Interspecific Competition Effects on Plants: Growth Rate,
Plant Height, Plant Weight, and Plant Densities
Amy Salamone
Tenessee Technological University, Cookeville, Tennessee
Abstract (might want to consider moving your headings to the left rather than the center
of your paper, other wise looks good besides the errors on pg. 5)
In this interspecific competition experiment, the objective was to find out whether
or not competition had a negative effect on the growth factors of marigold or alyssum
plants. The initial hypothesis was that interspecific competition would have a negative
effect on the growth factors of either the marigold or the alyssum. The data in this
research report shows that interspecific competition will have a negative effect on the
growth factors of plants. The data clearly shows that there is a slight difference in the
growth factors between the alyssum and the marigold plants. Overall, the research done
in this experiment reveals that competition will have an effect, either small or large, on
growing plants.
Key Words
Interspecific Competition, Growth Rate, Marigold, Alyssum
Introduction
Understanding the motivations that cause plants to distribute themselves on local,
regional, and global scales is one of the major issues ecologists are faced with (Crain et al.
2004). Plants distribute themselves by many determining factors. These factors include,
but are not limited to, dispersal techniques, demographic concerns, and biological factors
(Skov and Svenning 2003). The makeup and arrangement of plant communities are
influenced by positive and negative interactions between plants, the balance of which
may change in concentration throughout time, depending on competition for resources
(Armas and Pugnaire 2005). Competition affects all organisms that experience it.
Competition reduces individual plant growth regardless of whether it is for soil or light
(Cahill 2003).
One type of competition is interspecific competition. Interspecific competition
deals with two or more different species competing for a shared resource. Understanding
the means that control plant competition is a vital step in calculating the outcome of
interspecific competition (Raynaud and Leadley 2004). Interspecific competition can be
due to a limited supply of nutrients located in the soil (Day et al. 2003). Day et al 2003
lets us conclude that plants grown under higher nutrient levels have higher yields. With
the work of Pascual 1996, we can assume that reduced resources will increase the
intensity of competition (Pascual 1996). According to the results of Elderd and Doak’s
2006 experiment with two species of Tribolium, interspecific competition can restrict the
realized niche of both species (Elderd and Doak 2006). Both of these experiments suggest
that the realized niche will be restricted as the supply of resources declines. My objective
in this experiment is to find out whether or not interspecific competition will have an
effect on the marigold or the alyssum. With regard to this interspecific competition
experiment, I hypothesize that competition will cause there to be a difference in the
growth rates of either the marigolds or the alyssum. Consequently, my null hypothesis is
that competition will not have any effect on the growth rates of either species of plants.
Methods and Materials
I gathered 4 pots, and planted the following densities of seeds into their specific
pots: (1) 4 Alyssum and 4 Marigold; (2) 4 Alyssum and 24 Marigold; (3) 24 Alyssum and
4 Marigold; (4) 24 Alyssum and 24 Marigold. I made sure that the seeds were evenly
distributed inside the pot. Then soil was placed over the seeds and then watered.
I labeled each pot with the amount of seeds in each pot. Each week I measured stem
length and number of leaves per plant. Each time measurements were taken I filled a
foam cup with water and watered each pot. At the end of the 6th week, detailed
information was then gathered 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 that was
kept in a barrier to keep air from skewing the results (Day et al. 2003). The height
measurements were taken by simply using a ruler. I then weighed all of the plants from
one species and then divided that number by the total number of that species to get the
mean plant weight of that species.
Results
Number of Survivors
25
20
15
Alyssum
Marigold
10
5
0
4M:4A
4M:24A
24M:4A
24M:24A
Figure 1: Number of survivors of each species arranged by which density they were
planted at.
Total Weight (mg)
1200
1000
800
Ally
600
Mari
400
200
0
4M:4A
4M:24A
24M:4A 24M:24A
Figure 2: Total weight in milligrams of each species organized by the density they were
planted at.
Growth Rate (cm per day)
0.1
0.08
0.06
Ally
0.04
Mari
0.02
0
4M:4A
4M:24A
24M:4A 24M:24A
Figure 3: Mean growth rate in centimeters per day of each species organized by which
density they were planted.
Average Height (cm)
3.5
3
2.5
2
Ally
1.5
Mari
1
0.5
0
4M:4A
4M:24A
24M:4A
24M:24A
Figure 4: Average height in centimeters per species arranged by the density they were
planted at.
Discussion
The interspecific data results for the number of survivors shows that the number
of seeds planted is directly proportional to the number of survivors. When there is the
same number of Marigolds and Alyssum(s) planted, Alyssum(s) have a slightly higher
survival number, as demonstrated by figure 1 (change to Fig. 1). When there are 4
Marigolds and 24 Alyssum, Alyssum have a much higher survival number (Fig. 1). When
there are 24 Marigolds and 4 Alyssum, the Marigolds have a much higher survival
number (Fig. 1). These results are caused by the fact that when one species has a much
higher density than another then the higher one will usually out compete the lower one
(Raynaud and Leadley 2004). The bar graphs showing the total weight of Marigolds
versus Alyssum shows that Marigolds always weigh more (Fig. 2). The bar graphs
showing the average weight of the two plants shows that Marigolds will always have a
higher plant weight despite the densities of either (Fig. 2). Figure 3 shows that Marigolds
mostly grow at a specific rate. Figure 3 also shows that Alyssum will grow at a higher
rate when there are more of them planted. When one species outnumbers another species,
it will most always have a higher growth rate then the lesser species (Armas and Pugnaire
2005).
With regard to the t- tests, if P(T<=t) two-tail is greater than 0.05 then there is not
enough of a difference between the two variables for them to be significantly different
(Pascual 1996). The results of the interspecific t-tests show that all the values of P(T<=t)
two-tail are not great enough between the Marigolds and Alyssum for them to be
significantly different. This means that competition was either not taking place or it was
on such a minute level that the plants’ growth did not become affected. There are many
instances where one species will have such a minute effect on the growth factors of
another that it will not be evident during an experiment (Cahill 2003).
These results clarify that density may have an effect on all aspects of growth. The
effects of density may be related to competition in that the higher the density, the more
likely the plant population is to experience competition. With regard to my experiment,
the plants may have been competing for nutrients, water, or sunlight (Crain et al. 2004).
The density, in my experiment, had a slight effect on plant weight. This may be due to the
sharing of nutrients, because as density increases the nutrient allocation decreases (Elderd
and Doak 2006).
Conclusions

Competition is always taking place in every environment.

Interspecific competition has an effect, either minute or great, on the growth
factors of plants.

Plants may be competing for light, nutrients, or water, depending on where their
environment is located.

When density is higher, the plants will have a higher level of competition.
Future research may be done to find out the long term effects of interspecific
competition and the growth factors that are effected by long term competition.
Acknowledgements
I would like to acknowledge Dr. Chris Brown and graduate student Leah Lavoie
for their help in understanding competition from an ecological sense. They also aided in
analyzing the data and the methods and materials part of the experiment.
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