Exploring the Exploitative Competition between Collards and Radishes at the Interspecific and
Intraspecific Level
By: Shandrea Foster
Abstract
This study observed several types of competition including intraspecific, interspecific,
and exploitative competition. All of these types of competition are important because they are
the driving force behind variety in populations. The interactions that happen during competiton
are vital to the survival of different species on earth. This study mainly observed how density
and species affect the levels of competition that occur on two species of plants, which are
collard, Brassica oleraceae, and radishes, Raphinus sativa. The two hypotheses were: 1) Low
density plants would be more successful than the high density and 2) Radishes would be a
superior competitor over collards. Thusly, seeds of collards and radishes were planted in a
greenhouse and left to germinate for five weeks. After five weeks, the plants were observed,
measured, and cut into individual parts so that analyses could be performed to see if any
significance between the pots had occurred.
Using Anova single factor tests, two factor anova tests, and graphs, it was determined
that there was significance to the data and led to both hypotheses being accepted. The p-values
for the low density plants were below 0.05 except for in the collard data. The p-values were also
significant for the radishes and not the collards as well. These results are very important to the
ecological community because it examines the mechanism behind competition and the survival
traits that are used to make species a superior competitor. Furthermore, this experiment has
implications for the future and for similar experiments like it.
Introduction
One important aspect of life is the interactions that a species has with each other. While
some interactions do not affect the species, some interactions will. One important interaction that
is commonplace between species is called competition (Ricklefs 2010). Competition usually
arises when two or more species compete against each other, lowering the fitness of the species
involved, usually to obtain a limiting resource (Ricklefs 2010). Limiting resources can be
characterized as a nonrenewable or renewable substance that is either consumed by an organism,
or needed for survival (Ricklefs 2010). Some examples of limiting resources are sunlight, space,
water, and nutrients (Jekinson 2010). The two types of competition that were examined in this
study were interspecific and intraspecific competition. Intraspecific competition involves
competition between species that are the same, while interspecific competition is between two
different species (Ricklefs 2010).
From a biological perspective, competition is vital in nature because it provides increased
variety between populations and influences community interactions (Ricklefs 2010). The specific
mechanism or drive for this study is exploitative competition which means that a common
limiting resource indirectly affects the survival or fitness of the involved species. Another
definition includes: when an individual uses resources that limit a resource for the other
individual (Jekinson 2010). In any case it usually causes an increased ability to obtain resources
faster and more efficiently via growing faster or uptake of nutrient through its roots (Waller
1928). To explore the different types of competition and the mechanism involved, two species
of plants were observed which were radishes and collards. Radishes, Raphinus sativa, are
classified as a vegetable that has a purple-reddish tone to its roots and has green leaves with long
stems, which grow above ground (Wolford et al. 2013). Radishes are usually grown in the cool
months but several types can grow in different seasons as well. It is a fast maturing plant that is
easy to grow because it can sprout in almost any type of soil (Wolford et al. 2013). The best
conditions for radishes to grow in are where there is most soil that is well prepared (Wolford et
al. 2013).
One reason radishes are fast maturing is because of their taproot system. A taproot means
that one main root that usually grows vertically with the plant gathers the nutrients for the plant.
It makes radishes very efficient in getting nutrients because its taproot can gather more nutrients
than other plants with other types of root systems (Waller 1928). When radishes are planted they
are spaced about two to four inches apart from each other to allow optimal growth (Wolford et
al. 2013). Collards, Brassica oleraceae, on the other hand, are a very leafy green plant that is
known as “non-heading cabbage” because it does not form into a ball like cabbage does. Instead
it sprouts outward and has short stems that rise from the soil it is planted in. They are slow
maturing plants that are harvested about fifty to seventy months after being planted. When
allowed to mature, collards can grow to great amounts which are why they are planted about
eighteen inches apart from each other (Wolford et al. 2013). Collards have a fibrous root system
which means it has many roots that gather nutrients making it easy for the plant to reach deep
into the soil to get more nutrients (Waller 1928). In this experiment, we planted these two species
of plants to observe their competitiveness, which both plants have in their own right.
The experiment tested the competitiveness of the plants at two different levels: the first
comparing the success of growing in high and low densities, and the second comparing the
success of growth in same species pots and two different species pots. The importance of this
experiment was to observe the interspecific and intraspecific competition of the plants in order to
see the mechanism of exploitative competition (Jenkinson 2010). This type of study is important
because it identifies key elements of each plant that lead to its success or survival when faced
with competition. All of these aspects lead to the two hypotheses which are: 1) Low density
plants will be more successful than high density plants because there is more exploitation in low
density since there are more nutrients available (Aguiar et al. 2001) (MacIsaac et al. 1991). The
rationale behind this is since sunlight is a limiting resource, the leaves of the low density plants
will grow longer and bigger because they have more room to grow and to acquire sunlight with
the survival trait being rapid growth making them have more total biomass (Wolford et al. 2013).
