Brooke Troxell
Biology 301 laboratory Section 006
Density Effect and Competition Between Radishes (Raphinus sativa)
and Collards (Brassica oleraceae)
Abstract
A common question in ecology is how do intraspecific and interspecific
competitions influence the fitness of individuals in a population. In this
experiment, we address both types of competition by comparing 6
treatments of radishes (R. sativa) and collards (B. oleraceae) at different
densities and in both monotone and mixed species environments. I
hypothesized that high density would have a negative effect on plant growth.
The data collected supported this hypothesis and statistical analysis
supported that it was a significant difference. I also hypothesized that there
would not be a significant difference in growth between the radishes and
collards. However, the data collected refutes this hypothesis as statistical
analysis shows that there was a significant difference between the percent
survivorship of the radishes as compared to the collards.
Introduction
One of the essential questions of ecology is how intraspecific competition
(competition between individuals of the same species) and interspecific
competition (competition between individuals of different species) influence
the fitness of individuals in populations. Plants are often used to experiment
with this question as their populations and habitats are easy to control (Metz
et al, 2013). How intraspecific and interspecific competitions influence
fitness has real-world applications as farmers can use data to maximize crop
yield and calculate the effects of weeds on their crops (Armin and
Asghripour, 2011). For example, in 2011, Armin and Asghripour performed an
experiment that analyzed the effect of density on wheat crops and how the
weed wild oat affected the fitness of the wheat crop. In the experiment this
paper covers, we also attempted to address both intraspecific and
interspecific aspects of the ecological question by placing two different
species (radishes: Raphinus sativa and collards: Brassica oleraceae) in low
density and high density environments by themselves and with individuals of
the different species. I hypothesized that density would have a negative
effect on the growth of plants (including number of leaves, leaf weight, stem
weight, and underground biomass) because of intraspecific competition for
limiting resources (Metz et al., 2013). I also hypothesized that mixed
speciation would not have an effect on percent survival; there would not be a
significant difference of growth between the species in single or mixed
habitats because I did not think the species would have more of an influence
on competition for limiting resources than density. The null hypothesis is
that density would have no effect on growth or percent survival.
Materials and Methods
In this experiment we had 6 different treatments with 4 repetitions for each
treatment, for a total of 24 treatments. The 6 treatments were 8 radishes,
64 radishes, 8 collards, 64 collards, 4 radishes and 4 collards, and 32
radishes and 32 collards. Each treatment was planted in a 4 inch by 4 inch
by 4 inch cubic container and was exposed to the same amount of light and
water in a greenhouse. The seeds were distributed equidistantly in each pot
and then covered with a thin layer (about 1 cm) of soil. After six weeks of
letting the plants grow, we terminated the experiment and collected data.
We cleaned as much dirt out of the plants as possible, then separated the
underground biomass, stems, and leaves to find the total number of plants,
average underground biomass, average mass of stems, average mass of
leaves, and average number of leaves. We performed t-tests to determine if
density had a significant effect on the growth of the plants and to determine
if there was a significant difference in the survival of one species with
respect to the other.
Results
In order to examine the implications of intraspecific competition on individual
fitness, we compared the high and low density treatments of the individual
and different species (shown in figures 1, 2, 3, and 4).
Effect of Density of Number of Leaves per Plant
4.50
4.00
C-8 C-64 R-8 R-64 Mixed-8-C Mixed-64-C
3.50
3.00
2.50
Average number of leaves per plant
2.00
Mixed-8-R Mixed-64-R
1.50
1.00
0.50
0.00
Figure 1. The average number of leaves per plant of the comparative low
and high densities, respectively, juxtaposed.
Effect of Density on Mean Leaf Weight
1.40
C-8 C-64 R-8 R-64 1.20
Mixed-8-C Mixed-64-C
1.00
0.80
Average Leaf Weight (grams) 0.60
Mixed-8-R Mixed-64-R
0.40
0.20
0.00
1
Figure 2. The average leaf weight, in grams, of the treatments of the
comparative low and high densities, respectively, juxtaposed.
Effect of Density on Mean Stem Weight
0.40
C-8 C-64 R-8 R-64 0.35
Mixed-8-C Mixed-64-C
0.30
0.25
Average Stem Weight (grams) 0.20
Mixed-8-R Mixed-64-R 0.15
0.10
0.05
0.00
Figure 3. The average stem weight, in grams, of the treatments of the
comparative low and high densities, respectively, juxtaposed.
