Lab 2 Plant Competition

Email me

Back to:

Introduction:

One of the ways in which organisms may interact is competition. In this lab, we will investigate the interaction between seedling plants. We use plants for practical reasons. The primary advantage is that the growth of plants is very plastic. This means that the size a plant reaches is largely determined by environmental factors within the general framework set by its genes (peas will not grow into trees, no matter how advantageous the environment). Factors that are possibly important include such things as nutrients, light, soil characteristics (other than nutrients) moisture, and the density of other plants (both plants of the same species and other species). The focus of this lab will be on neighboring plant density as a factor affecting plant growth.

We will grow plants in two ways in order to detect the effects of competition with other plants. We will grow an herb, sweet basil, by itself but at different densities (number of plants per pot) and measure the size of the plants that result. Since competition affects individuals, we will concentrate on the effect of competition on the average size of basil plants. We would have to do other experiments to understand the effect intraspecific competition has on basil populations. Second, we will look at the relative strength of interspecific versus intraspecific competition by growing two different species of plants, zinnia and marigold, in pots with different densities of plants per pot. We will also vary the proportion of each species in the pots, as some will be mostly zinnia and some mostly marigold. Once again, WE WILL BE INTERESTED IN THE INDIVIDUAL PLANT'S RESPONSE, MEASURED AS THE AVERAGE SIZE OF PLANTS IN A POT (FOR EACH SPECIES)

Intraspecfic Competition:

Questions of interest:

Other things being equal, does the presence of other plants from the same species affect the growth of individual plants in a predictable fashion?

Hypothesis:

You will have to fill this in with a reasonable guess based on what you know about plants and the procedures described below. BE SURE THAT YOU STATE A HYPOTHESIS AS PART OF YOUR LAB WRITE-UP. I will need to read it in order to be sure you understand what the lab is all about. This step should be done WITH CARE.

 

Starting Procedure:

1. Obtain a set of six pots. Put an inch or two of tape on each pot as a label and label each pot with the number and type of seeds to be planted in each plus something to identify them as your pots. Use pencil as the pots will be sprayed regularly with water and left in sunlight. Ink can fade and wash away but the pencil marks should persist.
2. Fill each pot with soil until the soil comes to within 1 cm of the top. DO NOT TAMP THE SOIL DOWN, AS YOU CAN PREVENT THE SEEDS FROM SUCCESSFULLY GERMINATING. To settle the soil, you can tap the bottom of the pot on the table a couple of times.
3. Plant either 2, 3, 5, 10, 18, or 34 basil seeds in each pot. After germination, the plant populations pots will be thinned to 1, 2, 4, 8, 16, 32, or 64 plants per pot. The extra seeds are to ensure we have enough plants in each pot. Do your best to space them evenly over the top of the soil.
4. Top the seeds with more soil. Completely fill each pot with soil by gently dropping soil from your hand. (use a straightedge or piece of paper to level the top of the soil with the top of the pot). Once again, no tamping (tapping is OK).
5. Return the pots to the flats. The plants will be watered and tended for the next several weeks.

Interspecific Competition:

Questions of interest:

Other things being equal, does the presence of other plants from other species affect the growth of individual plants differently than the presence of plants from the same species and does this difference occur in a predictable fashion?

Hypothesis:

You will have to fill this in with a reasonable guess based on what you know about plants and the procedures described below. BE SURE THAT YOU STATE A HYPOTHESIS AS PART OF YOUR LAB WRITE-UP. I will need to read it in order to be sure you understand what the lab is all about. This step should be done WITH CARE.

Starting Procedure:

1. Obtain a set of 4 pots. Put an inch or two of tape on each pot as a label and label each pot with the number and type of seeds to be planted in each plus something to identify them as your pots.
2. Fill each pot with soil until the soil comes to within 1 cm of the top. DO NOT TAMP THE SOIL DOWN, AS YOU CAN PREVENT THE SEEDS FROM SUCCESSFULLY GERMINATING. To settle the soil, you can tap the bottom of the pot on the table a couple of times.
3. We will use marigold and zinnia for this portion of the lab. Plant the seeds in the densities of:
   • 4 M : 4 Z
   • 4 M : 32 Z
   • 32 M : 4 Z
   • 32 M : 32 Z.

