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Population Regulation in an Annual and a Perennial Grass

 

Plastic Responses, Birth Rates and Death Rates

 

by Serge Fortin

 

Biology 218

for Dr. David J. Blundon

Camosun College

Section W94001

April 14, 1994

Lansdowne Campus

 

2.0 Table of Contents

 

2.1 Introduction

2.2 Materials and Methods

2.3 Results

2.4 Discussion

2.5 Acknowledgements

2.6 Works Cited

 

2.1 Introduction

 

Any populations, which show the operation of clear limits on population size, are said to be regulated (Silvertown 1982; p. 110). Population regulation in plants, which are modular organisms, must be discussed as the regulation of biomass rather than numbers (Krebs 1994; p.340). "As a plant population increases in numbers and biomass, either reproduction or survival will be reduced by a shortage of nutrients, water or light; by herbivore damage; by parasites and diseases; or by a shortage of space" (Krebs 1994; p.340). Plants, being fixed in one location, will mainly compete for light or nutrients, and will follow a special rule, the -3/2 power rule or Yoda's law (the self-thinning rule) (Krebs 1994; p.340). This rule, which describes the relationship between individual plant size and density in even-aged populations, tries to fit mortality (thinning) for competition within the population to a theoretical line with a slope of -3/2 (Krebs 1994; p.340). The equation of the self-thinning rule is:

log ¯m = - (3/2) (log N) + K

where, ¯m = average plant weight (grams)
N = plant density (individuals per square meter)
K = a constant

The self-thinning rule, which has been suggested as an ecological law that applies both within one plant species and between different plant species, highlights the tradeoffs that can occur in organisms with plastic growth, such that the size of an individual can become smaller as density increases (Krebs 1994; p.340).

Recent evaluations of this rule have found many exceptions such as for the gymnosperm trees, which Weller (1987) found that more shade tolerant tree species had more shallow slopes than the predicted value (Krebs 1994; p.341). According to Weller, many particular plants don't fit to the -3/2 predicted slope of the self-thinning line (only 24/63 fitted to the slope). "The slope of the thinning line is variable, but this gives us further insight into species differences under strong competition for light and nutrients" (Krebs 1994; p.340). The relationship between mean plant weight and plant density expected when total plant biomass is at a maximum, and at a low density is a slope = -1 (Silvertown 1982; p. 118). "Populations of small plants at higher densities also increase in mean plant weight as they grow, but the mortality occurs before the carrying capacity is reached and before the increase in the total weight of the plant population ceases" (Silvertown 1982; p. 118). Dense populations which have reached a size at which mortality occurs demonstrate a relationship between log mean plant weight and log density which generally has a slope of -3/2 (Silvertown 1982; p. 118). For as long as the relationship between mean plant weight and density is governed by a line with a slope of -3/2, total plant weight will increase because mean plant weight is increasing faster than density is falling (Silvertown 1982; p. 119).

The weight of plant material that can be obtained from a unit area planted at a given density (yield) is derived from the -3/2 thinning law (Silvertown 1982; p. 127). The -3/2 law is:

y= w * d
y = Cd-1/2

where, y = Yield
w = Cd-3/2= mean weight per plant
d = plant density
C = interception of y axis

For a population with a self-thinning line of slope -3/2 and intercept C, this equation tells us the yield to be expected from a self-thinning population of density d. For populations which have reached the upper part of the self-thinning curve where the slope achieves the value a=1, we obtain a yield/density relationship: y = C. (Silvertown 1982; p. 127). "Population densities which are high enough to bring about self-thinning are too high as far as a farmer is concerned because the casualties of a self-thinning crop are of no economic value" (Silvertown 1982; p. 127). A straightforward asymptote relationship is found in various crops where the yield is measured in terms of whole plant weight or some vegetative part of this, such as the roots of beet crop or the tubers of a potato crop (Silvertown 1982; p. 127). Details of the method are given in Silvertown (1982).

Briefly, increasing density, which can be evenly or unevenly distributed among the individuals in a population, and intraspecific (among individuals in a population) competition for a limited resource has two major effects on the individuals in a population (Blundon 1994). It can cause mortality directly, or it can lead to reduced growth rates and fecundity (plastic responses); both mortal and plastic responses act to reduce population growth rates (Blundon 1994). This experiment was embarked upon the intention to know if these hypotheses are verifiable and conclusive. The objective of this experiment was to compare the effects of increasing density and light intensity on establishment, growth rates, allocation and net reproduction in an annual and a perennial grass.

 

2.2 Materials and Methods

 

January 13 to March 10, 1994, the experiment was conceived in a biology laboratory at Camosun College during a class period. The technique of collecting data and the used material were very simple. In conformity with the tables 1 and 4, the experimentation was divided in two parts, the preparation of the laboratory and the harvest of the plants. During the first week, 20 treatments in a factorial design, containing 2 species (Oats & Bromus), 5 sowing densities (10, 20, 40, 60, and 160 seeds), and 2 light intensities (low and high light), were prepared, and carried in a greenhouse at ambient temperature. After 8 weeks, plants were harvested, dried, weighted (roots and shoots separately), and compiled in the tables 1 and 4. Details of the method are given in Blundon (1994).

