Effect of Competition and Fertilization on Rye, Tomato, and Alfalfa plants in Relation to Height and Weight
Introduction
Plants, like most organisms, interact with each other using chemical signals and hormones. These interactions can be advantageous, for example in mutualism, where both species involved benefit from the interaction. These interactions can also be unfavorable to plants, for example in parasitism, where one species in the relationship is harmed, while the other benefits. Commensalism, another type of plant interaction, occurs when one organism benefits from the relationship while the other remains neutral. However, the specific interaction to be examined in this experiment occurs when the relationship is harmful to both plant species, an interaction known as competition.
According to David Tilman, competition occurs when one plant interferes with another plant’s attainment of resources, ultimately causing reduced growth. Competition also occurs when members of one population interfere with the needs of other members of another population for the same environmental resource. These essential resources include, and are not limited to sunlight, minerals, water availability, soil type, nutrients, and space. There are many different types of competition between species, but we will focus on two specific types: inter specific and intra specific competition. Inter specific competition is the competition between two or more different species that increases the population density of one species while leading to a decrease in the other species. Conversely, intra specific competition is the competition among individuals of the same species (Tilman, 1986). Competition increases and decreases the population density of different species, which by association means that competition influences the distribution and abundance of plant species. This alters the composition of plants in their respective communities (Bengtsson, Fagerstrm, Rydin, 1994). The use of fertilizer also affects the degree of competition that occurs. Fertilizers typically offer varied combinations of nutrients designed especially for use with specific types of plants, in specific settings. The vast availability of resources provided by fertilizer can reduce the competition within the two plant species, but it is important to consider the fact that certain types of fertilizers may only be beneficial for specific plants, and not others.
Materials and Methods
In our experiment, we planted 50 rye (R), tomato (T), and alfalfa (A) seeds in their respective pots to analyze intra specific competition. We then planted R, T, and A seeds containing pairs of species in a 50:50 ratio, and a combined density of 50 seeds i.e. 25 of each species (R/T, R/A, T/A) in order to analyze inter specific competition. Lastly, we planted three monoculture pots with densities of 25 seeds to compare the growth of these plants to the growth of the 50 seed pot monocultures, determining the effect of overcrowding and intra specific competition. After their germination period, we cut the stem of each plant just above the soil and measured the length and weight of each plant and recorded our data. The mean was used as a measure of central tendency, and is reported throughout our findings. P-vaules were computed to determine the significance of mean differences in height and weight.
Results
We found that fertilizer significantly increased the length of plants in all twelve pots, and significantly increased the weight of the plants in ten out of twelve pots. The alfalfa plants did not respond as strongly to the fertilizer as the other plant species. Figures 1 and 2 illustrate the differences in length and weight among fertilized and unfertilized plants, showing that the fertilized plants typically have a significantly larger value of height and weight.
Intra specific competition was demonstrated by analyzing the 25 seed monoculture in comparison with the 50 seed monoculture. In the unfertilized species, the 25 seed monocultures grew significantly taller two out of three times (see Figure 3), and significantly heavier three out of three times as shown Figure 4. These results were summarized in Table 1. Also illustrated in Table 1 was the fact that as fertilizer was added, there seemed to be less competition occurring in the 50 seed monocultures, as the 25 seed pot for R was no longer growing significantly taller, and the 25 A seed pot was not growing significantly heavier.
Inter specific competition was demonstrated by the pots containing a ratio of multiple species. From the multi-culture pots we found that rye plants outcompeted tomato plants, tomato plants outcompeted alfalfa plants, and rye plants outcompeted alfalfa plants. This means that there is a linear relationship between the competition of the three plants, i.e. in terms of competition, R>T>A.
Discussion
Fertilizer significantly increased plant growth in 22/24 pots because it contains many necessary nutrients for plant survival. Competition for resources such as minerals, and nutrients among the plants will decrease because they are provided in excess by the fertilizer, and as more of theses nutrients become available, the greater chance the plant will realize its potential growth and increase in height and weight (Berendse, 1994). Fertilizer does not completely abolish competition, as it does not provide resources such as sunlight, water availability and space. The results also indicate that the fertilizer does not necessarily work as well for all plant species. The rye and tomato plants showed a statistically significant increase in weight and height when exposed to fertilizer, however the alfalfa plant did not respond as strongly. Demonstrated in figure 2, in the case of 50 A vs. 50 A fertilized, the difference in weight was not significant (however the results approached significance). Conversely, in the case of 25 A vs. 25 A fertilized, the unfertilized plant grew significantly taller than the fertilized plant. These two findings may suggest that this specific type of fertilizer may not have been as beneficial for the alfalfa plant as it was for the other plants, which may lead to an unaccounted variable in the experimental design.
