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The effect of competition for space on Lemna minor population growth
by
Michelle Aucoin
201101357
Submitted to: Mr. John Bueglas
Biology 203: Introductory Ecology
Lab Section: Monday
Department of Biology
St. Francis Xavier University
12 November 2012
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Abstract
Plants require space as a resource for growth, reproduction, and survival. The purpose of
this experiment was to examine the population growth of a small aquatic plant, Lemna minor,
and to determine how large the effect of a reduced space was on the growth of this plant. Two
groups of Duckweed were compared with five replicates of each set. There was a control group
and an experimental group, which held 60% of the space of the control.
My research group and I found that the mean number of thalli in the control group, after
two weeks, was not significantly greater than the mean of the treatment group. However, the
growth rate of Duckweed in a smaller space, 0.106, was significantly different than the growth
rate in a larger space, 0.138. Population growth rates were affected by the consumption of space,
a limiting resource. Duckweed populations did not reach the carrying capacity in the duration of
the experiment. Further research should begin with a higher initial population size of thalli and
test several habitat sizes.
Introduction
Lemna minor, small duckweed, is an aquatic plant of the waters of North America and
Eurasia. This floating plant is simple in character, small in size, and rapid in growth, making it a
popular subject of research in Biology. Duckweed is found on the surface of nutrient rich water
(Frédéric et al. 2006). Lemna minor reproduces asexually where each plant, or thallus, contains a
small leaf with one rootlet hanging into the water.
Growing in optimal conditions, Lemna displays rapid population growth. Growth is
controlled by many factors, including nutrients, light, temperature, and space (Driever, van Nes,
and Roijackers 2004). Therefore, conditions of the habitat in which plants live can impose strong
competition among the plants (Lech et al., 2010). In this experiment, I and my colleagues will
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examine the effect of one environmental variable, space, on Duckweed. We will study the rate at
which Duckweed grows in a reduced space, competing for area in a medium. The population
growth will be rapid, until space becomes a limiting factor, at which point the growth will
decrease. Therefore, populations growing in different space measurements will have different
growth rates.
Materials and Methods
My colleagues and I grew Lemna minor for two weeks under 430-W light that mimics
natural sunlight’s spectrum, and room temperature of approximately 24°C. Small Duckweed was
collected from Gasperaux pond in Antigonish County, Nova Scotia, Canada. We placed 60 thalli
of Lemna minor in each of ten 100-ml beakers. As a control, Duckweed grew in the area of five
beakers while the other five contained a small tube, open at both ends, inside the beaker that held
60% of what the beaker, or control, carried. We placed 90-ml of growth culture medium in all
beakers. Growth medium for the Duckweed was dechlorinated, filtered Antigonish tapwater
(Table 1). This nutrient rich growth medium was used to grow Duckweed in optimal conditions
(Taylor 2012). For the five experimental beakers, thalli were contained inside the small tube.
Each day my colleagues and I counted the number of thalli present in the beakers.
Evaporation, by growing under the light, of the medium occurred and therefore distilled water
was added each day to prevent major loss of water and nutrients.
By the eleventh day, the experimental thalli enclosed in the reduced space became very
difficult to count as the thalli were growing on top of one another. Thalli were growing up the
sides of the tube above the surface of the medium. My colleagues and I counted only the
duckweed at the surface and above.
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Table 1. Composition of Lemna minor growth medium. The ingredients listed are
added to the dechlorinated, filtered Antigonish tapwater (Taylor 2011).
Chemical Name
Formula
Concentration (mg/L)
Potassium Nitrate
KNO3
350
Calcium Nitrate
Ca(NO3)2 4H2O
295
Potassium Phosphate
KH2PO4
100
Magnesium Sulfate
MgSO4 7H2O
100
Calcium Carbonate
CaCO3
Ferric Chloride
FeCl3 6H2O
0.76
Zinc Sulfate
ZnSO4 7H2O
0.18
Manganous Chloride
MnCl2 4H2O
0.18
Boric Acid
H3BO3
0.12
Ammonium Molybdate
(NH4)6Mo7O24 H2O
0.04
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With the information collected, my colleagues and I analyzed the data of the growth of
Lemna minor for fourteen days. We used Microsoft Excel to produce a scatterplot of the data and
linear equations of both conditions. Also, we ran a two-sample t-test assuming equal variances to
test for a difference between the means. Subsequently, we produced a graph of Ln-transformed
data to receive growth rates and a graph of lambda-transformed data to receive carrying capacity.
