Bio 236: Plant Biology

Lab Nine - Competition



Introduction

Competition between organisms is an interaction brought about by a shared requirement for a resource in limited supply. Darwin pointed out in The Origin of Species that, for any species you might pick, many more offspring are produced than can possibly survive. Consider the number of acorns produced by an oak tree during its lifetime versus the number of those acorns that actually make it into adulthood. Under such circumstances, Darwin argued that competition for limiting resources is inevitable. Since Darwin's time, ecologists have considered competition one of the most important factors determining the distribution and abundance of organisms.

The presence and magnitude of competition can be measured by the effect of competition on the survivorship, growth, and/or reproduction of the organism in question. Competition can be either intraspecific (among individuals of a single species) or interspecific (among individuals of different species).

Photosynthetic terrestrial plants require light, water, and nutrients for their survival and reproduction; therefore competition for any and all of these resources could occur. Competition among terrestrial plants can occur both above ground (for light) and below ground (for nutrients and water).

Purpose

To assess the effects of competition on the growth rate of both vascular and nonvascular plants. Competition is an interaction in which both interacting species are harmed.


GETTING STARTED:

During Lab 6 you will set up your competition experiment. You will collect your results in Lab 8 and report your findings during Lab 9.

The important variables for the experiment are the number of plants per pot and the ratio of plants of different species. We will use two agricultural species in the experiment: peas (Pisum sativum) and maize (Zea mays L.).

The lab will be divided into groups, and each group should plant ONE POT for each of the 11 treatments shown below:

Pea Monocultures


Maize Monocultures


Two Species Pots


Procedure

  1. Get 11 pots and some seed for planting.
  2. Now label each of your 11 pots; include on EACH LABEL the number of plants of each species, and a group name. These labels represent the TARGET for the number of plants the pots will contain.
  3. Unfortunately germination of seeds is never 100%. To overcome this problem we will OVERPLANT, and next week you will remove any extra seedlings that appear. FOR EACH POT, PLANT AT LEAST 2 MORE SEEDS OF EACH TYPE THAN THE TARGET NUMBER.
  4. Make sure that the seeds are spread evenly in the pot; in the two-species pots the seeds of the two types should be interspersed.
  5. These plants will be taken to the greenhouse, where they will be watered regularly. They'll be brought to lab next week so that plant numbers can be reduced to the target. The experiment will be completed during the eighth laboratory. The writeup is due March 5, 17:00 on Fabio.

Interpreting Replacement Series Experiments

In replacement series experiments, interspecific competition is detected by comparing mixed-species pots with control pots containing only one species. For example, consider mixed-species pot J in the competition experiment. Pot J contains eight plants (as do all the mixed-species pots), four maize plants and four peas. To see if the presence of beans has had any effect on the maize, you could compare the weight of the four maize plants in pot J to the four maize plants in pot F, which are growing without competitors. Similarly, the effect of interspecific competition on mung bean could be measured by comparing the weight of the four peas in pot J to the four of them in pot B.

It should be obvious from the experimental design that many such comparisons could be made. To be able to see the results of many comparisons at once, botanists summarize the results of replacement series experiments in replacement diagrams. An example of a replacement diagram is shown in Figure 1; this diagram is based on an actual competition experiment between two species of oats (Avena fatua and A. barbata). The experiment was similar to the one you performed, except that the measure of yield was flower production rather than biomass.

To interpret the results shown in Figure 1, it's useful to key in on one species at a time. Look first at species "B", shown as filled-in dots in Figure 1. There are two curves shown for B; the solid line represents the results without interspecific competition, and the dashed line represents the results with interspecific competition. The number of B plants per pot is shown in the bottom row of numbers below the X axis, and is the same for the solid and dashed lines; but for the dashed line, the B plants were grown with the number of A plants shown in the top row below the X axis. The difference between the two curves measures the effect of competition from the presence of the extra plants in the pot. For example, consider the case where there were 64 B plants per pot. In the absence of A plants, these pots produced an average of about 510 flowers per pot; but when 64 A plants were also present, interspecific competition reduced the yield of B plants to only about 190 flowers per pot. If you compare the solid and dashed lines for species B overall, it is clear that the dashed line is consistently well below the solid line; this shows that interspecific competition caused a large reduction in yield of species B.

It is possible to determine the effects of interspecific competition on species A by comparing the dashed and solid curves connecting the open dots. Here you can see that there is some reduction in yield caused by interspecific competition, but the effect is not as great as in species B (i.e. the dashed line is not so far below the solid line).

