The Effects of Predation on the Carrying Capacity of a Paramecium Population
Debra Jones
Abstract:
Paramecium tetraurelia are unicellular ciliate protozoa that are found in both fresh and
brackish water. Paramecium produce asexually, and they are known to conjugate and divide
readily. Didinium is a genus of other unicellular ciliate protozoa. The organisms in this genus
prey almost exclusively on Paramecium. The purpose of this experiment was to determine
whether the presence of didinium affected the carrying capacity of P. tetraurelia.
Six samples of paramecium were created. All six contained 1.3 mL of solution that held
about 20 paramecium per mL. Three samples were kept as control, and contained only 18.7 mL
of stock solution. The other three tubes were used to test the effect of Didinium predation on the
Paramecium population. These three tubes contained 1 mL of solution that contained Didinium,
and 17.7 mL of stock solution. Samples from each of the six tubes were fixed and counted two
times each day to monitor the Paramecium population. After the seventh day, the numbers that
were collected were averaged and graphed in order to determine the carrying capacity and the
intrinsic rate of growth.
The data suggested that the populations without Didinium had both a larger carrying
capacity and a larger intrinsic rate of growth. The data also suggested that the populations that
contained the Didinium were likely to go extinct. The data supports the hypothesis that
populations of Paramecium have both a higher carrying capacity and a larger growth rate than
the populations that contain the predator Didinium.
Introduction:
Organisms of the genus Paramecium and Didinium are protists that belong to an intricate
food web that is a key component to many ecosystems (Minter et al., 2011). The Didinium feed
almost exclusively on Paramecium. The interaction between Didinium and Paramecium has
been used as a model for many other predator prey relationships, and has helped scientist observe
predator-prey coexistence (Hewett, 1983).
The objective of this experiment was to determine how predation in a population affects
both the carrying capacity and the intrinsic growth rate. This was done by counting Paramecium
from control populations containing solely Paramecium and from test populations that contain
both a predator- Didinium- and prey- Paramecium. The population that contains the predator
Didinium is likely to have a much lower carrying capacity and intrinsic growth rate than the
population that contains solely Paramecium.
Materials and Methods:
Part I: Paramecium Observation and Intraspecific Speciation:
To become familiar with Paramecium tetraurelia, fixed samples of the protist that were
prepared by UVM staff were observed. There were six different samples that contained P.
tetraurelia that were allowed to grow for 2, 3, 4, 5, 6, and 7 days each. The population size of
each sample was discerned by counting the number of P. tetraurelia under a dissecting
microscope in 20, 100, 200, 20, 10, and 50 µl of solution respectively. This process was done
twice. Next, the population in mL was calculated by dividing the number of Paramecium
counted by the amount of µl that they were in, then multiplying that value first by 1.6, and then
by 1000. Using these calculated values, a growth curve was constructed, and the population
growth parameter was calculated.
Part II: Predation Experiment:
Next, the effects of predators on the population growth of P. tetraurelia was observed.
This was done by first making 3 control solutions of 1.3 mL of Paramecium, and 18.7 mL of
stock culture and 3 test solutions of 1.3 mL paramecium, 1 mL of the predator Didinium, and17.7
mL of the stock culture. The Paramecium population in the six solutions was allowed to grow
for one week. During the week, the number of Paramecium was counted beginning on day 2 and
continuing until day 7.
Each day, a 300 µl sample was taken from each of the six solutions and fixed using 180
µl of shocking solution. From these newly fixed samples, the number of Paramecium was
counted twice for each sample. These numbers were again used to calculate the number of
paramecium per mL. The averages for each sample were calculated. Then using those numbers,
the average population for both the control and the predators were calculated for each day.
Part III: Population Curves:
Next, the average population numbers for the control and the test group were used in
order to set up a growth curve for both the control and the test populations. From the averages,
another curve was plotted by taking the natural log of those averages. This was the intrinsic rate
of increase for the population.
Results:
The first three days of the sample culture showed very slow increase in population. After
day four, there was fast, significant increase in population (Graph 1). The population seemed to
peak during day six at approximately 13,600 Paramecium per mL. This was the population’s
carrying capacity (K). The population then rapidly decreased during day seven (Graph 1, Table
1). The natural log of the average population of Paramecium was taken in order to determine the
intrinsic rate of growth (r). This value was found to be 0.7619 Paramecium per day (Graph 2).
A similar pattern was noticed during the predation experiment. Both the control groups
and the test groups showed a slow initial population increase. The first three days showed similar
population numbers. However, on the fourth day, the control population showed a rapid increase
in Paramecium, followed by a peak at K equal to 16653 Paramecium per mL at day five, and a
rapid decrease in population size after the initial peak. The tested population showed a similar,
although somewhat less extreme, pattern. The first two days showed a slow increase in
population, followed by a large population jump, a peak at 8,027 Paramecium per mL during day
four, and a quick decline in Paramecium numbers (Graph 3). Again, the natural log of the
averages of each population was taken, and the intrinsic rate of growth, r, was determined to be
0.7237 Paramecium per day for the control population, and 0.1083 Paramecium per day for the
population that contained the Didinium (Graph 4).
After the carrying capacities were reached, there was a rapid decline in population of
Paramecium. In the control population, this decline is followed by another jump in population.
This pattern is not shown in the test population. Instead of a population increase after the initial
drop, the number of Paramecium in the population continued to decline.
Discussion:
The results collected support the hypothesis that Paramecium population that had the
predator Didinium in it would have a lower carrying capacity than the control population of
solely Paramecium. The average carrying capacity for sample cultures was13,600 Paramecium
per mL (Graph 1). The carrying capacity for the control populations of Paramecium was
similarly high, at K=16,653 individuals per mL. For the sample culture, the population hit its
carrying capacity on the sixth day, while for the control population, the carrying capacity was hit
slightly sooner, at day five (Graph 3).
