Name ________________________________
APES
Period ________
Duckweed Population Lab
Background Observations: One of the basic units of study in ecology is the population, a group of individuals of
the same species occupying the same area at the same time. Population dynamics refers to the changes that
occur in the umber of individuals in a population. Given unlimited resources, natural populations tend to show
exponential growth. In reality, however, biotic and abiotic factors limit population growth. The maximum
number of individuals an area can support is the environment’s carrying capacity. When a population
exceeds the carrying capacity, the environment cannot support all the individuals. In this lab, you will observe
the growth of duckweed (Lemna minor) to determine how population density changes over time.
Models of population growth help scientists understand how a population's size changes over time.
Exponential Population Growth
When resources are unlimited, the number of individuals in a population grows exponentially: 1, 10, 100, etc. In
exponential growth, the number of individuals increases rapidly and without bound. The rate of increase varies
from one environment to another. There exists, at least in theory, an environment that is perfect in all respects
for a population and in which it attains its maximum rate of increase. This rate is the population's biotic
potential, rm. Most environments, however, limit growth and the population's intrinsic rate of increase is
necessarily less than the biotic potential.
Exponential population growth does not continue indefinitely. If it did, one population would quickly
cover the surface of the earth. Growth is limited by the availability of resources, such as light, nutrients, and
space. The abundance of resources determines the environment's carrying capacity, K, the number of
individuals of a species the environment can support indefinitely.
Sigmoidal Population Growth
As a population approaches its environment's carrying capacity, population growth slows to almost
zero. Although individuals in the population may die and new individuals born, the number of
individuals in the population remains nearly constant. This pattern is called logistic population growth
and is shown in the following figure.
Notice that the population increases exponentially at first, when resources are in such supply they
are effectively limitless. When the number of individuals equals half the carrying capacity, the rate of
increase is at a maximum. Once this number is attained, the rate of increase slows, and the graph
approaches a horizontal line whose horizontal value is the carrying Capacity. This type of curve is
called an S curve, or sigmoidal (after the Greek name for the letter s) curve for its shape.
Lemna minor
Lemna minor is a member of the family Lemnaceae (duckweed family). This tiny organism is ideal for
population growth experiments because it reproduces quickly, requires minimal space to grow, and
requires no maintenance. The free-floating, freshwater plant consists of a green elliptical frond with
one root and is found in still waters, from temperate to tropical zones. The plant reproduces by
vegetative budding, although it may flower. On average, fronds live for four to five weeks.
Due to its rapid growth rate, duckweed finds wide use in governmental and commercial applications.
Name ________________________________
APES
Period ________
For example, the US EPA requires companies that make pesticides to determine whether their
chemicals affect the growth of aquatic plants. Many companies use duckweed as a test plant. In
the test, the pesticide is applied to duckweed's growing medium and any effects on the duckweed's
growth rate are taken as a measure of the pesticide's toxicity.
Some companies use duckweed to remove nitrogen and phosphorus from their wastewater. Nitrogen
and phosphorus are plant nutrients that in high concentrations (as in wastewater) promote rapid plant
growth. If wastewater were released into the environment untreated, new plant growth would clog
w at erw ays and cause eut r op h i cat i on . To remove nut rient s from t he wastewater, Lemna
plants are grown in it. As the plants grow, they naturally take up nitrogen and phosphorus from the
wastewater. When the duckweed plants die, they are harvested, composted, and used as mulch. The
treated wastewater continues to the next stage of purification.
Lemna minor grows best in environments that most closely approximate its natural habitat. Although
the plant can adapt to extreme conditions, duckweed prefers a pH range of 6.5 to 8.5, low salinity,
temperatures around 25° C, and plenty of light. In the experiment, you vary the amount of fertilizer in
the water.
Directions:
In your groups, fill your test tube about halfway full. Using the forceps, pick out four individual duckweed plants.
Place the duckweed in the test tube and place the test tube in the rack. You will count the number of
duckweed plants in your tube each day for the next couple of weeks.
Data:
Date
Days Elapsed
Number of Individuals
Observations
Name ________________________________
APES
Period ________
Pre-Lab questions: (16 points)
1. If a population has unlimited resources, what kind of growth does it exhibit?
2. Define carrying capacity.
3. List three examples of limiting factors.
4. What environmental conditions do duckweed prefer?
5. How will you identify the carrying capacity for the populations?
6. Name some biotic and abiotic factors that may alter the population of Lemna. Label the factors N for
natural and H for human induced.
