Plant Competition Experiment

Overview

Competition is species interaction that has major impacts on species and communities. Organisms compete over shared resources, meaning the requirements they need to survive and grow must overlap. Students will use a replacement series experiment to study competition between two plant species.

Background

Biology 19.2 (intraspecific competition)

Biology 19.4 (competitive exclusion)

Objectives

Students should be able to

Define terms including niche and competition
Describe various types of competition
Explain competition using modifications to the logistic growth equation (the Lotka-Volterra equations for competition)
Carry out a replacement series experiment, analyze, and graph results to consider competition between two species

Introduction

In ecology, a niche is defined as the the physical and biological conditions that a species needs to grow, survive, and reproduce. The niche of an animal means its place in the biotic environment and its relations to food and enemies (Elton 2001). For example, the flightless dung beetle has an ecological niche that comprise of occupying animal feces as a food source. Yet the dung beetle are not the only species to eat animal feces in nature. Competition can occur when the niches of organisms overlap.

There are many forms of competition. These can be categorized into direct and indirect interactions. Interference competition occurs when organisms are directly fighting for scarce resources. Resources that may be competed for in this way include but are not limited to food, habitat, and sexual partners. For example, allelopathy is a form of interference competition where plants release chemical herbicides that interferes with the growth or the establishment of other plants. In contrast, exploitative competition occurs when organisms (who uses the same resource) fight by making resources less available to the competitor.

Competition can also be divided between intraspecific and interspecific interactions. Intraspecific competition is competition between individuals of the same species and is best modeled by logistic growth. The logistic growth equation, dN/dt =rN(K-N/K), describes that the rate of population growth is dependent on the intrinsic growth rate (r), current population size, and the carrying capacity (K). The carrying capacity is the maximum population size that the environment can sustain, which is depicted as a decrease in population growth rate to a plateau in the logistic growth curve (Figure 1 on the right). Therefore, intraspecific competition increases as the current population approaches carrying capacity because individuals are competing for limited resources.

Figure 1. Logistic growth curve.

(Modified from the source of figure 1:

CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]

Interspecific competition is competition between individuals of different species and is best modelled by the Lotka-Volterra model of competition. Competitive Lotka-Volterra equations are similar to Lotka-Volterra equations for predation with the difference that the latter model uses the exponential equation as the base population model instead of the logistic equation. There are two equations in the competitive Lotka-Volterra model. We can model the growth of species 1 as: dN1/dt=r1N1(K1-N1+α12N2/K1), and species 2 as: dN2/dt=r2N2(K2-N2+α21N1/K2). The competitive coefficients, α₁₂ is the per capita effect of species 2 on 1, and α₂₁ is the per capita effect of species 1 on species 2. Note that intraspecific competition is still present in the equations. By plotting the zero growth isoclines of both equations together, you can predict the outcome of the competition, coexistence or one species go locally extinct (Figures 2 A-C).

Figure 2A. One of the possible outcomes of interspecific competition is coexistence between the two species. Yapparina [CC0]

Figure 2B. One of the possible outcomes of interspecific competition where species 1 is the better competitor and species 2 goes to extinction. Yapparina [CC0]

Figure 2C. One of the possible outcomes of interspecific competition where species 2 is the better competitor and species 1 goes to extinction. Yapparina [CC0]

A classic example of interspecific competition is Gause’s experiment on two Paramecium species, P. caudatum and P. aurelia (Figure 3). Both species grew well in separate cultures with the same medium added daily. However, when both species are grown under the same culture, P. aurelia outcompetes P. caudatum. Gause’s experiment is also an example of the competitive exclusion principle (also called Gause’s principle). This principle states that species cannot coexist when occupying the same niche. When two species occupies identical niches, one will outcompete the other. Therefore, species can coexist if they occupy slightly different niches.

Figure 3. Result of Gause's experiment on Paramecium. CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]

The replacement series experiment designed by de Wit (1960) has been used to explore many issues in ecology such as species coexistence, exclusion, coadaptation, niche overlap, niche differentiation, abundance, productivity, and diversity (Jolliffe 2000). Although faced with criticisms, this experiment has been proven useful in assessing the presence and the magnitude of competition between two plant species (Rodriguez 1997). Experiments using this design vary the proportion of plants in a mixture while maintaining the overall total density of plants. Plots include monocultures of both species and polycultures of varying proportions. Data (e.g., total dry weight of each species) can be obtained from these experiments used to compare intra- and interspecific competition using de Wit diagram. Under the null assumption that inter- and intraspecific competition are equivalent, yield for each species should vary directly with proportion and the total combined yield should be equivalent across all containers. High interspecific competition occurs when the combined yield is less than the yield of each plant grown separately (Figure 4a). Low interspecific competition occurs when the combined yield is greater than the yield of each plant grown separately (Figure 4b). This is the result of niche partitioning or facilitation where species differentiate their niches in order to avoid direct competition of resources, which helps both species to coexist. Another example of a de Wit diagram and more information on the technique can be found in Joliffe' s 2000 essay on replacement series (Figure 5).

Figure 4a. de Wit diagram of high interspecific competition.