The tested Paramecium population that contained the Didinium had a much lower
carrying capacity than the control and the sample cultures. The carrying capacity for the tested
population was determined to be 8,027 (Graph 3). This carrying capacity was over 5,000
Paramecium per mL shorter than the sample cultures, and over 8,000 Paramecium per mL lower
than the control population. The carrying capacity for the test population was also reached
sooner than the other two populations. K was reached at four days; that was two days sooner than
the sample population, and one day sooner than the control population (Graphs 1 and 3).
The intrinsic rate of growth for the averages of each population was found. The control
and the sample cultures had similar r values at 0.7237 Paramecium per day (Graph 4), and
0.7619 Paramecium per day respectively (Graph 2). The intrinsic rate of growth for the test
population was much lower, at a value of 0.1083 Paramecium per day (Graph 4). These values
show that the control and the sample populations were increasing at a much higher rate than the
test population was.
The growth curve patterns and the intrinsic rates of growth for both the test and the
control populations suggest that a population without Didinium both expands at a faster rate and
has the capacity to support more Paramecium than one that contains the predator. The control
Paramecium population also seemed to increase again after its initial drop, while the population
that contained the predators continued to decline. This pattern is similar to that of Paramecium
populations in another study that were subjected to resource enrichment, dispersal, and
predation. Due to predation, the Paramecium population quickly became extinct (Cadotte et
al.,2006).
One possible source of error that could have occurred during testing was accidently
including the Didinium in the Paramecium count. This would cause the results to be higher than
what they are supposed to be. However, the differences between the test and the control
population were so large that any small counting error would be inconsequential.
This experiment could be furthered by also recording the Didinium population. In future
experiments, the carrying capacity and the intrinsic rate of growth for both the Paramecium and
the Didinium could be recorded and compared in order to view both sides of the predator-prey
relationship.
Work Cited:
1. Cadotte M, Fortner A, Fukami T. The Effects of Resource Enrichment, Dispersal, and
Predation on Local and Metacommunity Structure. Oecologia [Internet]. 2006 Aug [cited
2013 April 27]; 149(1):150-157. Available from: http://www.jstor.org/stable/20445981
2. Hewett S. Predation by Didinium Nasutum: Effects of Predator and Prey Size. Ecology
[Internet]. 1988 Feb [cited 2013 April 26]; 69(1): 135-145. Available from:
http://www.jstor.org/stable/1943168
3. Minter E, Fenton A, Cooper J, Montagnes D. Microbiology of Aquatic Systems. PreyDependent Mortality Rate: A Critical Paramater in Microbial Models. Microbial Ecology
[Internet]. 2011 Feb [cited 2013 April 27]; 62(1): 155-161. Available from:
http://link.springer.com/article/10.1007%2Fs00248-011-9836-5
Count 1
2
720
640
3
1488
1744
4
184
144
5
3200
3040
6
12960
14240
7
1984
3360
Table 1 The population of
Paramecium per mL of the sample
cultures.
Individuals per mL
10000
Series1
0
2
4
6
8
680
1616
328
3120
13600
2672
6.522092798
7.387709239
5.793013608
8.045588281
9.517825072
7.890582535
Ln of Avg. Sample Cultures
15000
5000
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Ln of Avg
Table 2 The average number of
Paramecium per mL of solution, and the
natural log of those averages.
Growth Curve for Sample
Cultures
0
Avg
Individuals/mL
Count 2
Ln of Avg Individuals per mL
Day
10
8
6
y = 0.7619x + 3.8312
4
Series1
2
Linear (Series1)
0
0
2
Days
4
6
8
Days
Graph 1 The growth curve of the average
population of the sample cultures of
Paramecium per mL for each day.
Graph 2 The natural log of the average
population of the sample cultures.
Control
With Didinium
Day Tube A
Tube B
Tube C
Tube D
Tube E
Tube F
Count
Count
Count
Count
Count
Count
Count
Count
Count
Count
Count
1
2
Count 1 2
1
2
1
2
1
2
1
2
2
3200
2560
3600
3360
1840
1520
2720
480
1360
2720
2880
800
3
4320
6720
6160
4640
3440
3520
4800
5440
8640
6480
7280
6400
4
12640
12960
11040
12640
13120
12640
6560
6240
8160
7040
11840
8320
5
13680
13440
19680
14560
28640
9920
5760
3280
3040
5440
6080
3440
6
5360
3840
4960
3120
3360
3760
160
160
0
0
1920
1840
7
6080
3200
5760
15040
10240
11200
80
320
160
80
80
0
Table 3 Number of Paramecium per
mL of six samples.
Control
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Ln Avg Treatment Control
With Predators
2680
4800
12507
16653
4067
8587
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
1827
6507
8027
4507
680
120
Table 4 Average populations of
Paramecium for the three control
groups and the three test groups.
With Predators
7.893572
8.476371
9.434044
9.720346
8.310661
9.058005
Table 5 The natural log of the
averages for the control and the test
populations.
Paramecium Population
Paramecium per mL
20000
15000
10000
Control
5000
With Predators
0
0
2
4
6
8
Days
Graph 3 Population growth curves for the control Paramecium
populations, and the test Paramecium populations.
Ln( Population Number)
Ln (Paramecium Population)
12
10
Series1
8
y = 0.7237x + 5.0891
Series2
6
4
y = 0.1083x + 6.4241
Linear
(Series1)
2
0
0
7.510430556
8.780633799
8.990566138
8.413387023
6.522092798
4.787491743
2
4
Days
6
8
Linear
(Series2)
Graph 4 The natural log of the average values of both the control and
test populations of Paramecium.