Duckweed Population Lab
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 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.
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.
You will count the number of duckweed fronds in your beaker each class day over the course of the unit.
Table 1. Number of Duckweed fronds per day in non-manipulated water source.
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.
Data Collection & Analysis Aspect 2
Data Collection & Analysis Aspect 3
Conclusion & Evaluation Aspect 1