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Intraspecific and Interspecific competition was examined in relation to nutrient availability. Replicate groups were categorized by seedling density and the availability of fertiliser. Three quantities of Osmocote fertiliser were applied to three corresponding seed densities consisting of Canola and Wheat in nutrient impoverished substrate. Competitive performance was quantified by measuring above-ground biomass (height, weight, seedling percentage). Competition had a negative effect on seedling survival and growth. Interspecific competition and intraspecific competition similarly influenced results.
Competition is an interaction between individuals which share a requirement for a limited resource (Begon et al. 1996). It may be between species (interspecific) or within species (intraspecific). Terrestrial productivity is constrained by the availability of nutrients, with Nitrogen and Phosphorus having the most limiting affect on plant growth (Lambers, Chapin & Pons 1998). This experiment was designed to evaluate nutrient competition between plants, with replicate groups being categorized by seedling density and the availability of fertiliser. Canola (Brassica napus) and Wheat (Triticum aestivum) were used for their practicality and growth rates. The influence of interspecific competition can be observed by studying mixed Wheat and Canola, and also intraspecific competition by comparing the performance of Canola at high and low density.
Competition will lead to reduced growth and mortality
Interspecific competition will have a more adverse effect on growth rates and survival than intraspecific competition.
Three quantities of Osmocote fertiliser were applied to three corresponding seed densities consisting of Canola and Wheat in nutrient impoverished substrate. This included the recommended quantity of 5g, 2.5g and a third treatment containing no fertiliser. Canola seeds were then separated into high density and low density pots, containing 48 and 16 seeds respectively. 8 wheat seeds were also distributed into a series of pots containing double the amount in Canola to compensate for the lower propensity of Canola to germinate. To ensure experimental accuracy seeds were counted, allocated evenly in labelled pots and covered in an equal amount of soil. The arrangement of pots once was randomised once in the greenhouse to mitigate the effect of unintended variability between the treatments or the likelihood that one treatment would be favoured. Following growth, the abundance, weight and height of seedlings were measured to ascertain changes in biomass with corresponding nutrient levels. Analysis of variance tests were used to determine if there was a significant difference between the means that were calculated from the replicates.
Table 1. Illustration of Experimental Method and Replication
Figure 1. Seedlings present as a percentage of original seed number.
A notable regression across all densities is observed with increasing nutrient levels. Low density Canola (16C) has considerably more seedlings present as a percentage of original seed number across all three treatments and densities, excluding an exception where 48C at treatment 1contains a higher percentage. Seedling percentage at treatment level 0 is consistently high in all three Canola densities. From 0.5-1 a dramatic decline occurs in both 16C and 16CM(mixed) by 40% and 32% respectively. 48C and 16CM exhibit relatively similar data trends at 0 and 0.5 but the noteworthy decline at treatment level 1 in 16CM is not experienced to nearly the same degree in 48C.
Figure 2. Height of average tallest seedlings in Canola densities vs. Treatment level (mm).
* Wheat excluded to accentuate differences in Canola (0 = 188, 0.5 = 334, 1 = 352)
Fertiliser enhances the height of seedlings and there is an evident similarity in trend across all densities in correspondence to treatment level. Low density canola (16C) comprises of the tallest seedlings, followed closely by 48C and then 16MC. Heights are very comparable at treatment 0.
Figure 3. Average weight per Canola seedling (grams) at each treatment.
An increase in weight coincides with elevated fertiliser application. Major disparity in average weight per seedling is not observed until treatment level 0.5, whereby weights are very similar at treatment 0. Both 48C and 16CM are considerably lower in weight than 16C at 0.5. At treatment level 1 48C drops well below both other groups, and there is a marginal weight difference between 16C and 16CM.
Analysing the data shows that competition has had a negative effect on seedling survival with a positive increase in nutrient application. These nutrients enabled competition to be illustrated in interspecific and intraspecific densities, as only positive growth (height, weight) was available to those plants which competed adequately; however that is not to say comparatively reduced growth did not constitute more intense competition. I.e. A reduction in growth positively indicated competition, whereby each individual received fewer resources, decreasing growth rates and increasing mortality.
The regression of seedlings present (as a percentage of original seed number) across all treatments can be attributed to a variety of factors. It is likely as nutrient availability increases plants become more vigorous and will out-compete rivals. As there is a lack of nutrients at 0, a lack of competitive pressure translates into high survivorship for seedlings; however seedlings at 0 were evidently smaller than at greater nutrient levels. The consequent decline in seedlings does not correspond to a lack of biomass. For instance the change from 73% to 33% of seedlings present in 16C, may be due to a smaller amount of larger individuals overwhelming a larger amount of smaller individuals. Another explanation may be that because of the low density of plants, the nitrogen levels were excessive and toxic to emerging seedlings (Grant et al. 2000), killing a percentage of these individuals. This may be reinforced by the higher percentage of seedlings in 48C at treatment 1, whereby the spread of these nutrients across a larger sample size could have nullified any toxic effects if they existed, which is unlikely. In figure 1. Interspecific (16CM) and intraspecific (48C) competition have similar effects on seedling percentage at treatments 0 and 0.5. However, at treatment 1 the severe decline in percentage seen in 16CM is not experienced by 48C, indicating that interspecific competition has had a greater affect than intraspecific competition.
