Plant Competition - Assignment Example

Plant Competition Introduction Competition, from a biological sense refers to a type of interaction between two or more organisms wherein the survival of one is affected by the presence of the other (Went, 1973; Bengtsson et al., 1994). In plant communities, competition among and within different plant species is recognized as a strong force that affects species composition and distribution, and may occur due to limited resources such as water, nutrients, and light (Went, 1973; Tschirhart, 2002). Plant competition can be categorized into two types: intraspecific and interspecific. Intraspecific competition occurs when individuals of the same species compete for the same resources in a particular area (Freedman, 2011) For instance, two trees growing closely with each other or two flowers planted on the same pot will vie for the same nutrients, water, and space, reducing the amount of resources that each individual receives. Hence, it is logical to think that as the density of the plant increases, the more intense the competition becomes. In fact, this was demonstrated by Kothari et al. (1974) on Dichanthium annulatum, a dominant perennial grass species. It was observed in the study that as the number of plants increased from 17 to 135 individuals per meter-squared of land, the mean dry weight and nitrogen content per D. annulanum significantly decreased as compared to the other set-ups with lower plant densities. Meanwhile, interspecific competition refers to the interaction between two different plant species vying for the same resources (Freedman, 2011). Crops interspersed with weeds would be a good example of interspecific competition. Those species equipped with the least capacity to compete for the same environmental supply has to adapt or die eventually (Went, 1973). One of the earliest experimental investigations which catalogued the existence of competition within the floral community was conducted by Clements et al. (1929). Clements and his team planted sunflower, wheat, potatoes, and other plants species in varying distances with each other. Height (cm), leaf area (cm2), and dry weight (g) were then taken 80 days after planting (Clements et al., 1929). Results of the experiment indicated that the closer the plants are to each other, the more apparent growth inhibition becomes. Interestingly, increasing the number of plants per plot resulted to an overall production reaching a maximum value, which did not change even if spacing was decreased (Clements et al., 1929). It was also noted that growth of all plants within the same plot were equally inhibited (Clements et al., 1929). However, a different finding was observed by Wan et al. (2006) with the growth of Leymus chinensis, a C3 grass species and Chloris virgate, a C4 grass in a mixed pot culture. The researchers cultivated L chinensis in a 21 cm-diameter pots with 2 individuals per pot (monoculture) or mixed with C. virgate. Assimilation rate, quantum efficiency, light-saturated assimilation rate were then recorded for each set-up (Wan et al., 2006). Results revealed that interspecific competition significantly reduced the measured parameters for the C3 species. However, the presence of the C3 plants had no effect on the C4 species (Wan et al., 2006). The result suggested an asymmetric competition between a C3 and C4 species, with the negative effect taking its toll on the C3 plants only. Njambuya et al. (2011) also provided evidence in support of Wan et al. (2006) that indeed, asymmetric competition occurs. But Njambuya and her team discovered a significant finding: the response of the mixed culture of Lemna minuta, an invasive species and Lemna minor, a native species is also affected by the amount of nutrients supplemented into the culture. In the presence of high nutrient availability, the invasive species exhibited higher Relative Growth Rate (RGR) as compared to the native species (Njambuya et al., 2011) However, when under low nutrient conditions, the native species showed higher RGR relative to the invasive species. Millets, a diverse group of grains from the Family Poaceae, are consumed primarily as a staple food and are grown mostly in arid and semi-arid environments (Obilana, 2003). This crop, consisting of nine species represents a major source of energy and proteins in the semi-arid tropics (SAT) like Africa (Obilana 2003). In fact, according to Obilana (2003), millets are nutritionally superior to most cereals. On the other hand, tomato (Solanum lycopersicum), which belongs to the Family Solanaceae is a succulent vegetable believed to have originated from South America but is now seen in almost all parts of the world (Alvarez, 2005). Apart from its numerous nutritional contents, tomatoes are most popular for being rich in lycopene, an antioxidant that prevents the risk of heart attacks and certain forms of cancer (Alvarez, 2005). In this study, millet (what species of millet did you use) and tomatoes were selected as plants species to demonstrate interspecific competition using two experimental set-ups. The purpose of this experiment is to determine the effect of the density and weight of the competitor species on the mean wet weight of the indicator species. Specifically, it is hypothesized that increasing the density and weight of the competitor species will decrease the mean wet weight of the indicator species for both set-ups. Materials and Methods In this study, millet (what species did you use?) and tomatoes (S. lycopersicum) were selected as plant species to investigate the occurrence of interspecific competition. Density and weight of the competitor species were varied to determine the effect of the competitor on the mean wet weight of the indicator species. Two experimental set-ups were prepared. In set-up A, millet served as the indicator species while tomatoes served as the competitor species. In