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 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 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) 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 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) 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 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: Where: N=Total number of species/taxonomic 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 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 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: The “t” value for the above was 3.290 significantly within our accepted margin Next, the Chi-Square test was performed, it’s information is given by the X^2=E{(observed-expected)} / expected Where: expected value is given by {(row total x column total) / grand total} and “E”.
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.