   Do your best to space them evenly on the soil. Notice that each species experiences high and low amounts of interspecific and interspecific competition

4. Top the seeds with more soil. Completely fill each pot with soil by gently dropping soil from your hand. (use a straightedge or piece of paper to level the top of the soil with the top of the pot). Once again, no tamping (tapping is OK).
5. Return the pots to the flats. The plants will be watered and tended for the next several weeks.

Gathering the Data:

In gathering the data from the intraspecific competition experiment, you gather detailed information from each pot (number of plants, weights of plants, number of leaves, length of stem). In the interspecific experiment, all you do is count the number of plants of each species and then take the total weight of all plants in each pot.

You must weigh the plants quickly, as they will lose weight, due to water loss, as soon as you remove them from the soil. They can lose so much water that you can completely change the outcome of the work. So, work as a team. To gather the interspecific data, each team member should clip the plants from a pot and immediately weigh the plants. In the interspecific experiment, one person should clip the plants from a pot as another tallies the species each belongs to and separates the plants into piles of different species and then immediately weighs the plants from that pot as soon as they are done clipping and tallying (the tendency in previous labs was to clip all pots, separate the plants and then weigh them all at once, but this takes too much time.

Intraspecific Competition:

1. Count and record the number of plants in a pot and the number of leaves on each plant in the pot. Record the number of plants that produced a flower bud (if any have done so in the time we have for this exercise).
2. Cut off all shoots at ground level.
3. Weigh all of the plants from a pot together. Calculate the average weight by dividing this total by the actual number of plants in the pot (not the number of seeds you planted).
4. Remove the buds and leaves from each plant stem (by pot once again!), combine them on one weighing pan, and weight them together to obtain the total leaf, bud, and stem weight in each pot. Don't worry if there are no buds. Notice that you don't have to weigh the stems, as you can get their weight by subtracting the leaf and bud total (or just the leaf total if there are no buds) from the total plant weight for the pot.
5. Measure the length of each stem.
6. Return the pots to the flats.
7. Designate a member of the group who will enter the data onto a spreadsheet and email the results to the instructor by noon of the following day.

Interspecific Competition:

1. Cut off all shoots from a single pot at ground level and hand them to another student who will divide them into separate species. Record the number of each that germinated.
2. We will again ignore the below-ground portion of the plant, as we have no reliable way to separate the roots by species and it is impossible to separate complete plants without tearing apart the root ball.
3. Weigh all of the plants from each species and divide by the total number of plants of that species in that pot to get a mean plant weight for each species in each pot.
4. Designate a member of the group who will enter the data onto a spreadsheet and email the results to the instructor by noon of the following day.

Data Analysis: You must do the sections beginning with "To get an A ...," or your maximum score will be B+

Intraspecific Competition: Please answer the following questions with graphs and tables constructed from both your data and the data from the rest of the class.

1. You can look at the effects of intraspecific competition in several ways. The graphs, charts and calculations suggested below are intended to do just that. first you should plot the mean plant size versus density of plants in the pot and do a second plot of total plant biomass in the pot verss plant density in the pot:
   • How does competition affect the mean weight of plants in a pot?
   • What is the relationship between total biomass and density? Compare this to the mean weights. Does total weight tell you anything about intraspecific competition?
2. Plot the average stem length versus number of plants in the pot(include standard deviation error bars).
   • What does this plot tell you about intraspecific compeition?
3. Do plants change the proportion of biomass allocated to leaf, shoot and root as density increases? Since the number of plants in a pot was not strictly controlled, we can not make a sensible bar chart. Use a scatter plot of density versus the three proportions (stem, leaf, and bud) and examine the graph to draw your conclusions.
   • How does density affect the proportional allocation of biomass to the various parts of the above-ground portion of the plant?

• To get and A, you must do the following analysis. A long time ago, Kira et al. (1953) proposed that the relationship between mean plant weight and density could be described as:

w = K p^-a

where w = mean plant weight and p = density. K and a are constants used to fit this relationship to different plants and must be estimated from the data. We can do this, but must first linearize the relationship by taking the log of both sides:

log(w) = log(K) - a log(p)

This equation is in the form of y = mx + b, the standard linear equation (the slope is negative here), If you plot this, with your y values = log(w) and your x values = log(p), the points should form a line. You will, of course, not get a perfect line, but you can estimate the real relationship by estimating the line. Your spreadsheet program will do this estimation for you if you ask for a linear trend line (be sure to get the equation for the line). Use the equation to get your estimates of a and K. Use a and K to get estimated mean plant sizes for your densities. Add the expected data to the graph of mean plant wt. versus density as a second variable (you may want to add more densities to get a smooth line). How does the expected line agree with the observed data?

