The Effects of Intraspecific Competition among
Varying Groups of Marigolds
Tiffany Landis
Microbiology Student
TTU Box 6193
Abstract
Intraspecific competition is competition between organisms of the same species. In
nature plants will out compete on another for the same shared resource leading to
smaller plant size and plant densities. Intraspecific competition does not, however, lead
to extinction of a species. If gardeners, farmers and botanists knew the optimal density
to plant the marigolds in to get optimal plant size they would be able to grow more and
better quality marigolds. In my experiment with marigolds I looked to Intraspecific
competition to determine if plants with more competitors will end up smaller than plants
grown with fewer competitors. The idea being that competitors will take up light and
nutrients and leave a lot less of those nutrients for other plants. My null hypothesis being
that no matter the density, plant weight and height will reflect no difference among the
varying densities. I conducted my experiment my purchasing six pots and marigold seeds.
I then planted them in varying densities: 3, 5, 10, 18, 34, or 68 seeds. After germination,
the plant populations were thinned to 2, 4, 8, 16, 32, or 64 plants per pot. Each week I
measured the number of leaves on each plant in each pot, heights of the plant stems, and
number of flowers on each plant. The results were not conclusive enough to say that
competition affected their growth. According to various tests, density had no effect on
plant growth.
Key Words: Intraspecific Competition
Introduction
Competition for resources is what drives natural selection. As Darwin stated
“Only the fit survive.” Those species that are best suited for survival are the ones that
can out compete with other species for the same-shared resources. “The purpose of
making plant competition studies is ultimately to be able to predict the dynamics of
natural plant communities.” (Damgaard, 1998 ) Intraspecific competition occurs
between members of the same species. They will both compete for the same limited
resource. A consequence of intraspecific competition would be that when the population
becomes more dense the survival and/or reproduction rates of the individuals declines.
For many plants, an increase in density results in a decrease in reproduction, total
biomass, height and/or survival (Antonovics and Levin, 1980). “Competition, either
among members of the same or different species may have broad effects on plant
community structure. The more crowded a population the more competition there is for
resources.” (Tansley, 1917) In the intraspecific competition, I was testing to determine
if competition had an effect on how the plants grew in height, weight and number of
leaves within the same species. In many cases intraspecific competition will occur
because of the need to compete for the same shared resource, such as the need for space.
“Spatial arrangement of plants in a community can be an important determinant of
species coexistence and biodiversity” (Stoll et al, 2001). In this study plants were grown
in two densities either randomly or intraspecifically aggregated. Stoll and his colleague
Prati concluded that the Competitively superior species had a lower biomass in the
aggregated distribution than in the random distribution at high densities. “The data
supported the hypothesis that the spatial distribution of plants profoundly affects
competition in such a way that weaker competitors increase their fitness while
stronger competitors are suppressed when grown in the neighborhood of co specifics”
(Stoll et al, 2001). One would expect that since the plants are competing for the same
shared resource that competition would be strong and that some plants would grow to an
immature size, die or just not germinate. “Plants biomass and seed production were
always higher in plants growing with reduced competition” (Aquiar et al., 2001). In the
aforementioned study, Aquiar and his colleagues worked with a short grass steppe
species. They found that the short grass will compete for the same resource in order to
survive, thus they predicted that competition for that shared resource would be intense.
Their study proved that plants would compete for shared resources, affect plant biomass,
and seed production. In my experiment with marigolds I looked to intraspecific
competition to determine if plants with more competitors will end up smaller than plants
grown with fewer competitors. The idea being that competitors will take up light and
nutrients and leave a lot less of those nutrients for other plants. My null hypothesis being
that no matter the density, plant weight and height will reflect no difference among the
varying densities. I hypothesized that as plants increased in density so would competition
between the species. My null hypothesis being that no matter the densities there will be
no difference in plant growth. I came to these conclusions on the mere knowledge that
has space is limited and the need for resources is high there will be rigorous competition
for those resources. As one species competes, the other must also in order to survive. I
knew that the most fit will survive and through the experiment I conducted and my
classmates I will try to provided evidence to support my hypotheses.