set-up B, tomatoes were chosen as the indicator species while millet was the competitor species. Each set-up was composed of 4 density treatments in triplicates. Planting Twelve 4-inched pots, corresponding to four density treatments with three replicates each were labeled with indicator name, competitor name, competitor density, replicate number, and lab section. Each pot was then filled with soil to the inner rim. Competitor plant densities were 0, 10, 20, and 30. Since it is not always possible to ensure that all the seedlings will grow to maturity, 0, 13, 24 and 35 competitor seeds were planted in each density treatment respectively. Meanwhile, each plot was planted with 3 indicator seeds even if the actual number of indicator species required is only two per plot. The seeds were planted evenly across the soil at a depth three times the seed size. The pots were then placed in plastic trays labeled properly with the treatment. The pots were checked weekly for germination and survival. Pots with seedlings exceeding the appropriate density required for the experiment had the extra plants removed. Harvesting All plants were harvested and weighed seven weeks after planting. Each plant was carefully pulled out of the soil, removing first the indicators followed by the competitors. The surviving indicators and competitors were then counted. The roots from each plant were clipped and the total wet weight of the all indicators and competitors were measured. The mean wet weight of the indicators was computed. Analysis The reciprocal mean wet weight (1/W) for the indicator species in each pot was calculated. Values for 1/W were the plotted against final competitor density and final competitor weight separately Regression analysis was conducted and the reciprocal yield equation was calculated. The slopes of the two reciprocal yield equations were compared. Results The first experimental set-up seeks to determine the effect of increasing the density of tomatoes (competitor) to the mean wet weight of millet (indicator). Shown in Figure 1 are the reciprocal mean weights (1/W) of millet plotted against the four density treatments. . Figure 1. Reciprocal mean wet weight (1/W) of millet (indicator) as density of tomatoes (competitor species) increases. Values plotted represent the mean of three replicates. Results revealed that as the density of tomatoes (competitor species) increases, 1/W values of millet also increase. Analysis of variance (ANOVA) showed that there is a significant difference in the 1/W values among the treatments as indicated by a p-value < 0.05. Moreover, the linear equation generated from the regression analysis showed an R2 value of 0.45, indicating that 45% of the result of the experiment can be accounted for by the treatments. The linear equation also yielded a positive slope, implying the presence of competition in the set-up. Therefore, there is sufficient evidence to reject the null hypothesis that the density of the competitor species (tomatoes) does not significantly affect the mean wet weight of the indicator species (millet). Meanwhile, Figure 2 shows the 1/W values of millet (indicator) plotted against the weight of the competitor (tomatoes). Results showed that 1/W values increase as weight of the competitor species increases. Analysis of variance (ANOVA) showed that there is a significant difference in 1/W values of millet across all treatments as revealed by a p-value < 0.05. Furthermore, the treatments account for 37% of the increase in 1/W as indicated by the R2 value. The linear equation showed a positive slope, indicative of competition between millet and tomatoes. There is enough evidence to reject the null hypothesis that the weight of the competitor species (tomatoes) does not significantly affect the mean wet weight of the indicator species (millet). Figure 2. Reciprocal mean wet weight (1/W) of millet plotted against increasing weight of the competitor species (tomatoes). Values plotted are means of three replicates. Set-up B seeks to determine the effect of millet as competitor species to the wet weight of tomatoes, the indicator species. Figure 3 shows the trend of 1/W values of tomatoes as the density of the competitor increases. Best-fit line of the plot revealed that 1/W values decrease as the density of millet increases. However, the 1/W values did not significantly differ across the four density treatments (p-value>0.05), which according to the R2 value, only accounts 6% to the obtained wet weight values of the tomatoes. Linear regression analysis yielded an equation with negative slope, indicating the absence of competition. Hence, there is enough evidence to accept the null hypothesis that density of the millet (competitor species) does not significantly affect the mean wet weight of the harvested tomatoes. Figure 3. Reciprocal mean wet weight (1/W) of tomatoes (indicator species) against varying densities of millet (competitor species). The values plotted are means of three replicates. On the other hand, Figure 4 shows the effect of increasing the weight of millet on the wet weight of tomatoes. Results revealed that increasing the weight of millet correspondingly decreases the 1/W values of the tomatoes. However, ANOVA showed no significant difference in the 1/W values among the treatments (p-value>0.05). In addition, only 8.4% of the wet weight outcome of the tomatoes is accounted for by the treatments. The equation derived showed a negative slope, indicative of absence of interspecific competition. This leads to the acceptance of the null hypothesis that the weight of millet (competitor species) does not significantly alter the mean wet weight of tomatoes (indicator species). Figure 4. Reciprocal mean wet weight (1/W) of tomatoes (indicator) against varying weight (UNITS PLS) of millet (competitor species). Values plotted represent the mean of three replicates. Discussion/Conclusions