Figure 4b. de Wit diagram of low interspecific competition.

Figure 5: From Joliffe 2000.

Plant Competition

Methods

You'll now apply these ideas about describing how intraspecific and interspecific competition affect mung bean and wheat plants by performing a replacement series experiment.

Experiment Design

You also need to understand how science works in order to carry out an experiment. A replacement series tests if there is a difference between intra- and interspecific competition. You should be able to create a hypothesis about the difference between these types of competition (What is the null hypothesis)? After a few weeks (science takes time!) we'll collect and analyze data and use our new knowledge to consider if our hypotheses did a good job describing how the world really works. Hypotheses are never true or false! We just compare two (or more) and decide which one is currently the best at explaining what we observe.

Materials (per group)

6 small pots
1 tray (holds up to 12 pots or 2 groups of plants)
Potting soil (near the sink)
Mung bean seeds
Wheat seeds
Tap water for watering
Electronic scale (accurate to 0.01)
Weighing dishes
Label tape
Scissors

Procedure

PART 1 - Experiment Set-up

1. Remove any leftover tape on pots and tray. Write your group name on labeling tape and label your tray (or the side of your tray), make sure it is visible enough for easy identification.

2. Label each pot using labeling tape with the following identification, feel free to double up on the labels just in case it falls off:

3. Fill the pots with soil to within 1 inch of the top (the upper ring around the pots is usually a good stopping point). Make a point to add the same amount of soil to each pot.

- If using the dry garden soil, add water to each pot until the soil is moist. Adding the seeds before the water may result in seeds being pushed further down in the soil.

4. Add a total of 10 seeds to every pot, varying from 10 wheat seeds (and thus 0 bean seeds) to 10 bean seeds (and thus 0 wheat seeds). Additional seeds may be added if germination rates are low; if this is done, pots should be thinned to 10 plants after seeds have sprouted. Analyze the following figure to make sure you understand the design before continuing.

5. Start with the 100% W 0% B pot, add only 10 wheat seeds in the pot. Next do the 0% W 100% B pot, adding only 10 bean seeds.

6. Add the appropriate amount of bean and wheat seeds to the remaining pots. Plant seeds in randomized pattern and distributed throughout the pot, make sure the seeds are covered with soil just below the surface of the soil. If needed more water can be added to ensure the seeds are moist enough to germinate.

7. Place all pots in the tray after the seeds are planted. Randomize pot positions within tray (e.g., don't have all the 100% W pots in one row!)

8. Place tray under growth lights or greenhouse bench and grow in room temperature for 2-3 weeks.

9. Check the tray frequently and water your plants.

PART 2 - Data Collection and Analysis

10. After 2-3 weeks, cut all plants just above the soil surface and brush off the soil.

11. Using the weigh dish, weigh the plant material and record the weights. Data can be entered in the table below and also in a copy of the spreadsheet shared below (in the Individual Groups tab). (Don’t forget to tare the weight of the weigh dish beforehand.)

de Wit Diagram Template

12. Discard all dried plant material, soil, and labeling tape after recording. Make sure to clean your table before returning pots, tray, and scissors to instructor.

13. Class wide data can also be entered in the shared google sheet and used to calculate the class mean for both bean and wheat weight for each treatment group.

14. Graph the dry weight of both plants in the de Wit diagram below using the class means. The DeWit Diagram tab of the shared spreadsheet will also help with this step! Label the scale of the y-axis and find the weight of the combined in the corresponding treatment groups.

15. Answer the questions below after completing the diagram.

Tips

Adding multiple data series to a Google Drive graph

To add multiple data series, let the first column be your desired independent variable (for x-axis). Dependent data series can be placed in adjacent columns. All columns can have identifying labels. Select all the columns you desire to graph, then insert a chart using the same procedure from the Summarizing Data Lab. The only modification required is that you may need to specify which series is the X-axis depending on your browser defaults.

Select all columns you desire graphed and then insert chart.

If needed, designate the appropriate series as the x-axis.

Review Questions

State the null and alternative hypothesis of this experiment.
Compare the dry weights of mung bean plant of varying percentages. Is interspecific competition stronger or weaker than intraspecific competition?
Compare the dry weights of wheat plant of varying percentages. Is interspecific competition stronger or weaker than intraspecific competition?
Is there niche overlap or niche partitioning between mung bean and wheat plants? How can you tell from the de Wit diagram?
Would you recommend planting mung bean and wheat plants together in the same field? Explain your reasoning.
Based on the class results, would you expect the competition coefficients ɑ and β of the Lotka-Volterra model of competition to be equal or unequal to each other? If not, how do the competition coefficients differ?

References

Elton, C.S. 2001. Animal Ecology. University of Chicago Press. p.64.

de Wit, C.T. 1960. On competition. Verslagen Landbouwkundige Onderzoekingen, 66, 1±82.

Jolliffe, P.A. 2000. The replacement series. Journal of Ecology 88: 371-385.

Rodriguez, D.J. 1997. A method to study competition dynamics using de Wit replacement series experiments. Oikos 78(2): 411-415.

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