Methods and Materials
I obtained six pots and labeled them according to the number of seeds in each
pot. The plants were kept in a sunroom and watered regularly. Each of the six pots was
filled with soil until it came to about one centimeter from the top of the pot. I obtained
marigold seeds and planted the following densities of seeds in each pot: 3, 5, 10, 18, 34,
or 68 seeds. After germination, the plant populations were thinned to 2, 4, 8, 16, 32, or 64
plants per pot. I then placed more soil on top of the seeds and watered them. Each week I
measured the number of leaves on each plant in each pot, heights of the plant stems, and
number of flowers on each plant. We then gathered detailed information from each pot
number of plants, weights of
plant stems, number of leaves and length of stems. I
obtained stem weights of the plants by clipping out the plants at soil level, clipping off
the leaves and placing them on a scale.
Results
The results of my experiment were flawed because the two, four and eight seed
densities did not grow this involving Intraspecific competition. The pot with sixteen
plants grew in size so competition was less among individuals. In the pot with thirty two
plants self-thinning could be seen; some individuals stayed relatively the same size
throughout the experiment. In the plot with sixty-four plants, they did not grow much in
height or weight the growth seemed to be stunted after the fourth week. Throughout the
experiment their was a high survival rate of about ninety percent. Intraspecific
competition was against shared resources where it could have affected plant growth, but
in this case it did not.
Density vs Percent Survival
120
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Density vs Total
Plant Weight
Percent Survival
Total Plant Weight
Density vs Total Plant Weight
100
80
Density vs Percent
Survival
60
40
20
0
0
20
40
60
0
80
50
Figure 1
Figure 2
Density vs Average Stem Length
60
50
40
Density vs Average
Leaf Weight
30
20
10
0
20
40
60
80
Average Stem Length
Average Leaf Weight
Density vs Average Leaf Weight
0
6
5
4
Density vs Average Stem
Length
3
2
1
0
0
Density
Figure 3
100
Density
Density
20
40
Density
Figure 4
60
80
Discussion
After completion of my experiment with the marigolds my data concluded something that
I had not expected. The density of the marigolds did not effect intraspecific competition
among the marigolds. These results are not conclusive enough to support my hypothesis.
Thus, I have been able to accept my null hypothesis which was being that no matter the
densities there will be no difference in plant growth. The results were not conclusive
enough to say that competition affected their growth. Studies done by scientists as
mentioned in the introduction were conducted over a longer period and proved that
competition intra- had an effect on plant growth.(Damgaard, 1998 )
The experiments
that I conducted were over a shorter period and thus we could not prove that
competition intra had a significant effect on plant growth. However, through my
various researches on this topic I found that competition can be avoided and that the
plants through natural selection would be allowed to converge. (Stoll et al, 2001)
Avoidance of competition may imply that the species have gotten used to each other’s
presence. (Tansley, 1917) As a country of farmers this information will be able to
impact the various crops grown in the country. Making a more structured crop
community being able to dynamically grow plants in a high density environment.
(Antonovics and Levin, 1980). Then again all these researchers have been able to
conclude that competition impacts plants in many different ways. (Aquiar et al., 2001)
Conclusions

Density did not affect the amount of intraspecific competition among the
marigolds.
Acknowledgments
I would like to thank Dr. Brown for his guidance throughout my entire experiment. Also,
to Dr. Morgan for helping me some with my website.
Literature Cited
Aguiar, M., Lauenroth, W. and Peters, D. 2001. Intensity of intra- and
Intraspecific competition in coexisting short grass species. Journal of Ecology
89: 40.
Antonovics, J. and Levin, D. 1980. The ecological and genetic consequences of density
dependent regulation in plants. Ecology 11:411-452.
Damgaard, C. 1998. Plant Competition experiments: testing hypotheses and
estimating the probability of coexistence. Ecology 79: 1760-1767.
Stoll, P. and Prati, D. 2001. Intraspecific Aggregation Alters Competitive Interactions In
Experimental Plant Communities Ecology 82: 319.
Tansley, A. 1917. On competition between Gallium saxatile L. and Gallium sylvestre
on different types of soil. Journal of Ecology 5:173-179.