When all of the data from your lab are averaged together, you will be able to make two graphs like the one in Figure 1. The measure of yield will be biomass rather than flower number. Your graphs will have the added sophistication of SE bars around each mean, making their interpretation easier. Note that for the interspecific competition treatments (dashed lines) in Figure 1, there are always 128 plants per pot. Only the ratio of the species changes as you move along the X axis. This is exactly the way our experiment was set up; for a particular density, every interspecific competition pot in the lab was set up with the same total number of plants.

Data Collection & Analysis For Competition Experiment

Find the pots that you planted the first week of lab. Before you do anything to the plants, note carefully any differences among plants at different densities. Does it look like one of the two species is "winning" in interspecific competition? Are plants from different densities different in height, number of leaves, or size and shape of leaves? Look for tillering (the production of side shoots at the base of the stem). Does rate of tillering vary with competition?

The overall shape of a plant is sometimes called its architecture. Sketch a typical bean shoot and maize shoot in your notebook. Consider how the different architecture of these plants might affect competition for light. How do the species differ in the size of their leaves and in the way the leaves are arrayed in space? Where would most of the light be intercepted for each species?

The root systems of plants also have architecture. To examine roots, use pot A (2 peas) and pot E (2 maize). You will be weighing the shoots of these plants, so MAKE SURE TO KEEP TRACK OF WHAT PLANTS ARE FROM WHICH POTS AND DON'T THROW ANYTHING AWAY. For each of these pots, wash the soil from the roots so that you can clearly see the root system. Sketch the root system of each species in your notebook. Do you think that the roots were dense enough for competition for nutrients or water? Do the species have comparable overall root density? Are the roots at the same density at every soil depth?

The data you will use to measure the outcome of competition is simply each pot's aboveground dry weight. For each pot, carefully clip all of the seedlings AT THE SOIL SURFACE. For the three pots you used to look at roots, cut the shoots at a similar location. For the mixed-species pots, sort the seedlings into a pile for each species. Put the samples in aluminium weigh boats, labelled on top and bottom. Put into the oven. Dry overnight or until the samples no longer lose weight. Weigh each sample separately, and record for each pot the treatment and the weight (in grams, with 2 places after the decimal).

Pool the data for the experiment for the entire lab. For each treatment, calculate the mean and the standard error (see the Appendix). Then make a replacement series diagram; be sure to include standard error bars for comparisons among means. Note that each two-species pot will give two points on the diagram, one for each of the species.


Questions

  1. Examine the replacement series diagrams from your lab section. Is there any evidence of interspecific competition? Were both species equally affected? How does this interpretation fit with your qualitative observations of the results? Did the outcome of competition vary with density?
  2. Is there anything about the architecture of either the shoots or roots of these two species that would be a possible explanation for the outcome of the competition experiment?
  3. The competitive effects that you observed could be due to competition for resources between shoots, roots, or both. Design an experiment that would allow you to separate aboveground from belowground competition.
  4. It is also possible to use replacement series diagrams to detect intraspecific competition. This is done by looking at the treatments with only a single species (the solid lines in Figure 1 show considerable intraspecific effects. Were the species we used affected by intraspecific competition?
  5. There is a clearer way to represent the effects of intraspecific competition discussed in the previous question. It is possible to use the data from the replacement series diagram to plot AVERAGE plant weight (y-axis) as a function of plant density (x-axis). If there were no effect of intraspecific competition, plants from all treatments should have the same weight on average. To make this graph, use only the data from the monocultures. For each plant density, divide the total weight per pot by the number of plants per pot to give the average plant weight. You will need SE bars on the graph, but (happily!) you will not need to go back to the raw data to recalculate SE's. Quite simply, the SE of the average plant weight is the SE of the total weight divided by the number of plants. For example, if the pooled class data for treatment G (6 maize plants per pot) gave a mean + SE of 2.35 + 0.24 g of maize per pot, the data on a per plant basis would be 0.39 + 0.04 g per plant. In your lab notebook, plot the data in this way for both pea and maize. Is the plant weight affected by plant density?
  6. If plants from all treatments had the same average weight, could you conclude that competition did not occur? Why or why not?

    Figure 1. An example of a replacement series diagram, based on competition between two species of oats: Avena fatua (species A) and Avena barbata (species B). Open dots show yield of species A and filled-in dots show yield of species B. Yield data from single species pots (i.e. without interspecific competition) are connected by solid lines and from two-species pots (i.e. with interspecific competition) are connected by dashed lines. The downward displacement of the dashed lines for each species is a measure of the effect of interspecific competition.

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