THE EFFECTS OF INTRASPECIFIC AND INTERSPECIFIC

Abstract
THE EFFECTS OF INTRASPECIFIC AND INTERSPECIFIC
COMPETITION ON PLANT GROWTH
Rashmi Jasrasaria and Debleena Mitra
Competition between plants of the same species (wheat and wheat) and plants of
different species (wheat and pea) was examined to determine an optimum seed density
and whether two kinds of plants could be grown together. Different numbers of wheat
seeds were planted per starting pot to test the impacts of intraspecific competition on an
individual plant, and wheat seeds were planted with pea seeds in different densities to test
the impacts of interspecific competition. There was no significant difference among the
stalk length of the wheat control, the wheat grown with other wheat, or the wheat grown
with peas. However, the average root length of the wheat control was longer than the
root length of wheat grown in competition. Many of the pea seeds failed to germinate,
but from the data present it was seen that peas, like wheat, did not follow a pattern for
shoot length, but root length was greater in the pea control than it was in the higher
concentrations of peas and wheat. To optimize the growth of an individual wheat or pea
plant, the two can be started together but should be grown in low densities.
Introduction
Competition among plants involves a struggle for limited resources such as
sunlight, water, and nutrients in the soil. Competition is an interaction that is harmful to
both individuals involved. Plants better adapted to obtain resources can out-compete
other plants and maintain their growth, so plants can be grown in competition without
major effects on their growth if they have different individual adaptations.

Intraspecific and interspecific competition between two species of plants were
examined in this study to help a group of organic farmers determine the best planting
arrangement, including the optimum density of seeds and the optimum combination of
plants of different species. The farmers would like to maximize their crop of healthy
seedlings in a limited amount of space. The purpose of this study was to find the balance
between competition and healthy growth.
Competition is a very important part of plant interactions. Plants undergo two
types of competition: intraspecific, among their own species, and interspecific, with
plants of another species. The two kinds of competition have distinct effects.
Intraspecific competition is the most aggressive because plants of the same species have
the same needs and same resource-obtaining structures (Alexander et al., 1970). There
are three principle effects of intraspecific competition outlined in a series of papers by
Yoda et al., 1963: the average size of a plant decreases as density increases, the size
structure of the population becomes hierarchical and self-thinning or density dependent
mortality occurs (Park et al., 2003). For interspecific competition, many different
experimental designs have been employed, for researchers have different aims (Park et al.,
2003). The replacement series is one design where the proportions of one plant to
another are changed, but the total density is kept constant. According to Park et al., 2003,
many argue this does not separate the effects of intra and interspecific competition, so is
not an adequate design. Another design is the additive design where both density and
proportion are altered, and this yields a simple hyperbolic relationship between the crop
damage and number of weeds (Park et al., 2003).
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However, in this study, the goal was not to study crop-weed interspecific
competition, but intercropping, and whether two crops could be planted together to
produce higher, healthier yields in smaller space. In his 2003 Agronomy Guide, Preston
Sullivan said, “Intercrops can be more productive than growing pure stands.” He argues
there is more stability in diversity established in having more than one crop. The reason
to intercrop is to increase the productivity of the land, and when the density, maturity
dates, and plant architectures are taken into account, it can be very successful (Sullivan,
2003). In 2003, Sullivan described a South Carolina trial where corn and peas were
intercropped at three different corn densities. There was a definite yield advantage from
intercropping than from growing the two crops pure stand, but the peas suffered from
increased competition in the higher densities of corn (Sullivan, 2003). A balance must be
attained.
The specific purpose of this study was to find an optimum density for growing
wheat and to find out whether wheat and peas could be grown together to maximize yield
per area. It was hypothesized that there would be no difference in the stalk length (above
ground height of the plants) or the root length between pure wheat grown at a density of
two per pot and wheat grown at higher densities, and that there would be no difference
between the control wheat and the wheat grown with pea seeds. The purpose of the
experiment was to find out whether competition (intraspecific or interspecific) did indeed
effect the growth of individual plants.