2) Radishes will be more successful than collards because of an increase in exploitative
competition (Jenkinson 2010) (Wolford et al. 2013). The rationale being that water is a limiting
resource and the taproot system is able to gather the most water than the fibrous root system of
the collards, with survival trait being efficient absorption of nutrients because of rapid growth,
which was tested by total biomass (Waller 1928). The first hypothesis was measured using
normalized leaf data to acquire differences between low and high density leaves and the success
checked using the total biomass of the plants collected which were measured per plant. The
second hypothesis was measured using normalized stem data to observe the growth of the stems
and the success of the absorption of the roots using the total biomass of the plants, which were
measured per plant.
Methods and Materials
This experiment was conducted in the Greenhouse on the University of South Carolina
campus. The entire study was conducted in three separate steps, with the first being the set up at
the greenhouse. The greenhouse was very hot and humid and had green and clear windows on
the roof and walls which let in plenty of sunlight. The rest of the greenhouse was built with
cinderblocks which help to keep a constant insulation for the plants inside of it. There were
wooden tables for the plants to sit on and above them were big light bulbs that are used for
heating. Also, along each row of tables there were sprinklers which provide water for the plants
to grow.
The experiment commenced by spreading the seeds of the plant into their respective
seeds with the larger yellow seeds being collards and the small red seeds being radishes. The
counting out the seeds of each plant based on what density and species the pot needed. Then soil
was taken and put into the pot, filling it to about one inch from the top (Jenkinson 2010). The
seeds were then separated into eight seeds for low density and sixty- four seeds for high density
pots. In all pots, the seeds were evenly spaced between each other to allow equal growing
opportunity. In the intraspecific pots, seeds were alternated between radish and collard seeds in
both densities of pots (Jenkinson 2010). In total there were three levels of species at two separate
densities that included: collards only, radishes only, and collards and radishes, making a total of
six different treatments for the pots to receive. A total of twenty-four pots, four replicates of each
of the six treatment designs, were labeled appropriately with treatment, group name, and section
number .Once the seeds were planted, the pots were filled with one centimeter of soil over the
top of them and left to germinate for five weeks and watered every eight hours (Jenkinson 2010).
The second step was conducted again in the greenhouse and it involved collecting the
data from the experiment. Plants were removed from the pots with extreme care by squeezing the
pots to break up the soil and then holding the plants downward until they fell out of the pot. Then
the plants were removed trying not to break the roots away from the plants. With fingers, the soil
was pushed away from the plant roots and then washed off in a bucket of water. Once all of the
plants were freed from the pots they were taken and counted and if the pot was mixed they were
also separated into their different species (Jenkinson 2010).
Once separated, the cotyledons of the plants ,which are the sprouting leaves, were cut
away from the plant and discarded. Then the below ground biomass, roots of the plant, was cut
off and put in a weigh boat and measured. The leaves of the plants were cut off and counted and
were put into weigh boats and measured. Then the stems on each plant were measured five
separate times, divided by five to get an average of the stem length, and weighed. All of the
measurements were taken and recorded on the data sheet provided. Once everything was
weighed and measured, the plants were discarded in the compost pots, dirt was put into a bucket,
and water was thrown out (Jenkinson 2010). The final step of the experiment was conducted in
the laboratory where all of the data from each group was compiled and data analyses were run on
the data. The testing required single factor ANOVA tests on each species to compare the success
rate at each density, which tells whether there is a significant difference between the groups
(Microsoft 2012). Then a two factor ANOVA test was ran on the mixed pot data to see if there
was any significant difference between the densities and the interactions between species
(Microsoft 2012). Then the data analyses were repeated using the survival traits of the plant
species. The data was normalized using equations which included average number of leaves (# of
leaves / # of plants), average stem lengths (5 stem averages / 5), and average biomass (total stem
weight + total leaf weight + total root weight / # of plants) (Jenkinson 2010).