Intra specific competition was a large factor that influenced the growth of all three plant species. On average, the unfertilized 25 seed monoculture plants grew heavier and taller than those in the 50 seed pots of the same species. This allows us to conclude that overcrowding, and/or a higher rate of competition for resources occurred in the 50 seed pot, demonstrating intra specific competition. We can also infer that fertilizer reduces the amount of intra specific competition, because in the presence of fertilizer, the 25 seed monoculture plants grew significantly taller only one out of three times and significantly heavier only two out of three times as can be seen in Figure 3 and Figure 4. This reduction in significance indicates that more often than in the unfertilized plants, there was similar growth data (length and weight) between the 25 and 50 seed pots, and thus a reduction of intra specific competition. The changes in significance between the fertilized plants and unfertilized plants are labeled in bold in Table 1. Again, this can be attributed to the fact that fertilizer provides essential nutrients for the plants, allowing the amount of competition for minerals and nutrients within the species to decrease. The alfalfa plant exhibited different characteristics than the other plants as demonstrated in Figures 3 and 4. The 25 seed and 50 seed alfalfa plant were not significantly taller or heavier in the 25 seed plant, which may indicate that alfalfa plants are less sensitive to overcrowding, and are able to thrive in larger communities i.e the 50 seed pot. In addition, while the other species were experiencing large length and weight differences in their data, the alfalfa plant seemed to grow slightly longer in fertilizer, but it Figure 4, it is noticeable that the fertilized 25 A plant weighed less than the 25 A unfertilized plant. This could indicate that the alfalfa plants respond differently to this specific type of fertilizer than the other plants.
Inter specific competition was also represented by the pots with combinations of different species, and densities. In the fertilized plants, it can be seen in Figure 5 and Figure 6 that the Rye plants were the least affected by intra specific competition as the heights and weights of the combination pots (25R/T and 25 R/A) and did not significantly vary from the original 25 R seed monoculture data. However, in the alfalfa plants, both combinations (25 R/A and 25 T/A), had heights and weights that were significantly shorter and lighter than in the original 25 A pot. This suggests that alfalfa is the weakest competitor and is dominated by both the rye and the tomato plants. The tomato plant does not show a significant change in height and weight when compared with the alfalfa plant (25 T/A), however the alfalfa plant is significantly shorter in this combination, so it can be assumed that the tomato plant outcompetes the alfalfa plant for resources. When the tomato plant is combined with rye, it is significantly shorter, and lighter, therefore the tomato plant outcompetes the alfalfa plant, but is outcompeted by the rye plant. Due to these findings, there is a linear relationship between the competition of the three plants in a way that rye dominates both tomato and alfalfa and tomato dominates alfalfa (R>T>A). It is recommended when planting these species in an agricultural setting, that alfalfa be planted in monoculture areas, without any other plants to compete with, otherwise it could be dominated. Rye and tomato plants appeared to thrive when there was less intra specific competition, so it would be wise to grow them in smaller sample sizes of seeds. The data suggested that the rye plants actually grow slightly taller when combined with tomato plants, and slightly heavier when grown with alfalfa plants than they did in their own 25 seed monoculture. This could suggest that rye plants thrive in the presence of other species, and it could be beneficial to plant them with these other species to maximize growth.
Competition, as defined in the introduction, is not directly correlated with the height and weight of plants, so, the use of these measurements as a means to determine competition is slightly inaccurate. The growth measurements do not necessarily indicate competition has or has not occurred, and can lead to experimental error. However, they are factors that are likely linked to the effects of competition, and therefore we can infer that through this data, that competition occurs. There are also a few more limitations of our experimental design. Environmental factors such as soil type, predators, disease, and overcrowding are unaccounted for in our experiment, and therefore the best competitor (rye) in the controlled experiment wouldn’t necessarily survive the best in the wild. Because the plant is able to grow well in an isolated environment, doesn’t necessarily mean their ability to grow in a natural environment would be as strong. Certain types of fertilizer do not respond as strongly to some species. This was demonstrated by the alfalfa plant having a weak response towards the fertilizer, while the other plants in this experiment seemed to thrive in response to it. The type of fertilizer used when planting these crops in the wild should also be carefully selected, in order to ensure the best growth for the desired plant.
Literature Cited
Bengtsson, J., T. Fagerstrm, and H. Rydin. 1994. Coexistence Competition Plant Review. Trends in Ecology and Evolution, Vol.9: 246-250.
Berendse, Frank. Competition between Plant Populations at Low and High Nutrient
Supplies. 1994. Oikos 71.2: 253-60.
Tilman, David. 1986. Plant Ecology. Blackwell Scientific Publications, Oxford, England. Chapter 2 Resources, Competition and the Dynamics of Plant Communities: 51-75.
