Download High school lesson plan

Earthworms - invasive or not?
LESSON PLAN
Page 1 of 4
EARTHWORMS - INVASIVE OR NOT? DEPENDS ON WHERE YOU STAND!
Overview
Humans have and are changing the environment around us in many ways. Often, it is difficult to
tell whether the changes we impose are good or bad. In this lesson, students will learn how
glaciation removed all earthworms from the Great Lakes region, and how humans are the cause
of the presence of earthworms today (making them exotic species, like the zebra mussel). This
lesson will challenge the common preconception that earthworms are always good for an
ecosystem. Students will sample for earthworms in three different ecosystems using a mustard
extraction technique. Students will learn that the activities of earthworms, their population
density, and the ecosystems in which they are present determine whether they are simply exotic
and benefit or don’t harm the ecosystem, or invasive and damage the ecosystem.
Objectives
At the conclusion of this lesson, students will be able to:
 Define the term “invasive species” and give examples of invasive species in Michigan
 Discuss the positive and negative effects of invasive species on ecosystems, using
earthworms as an example
 Demonstrate a method for randomly sampling earthworms
 Generate hypotheses and generate data that tests these hypotheses
Length of lesson
15 minutes for introduction
50 minute (x3) for each site sampled
30 minutes for sample processing and data analysis
Grade Level
High School
Standard covered
B1.2B Identify and critique arguments about personal or societal issues based on scientific
evidence.
B3.4 Changes in Ecosystems - Although the interrelationships and interdependence of
organisms may generate biological communities in ecosystems that are stable for hundreds or
thousands of years, ecosystems always change when climate changes or when one or more
new species appear as a result of migration or local evolution. The impact of the human species
has major consequences for other species.
B3.4C Examine the negative impact of human activities.
B3.4x Human Impact - Humans can have tremendous impact on the environment. Sometimes
their impact is beneficial, and sometimes it is detrimental.
B3.5 Populations - Populations of living things increase and decrease in size as they interact
with other populations and with the environment. The rate of change is dependent upon relative
birth and death rates.
B3.5C Predict the consequences of an invading organism on the survival of other organisms.
Materials
Earthworm sampling kit
1 gallon jugs (2)
** Written by David Costello and Lauren Kinsman (Sept. 2008)
Earthworms - invasive or not?
LESSON PLAN
Page 2 of 4
Beaker to measure 100mL of mustard slurry
Plastic bags/cups to hold earthworms (4 for each ecosystem)
Trays for sorting earthworms (1 for each group)
Definitions
Native (Indigenous) species - A species living within its native range (Michigan examples:
white pine, beaver, and painted turtle)
Exotic (Non-native, alien) species - A species living outside of its native range. This species
has entered this new area due to the activities of humans. (Michigan examples: honey bee,
apple tree, and domestic cat)
Invasive species - An exotic species whose establishment and spread threatens ecosystems,
habitats, or species with environmental and/or economic harm. (Michigan examples: zebra
mussel, rats, and chestnut blight)
Background
As humans move themselves and their stuff to new places, they often bring with them
(either intentionally or accidentally) plants, animals, and diseases from their homeland. Some of
these exotic species populations grow very large and can have detrimental effects on their new
ecosystems. Some of these effects can be quantified in dollar amounts (e.g., Great Lakes
power plants spend $60 million a year controlling zebra mussels!) and the species with the most
severe economic effects are often the most well known. Some examples of species with large
economic impacts include zebra mussels, chestnut blight (destroyed all large American
Chestnuts), and rats (a pest that consumes grain on farms). Many other invasive species have
become notorious not because they cost the economy directly, but because they are damaging
ecosystems and threatening biodiversity.
European earthworms are impacting northern forests in just such a way; they are reducing
leaf litter on the forest floor and changing the understory plant community. In Minnesota,
European earthworms are responsible for the disappearance of the rare goblin fern, which can
only grow in very thick leaf layers. Although earthworms are considered bad for forests, they
are often beneficial in farm fields. The soil in farm fields is often very compacted, and
earthworms help to till the soil, decompose dead organic material, and move water deeper into
the soil. Less is known about how earthworms are influencing other critical ecosystems, such
as riparian (streamside) areas. This lesson about European earthworms in America will teach
students to think about why species are viewed as invasive and understand how the same
organism can interact with different ecosystems in sometimes opposite ways.
Activities
1. Discuss what the students know about earthworms. Start the lesson by showing a
picture of an earthworm and ask the students to share any information that they know about
this organism. First, have each student individually write down three things they know about
earthworms on a piece of paper. Then, discuss what the students have written down.
Probe the students to address what earthworms eat, where they are commonly found, and
also if they are good or bad for soil. This exercise should highlight the preconceptions that
the class has about this organism before the lesson. Write these suggestions on the board
and throughout the lesson challenge some of their suggestions if they are inaccurate.
** Written by David Costello and Lauren Kinsman (Sept. 2008)
Earthworms - invasive or not?
LESSON PLAN
Page 3 of 4
2. Present introductory material on invasive species. Define “native species”, “exotic
species”, and “invasive species” and ask the class for examples of each of these. Discuss
the effects invasive species have on ecosystems; be sure to cover economic costs of
cleaning up invasive species, reduction in biodiversity, and alteration to ecosystem function.