 

2.3 Results

 

Obtained data were compiled in the tables 1 to 4. Table 1 and table 3 represent data that were compiled by all the groups. In these tables (Oats and Bromus), there are seed numbers, number of established plants, total net production or dry weight production (gr.) divided in roots and shoots, establishment (%), and the fertility (% of established). The establishment is the percentage of the number of established plants in function of sowing densities. The fertility (% of established) was abandoned because plants were not old enough to produce flowers. Table 2 and table 4 are the average of all data that were compiled in table 1 and table 3.

Figure 1 and figure 2 represent the effect of density on establishment success. The number of established plants in function of the seed number was plotted for each species, with both light intensities. Fig. 1 and fig. 2 show two different curves, a straight line and a logarithmic (maybe a sigmoid) curve. Oats and Bromus curves are logarithmic curves at low light intensity and straight lines at high intensity. Consequently, the mortality rate is higher at low light intensity than at high light intensity. The Oats mortality is higher than Bromus mortality.

Figure 3, figure 4, figure 5, figure 6 show the effect of density on total quantity of net production by each population. The dry weight production in function of density of established plants for both species was plotted for each species and each light intensity. Oats and Bromus curves are logarithmic curves at both intensities. Consequently, at the beginning, net production increases continuously with increasing density, but at the end, each curves has a straightforward asymptote. The response is the same for both light intensities, but the dry weight production rate at low light intensity is lower than at high light intensity. The Bromus dry weight production rate is lower than the Oats dry weight production rate.

Figure 7, figure 8, figure 9, figure 10 show the effect of increasing density on the average weight of individuals in each population. The total net production was divided by the number of established plants in each population, and a graph log-log, containing this division, was plotted in function of the density. The results of each curve are a straight line, but a logarithmic curve was drown to know the exact formulas. Contrary to others that have a negative straight line, Oats at low light intensity has a straight line equal to zero. The growth of individuals of Oats is more suppressed by increasing densities than Bromus. Oats at low light intensity doesn't seem to be affected by the competition, while Oats a high light intensity seems to be the most affected by the competition. In general, at low light intensity, the competition is low, while at high light intensity, the competition is more important. With our data, there is any relationship between root and shoot ratios.

 

2.4 Discussion

 

According to the initial hypotheses, this experiment seems to be very conclusive and verifiable. The numerous figures show the direct effects of the population regulation. Despite of the fact that intraspecific competition has few effect at low light intensity, it decreases the population growth rate and causes mortality directly. Unfortunately, it was impossible to conclude something about the fecundity and the relationship with root and shoot ratio. Because plants were too young, the fecundity was indeterminable. The -3/2 power rule or Yoda's law wasn't conclusive because most plants have a slope near 0. This experiment was directly in accordance with Silvertown who said: "populations of small plants at higher densities also increase in mean plant weight as they grow, but the mortality occurs before the carrying capacity is reached and before the increase in the total weight of the plant population ceases." A straightforward relationship was found too where the yield is measured in terms of whole plant weight. At low intensity, the competition is low or absent, while at high intensity, the competition is high and more affected by the processes of regulation. Oats is the less and the most affected by the competition and the processes of regulation, dependent of the light intensity. There was some problems with the plants pots, because some roots could reach directly the water outside of the pot, and by this fact escape to the competition. An interesting improvement of this method should be to buy better oats seeds and try to find a better solution to eliminate the contact between roots and water outside of the pot.

 

Table 1. Effects of Density on Establishment Success

 

Avena (Oats)

 

Group 1: Low Light Intensity

Seed No.

10

20

40

80

160

# Established Plants

0

2

10

9

18

Total Net Production (dry g.):

Roots

0

0.05

-

0.45

0.46

Shoots

0

0.21

-

1.16

1.35

Establishment (%)

0.00

10.00

25.00

11.25

11.25

Fertility (% of Established)

-

-

-

-

-

 

Group 5: Low Light Intensity

Seed No.

10

20

40

80

160

# Established Plants

3

7

11

26

31

Total Net Production (dry g.):

Roots

0.06

0.31

0.64

1.50

1.23

Shoots

0.23

0.65

0.98

1.43

1.47

Establishment (%)

30.00

35.00

27.50

32.50

19.38

Fertility (% of Established)

-

-

-

-

-

 

Group 2: High Light Intensity

 

Seed No.

10

20

40

80

160

# Established Plants

4

7

6

13

31

Total Net Production (dry g.):

Roots

0.94

0.79

-

1.57

2.29

Shoots

2.18

1.70

-

1.84

3.33

Establishment (%)

40.00

35.00

15.00

16.25

19.38

Fertility (% of Established)

-

-

-

-

-

 

Group 6: High Light Intensity

 

Seed No.

