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Simpson’s Theory Istinguish Between Two Communities

Introduction

All organisms within a community interact with each other as well as the immediate environment surrounding them for the sake of survival. The scientific study analyzing these interactions is termed ecology. Ideally, ecologists answer several questions concerning an organism including how organisms affect each other, the way the environment influences it, and how an organism is adapted to survive in its niche. As regards the study of the environment, the preliminary step involves the description of the niche. As such, ecologists quantify their descriptions using appropriate mathematical concepts depending on the experimental design.

Akin to an organism which has an atom as the smallest unit that eventuates in the formation of a complex unit called an organ system, is the environment that can be thought of assuming a similar pattern. As such, the atom in this case is the organism which belongs to a particular species. These organisms merge to form populations, defined as the individuals hailing from a particular species and colonizing a specific area. The different populations within the environment interact with each other in what is termed as the community. Eventually, the entire system that constitutes the aforementioned elements is called an ecosystem. As such, we have different ecosystems including marine and desert among others.

Importantly, the characteristics of a community within an ecosystem are dictated by several extrinsic factors including the climate, the geography of the place, the history and the geology of the bedrock. Stockton in New Jersey Pinelands represents a typical example of how these factors influenced the characteristics of the community. In a synopsis, the ecosystem is characterized by sandy soil that is deficient in nutrients and is acidic in nature. Moreover, the bedrock has a higher water table that negatively affects the drainage system. As such, the berries (cranberries and blueberries) that understandably thrive in the Pinelands have been forced to adapt to this harsh environment. On the other hand, the natives exploited the resources available in this ecosystem for economic gains. As such, they extracted silica from the sand to manufacture glass, used the trees for construction, and they fished from the then shallow sea of New Jersey.

In a community, the relative abundances of the populations can help in understanding the characteristics of the same. This can easily be determined by plotting a dominance density curve that correlates the number of individuals in a species and the descending rank with respect to the abundance. As such, the plots (A and B) below (graph 1) can be traced. To this end, plot A shows that the community has evenly distributed populations devoid of a dominant species. In the contrary, plot B exhibits a community conquered by a few dominant species. Notably, prevalence does not always imply that a population constitutes a keystone species. A less populous species can influence the activities of the entire community in a big way, qualifying to be a keystone species.

a dominance density curve for two communities
Graph 1 showing a dominance density curve for two communities (A and B)

Alternatively, the existence of a dominant species can be determined us the Simpson’s Dominance factor, I. Simpson’s factor is a measure of probability that when two organisms are randomly picked from a pool in a community there is a high chance that they will be of a common species. Simpson uses the formula below to determine this.

Formula

Where, ni and N represent the total number of a specific species and the total number of members in the entire community respectively.

According to Simpson, the probability of picking two individuals that doesn’t hail from a common species represents the communities’ diversity (D) i.e.

D = 1 – I

Following the above literature, the main objective of this study is to make a visit to the Pinelands and study two communities in order to establish the similarities and differences between them. As such, the criteria used to distinguish communities will be established, the sharpness of the boundary will be analyzed, the physical elements that discriminate these communities will be established, and the organisms that colonize these communities and how they interact with the flora will be studied.

Method

In this laboratory, Pine Barren Field was the area that was studied to establish the characteristics of two communities. To begin with, the method employed for analysis in this study was the Quadrat Method. As such, the area under study that is in form of a quadrant with an area of 100 m2 was randomly selected. This area was then stratified along one of the sides such that a path of 1 m was formed. The populations of different species along this strip was established and recorded for analysis. Importantly, leaves and twigs of the species that could not be identified on the field were collected for identification in the laboratory. These plant parts were appropriately labeled on the field and their respective counts recorded to avoid errors. The figure below (fig. 1) demonstrates how the experiment was performed.

a stratified quadrat
Figure 1 showing a stratified quadrat (10 by 10 m)

From the data, the dominant species were established owing to their prevalence. Also, the keystone species were also established from the data collected and observation made. Moreover, the data collected was used to determine the species’ densities as well as the relative densities for the same. Finally, both the “Simpson’s Dominance values and the Simpson’s Densities values were calculated for each community visited” (McGinley 68).

