In this experiment, intraspecific competition was tested which is when members of the same species compete for a same resource. The experiment tested was to see if intraspecific competition using differing densities affected the growth rate of Brassica rapa plants. Brassica rapa are a type of mustard plant also considered the “fast plants” because they grow within a 6-12 week period. They were used for this experiment due to their fast and easy growing process. Past experiments were observed beforehand and according to those, one density would further the survival rate more than the other. The Brassica rapa seeds were planted and observed over time to study the growth rates between a high and low density. The lower density was expected to have the higher survival and growth rate before starting the experiment. The findings in this particular experiment came out to show extremely similar results to previously tested experiments. While the height did not differ between high and low densities, the survival rate and number of flowers produced was significantly different.
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Competition plays a huge role in life’s organisms, especially when it comes to plants. A more specific type is intraspecific competition, in which members of the same species compete for the same resource, which is what this experiment involves. Intraspecific competition is important because when plants of the same group are competing for the same resources in the same living place survival rate can decrease greatly, their need for the immediate resource may run out due to other competing members (Aspbury et al., 2013). The purpose for compiling this experiment was to determine if this intraspecific competition affected the growth and development of the Brassica rapa plant at differing high and low densities. The Brassica rapa was used because it can be easily tested, and completes their life cycle within six weeks after planting (Aspbury et al,. 2013). Predictions could be made based off past experiments that used Brassica rapa as well. According to these similar experiments one would conclude that the high density seeds produce less plants, however seem to produce a greater biomass (total mass of once living material) amount. Whereas low density would do just the opposite and produce a lower biomass level going on to produce a larger number of flowers, increase in height, and so essentially obtaining a greater survival rate than that of the higher density. The hypothesis states that the low density would essentially have a higher growth and survival rate than that of the high density. Of course all these experiments differ in terms of density variability, but all testing the same issue of high vs. low density on Brassica rapa. Methods
The experiment was conducted by first obtaining Brassica rapa seeds (using the Brassica rapa plant because it grows within a 6-12 week period, AKA: the “fast plant”). Two treatments were then carefully put together, one pot consisting of low density seeds while the other contained high density seeds. Two low density seeds, and ten high density seeds were then planted. After planting the differing seeds in separate pots the pot was then filled about halfway with Miraclegro potting soil. After that six fertilizer balls were added to each and watered it just enough to dampen and pack in the soil. We then fill the rest of the pot up with soil and water it again. After the soil is dampened and the pot is completely filled, small holes were poked for all the seeds making sure they are spaced out evenly. Then made sure all the seeds were covered in the soil and watered them once again. The pots were labeled by which one has high density and which has low, so that we were able to observe the growing rate and survival for each, recording them in a data table. We then place the labeled pots on a thin layer of gravel in a plastic box making sure there is enough water filling the box. We kept them under a warm light making sure they were no closer than 6-8 centimeters from the light. This helps them grow in what would be considered normal growing conditions for a plant. Using a chi – square test which involves the comparison of frequency distributions, the results were then analyzed. Each week we observed and recorded the results for height, number of pods (flowers), and density.
Mean Proportion Surviving:
From our analysis after three weeks, our results indicated there was a significant difference in proportion surviving between the low-density and high-density treatments (unpaired t test: t: 4.908, DF=499.451, P= .0001). Plants in the low-density treatment had significantly higher proportion surviving than those in the high-density treatment (Figure 1: mean +/- s.e. proportion surviving: low-density= .837+/- .017; high-density= .721+/- 0166).
Mean Flowers Per Plant:
From our analysis after four weeks, it was found that there was a significant difference in the average amount of flowers per plant between the low and high-density treatments (unpaired t-test: t= 3.748, DF= 353.359, P= .0001). Plants in the low-density treatment had significantly more flowers per plant than those in the high-density treatment (Figure 2: mean+/- s.e. mean plant height: low-density= 2.427+/- .219; high-density= 1.507+/- .109).
Meant Plant Height:
From our analysis after six weeks, our results indicated there was not a significant difference in mean plant height between the low and high-density treatments (unpaired t test: t= .020, DF= 410.77, P= .984). Plants in the low-density treatment had nearly the exact same mean plant height than those of the high-density treatment (Figure 3: mean+/- s.e. mean plant height: low-density= 13.372+/- .337 ; high-density= 13.385+/- .543).
The lower density was expected to produce more surviving plants and according to the recorded results it did, as well as produced more numbers of flowers. These results support the hypothesis that states the plants under low density conditions would survive better over plants under high density conditions. This basically means that Brassica rapa plants survive better under lower density conditions. On the other hand, the height stayed relatively the same for both high and low density trials, therefore the null hypothesis was partially accepted. (Miller, 1995). While the height did stay the same for our plants alone an unusual finding was discovered. In comparison to the other classes that did this same experiment, height levels differed in that the Wednesday 2-5 pm class had an overall much lower measured height. This could be due to a number of things such as different amounts of light received, or even not being watered as frequently as the other classes. Other outside studies, which dealt with intraspecific competition, show that it highly affects plant size as well as number of leaves (Shahid et al., 2009). Number of leaves in this case, could correspond with number of flowers on the Brassica rapa plant. The findings in the experiment prove how much the plants were essentially affected.
Aspbury, A.S., Gabor, C.R. 2013. Laboratory Exercises for Organismal Biology. 15-19
Miller, T.E. 1995. Evolution of Brassica rapa Populations in Interspecific and Intraspecific Competition, Evolution. 49. 1125-1133.
Shahid, S.S., Waqar, A., et al. 2009. Intraspecific Competition and Aggregation in a Population of Solanum Forskalii Dunal in a Semiarid Habitat: Impact on Reproductive Output, Growth and Phenolic Contents, Pak. J. Bot. 41. 2751-2763.
Figures and Legends
Figure 1: proportion surviving in low and high-density treatments of Brassica rapa after three weeks.
Figure 2: Mean Flowers per plant in low and high-density treatments of Brassica rapa after four weeks.
Figure 3: Mean plant height in low and high-density treatments of Brassica rapa after six weeks.