10

20

40

80

160

# Established Plants

3

3

9

13

22

Total Net Production (dry g.):

Roots

0.69

0.81

1.51

2.32

3.95

Shoots

0.98

1.00

1.99

2.53

2.73

Establishment (%)

30.00

15.00

22.50

16.25

13.75

Fertility (% of Established)

-

-

-

-

-

 

Table 2. Effects of Density on Establishment Success

 

Average: Low Light Intensity

Seed No.

10

20

40

80

160

# Established Plants

1.50

4.50

10.50

17.50

24.50

Total Net Production (dry g.):

Roots

0.03

0.18

0.64

0.98

0.85

Shoots

0.12

0.43

0.98

1.29

1.41

Establishment (%)

15.00

22.50

26.25

21.88

15.31

Fertility (% of Established)

-

-

-

-

-

 

Average: High Light Intensity

Seed No.

10

20

40

80

160

# Established Plants

3.50

5.00

7.50

13.00

26.50

Total Net Production (dry g.):

Roots

0.81

0.80

1.51

1.94

3.12

Shoots

1.58

1.35

1.99

2.18

3.03

Establishment (%)

35.00

25.00

18.75

16.25

16.56

Fertility (% of Established)

-

-

-

-

-

 

Table 3. Effects of Density on Establishment Success

 

Bromus

Group 5: Low Light Intensity

 

Seed No.

10

20

40

80

160

# Established Plants

3

12

23

46

37

Total Net Production (dry g.):

Roots

0.07

0.17

0.26

0.91

0.25

Shoots

0.06

0.33

0.29

0.56

0.31

Establishment (%)

30.00

60.00

57.50

57.50

23.12

Fertility (% of Established)

-

-

-

-

-

 

Group 3: High Light Intensity

 

Seed No.

10

20

40

80

160

# Established Plants

7

16

26

51

116

Total Net Production (dry g.):

Roots

0.41

0.32

1.16

0.35

1.71

Shoots

0.22

0.39

0.82

0.90

1.58

Establishment (%)

70.00

80.00

65.00

63.75

72.50

Fertility (% of Established)

-

-

-

-

-

 

Group 4: High Light Intensity

 

Seed No.

10

20

40

80

160

# Established Plants

8

14

31

50

134

Total Net Production (dry g.):

Roots

0.67

0.72

3.05

2.79

3.85

Shoots

0.71

0.86

1.56

1.45

2.20

Establishment (%)

80.00

70.00

77.50

62.50

83.75

Fertility (% of Established)

-

-

-

-

-

 

Table 4. Effects of Density on Establishment Success

Average: Low Light Intensity

Seed No.

10

20

40

80

160

# Established Plants

3

12

23

46

37

Total Net Production (dry g.):

Roots

0.07

0.17

0.26

0.91

0.25

Shoots

0.06

0.33

0.29

0.56

0.31

Establishment (%)

30.00

60.00

57.50

57.50

23.12

Fertility (% of Established)

-

-

-

-

-

 

Average: High Light Intensity

Seed No.

10

20

40

80

160

# Established Plants

7.50

15.00

28.50

50.50

125.00

Total Net Production (dry g.):

Roots

0.54

0.52

2.11

1.57

2.78

Shoots

0.46

0.62

1.19

1.17

1.89

Establishment (%)

75.00

75.00

71.25

63.13

78.12

Fertility (% of Established)

-

-

-

-

-

Fig. 1:

Effect of Density on Establishment Success

(Oats)

Fig. 2:

Effect of Density on Establishment Success

(Bromus)

Fig. 3:

Effect of Density on Net Production

(Oats - Low Light Intensity)

Fig. 4:

Effect of Density on Net Production

(Oats - High Light Intensity)

Fig. 5:

Effect of Density on Net Production

(Bromus - Low Light Intensity)

Fig. 6:

Effect of Density on Net Production

(Bromus - High Light Intensity)

Fig. 7:

Effect of Increasing Density on Weight of Individuals

(Oats - Low Light Intensity)

Fig. 8:

Effect of Increasing Density on Weight of Individuals

(Oats - High Light Intensity)

Fig. 9:

Effect of Increasing Density on Weight of Individuals

(Bromus - Low Light Intensity)

Fig. 10:

Effect of Increasing Density on Weight of Individuals

(Bromus - High Light Intensity)

 

2.5 Acknowledgements

 

I would like to thank my classmate Jason, who helps me to do this experiment and all students in my class that contributed to the success of this experiment. I would like particularly thank Dr, David J. Blundon for his precious help and for its laboratory manual in which I took a lot of information and ideas.

 

2.6 Works Cited

 

Blundon, David J. 1994. Ecology: Laboratory Manual. Camosun College, Victoria, Canada.

Krebs, Charles. J. 1994. Ecology. HarperCollins College Publishers, New York, USA.

Silvertown, Jonathan W. 1982. Introduction to Plant Population Ecology. Longman Group Limited, New York, USA.

 

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