Download Duckweed Population Lab

8th Grade Environmental Science
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. These models have
applications in microbiology, wildlife management, pest management, agricultural productivity, and toxicology.
Consider two simple models of population growth: exponential and sigmoidal.
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.
Concepts to REMEMBER:
Exponential population model determines a
population’s growth rate in optimal conditions
when resources are unlimited (J-shaped curve).
Logistic population model determines a
population’s growth rate relative to the
environment’s carrying capacity (S-shaped
curve).
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. 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.
8th Grade Environmental Science
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 water wa ys a nd
c aus e eutroph ic ation . T o r em ove nutr ients f rom 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. 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. Record how many fronds (leaves) each plant has.
Create a table like below to record your results.
Data: SAMPLE
Date
9/15/2015
9/17/2015
Days Elapsed
2
2
Number of Individuals
5 ( 2-1frond 3-2frond)
Observations
One is turning yellow
6 ( 3-1 frond 3-2 frond)
Conclusion:
1. Has this population reached the carrying capacity of the test tube?
2. What are the limiting factors for population growth of Duckweed?
3. Based on your data and calculations, is Lemna an r-species or K-species? Explain your answer.
4. What would happen in a lake or pond if Duckweed completely covered the surface? What environmental effects might
this have?
5. Can any organism exhibit exponential population growth forever? What happens to a population that is above its
carrying capacity?
6. 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?
7. Create a graph illustrating how your population changed over time.