Plant Dispersion Patterns

To define the special structure of a population, scientists look at three properties: distribution, density, and dispersion. Distribution refers to a population’s range over a geographical landscape. Furthermore, density reflects the number of individuals per unit of area or volume. Lastly, dispersion is defined by how members of a population arrange themselves in their area. Each property tells something different about the environment, like availability of suitable habitat or resource availability. Dispersion tells about the abiotic and biotic factors present and how they influence the population. Some of these factors include wind, moisture, sunlight, competition, and predation.

The influence of abiotic and biotic factors causes three dispersion patters. With clumped dispersion, individuals form tight aggregations, or clumps. Clumping may be due to a few different events. Some populations clump for social interaction, like fishes swimming in a school to optimize hunting and keep better protection from predators. Some populations may just be responding to the clumping of resources, or they may be influenced by their own reproductive cycle and not have the ability to move far from the parent. In uniform dispersion, the individuals of a population are spaced evenly. Sometimes this happens in plants, who compete for root space; they may have a minimal distance from one another to better survive in their environment. Lastly, random dispersion occurs when individuals of a population are spread out independently of one another. This usually happens when none of those abiotic or biotic factors are influencing the population, which makes random dispersion patterns rare in nature.

Random dispersion patterns are considered to be the norm for populations, making it a null model. When scientists test the dispersion pattern of a population, they are testing to see if it deviates from the null hypothesis and fits an alternative hypothesis. In this lab, students are testing the dispersion pattern of dallisgrass (Paspalum dilatatum) in the Confederate Cemetery by using the Poisson distribution formula and chi-square analysis. Just this week, students collect data. If we perform random sampling of the Confederate Cemetery, we will find that dallisgrass exhibits uniform dispersion because it competes with other grasses and plants in the area.

In order to test the hypothesis, student groups divided into their four groups and traveled to the Confederate Cemetery. The cemetery is nestled on the north corner of the University of Tennessee at Chattanooga campus, directly across from the biological and environmental sciences building. It is regularly maintained, and some students walk through it as a shortcut to class. At the cemetery, the student groups obtained a 1m² quadrat. One group member generated a random number, and another walked that many steps in one direction. A second random number was generated, and the person with the quadrat walked that many steps in another direction and placed it at their feet. They then counted the number of dallisgrass individuals within the quadrat. Students repeated these steps nine times for a total of ten sampled quadrats.

Our group found four individuals in quadrat one, two individuals in quadrat two, three individuals in quadrat three, one individual in quadrat four, and two individuals again in quadrat five. In quadrat six, the numbers jumped to a total of fifty-four individuals found. In quadrat seven, five individuals were found, and in quadrat eight, eight individuals were found. Lastly, in quadrats nine and ten, seven individuals were recorded in each. These results are summarized below in Table 1. Overall, the group found an average of nine individuals per quadrat.

Table 1. Number of Individuals of Dallisgrass Found in 1m² plots.

Quadrat	Number of Individuals
1	4
2	2
3	3
4	1
5	2
6	54
7	5
8	8
9	7
10	7

Table 1 shows an odd pattern of dispersion. Most plots, like one through five, contained only a few individuals. Plots eight through ten had a few more individuals, but then the sixth quadrat stands out with fifty-four dallisgrass individuals. Because it is so different from the other numbers, we regard it as an outlier, and the new average of individuals found is 4.33. Disregarding our outlier, I’d say that this pattern is likely a clumped pattern that results from the clumping of resources. This strays from the proposed hypothesis. Perhaps the plants are competing for something other than root space. Sunlight, for example, may be a “clumped” resource because of the shading of trees. Some areas will have greater sunlight than others, so more dallisgrass would grow there. The class dataset suggests the same dispersion pattern, clumped. In order to evaluate if this interpretation is correct, we will perform statistical analysis next week with the Poisson distribution test and follow it with chi-square analysis.

              All in all, it is important to consider sample size when conducting field surveys and analyzing data. Each individual group only looked at ten quadrats, making a total of forty for the class set. That is only a small fraction of the size of the Confederate Cemetery. Similarly, the patterns seen may change with the spatial scale. What may seem clumped on a small scale may look more like uniform dispersion on a larger scale. Thus, we must keep that in mind when interpreting the data.