Plant height was measured to estimate the biological health and vigour of replicates. The application of fertilisers had a positive influence on plant height in all densities. At 0, across all densities plant heights are remarkably similar; however the introduction of 2.5 grams (0.5) of fertiliser coincided with a significant increase in height. This suggests that the height and therefore the health of the plant is strongly limited by nutrients, this is reinforced by the uniformity of trends across densities as well as a continuation of height from 0.5 to 1. Once again assuming 16C represents the most ideal situation in this experiment, it is observed that interspecific competition (16MC) has been noticeably more detrimental to plant heights than intraspecific competition (48C).
Average weight per Canola seedling was positively influenced by fertiliser application. A high number of light, small individuals exist at treatment 0 in response to successful germination but a notable lack of competitive exclusion. The steady increase in weights correlates to increased nutrient uptake. Intraspecific and Interspecific competition similarly influence seedling weight at treatment 0.5. However, at treatment 1, the average seedling weight of 16CM increases significantly, indicating that interspecific competition has a lesser effect at treatment level 1 than intraspecific competition, which exhibits lesser growth. This is contrary to figure 1 which suggests that taller and possibly more vigorous plants exist in 48C. In 48C the average weight is calculated from the surviving 46% of plants, but only 20% from 16CM, meaning fewer but larger (or at least heavier) competitors exist. It is therefore difficult to quantify competition in this case as more individuals are surviving under intraspecific competition than interspecific competition in high-nutrient environments, but a lesser amount of individuals which happen to be heavier (but not taller) are present interspecifically. The graphed trend of 16CM at treatment 1 looks anomalous and the data is equally dubious, with Canola weights ranging from 17g to 2g to create the average. This degree of variation does not exist in any other averaged replicate group and human error at the stage of data collection may be to blame. Total biomass is an unsuitable measure of competition as 48C contains much more initial plant material than other densities.
Both interspecific and intraspecific competition has had an adverse affect on Canola and has caused mortality and reduced growth. The results generally suggest that interspecific competition is more deleterious to Canola than intraspecific competition, however as some data refutes this statement it cannot be considered absolute, and the hypothesis is therefore not entirely supported.
A similar study (Daugovish et al. 2002) found that 'Canola per plant biomass was affected more by interspecific competition with wild oat than by intraspecific competition'.
The superiority of Wheat in terms of height and survival can be attributed to its biology, whereby it attains height quickly and vertically with optimum growth requiring less fertilization than Canola, thus accounting for the detrimental impact it imposed on competing Canola.
Interspecific competition would have been considerably less influential if Canola was a better competitor or wheat was less successful. Canola requires 25% more Nitrogen, Phosphorus and Potassium and 500% more Sulphur than Wheat to achieve comparative yields and maintain a photosynthetically efficient leaf area (Hocking et al. 2000). Canola was shown to be a poor competitor by Daugovish et al. (2002), which implied Canola had the slowest canopy elevation and growth rate of three competitors.
Conversely, Li et al. (2001) shows that Wheat is highly aggressive and has a greater capability to acquire nutrients and nitrogen than other competitors. This is also the case in Mason (1998), where Wheat achieved higher yield at lower levels of applied nitrogen input compared to Canola, but absorbed more nitrogen than Canola at all stages of growth. The domination of Canola by Wheat at treatment 0 was present in all acquired data, but is only illustrated in figure 1 (Seedlings present as a percentage of original seed number), where high germination rate and survival of Wheat (90%) is apparent. In nutrient poor habitats competitive ability is dictated by traits which reduce nutrient losses and maintain efficiency (Aerts 1999), and it follows that Wheat uses nutrients more efficiently than Canola.
Competitive interactions are influenced by natural selection, which has developed traits to allow species to compete (Aerts 1999). In both intraspecific and interspecific interactions, strong seedlings have stunted the growth of others, and weak competitors have experienced mortality, meaning natural selection in response to competition is demonstrated in this experiment (Begon et al. 1996). For example, Canola seedlings containing extra proton pumps for increased uptake kinetics will be favoured by exploitative competition for nutrients, and therefore will have a higher reproductive output (Aerts 1999) (Begon et al. 1996). Therefore competitive ability evolves by natural selection and the ultimate effect of competition to an unsuccessful individual is a decreased contribution to the gene pool (Krebs 2008) (Begon et al. 1996). It follows that communities would change over time in response to selection pressure brought about by competition.
As plants were cut at the soil surface in this experiment, and vitality was approximated from above ground height, the role and quantification of 'below-ground' plant material was dismissed, thus representing a potential failure in experimental methodology. According to Aerts (1999) 'below-ground' competition affected species more than 'above-ground' competition, with competitive outcomes being largely determined by root competition at both low and high nutrient availability. In future these below ground structures should be observed and quantified if at all possible in order that competition can be more readily evaluated. Similarly, 'actual' nutrient uptake could not be measured, and therefore not equated to competition; rather uptake could only be approximated by using other measurements of above ground biomass.
The effect of competition, both interspecific and intraspecific, was assessed by altering plant densities and allowing the performance of varying fertiliser treatments to be quantified by abundance and weight following germination and growth. Competition affects the fitness of individuals, and consequently their contribution to the gene pool. Changes in experimental methodology should be employed to quantify other facets of competition. The hypothesis that interspecific competition is more intense than intraspecific competition seems plausible but cannot be confirmed across all available data.