Materials and Methods
The experiment began on September 28, 2004. After planning out the
experimental design, 42 small starting pots were obtained and filled three-fourths of the
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way with potting soil. Then seeds were placed inside each pot according to the
arrangement shown in Figure 1. 94 wheat seeds from a brown paper bag were used and
44 Alaskan pea seeds from a plastic bag full of seeds were used. The seeds were spaced
evenly within the pots. Then the pots were filled to the top with soil and watered. The
36 connected pots were covered with plastic and placed under one plant light inside a
classroom close to the windows, and the other 6 were placed under the adjacent plant
light and were covered, but air could still pass under the cover, for it was not a perfect fit
over the smaller number of pots. Every school day, around 12:40 PM, the plants were
watered, and extra water was given before a weekend, so the plants would not dry out.
Figure 1
2 W 2 W 2 W
2 W 2 W 2 W
2 W 2 W 2 W
2 P 2 P 2 P
2 P 2 P 2 P
2 P 2 P 2 P
2 W 2 W 2 W
2 W 4 W 4 W
4 W 6 W 6 W
8 W 10 W 1P, 1W
1P, 1W 1P, 1W 1P, 1W
1P, 1W 2P, 2W 2P, 2W
Once the plants were too tall and the wheat stalks were
2P, 2W 3P, 3W
3P, 3W 4P, 4W
5P, 5W EXTRA
hitting the cover, the covers were
removed. This was after about 11 days.
They continued to be watered. On
The arrangement of seeds planted.
W= wheat seed, P= pea seed, each cell=
one starting pot. 36 were all together in
one tray, and the other 5 were in the
neighboring tray. The top 9 are wheat
controls. The next 9 are pea controls.
The next 11 cells show wheat
intraspecific competition at different
densities, and the last 12 cells show pea
and wheat interspecific competition.
October 13,
the wheat
controls and
pea controls
were uprooted
and measured.
The individual plants were carefully pulled out of the pots,
and the soil was washed off in a beaker of water. Then,
using a metric ruler, the stalk length and root length of each plant was measured and
recorded. The number of leaves and the number of tendrils (on the peas) were also noted.
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The rest of the plants were uprooted and measured on October 14, 2004. After all the
data was collected, the plants were thrown away, and the pots were cleaned out. The data
was entered into Microsoft Excel and analyzed using statistics to make bar graphs. The
experiment had been completed.
Results
Of the 138 seeds planted, only 97 germinated and grew. Of those 97, only 15
were pea plants. There is clearly a lack of pea data in the interspecific competition
results; however, some trends are still visible. In the intraspecific competition
comparison, the average root length of the wheat control was greater than the mean root
lengths of wheat grown at increased densities. Of the 12 individual wheat seedlings,
grown four per pot, six had root lengths beneath the 95% confidence interval of the wheat
control (10.985-25.605 cm), and this trend continued, for six of the ten wheat seeds,
grown five per pot, had root lengths outside of the wheat control confidence interval.
Wheat grown six and eight per pot did better with higher average root lengths than wheat
grown four and five per pot.
root length in cm
30
25
20
15
10
5
0
Wheat Intraspecific Competition: Root Length Means
and 95% Confidence Intervals
control (2) 4 6 (act 5)
1 8 (act 6) 10 (act 8)
number of seeds per pot
act = # that actually
germinated
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There is no such distinction between the stalk/shoot length of the wheat control and
wheat grown at higher densities. The graph of the means is flat, and the confidence
intervals are large. Shoot lengths were constant throughout the study of intraspecific
competition.
shoot length in cm
30
25
20
15
10
5
Wheat Intraspecific Competition: Shoot Length
Means and 95% Confidence Intervals
0
control (2) 4 6 (act 1 5) 8 (act 6) 10 (act 8)
act = # that actually
germinated
number of seeds per pot
This trend of constant shoot length persisted in the interspecific study as well.
Wheat stalk lengths did not vary very much when wheat was planted with peas. It can be
seen that the means are very close in the summary table of statistics (Figure 2).