Effect of Density on Below Ground Biomass
1.20
C-8 C-64 Mixed-8-C Mixed-64-C 1.00
R-8 R-64
0.80
Av erage Below Groung Biomass (grams) 0.60
Mixed-8-R Mixed-64-R
0.40
0.20
0.00
Figure 4. The average below ground biomass, in grams, of the treatment of
the comparative low and high densities, respectively, juxtaposed.
In all of our criteria of growth and survival (number of leaves per plant,
average leaf weight, average stem weight, and average below ground
biomass), the lower densities treatments did better than the high density
treatments, even in the mixed species treatments except for when the
collards were mixed with the radishes (see figures 1, 2, 3, and 4). To
determine if these differences were significant, we performed t-tests for each
comparison of low to high density (see Table 1). To examine the effects of
interspecific competition, we compared the percent survival of radishes
versus collards (see figure 5). The radishes did better in the experimental
conditions than the collards in both monotone and mixed species treatments
(see figure 5). To determine whether these differences were significant, we
performed t-tests comparing the survival of radishes and collards of similar
densities (see table 2).
Percent Survivorship
C-8
1.00
C-64
0.90
Mixed-8-C
0.80
Mixed-64-C
0.70
R-8
0.60
Percent Survival 0.50 R-64
0.40 Mixed-8-R
0.30 Mixed-64-R
0.20
0.10
0.00
Figure 5. The percent survival for each treatment with comparative density
treatments juxtaposed.
Table 1. P-values calculated by performing a t-test to determine if a
significant difference exists in the growth of low and high densities.
T-test
# leaves per plant p-value
C-8 vs. C-64 0.006115
R-8 vs. R-64 2.76E-11
MC-8 vs. MC-64 0.488943
MR-8 vs. MR-64 0.000318
mean leaf weight
C-8 vs. C-64 8.73E-05
R-8 vs. R-64 1.6E-10
MC-8 vs. MC-64 0.000255
MR-8 vs. MR-64 3.61E-06
mean stem weight
C-8 vs. C-64 0.001058
R-8 vs. R-64 0.034275
MC-8 vs. MC-64 0.003282
MR-8 vs. MR-64 0.049792
below ground
biomass
C-8 vs. C-64 0.02915
R-8 vs. R-64 1.22E-05
MC-8 vs. MC-64 0.487929
MR-8 vs. MR-64 0.000105
Table 2. P-values calculated by performing a t-test to determine if a
significant difference exists between percent survival of radishes and
collards.
T-test P-
value
5.41337E-
C-8 vs. R-8 10
2.68476E-
C-64 vs. R-64 18
Mixed-8-C vs. Mixed-8- 2.76752E-
R 08
Mixed-64-C vs. Mixed- 3.30926E-
64-R 16
Discussion
I hypothesized that the individuals in the lower density treatments would
survive better than the individuals in the high density treatments. This
hypothesis is supported by the significant difference between each
comparative low and high density treatments, even with mixed species (see
figures 1, 2, 3, and 4 and table 1). Therefore, the difference in the different
density treatments shows that access to and competition for limiting
resources, both intraspecifically and interspecifically, have a significant
impact on individual growth and fitness (Metz et al., 2013). However, the
collards, when mixed in with radishes, did not vary with respect to density. I
also hypothesized that there would not be a significant difference in the
survival of the two different species of plants. However, this hypothesis is
refuted by the significant difference between the radish percent survival and
the collard percent survival (see figure 5 and table 2). The data showed that
the radishes did better than the collards over the course of the experiment
even when not exposed to interspecific competition. This is also
demonstrated by the fact that the collards, when mixed in with radishes, had
very low survivorship not dependent on the density (see figures 1, 2, 3, 4,
and 5). Therefore, even though there is a significant difference supporting
the radishes victory of interspecific competition, the overall poor
developmental quality of the collards makes me question the integrity of the
design of the experiment to the ideal conditions for collards. More
experimentation can be done with an environment that is more ideal for both
collards and radishes so that a fiercer interspecific battle for limiting
resources can be observed.
Acknowledgements
I would like to give credit to Amanda Schlemmer, Audrey Baudendistel, and
Greg Sippel for their assistance with initiation and maintenance of the
experiment. Amanda Schlemmer and Greg Sippel also helped greatly with
the collection and analysis of data.
Literature Cited
Armin, M. and Asghripour, M. 2011. Effect of plant density on wild oat
competition with competitive and non-competitive wheat cultivars.
Agricultural Sciences in China. 10. 1554-1561.
Metz, J., Seidel, D., Schall, P., Scheffer, D., Schulze, E., Amner, C. 2013.
Crown modeling by terrestrial laser scanning as an approach to assess
the effect of aboveground intra- and interspecific competition on tree
growth. Forest Ecology and Management. 310. 275-288.
