case Bernaldo creek had a Simpson’s Index of .017712946 and
Alazan creek had a Simpson’s Index of .0092367032. That’s a difference of
.0084762429, or a 91 % greater chance of getting two of the same organisms.
This shows a significantly greater level of diversity for Bernaldo creek than for
Alazan.
Shannon’s Index in determined by the equation:
H’=3.3219[log N - 1/N E(Ni log Ni)]
With “N” being the total number of individuals in
the sample, “Ni” being the number of individuals in
each species/taxonomic group, and “E” being the
summation of all logs. (Cox)
Bernaldo creek had a Shannons Index of 2349.0908. Alazan creek had
a Shannon’s Index of 1876.1630. That’s a difference of 473.9278, or
approximately 40%. The proves that the diversity in Bernaldo creek is higher
than the diversity of Alazan creek.
“The null hypothesis that the two Shannon diversity indices come from
communities equal in diversity can be tested by a test a “t” test.” This test is
used to calculate chance of a type one error. The equation for this is:
t = (H1 - H2) / Sd
The “t” value for the above was 3.290 significantly within our accepted margin
of error .0005. (t=3,290P,.0005)
Next, the Chi-Square test was performed, it’s information is given by the
equation:
X^2=E{(observed-expected)} / expected
Where: expected value is given by {(row total x
column total) / grand total} and “E” is the
summation. (Cox)
In this case (X^2 = , P,.005)
Discussion
The evidence collected in our study significantly proves the hypothesis.
All of the confidence intervals were met and exceeded in each test with out
exception. This was also the case with each of my peers that performed this
experiment. Although this increases the general knowledge on the impact of
detrital input into a system there is more to be learned.
There were three main sources of potential errors in this experiment.
First, there needed to be more samples taken from more places and at
different times of the year. Four samplings from one spot on the creek are not
enough to draw conclusions about the entire system. What if the area tested
was near some source of point pollution? This could have an effect on the
immediate area but cause no down stream effects due to rapid break down, or
simple dilution. What if the area picked was more diverse during summer?
Next, an unforeseen problem occurred when the crayfish began eating many
of the “bugs” in the collection jars. This caused many of our
species/taxonomic groups to be under represented because they got eaten
before they could be counted! What about animals that were so small they
slipped through the holes in the nets? What about burrowing worms? None
of these animals are represented in the sampling. Had these species/groups
been represented some of the statistics might have been a little different.
In the future this test should be modified in a manner to correct some of
the afore-mentioned problems. With those modifications a person could build
an even stronger case to support the hypothesis.
Word Count: 1682
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