In this experiment, it was observed that in a mixed pot culture containing millet and tomatoes, increasing the density of tomatoes while maintaining the number of millets per pot significantly decreased the mean wet weight of millet. The same pattern was also observed as the weight of tomatoes increased. The presence of tomatoes in numbers relatively greater than millet was found to negatively affect the growth of millet. This result is actually consistent with the given alternative hypothesis that increasing the density and weight of the competitor species, in this particular case, the tomatoes, will decrease the mean wet weight of the indicator species (millet). In fact, these results agree with the earlier findings of Rejmanek et al. (1989). Rejmanek and colleagues planted Japanese millet together with tomatoes in mixtures with varying proportions of the two species and found that as the proportion of tomatoes relative to the millets increased, the whole-plant dry weight of millet decreased, suggesting growth inhibition (Rejmanek et al., 1989). This outcome is quite logical because the more individuals share the same resources within a particular area, the immediate supply of these resources would tend to fall below the combined demands of all the users. Hence, each individual gets a smaller portion of the available nutrients, eventually leading to retardation of growth (Went, 1973). This phenomenon is called Law of Diminishing Returns (Tschirhart, and Finnoff, 2009). Interestingly, increasing the density and weight of millet in set-up B did not significantly alter the mean wet weight of tomatoes, suggesting that the presence of millet in the culture did not affect the growth of tomatoes. This observation may seem to contradict the concept of diminishing returns mentioned earlier. However, if the individual characteristics of the plant species selected in this study are taken into consideration, these results might make sense. It must be noted that millet makes use of C4 photosynthetic pathway, which is different from that of the tomatoes. C4 plants make use of a 4-carbon molecule as a starting material for photosynthesis by virtue of phosphoenolpyruvic acid carboxylase, increasing cellular concentrations of CO2 (Robinson, 2011). In hot conditions, C4 plants are capable of twice as much photosynthesis per gram of water as compared to C3 plants (Robinson, 2011). Hence, millet can maintain its physiologic consumption for a resource at a minimum without compromising its growth and without compromising the growth of tomatoes. One aspect of the experimental design which may have affected the result is the fact that the total number of plants was not standardized per plot. Note for instance, that in the density treatment where 10 plants served as the competitor species and another 2 individuals accounted for the indicator species, the total number of plants in the pot was 12, while in other density treatments the total number of plants per plot are 2, 22, and 32. It would have been much better that the total plant density per plot was made constant and the proportion of competitor to indicator was varied. The over-all plant density per plot may have a confounding effect on the competition outcome. Investigating the role of nutrient loading on the outcome of millet-tomato competition is one interesting area worth pursuing. To determine which species is capable of sustaining growth when the conditions are unfavorable, mixed pot cultures of tomatoes and millet will be grown in high, medium, and low nutrient-enriched soil. In addition, determining how tomatoes or millets respond in a pot culture with existing or mature competitor species is something that can be included in future studies. Note that in the present experiment, the indicator and competitor species were grown together at the same time. Will there be a difference in the outcome of competition when millet for instance, is grown ahead of tomatoes and vice versa? Literature cited: Alvarez, A. 2005. Tomato nutrition and tomato soup nutritional fact. Accessed at: http://www.philippineherbalmedicine.org/tomato.htm. Date accessed: April 19, 2011 Bengtsson, J., Fagerstram, T., and and Rydin, H. 1994. Competition and coexistence in plant communities. TREE. 9(7): 246-250 Clements, F., Weaver, J., and Hanson, H. 1929. Plant competition: an analysis of community functions. Carnegie Inst. Wash. Publ. 398 Freedman, B. 2011. Competition - Competition as an ecological and evolutionary factor. Accessed at: http://science.jrank.org/pages/1652/Competition.html. Date Accessed: April 19, 2011 Kothari, A., Pandey, H., and Misra, K. 1974. Intraspecific competition in grassland species I. Dichanthium annulatum (forsk) stapf. Agro-Ecosystems. 1: 237-247 Njambuya, J., Stiers, I., and Triest L. 2011.Competition between Lemna minuta and Lemna minor at different nutrient concentrations. Aquatic Botany. 94:158–164 Obilana, A. 2003. Overview: importance of millets in Africa. AFRIPRO. 23-46 Rajmanek, M., Robinson, G., and Rejmankuva, J. 1989. Weed-crop competition:experimental designs and models for data analysis. Weed Science. 37(2): 276-284 Robinson, R. 2011. C4 and CAM plants. Accessed at: http://www.biologyreference.com/Bl-Ce/C4-and-CAM-Plants.html. Date accessed: April 20, 2011. Tschirhart, J. 2002. Resource competition among plants: from maximizing individuals to community structure. Ecological Modelling. 148 :191–212 Tschirhart, J., and Finnoff, D. Plant competition and exclusion with optimizing individuals. Journal of Theoretical Biology. 261: 227-237 Wan, S., Niu, S., Zhang, Y., Yuan, Z., Liu, W., and Huang, J. 2006. Effects of interspecific competition and nitrogen seasonality on the photosynthetic characteristics of C3 and C4 grasses. Environmental and Experimental Botany 57: 270–277 Went, F. 1973. Competitiong among plants. Proc. Nat. Acad. Sci. 70(2): 585-590 Read More