Interspecific Competition: Wheat Stalk Lengths (cm)
control (2 wheat) 1 wheat, 1 pea 3 wheat, 2 peas 5 wheat, 2 peas
mean 17.11905 17.55 18.8 17.3
std dev 2.11533 0.353553 3.325658 0.223607
66% interval 15.00372 17.19645 15.47434 17.07639
19.23438 17.90355 22.12566 17.52361
95% interval 21.34971 16.84289 12.14868 16.85279
12.88839 18.25711 25.45132 17.74721
Figure 2
Wheat root length, though, did decrease as pea density increased, but it was
constant between the wheat control and the one wheat grown with one pea. There were
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two of the six wheat grown as 3 wheat seeds with 2 pea seeds per pot that were beneath
the wheat control’s confidence interval. One of these plants had a root length of only 4.5
cm. In the 5 wheat with 2 peas pot, four of the five wheat seedlings had roots too short to
be within the confidence interval of the control, and one plant had a root length of only
5.5 cm.
rootl length in cm
30
25
20
15
10
5
0
Wheat Interspecific Competition: Root Length Means
and 95% Confidence Intervals
control
(2 wheat)
1 wheat, 3 wheat,
1 pea
1
2 peas
number of seeds per pot
5 wheat,
2 peas
From the available minimal pea plant data, the same results predominated. Root length
decreased as wheat density increased. However, it actually increased with the addition of
one wheat plant. The pea control mean root length was 10.5 cm, and it had a 95%
confidence interval from about 6 to 15 cm. The 1 pea, 1 wheat peas (of which there were
only two) had about 14 cm long roots, but this upward trend could have been exaggerated.
root length in cm
20
15
10
5
0
Pea Interspecific Competition: Root Length Means
and 95% Confidence Intervals
control (1) 1 pea,
1 wheat
2 peas, 3
1
wheats
number of seeds per pot
7
2 pea,
5 wheats

Pea shoot length was varied, and the control had a huge confidence interval of about 5.8
to 41.6 cm because one of the six control peas had a stalk length of 5.5 cm. This may
have thrown off some of the statistics along with the low number of data points, for if this
low value was not included in the calculation of the mean, the mean would be 27.3 cm
instead of 23.6 cm.
In general, root length is greater when the plants are growing in low densities
even if the plant species are mixed, and shoot/stalk length is not affected very much by
competition. The number of leaves on the wheat plants was always two, except for a few
individual plants in intraspecific competition that had only one leaf. For the peas,
number of leaves varied directly with the height of the stalk. Taller plants had more
leaves, and tendrils followed this model as well. In one instance of interspecific
competition between three wheat and two peas, one of the pea tendrils was knotted
around the tip of a wheat leaf, but it did not seem to be harming the wheat plant in any
way. Although many peas did not germinate, general trends were visible in the
experiment results.
Discussion
Through looking at the data, certain conclusions are reached. The previous null
hypothesis that there is no significant difference between the average individual wheat
plant grown alone, grown in greater density and grown with a different species plant must
be rejected. There are several differences between the wheat control and wheat grown
with other wheat at high densities. First, the root lengths of the wheat control are higher
than the wheat grown at a greater density. Shoot length may be very similar, but the
roots are the major resource obtaining structures in plants. Most of the competition must
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e for the water and nutrients in the soil because the leaves are responsible for taking in
sunlight, and most of the wheat seedlings had two leaves that were about the same size.
Wheat grown with peas also yielded the same difference. Roots were longer in wheat
grown at lower densities of two plants per pot, be they two wheat plants or one wheat
plant and one pea plant. As soon as the number of plants per pot was increased to 5 or 7,
the root length decreased. Clearly, wheat grows better in lower densities, but it can easily
be grown with another species. For farmers looking to grow wheat, an optimum density
is two wheat seedlings per starting pot, and wheat can be grown with peas, for the root
length stayed constant.
The null hypothesis that there is no significant difference between the root length
and stalk length of peas grown alone in low density and peas grown with wheat cannot
fully be rejected or accepted because there is not adequate data to analyze. There are
only six data points for the control, two for the one pea/one wheat, none for two pea/two
wheat (only the wheat grew), four for three pea/three wheat (which turned out to be two
pea/three wheat), one for four pea/four wheat (which turned out to be one pea, three
wheat, so it was not included in the analysis), and two for the five pea/five wheat (which
turned out to be two pea/five wheat). With data sets of only two points, the conclusions
reached are probably exaggerated or non-existent. There was a difference in the root
length of the pea controls and interspecific competing peas. Root length decreased with
increased density, but actually increased when one pea plant was grown with one wheat
plant. It actually may be beneficial to farmers to grow wheat and Alaskan peas together,
for the wheat’s root length was not affected, and the pea’s root length increased.