Results

The results of the data collected from the field are shown the table 1 below.

Community 1
Species Population (ni) species density Relative Density (%) Simpson’s Dominance (I) Simpson’s Diversity (D)
White Oak 7 0.7 0.1 0.22 0.78
Shrub Oak 8 0.8 0.1
Low Blue Berry 900 90 7.0
High Blue Berry 1000 100 7.7
Moss 22 2.2 0.2
Greenbrier 1 0.1 0.0
Bear Berry 4 0.4 0.0
Species A 3600 360 27.8
Species B 1500 150 11.6
Species C 2000 200 15.5
Species D 3900 390 30.1
Community 2
Species population species density Relative Density (%) Simpson’s Dominance (I) Simpson’s Diversity (D)
Pitch Pine 7 0.7 23.3 0.63 0.27
Post Oak 23 2.3 76.7

Table 1 showing populations of different species from communities 1 and 2

The population density is obtained using the formula below:

  • Population density= (population of the species/area).
  • For instance, for white oak, population density= 7/ (1×10) = 0.7 species/m2.

The results for the other species are presented in table 1 above.

  • The relative density = (density of species/total density x 100). Therefore, for the white oak:
  • The relative density= 0.7/1294.2×100 = 0.1%.

The Simpson’s Dominance (l) is obtained using equation (1) above. Thus,

  • For community 1, I = 36217672.0/ (12942(12942-1) = 0.22
  • For community 2, I = 548/30(30-1) = 0.63

The corresponding Simpson’s Diversity for the above communities is:

  • Community 1, D = 1- 0.22 = 0.78
  • Community 2, D = 1- 0.22 = 0.27

Discussion and conclusion

The main objective of this experiment was to distinguish between two communities with respect to species dominance or diversity courtesy of Simpson’s theory. The results obtained from the experiment led us to us make several inferences regarding the sharpness of the boundary between the two communities, the physical elements that discriminate these communities, and the organisms that colonize these and and how they interact with the flora will be studied. From the results obtained, it is evident that the two communities are dissimilar with respect to both dominance and diversity. Ideally, community 1 illustrates a unit that is less dominated by a few species. As such, the probability of picking two consecutive species at random that hail from one species is very low (0.22). This is echoed by the corresponding value of Simpson’s Diversity (0.78), which implies that the community is much diversified. On the other hand, community 2 with Simpson’s Dominance of 0.63 shows that there is a high likelihood of picking two consecutive species that hails from the same species. This is affirmed by the Simpson’s diversity value, 0.27, which implies that the community is les diversified. As such, the probability of coming across Post oaks is much higher in community 2 than it is to come across any of the species in community 1. The results do also imply that a dominance curve for community 1 would assume a shape close to curve A in the graph 1 above. The opposite will be true (will assume trend B) when a dominance curve for community 2 is plotted.

Among the physical elements that render these communities to be different include the geological bedrock, topography and the general morphology of the native species (Campbell and Langmann 56). In that respect, post oak is commonly found in swampy places and low altitude areas. They are tall and canopied, competing favorably for sunlight that is used to manufacture food. They provide shade to the shorter species, limiting their growth which impacts negatively on species diversification. The organisms that inhabit this community include the cottontail rabbit, quail squirrel and juncos among others. Both members of community 2 inhabit harsh environment that supports few organisms as shown by the results obtained. The opposite is true with regards to community 1 which supports diversification. In this community it can be concluded that there is fair completion and there is no evidence for the existence of a keystone species. In community 2, pitch pine could be the keystone species since it absence would adversely affect the population of the community.

In a conclusion, the objectives of the experiment were met, with community 1 (I =0.22) representing a diversified community while community 2 represents a (I =0.63) a community dominated by few species.

Reference

Campbell, Reece, and Langmann Mitchhell. Biology. New York: Random, 2006. Print.

McGinley, Martin. Community ecology. New York: St. Martin’s, 2011. Print.

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