Interspecific Competition:

There are many possible ways to analyze the data from this portion of the lab. You want to see how the average size of a plant changes as its neighbors are more and more from another species. One of the most visually interpretable methods was devised by De Wit (De Wit, 1961; Harper, 1967). The diagrams are called De Wit replacement plots and are easy to make. Make a separate graph for each species. Plot the average plant weight for each density (total density here is approximately 10, 55, and 100 plants). There should be two points at the intermediate density, one for the 5:50 pot and one for the 50:5 pot. Notice that this assumes that all of the seeds have germinated, an unlikely occurrence. Your plot will likely have different densities and, on the X axis, you should plot the total number of plants you actually measured, not the number of seeds planted. In the example below, dotted lines indicate the interspecific effect and solid lines the intraspecific effect (THIS IS NOT SO EASY TO SEE SO BE SURE THAT YOU UNDERSTAND WHY THIS IS SO). Look hard at the graph below and read the interpretation of in underneath.

The solid line from the dot at 20 plants per pot to the dot labeled 100M:10Z represents the change in marigold size when the number of marigold seeds go from 10 to 100 while the number of clover seeds remains constant at 10. So, the solid line is one estimate of the effect of interspecific competition on marigold. The dotted line above it is just the opposite. Here, it is the marigold plants that are constant at 10 in each and the zinnia goes from 10 to 100 plants in the pot. So the dotted line represents the interspecific effect of zinnia on marigold seedling weight. In this diagram, the average weight of Marigold plants is reduced more by being in a pot with 110 plants if 100 of them are marigolds than if 100 of them are zinnias. I can see this because the slope of the intraspecific (solid) line is more negative than the slope of interspecific (dotted) line. Remember what slope means. The second set of lines (from 100:10 to 200) will give you the same conclusion. Plot your data and draw conclusions from what happened in your pots (this is hypothetical here).

What you must do, from the two graphs from your data, is to draw a conclusion about which has a greater effect interspecific competition or intraspecific competition? Of course, your conclusion must be reasonably argued FROM THE DATA.

Thinking about the experiment:

1. What is the most likely mechanism by which one plant affects its neighbors in the intraspecific experiment? Mechanism here refers to the actual way in which plants affect one another's growth. There are many possibilities and explain why you made your choice.
2. Is the mechanism of interaction likely to be the same in both the intra- and interspecific experiments?
3. Design an experiment to test your idea from Question #1. Describe it in enough detail to show that you have drawn on what you have learned in other classes in order to develop this experiment.
4. Is the mechanism you have chosen an instance of exploitation competition or one of interference competition? Whichever you choose, describe a possible mechanism of interaction that belongs to the opposite type of competition.

Classic References:

Black, J. N. 1960. An assessment of the role of planting density in competition between the red clover (Trifolium pradense L.) and lucerne (Medicago sativa L.) in the early vegetative stage. Oikos 11:26-42.

Clements, F. E., J. E. Weaver, and H. C. Hansen. 1929. Plant Competition. Carnigie Institute, Washington, D. C.

de Wit, C. T. 1961. Space relationship within populations of one or more species. Society of Experimental Biology Symposium 15:314-329.

Harper, J. L. 1967. A Darwinian approach to plant ecology. Journal of Ecology 55:247-270.

Kira, T., H. Ogawa, and N. Sakazaki. 1953. Intraspecific competition among higher plants. I. Competition-yield-density interrelationship in regularly dispersed populations. Journal of the Polytechnic Institute of Osaka City University D 4:1-16.

Link to dataset if a greenhouse disaster occurs:

Plant Competition Lab Dataset

Materials (startup week in bold):

Sweet Basil, Zinnia and Marigold Seeds, Pots (11 x #of groups), Straight edges, Soil, Paper Tape, Scissors or razors, Balances, Rulers, Buckets, Soil Storage Container

Last updated January 21, 2003