Results
The first anova showed that the p-value was 0.019115 when comparing the high and low
density radish plants to each other (Table 1). Table two compares the intraspecific densities of
collards to each other and calculated the p-value to be 0.190661. The graph shows that the
general trend between the intraspecific densities of radishes and collards to be that, at low
density radishes had a higher number of leaves than collards and at high density, both species
had about the same number of leaves between them ( Figure 1). Anova test were also done on the
success of the low and high density plants when comparing the survival trait of rapid growth.
Table three had success data of the intraspecific total biomass of the radishes with a p-value of
5.66E-06and table 4 showed the total biomass of the collards with a p-value of 0.145581. The
general trend of the graph showed that high density plants had higher total biomass than the low
density pots (figure 2).
Table 1: Single Data Anova Results for Intraspecific Radishes at High and Low Densities –
Table 1 shows the variation between the high and low density groups of the radish plants. The plants were
tested to see the significance between high and low density plants when examining the number of leaves
they grew.
ANOVA
Source of Variation
SS
df
MS
F
P-value
F crit
Between Groups
5.295599
1 5.295599 6.134814 0.019115 4.170877
Within Groups
25.89613
30 0.863204
Total
31.19173
31
Table 2: Single Data Anova Results for Intraspecific Collards at High and Low DensitiesTable 2 shows the variation between high and low density groups of the collards. The plants were tested
to see the significance between the high and low density plants when examining the number of leaves
them grew.
SS
4.882812
81.71354
Total
86.59635
Number of Leaves/Plant
ANOVA
Source of Variation
Between Groups
Within Groups
df
MS
F
P-value
F crit
1 4.882812 1.792657 0.190661 4.170877
30 2.723785
31
6
5
4
3
Radish
2
Collard
1
0
Low
High
Density
Average Number of Leaves of Radishes and Collards
at Low and High Density
Figure 1: Average Number of Leaves of Radishes and Collards at Low and High Density –
Figure 1 shows the average densities of both species of plants at an intraspecific level. The low density
plants had a higher difference between them than the high density plants did.
Table 3: Single Data Anova Results for Success of Survival Trait in Radishes- Table 3 shows
the significance of the survival trait which was rapid growth by analyzing the total biomass of the
radishes at low and high densities.
ANOVA
Source of Variation
Between Groups
Within Groups
SS
1107.702
1098.063
Total
2205.765
df
1
30
MS
1107.702
36.6021
F
30.26333
P-value
5.66E-06
F crit
4.170877
31
Table 4: Single Data Anova Results for Success of Survival Trait in Collards- Table 4
examines the significance of the survival trait of rapid growth on the total biomass of the collards at low
and high densities.
ANOVA
Source of Variation
SS
df
MS
0.731647
1
0.731647
Within Groups
9.831836
30
0.327728
Total
10.56348
31
Average Biomass/Plant
Between Groups
F
2.232482
35
30
25
20
15
10
5
0
-5
P-value
0.145581
F crit
4.170877
Radish
Collard
low
high
Density
Average Succes of Radishes and
Collards Using Total Biomass
Figure 2: Average Success of Radishes and Collard Using Total Biomass- Figure 2 shows the
average biomass of the intraspecific pots at high and low densities. The low density plants had the greater
difference between the two species than at the high density level.
The interspecific data was performed with two-factor anova tests and they compared the
radishes and collards that were in the same pots with each other. The first anova tested the stems
lengths of the collards and radishes against each other and the p-value was determined to be
1.23E-13 between the species (Table 5). The general trend of the graph showed that radishes at
both high and low densities had much higher stem lengths than the collards did (Figure 3). The
next anova was performed on the success of how well radishes and collards could absorb
nutrients, therefore the total biomass was examined and the p-value was determined to be 5.28E16 between the species (Table 6). The general trend of the graph examining this was radishes
showed a tremendous difference in total biomass between the radishes and collards, with
radishes having more biomass (Figure 4).
Table 5: Anova Two- Factor Test to Determine Significance of Interspecific Collards and
Radishes- Table 5 shows the significance of the stem lengths of the plants to see if there was any
difference between the species to see if one species was superior over the other.
Anova: Two-Factor With Replication
ANOVA
Source of Variation
Species
Columns
Interaction
2.732822
1 2.732822
Within
261.1562 60 4.352603
660.0234
df
MS
1
396.1344
1
9.77E-06
0.431263
F
P-value
91.01092 1.23E-13
2.24E-06 0.99881
4.001191
63
10
Stem Lengths
Total
SS
396.1344
9.77E-06
0.627859
8
6
4
Low
2
0
-2
Radish Mix
Collard Mix
Species
Average Stem Length Growth of Radishes
and Collards
High
F crit
4.001191
4.001191
Figure 3:
Average Stem Length Growth of Radishes and Collards - Figure 3 shows that stem lengths of
the radishes were a lot longer than the collards which shows they most likely were able to absorb the most
sunlight, blocking it from the collards that grew poorly.