Growing two plants in one area will maximize yield and the two plants that have slightly
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different niches could cooperate. The stalk length was too variable to place a trend upon,
so for that the null hypothesis can be accepted, meaning there is no significant difference
in pea stalk height based on competition. Eventually, though, plants with longer roots
will do better competitively than plants with shorter roots, for they can reach more
nutrients and have more surface area to absorb water, so if the experiment was continued
past 17 days, these plants with longer roots could have grown longer stalks as well.
The conclusions reached in this lab verify the statement put forth in 1970 by
Alexander et al. that intraspecific competition is more damaging, for plants of the same
species have the exact same needs and adaptations. The wheat root lengths were hurt
more by increased density among other wheat plants than among the same number of
peas. For example, the mean root length of 5 wheat seeds per pot was 9.86 cm, which
was less than the mean root length of 3 wheat and 2 pea seeds per pot (10.55 cm). It is
the same density, but that addition of peas instead of wheat yields longer roots. The
increase in root length in one pea grown with one wheat seed versus one pea grown alone
supports Sullivan’s statement that intercropping can be beneficial. However, this
increase could be inflated because there was not enough pea data.
When retrying such an experiment, there are several design changes that should
be made. First, fresh seeds that have a higher probability of germination should be used.
In this study, two year old Alaskan pea seeds that had been stored in a plastic bag had
been used, and not very many actually germinated. Also, learning how to lightly add pot
soil on top may help, for it may have been too packed for some seeds to grow in this
experiment. In addition, it would be more controlled to have all the pots on one tray
under the same light, so the resources being received are the same. To create more
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drastic results, the density of the wheat in intraspecific competition could be increased to
20 seeds per pot at most and 1 seed per pot at least, and then it could increase by fives in
between. This would give a wider range of data, and if most seeds germinated, there
would be more data points as well. These were some improvements that could be made
to the experimental design.
To test other aspects of interspecific competition, it would be interesting to vary
the proportions of one species to another in the same area to see if one species dominates
the other or find an optimum proportional relationship, suppose five wheat seedlings and
only one pea because the pea is heavier and larger. Another experiment that would
clarify some doubt about the benefits of interspecific competition would be to set up a
control of wheat, interspecific competition with one species, and competition with
another species to see if it depends what species the wheat is most cooperative or
competitive with.
The relevance of this experiment is slightly questionable, for a starting pot is a
very rough simulation of two crops being planted in rows in a field. It may be more
relative to study weed-flower competition, for this applies more to students who have
experienced domestic gardening and attracts more interest. A longer experiment that
lasted through plant maturity would also be more relevant, for plants in this study that
were benefiting at the stage where they were uprooted, may have been struggling as other
plants around them matured. It would also be helpful to learn some background about
each of the plant species that are used in this study, for then specific needs and
adaptations would be known and the interspecific study would be more relevant.
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The experiment was still helpful in portraying the differences between intra and
interspecific competition and portraying a practical application of descriptive statistics in
the real world. It was also interesting to develop an original experimental design to
figure out a real situation. Wheat and peas can be grown together just in low densities (2
seeds per starting pot).
Literature Cited
Alexander, Taylor R., R. Will Burnett, and Herbert S. Zim 1970. Botany. New York:
Golden Press, Western Publishing Company.
Park, Sarah E., Laurence R. Benjamin, and Andrew R. Watkinson 2003. The Theory and
Application of Plant Competition Models: an Agronomic Perspective. Annals of
Botany. Oxford Journals Online.
http://aob.oupjournals.org/cgi/content/full/92/6/741.
Sullivan, Preston 2003. Intercropping Principles and Production Practices.
http://www.attra.org/attra-pub/intercrop.html#Concepts.
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