Some useful examples of invasive species:
Zebra mussels - A poster child for invasive species in our region, the zebra mussel invaded
the Great Lakes in the late 1980’s and forever changed the lakes. Zebra mussels have both
major ecosystem and economic impacts in the Great Lakes. Zebra mussels form a thick
carpet of shells on any hard surface in a lake and consume suspended algae at extremely
high rates. Zebra mussels are not edible to most organisms and when the reach high
densities they change the food web dramatically. Species that rely on suspended algae
decline and decomposers that eat zebra mussel feces increase. Sharp zebra mussel shells
wash up on beaches and make them unusable. It is estimated that power plants spend $60
million a year just to clean zebra mussels out of water intake pipes!
Chestnut blight - Before the early 1900s the American chestnut was one of the most
common trees in the Eastern US. The chestnut blight is a bark fungus from Asia which has
killed almost all adult chestnut trees in the US. Before the blight, chestnut trees were
important members of the forest ecosystem because they supplied nuts for white-tailed
deer, turkeys, and other birds.
Brown tree snake - This snake is native to Australia and it has completely decimated the
bird populations on the island of Guam. The brown tree snake came to Guam after WWII,
probably as a stowaway on a cargo ship. Before this introduction Guam had no snakes,
now Guam now has one of the highest densities of snakes in the world (2000 per km2)! This
snake has been responsible for the extinction of 10 of 13 species of bird on Guam.
Purple-loosestrife - This wetland plant is originally from Europe, Asia, and Africa and now
can be found in wet areas throughout most of the US. This tall plant with bright purple
flowers is also commonly found in roadside ditches. This plant spreads very rapidly
(3,000,000 seeds a year!) and often chokes out other wetland plants (e.g., cattails). Purpleloosestrife provided less food, cover, and nesting spots than native wetland plants. Rare
waterfowl and other wildlife are often less common in areas dominated by purple-loosestrife.
3. Discuss the facts of non-native earthworms in Michigan. Start with a discussion of how
the glaciers created the lakes ~10,000 years ago and North American earthworms only
survived in the south. North American earthworms move very slowly, and the earthworms
we find in Michigan today are mostly from Europe. Discuss the role of earthworms in the
ecosystem as decomposers and burrowers in the soil. Mention that earthworms are found
in many ecosystems, and they are considered invasive in forests. Poll the class: where
have you seen earthworms? Have you ever seen them in the woods? Be sure to correct
any misconceptions that were brought up in the initial discussion of earthworms.
4. Introduce the sampling. Mention that the class will be sampling for earthworms in different
ecosystems, to generate data for a scientist. You could show the students my website
(http://www.nd.edu/~strmeco/david.htm) to put a face and name to the scientist. Mention
that the goal of this research is to figure out which ecosystem has more earthworms. Be
sure to continually remind the students of this question as the lesson progresses. Introduce
the three ecosystems (fields, forests, and riparian areas) and mention that the effects of
earthworms are different in each one of these habitats. Give an introduction of the mustard
sampling technique. Be sure that the class is familiar with random sampling.
** Written by David Costello and Lauren Kinsman (Sept. 2008)
Earthworms - invasive or not?
LESSON PLAN
Page 4 of 4
5. Generate hypotheses. Before sampling, have each group write down a hypotheses for the
question “Which ecosystem will contain more earthworms?” With each hypothesis, have the
students present their hypothesis and describe the evidence they used to determine that the
ecosystem will support more earthworms. The class does not have to agree on a single
hypothesis, remind them that this question does not have an answer, and they are helping
scientists to answer this question! In addition to answer the question of how many
earthworms, you can also suggest additional questions, for example: which ecosystem
contains larger earthworms, or are their more mature or adult earthworms?
6. Sample for earthworms. See the attached sheets “Sampling Instructions” and “Student
Worksheet” for instructions and handouts for using the mustard extraction technique.
7. Present the data. Once data is collected, have the students present their data to the class
and determine if their hypothesis was correct or not. Earthworms have a very patchy
distribution and are not evenly spread in the soil, and just by chance one group might hit a
hot spot of earthworms and get a very high value or one group may get very few
earthworms. Therefore, it is quite possible that the different group’s conclusions do not
agree. If your class is comfortable with replication and calculating averages, work as a class
to calculate averages for each ecosystem and make a decision about the hypothesis based
on data from the whole class.
8. Conclusion. Preserve the earthworms in ethanol (keeping each sample in a separate
container) and send the samples to David Costello. See “Sampling Instructions” for details
about preservation. Please send the “Teacher worksheet” along with the earthworm
samples.
Resources
USDA - National Invasive Species Information Center
http://www.invasivespeciesinfo.gov/
SeaGrant Nonindiginous Species Site
http://www.sgnis.org/
http://sgnis.org/publicat/slide/catalog1.htm (good site for photos of aquatic invasive species)
Earthworms of the Great Lakes by Cindy Hale
Great Lakes Worm Watch:
http://www.nrri.umn.edu/worms/default.htm
Canada Worm Watch
http://www.naturewatch.ca/english/wormwatch/index.html
** Written by David Costello and Lauren Kinsman (Sept. 2008)