Table 6: Anova Two-Factor Test that determines the Significance of the Success of the
Interspecific Collards and Radishes- Table 6 examines the significance of the success of the
survival trait which was nutrient absorption by measuring the total biomass of the plants.
Anova: Two-Factor With Replication
ANOVA
Source of
Variation
SS
df
Species
2691.383
Columns
291.1928
Interaction
251.3717
Within
1336.543
1
1
1
60
Total
63
4570.49
MS
F
P-value
F crit
2691.383 120.8214 5.28E-16 4.001191
291.1928 13.07221 0.000615 4.001191
251.3717 11.28456 0.001363 4.001191
22.27572
Average Biomass/Plant
20
15
10
5
Low
High
0
-5
-10
Radish Mix
Collard Mix
Species
Average Success of Radish and Collards
Mixes Using Total Biomass
Figure 4: Average Success of Radish and Collard Mixes Using Total Biomass - Figure 4
shows the success of the absorption of the nutrients by weighing the total biomass of the radish and
collards at the interspecific level. The radishes had a significantly higher biomass overall than the
collards.
Discussion
The interactions that take place in the natural world are necessary from a biological
standpoint. When interactions between species lower the fitness or causes an effect on a species
is known as completion. Competition is a necessary part of the natural world because it promotes
variation within communities (Ricklefs 2010). When competition arises between species, it
usually because a limiting resource which is a nonrenewable source that is vital to the survival of
a species. Some examples of a limiting resource include sunlight, water, and nutrients (Ricklefs
2010). Specifically this experiment examined interspecific competition, which is a species
competing against itself, and interspecific competition, which is when a species competes against
a different species. Both types of competition occur frequently in nature and have a necessary
part in the interactions between organisms (Jenkinson 2010).
The mechanism behind interspecific and intraspecific competition in called exploitation
competition, which is when a common limiting resource indirectly effects the survival of another
species (Ricklefs 2010). The two plants that were observed to show this mechanism were radish
and collards. Radishes and collards both grow in cool weathered months and have their own
survival traits. Radishes are fast growing and have a taproot which allows for efficient absorption
of nutrients. Collards are slower growing but have fibrous roots that allow it to go deep into the
soil to gather nutrients that some other plants cannot get to (Wolford et al. 2013). Based on the
traits, limiting resources, and densities, two hypotheses were formed. The first being that low
density plants will be more successful in growing because then have more nutrients to absorb
and will have more exploitative competition driving the plants to grow faster (Aguiar et al. 2001)
(MacIsaac et al. 1991). The second hypothesis was that radishes will be the more superior plant
because of its survival characteristics when faced with a competitor (Jenkinson 2010) (Wolford
et al. 2013).
The expected results of the intraspecific competition experiment was that low density
plants would fair far better than the high density plants because they would have more room to
grown and the high density plants would have less growth overall because of all of the plants
trying to get all of the resource that it could absorb (Jenkinson 2010) (Ricklefs 2010). While low
density plants could have grown slower because of fewer plants to compete with, it was
suspected that they would become even more competitive because there was more nutrient for
them to absorb (MacIsaac et al. 1991). The expected results of the radish and collard mixes were
that radishes would be the superior plant because it is known to mature a lot faster than collards
and to have great absorption ability (Waller 1928) (Wolford et al. 2013). They also initially grow
faster than the collards which made it clear that radishes would get most of the sunlight and
block the collards from getting as much sun on its leaves (Wolford et al. 2013).
The results of the experiment for the first hypothesis showed the p-value between the
radishes at low and high density to be 0.019115, which shows that there was a significant
difference between the numbers of leaves found on the radish species (Table 1). However, for
the high and low density collards the p-value was 0.190661 showed there was no significant
difference between the numbers of leaves found at the different densities (Table 2). Moreover,
the results for the success tested for the low density radishes showed a significant difference in
the total biomass of the plants with a p-value of 5.66E-06 (Table 3). This significance showed
that the survival trait of rapid growth was successful in helping the leaves absorb water from the
soil. There was no significance in the survival trait of the collards since the p-value was
0.145581, which means that neither low nor high density plants had difference in the amount of
biomass collected (Table 4).