We then ran regression analysis.
Results
In this experiment, thalli of Lemna minor grew rapidly and after two weeks it was
reported that in a reduced space the number of thalli will reach its highest amount and remain
constant before that of a larger space (Figure 1). The mean number of thalli in the larger space
treatment after fourteen days was not significantly greater than in the reduced space treatment
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(t=0.636, P>0.05).
Regression indicates that the mean growth rate (r) in the larger space treatment was
0.138. The mean growth rate (r) in the reduced space treatment was 0.106. The mean growth rate
(r) in the larger space treatment was significantly different (t=5.06, P<0.05) than the rate in the
reduced space treatment.
500
450
400
Number of Thalli
350
300
250
200
150
100
50
0
0
2
4
6
8
10
12
14
16
Time (Days)
Control
Treatment
Figure 1. Growth curve of Lemna minor under 430-W grow-lights at 24°C. Increase in number
of thalli from initial population of 60 (mean ± standard deviation of 5 replicates) for the Control
population and the Treatment population which grew in a space 60% of the control.
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The combined carrying capacity from the line of best fit of the geometric growth between
days of the control and reduced space was 472 (Figure 2).
1.4
1.2
Lambda
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y = -0.0005x + 1.236
R² = 0.2585
0.8
0.6
0.4
0.2
0
0
50
100
150
200
250
300
350
400
Population (Nt)
Figure 2. Daily geometric growth of Lemna minor with line of best fit shown. Carrying capacity,
K, for the population is 472 and rmax is 0.212. The p-value is 0.006.
Discussion
My research group and I showed that there is a significant difference in the growth rate of
Lemna minor based on the space provided for the plant to grow. The daily means of the two
populations in different space confinements grew at a very similar rate for the first nine days.
The means grew further apart with time after day nine, which is when space became a limited
resource for the reduced space population. Like any plant, Duckweed requires space for growth
(Cain, Bowman, and Hacker 2011). When space becomes a limiting resource, competition can be
intense. Therefore, population growth rates decrease when all space is filled by an organism
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(Cain, Bowman, and Hacker 2011). Population growth rates are affected by a consumption of
resources and competition between organisms more and more as they approach carrying
capacity.
Populations did not reach the carrying capacity. The combined population will continue to
increase until it reaches carrying capacity and then stabilize beyond the end of the experiment. It
is likely that the population of the control group and the population of the group with reduced
space would meet carrying capacity at different times. In order to reach the carrying capacity in
this time period and under these conditions, the initial population size should be increased.
The experiment presented accurate results compared to what other researchers have found
about competition of space among organisms. However, there still could have been error in the
methods. When Duckweed started to grow in the hundreds, thalli were very difficult to count.
One person was required to count each beaker containing Duckweed one time each day, but if
two or more researchers counted each day and then we took the average of both, this would be
more exact, reliable data. Nonetheless, as expected, reduced space does have an impact on
growth and reproduction. As a result, the hypothesis was proven that the growth rates of
Duckweed are significantly different. Further studies of research on the growth rate of Lemna
minor, including the environmental variable space, could use multiple different sizes of habitat
for the Duckweed to grow and compete within, instead of simply two different areas.
Literature Cited
Cain, M.L., Bowman, W.D., and Hacker, S.D. 2011. Ecology. Second edition. Sinauer Assoc.,
Sunderland, Mass., U.S.A. 648 p.
Driever, S.M., van Nes, E.H., and Roijackers, R.M.M. 2005. Growth limitation of Lemna minor
due to high plant density. Aquatic Botany. 81: 245-251.
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Frédéric, M., Samir, L., Louise, M., and Abdelkrim, A. 2006. Comprehensive modeling of mat
density effect on duckweed (Lemna minor) growth under controlled eutrophication. Water
Research. 40: 2901-2910.
Lech, K., Malgorzata, S., Konieczna, A., and Izdebska, K. 2010. The effect of Stratiotes aloides
L. and nutrients on the growth rate of Lemna minor L. Aquatic Botany. 92: 168-172.
Taylor, B.R. 2011. Introductory ecology: Laboratory manual 2012. St. Francis Xavier
University, Antigonish, NS, Canada.