7. What is the problem that you will be investigating?
8. Write a hypothesis for your experiment.
9. Identify the independent, dependent variables for your experiment.
10. List three controlled variables for your lab.
11. What does the EPA use duckweed for?
12. Duckweed can remove what two substances from wastewater?
Name ________________________________
APES
Period ________
Graphing: Graph your data below. Be sure to label the X and Y axis, and write an appropriate title. (6 points)
Title: ___________________________________________________
Post Lab Analysis: (10 points)
1. Has this population reached the carrying capacity of the test tube? How do you know?
2. What limiting factors affected the carrying capacity in this lab? Were they density-dependent or densityindependent limiting factors?
3. What would happen in a lake or pond if Duckweed completely covered the surface? What environmental
effects might this have?
Name ________________________________
APES
Period ________
4 Can any organism exhibit exponential population growth forever? What happens to a population that is
above its carrying capacity?
5. Do you believe there will be a point when the human population, which is growing exponentially, will reach
the carrying capacity of the earth? Why or why not?
Conclusion: Was your hypothesis supported or refuted? Justify your conclusion with data in an explanatory
paragraph. Describe at least three possible experimental design flaws and be sure to explain how each
source may have affected your results. (12 points)
Name ________________________________
APES
Period ________
Procedure:
Group 1 Obtain one beaker. Measure 30 mL of spring water in a graduated cylinder and pour into the
beaker. Using a toothpick, carefully transfer one colony of three fronds and two colonies of two fronds
(total of seven) to the beaker. Cover the beaker with plastic wrap and secure with a rubber band. Poke
ten small holes in each test tube's plastic wrap. Label the beaker "Spring Water 1, Period _____" . Place the
beaker under the grow light in your period’s green tray.
Group 2 Obtain one beaker. Measure 30 mL of spring water in a graduated cylinder and pour into the
beaker. Using a toothpick, carefully transfer one colony of three fronds and two colonies of two fronds
(total of seven) to the beaker. Cover the beaker with plastic wrap and secure with a rubber band. Poke
ten small holes in each test tube's plastic wrap. Label the beaker "Spring Water 2, Period _____" . Place the
beaker under the grow light in your period’s green tray.
Group 3 (fertilizer)..
Measure 30 mL of spring water in a graduated cylinder and pour into a beaker. Weigh out 0.45 grams
(0.5%) of Miracle Gro fertilizer on the electronic scale. Stir using a glass rod. Using a toothpick, carefully
transfer one colony of three fronds and two colonies of two fronds (total of seven) to each tube. Cover
the beaker with plastic wrap and secure with a rubber band. Poke ten small holes in the plastic wrap.
Label the beaker “Miracle Gro " and indicate the concentration of fertilizer used and your class period.
Place the beaker under the grow light in your period’s green tray.
Group 4 (fertilizer)
Measure 30 mL of spring water in a graduated cylinder and pour into a beaker. Weigh out 0.9 gram (1%),
of Miracle Gro fertilizer on the electronic scale. Stir using a glass rod. Using a toothpick, carefully transfer
one colony of three fronds and two colonies of two fronds (total of seven) to each tube. Cover the
beaker with plastic wrap and secure with a rubber band. Poke ten small holes in the plastic wrap. Label
the beaker “Miracle Gro " and indicate the concentration of fertilizer used and your class period. Place
the beaker under the grow light in your period’s green tray.
Group 5 (fertilizer)
Measure 30 mL of spring water in a graduated cylinder and pour into a beaker. Weigh out 1.8 grams
(2%), of Miracle Gro fertilizer on the electronic scale. Stir using a glass rod. Using a toothpick, carefully
transfer one colony of three fronds and two colonies of two fronds (total of seven) to each tube. Cover
the beaker with plastic wrap and secure with a rubber band. Poke ten small holes in the plastic wrap.
Label the beaker “Miracle Gro " and indicate the concentration of fertilizer used and your class period.
Place the beaker under the grow light in your period’s green tray.
Group 6 (fertilizer)
Measure 30 mL of spring water in a graduated cylinder and pour into a beaker. Weigh out 3.6 grams
(4%). of Miracle Gro fertilizer on the electronic scale. Stir using a glass rod. Using a toothpick, carefully
transfer one colony of three fronds and two colonies of two fronds (total of seven) to each tube. Cover
the beaker with plastic wrap and secure with a rubber band. Poke ten small holes in the plastic wrap.
Label the beaker “Miracle Gro " and indicate the concentration of fertilizer used and your class period.
Place the beaker under the grow light in your period’s green tray.
On days your instructor designates, count the number of fronds in your test tube and record the data in
your table. Count only live (green) fronds. Fronds that are totally yellow, clear, black, or white are dead.
The experiment may continue for 3 to 4 weeks.