The results for the interspecific data concluded that the stem lengths had a significant
difference when juxtaposed between the collard and radishes because the p-value was 1.23E-13
between the species, meaning that the stems of one of the species were significantly longer than
the other species stems (Table 5). There was also significance to the success test because the pvalue was 5.28E-16 which means that the survival trait of nutrient absorption was correct
because there was significance in the biomass of the plants (Table 6). The graphs of the data
made it clear that hypothesis one was accepted. Taking the graphs into account, low density
plants grew longer leaves and retained water better than the high density plants when looking at
the biomass (Figure 1) (Figure 2). From a biological standpoint this makes a lot of sense because
the survival trait of rapid growth made the less dense plants want to gather as many nutrients as it
could because of limiting resources (Wolford et al. 2013). This shows exploitation competition
between the less dense pots, which is a surprising result that it is much stronger in the low
density plants than the high density plants since the high density plants have fewer resources to
go around (Ricklefs 2010). The total biomass also showed the success of the survival trait, since
there was evidence of rapid growth, it would be logical that the biomass would weigh more
because they were more mature (Bonser et al. 1995).
Hypothesis two was also accepted because of the graphs of the data. The graphs showed
that radishes were superior in growing longer, which helped them to be more competitive by
blocking the sun from the collards which are lower to the soil level (Bonser et al 1995) (Figure
3). The stems of the radishes were significantly longer because of exploitation competition since
sunlight is a limiting resource (MacIsaac et al. 1991). The success of this was checked by the
total biomass graph which in short, shows that the taproot of the radishes was able to absorb
more nutrients than the fibrous roots of the collards so the radishes grew longer because they had
more nutrients making them into a superior predator over the collards (Figure 4) ( Waller 1928).
Radishes have a more efficient growing system than collards because they specialize in growing
rapidly and exemplify exploitative competition because the collards definitely had a decrease in
its fitness level and it was very hard for the collards to grow at all during this experiment
(Ricklefs 2010).
The main importance of this study was to examine the effects of competition because it is
a vital part of understanding many biological processes. This study also shows the survival traits
that arise within species to help the plant become a better competitor, as with the radishes
(Wolford et al. 2013). By looking at the mechanisms of how competition works helps to get a
better understanding of why it happens in nature. It teaches the ways in which organisms have
evolved in order to survive such a competitive world (Ricklefs 2010). Moreover, there are future
research ideas that could delve into the issue of competition even further. The first is seeing if
certain parts of an area can possibly predict how competitive the competition will be (Aguiar et
al. 2001). Another could be to test the environment health of the soil to the root systems of the
plants planted in those areas (Waller 1928).Lastly, is to see if biomass and competition increase
by a significant level with other organisms, such as plant with the same rapid growth as radishes
(Bonser et al. 1995).
Acknowledgements
Firstly I want to acknowledge all of my group members whom included: Blair, Gabrielle,
Courtney, and Carly. Blair worked very fast and efficiently to label all of the pots correctly.
Gabrielle worked as team leader and wrote down the data collected. Courtney worked hard at
planting the plants during the first initial step and also acted as team leader and delegated tasks to
everyone when we were gathering data. Carly also worked as team leader and kept everything on
track and helped with the clean up after all data was collected. Everyone worked very well
together and it helped us to finish quickly and correctly. I also want to acknowledge USC for
providing the greenhouse in which the experiment was carried out in and for providing the
resources as well. I also want to acknowledge Leslie Muggelberg for her patience and help in
compiling the data correctly for the Anova and graphs for this experiment.
References
Aguiar M, Lauenroth W, Peters D. 2001. Intensity of Intra- and Interspecific Competition in
Coexisting Shortgrass Species. Journal of Ecology: 40-47.
Bonser S , Reader R . 1995. Plant Competition and Herbivory in Relation to Vegetation
Biomass. Ecology: 2176-2183.
Jenkinson S. 2010. Interspecific and Intraspecific Competition. Biology 301 Lab. University of
South Carolina- Columbia.
MacIsaac H, Gilbert J. 1991. Discrimination Between Exploitative and Interference Competition
Between Cladocera and Keratella Cochlearis. Ecology: 924-937.
Microsoft. Microsoft Excel. Redmond Washington: Microsoft, 2012. Computer Software.
Ricklefs R. 2010. The Economy of Nature. Sixth Edition. New York: W. H. Freeman and
Company. 620.
Waller A. 1928. Roots of Vegetable Crops. Ecology: 96-99.
Wolford R, Banks D. 2013. Vegetable Directory. University of Illinois Extension. <
http://urbanext.illinois.edu/veggies/directory.cfm>. 19 Mar 2013.