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Duckweed Population Lab

Background:

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 number 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. These models have applications in microbiology, wildlife management, pest management, agricultural productivity, and toxicology.

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.  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 waterways and cause eutrophication. To remove nutrients from the 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.  

Growth:

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 (S-Curve)

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 1.  Further, this growth pattern can be divided into different phases (labeled A-D) for Duckweed.


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, K. This type of curve is called an S curve, or sigmoidal (after the Greek name for the letter s) curve for its shape.

Objectives:

Directions:

  1. Obtain five beakers and fill with 100ml of water from Mr. Rott’s freshwater salt aquarium.
  2. Label your beakers with your group member names (or team name) and class.
  3. Using a toothpick, carefully transfer one colony of three fronds and two colonies of two fronds (total of seven fronds) to the dixie cup.
  4. Place the labeled cups in the growing area.

You will count the number of duckweed fronds in your beaker each class day over the course of the unit.

Data:

Table 1. Number of Duckweed fronds per day in non-manipulated water source.

Date

Days Elapsed

Beaker 1: Number of Individuals

Beaker 2: Number of Individuals

Beaker 3: Number of Individuals

Beaker 4: Number of Individuals

Average Number of Individuals

Table 2. Qualitative observations of Duckweed fronds per day in non-manipulated water source.

Date

Observations

Data Analysis:

  1. Calculate the average population size for all trials per day of growth.
  2. Create a graph to show the average population size per day of growth (remember, you have only counted number of individuals on class days); your graph should include a title, axes labels, and error bars.

  1. Add a regression line to your graph.

  1. Print and staple the graph to this lab.

  1. Determine the growth rate (r) of Duckweed over the course of the activity using the natural growth formula: Nt=Noert  Show your calculations
  1. No: number of duckweed fronds that are present at the beginning of the time period of which you are determining the natural growth rate
  2. Nt: number of duckweed fronds that are present at the end of the time period of which you are determining the natural growth rate
  3. e: Euler’s number for natural decay (the inverse of the natural logLN)
  4. t: number of days in the period in which you are determining the natural growth rate
  5. r: the natural growth rate
  6. Example:

Analysis:

  1. Has your population reached the carrying capacity of the test tube?  Use data from your graph to justify your response (CE Question).

  1. What are three limiting factors for population growth of Duckweed?

  1. What would happen in a lake or pond if Duckweed completely covered the surface?  What environmental effects might this have?

  1. Can any organism exhibit exponential population growth forever, why or why not?  What happens to a population that is above its carrying capacity?

  1. Explain the following growth phases and why they exist:
  1. Exponential Growth Phase:

  1. Transitional Phase:

  1. Plateau Phase

  1. Given the rapid increase of human population growth, do you think the the human population will reach the carrying capacity of the earth?  Why or why not?

Assessment:

Data Collection & Analysis Aspect 2

Exceeds Proficiency

Meets Proficiency

Not Proficient

Data Collection & Analysis Aspect 3

Exceeds Proficiency

Meets Proficiency

Not Proficient

Conclusion & Evaluation Aspect 1

Exceeds Proficiency

Meets Proficiency

Not Proficient