BIOL 360 - Description of Plant Competition Experiment
Introduction
Competition occurs when one type of organism deprives another of essential
resources, resulting in a decline in survivorship or reproduction of the latter. When both
types of organisms consume significant amounts of essential resources, both may be
negatively affected.
Darwin (1859) believed that the more similar two organisms were the stronger
would be the negative effects of competition. He reasoned that similar organisms would
have the same resource needs and thus would deprive each other of essential
requirements. He said, “The struggle will generally be more severe between species of
the same genus, than between species of distinct genera.” Likewise, one may argue that
competition between members of the same species (intraspecific competition) should be
more severe that competition between different species (interspecific competition).
The class research project will test Darwin’s hypothesis. In greenhouse
experiments, you will study competition between the California poppy (Eschscholzia
californica) and annual ryegrass (Lolium multiflorum). The ryegrass is an introduced
species that is planted throughout the state for ground cover and erosion control. The
poppy is a native plant and the official state flower of California. If Darwin’s idea is
supported in this case, then competition with other poppy plants will have a stronger
negative effect on the growth of a poppy than competition with ryegrass. By choosing to
compare a native species with an introduced species we may also learn something about
conservation and environmental management.
Your will collect and analyze data on plant growth, survival and reproduction.
You will analyze these data and explain your analysis in a worksheet next week.
Initial Procedures
In the first week of the quarter, the biology greenhouse technician filled 4 inch pots with
soil. Nine seedlings were then planted in each pot. Five pots had poppy seedlings only, 5
pots had 3 rye and 6 poppy, 5 pots had 6 rye and 3 poppy, and 5 pots had 9 rye and no
poppy. The following table summarizes the experimental design:
Treatment
group
Number of
poppies per
pot
Number of
rye per pot
1- Poppy
monoculture
9
2
3
6
3
4- Rye
monoculture
0
0
3
6
9
This week, (about eight weeks after planting), the class will record the number of
surviving plants and weigh plant sizes. For annual plants in general, the heavier the plant,
the more seeds it eventually produces. Therefore, weight is a good indication of
reproductive output and we will measure plant weight as an indicator of plant fitness.
Next week, these data will be analyzed using the SPSS statistical software package.
The design of the experiment is a “replacement series.” It is used to determine whether
there is a difference between intra- and interspecific competition for each of the two
species. Notice that the total densities in each plot are constant (same number of seeds
planted), but that the relative proportions of the species change. This design allows us to
ask “If the neighbors surrounding an average poppy plant switch from other poppies to
rye, is that average poppy better off?” If we measure the rates of survival, growth, and
flower production per plant, we can ascertain which is stronger: intra- or inter-specific
competition.
Step 1 – Formulate a hypothesis
In the space below, propose a formal hypothesis for this experiment. If Darwin’s idea is
a universal generality, what relative levels of poppy/rye growth would you expect to
observe in each of the four treatments? If his idea does not apply in this case (null
hypothesis), what would you expect to observe?
Step 2 – Data Collection
The class will divide up into 5 teams. Each team will analyze all the pots in a replicate.
Process each pot as follows:
1. Remove the plants from the pots at soil level. Your instructor will demonstrate
how this is done.
2. Place the plants of one species in an envelop labeled with (1) your team’s
replicate number (2) the treatment group (e.g. “6 poppy/3 rye”) (3) whether the envelop
contains poppies or rye, and (4) whether it was the high density or low density series. BE
VERY CAREFULL THAT YOU CORRECTLY LABEL THE ENVELOPES AND
THAT YOU DO NOT MIX UP THE SAMPLES!
3. Give the completed envelopes to your lab instructor for drying. They will dry
the plant material and weigh each envelope. They will then record the weights for each
pot under the different experimental conditions in a master data sheet. These data will be
Xeroxed and handed out in the computer lab next week for analysis.
Bibliography
Buchanan, G.A., R.H. Crowley, J.E. Street and J.A. McGuire. 1980. Competition of
sicklepod (Cassia obtusifolia) and redroot pigweed (Amaranthus retroflexus)
with cotton (Gossypium hirsutum). Weed Science 28:258-262.
Cook, S.A. 1965. Population regulation of Eschscholzia californica by competition and
edaphic conditions. Journal of Ecology 53:759-769.
D’Antonio, C.M. and B.E. Mahall. 1991. Root profiles and competition between the
invasive, exotic perennial Carpobrotus edulis, and two native shrub species in
California coastal scrub. American Journal of Botany 78:885-894.
Darwin, C. 1859. The Origin of Species. John Murray, London.
Grace, J.B. and D. Tilman (eds.). 1990. Perspectives on Plant Competition. Academic
Press, San Diego.