S C I E N C E
sampler
Interactive
interdependence
Any middle school teacher can tell you that
adolescents can be self-focused and egocentric,
making the teaching of interdependence of
organisms in an ecosystem a monumental
challenge. After struggling to get my sixth
graders to understand how all organisms
are connected in an ecosystem, I enlisted
an interactive approach to the concept of
interdependence. Modifying and building on
a lesson created by the Vancouver Aquarium
Marine Science Centre, the British Columbia
Ministry of Education, and the Provincial
Learning Network (2000) that supports
National Science Education Standards Teaching
Standard A and Content Standard C (NRC
1996), I (literally) tied my students in knots to
create not only a human food web, but a web
of understanding.
During our study of populations and
ecosystems, we begin by building simple
food chains and food webs, and then trace
the flow of energy from one organism to the
next. After those concepts are developed,
students are challenged to discover how
all the organisms in an ecosystem are
connected and investigate the natural and
human-created factors that influence the
ecosystem’s carrying capacity. The timeline
for these activities often varies, depending on the amount
of extension students’ interest warrants. Typically, the
interdependence activities can be completed over the
course of two to four class periods of 45–60 minutes in
length. The first activity does require some advance
preparation in terms of arranging your classroom so there
is a large open area for students to gather. You will also
need to cut string or yarn into pieces approximately two
to four meters in length (the longer the string the better).
You will need approximately 20 pieces of string, which
can be reused from class to class. Heavy twine works best;
yarn tends to break as the students hold and pull on it.
We begin the activity by focusing on a Pacific aquatic
ecosystem with salmon as the primary organism. Using
the resources provided by the Provincial Learning
Network’s (PLNet’s) “Salmon Tales Field Trip” (2000),
we brainstorm as a class the organisms an adult salmon
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would encounter in its ecosystem. After generating our
list of organisms for a salmon ecosystem, which includes
organisms such as bears, killer whales, seals, shrimp,
plankton, krill, various small fish, birds, bugs, plants, and
humans, I begin to assign roles. I write the names of all
the organisms from our list on masking tape or stick-on
nametags and hand them out to students. Previously, I
repeated roles so every student in the class represented
an organism, but I found that the activity often become
unwieldy and lost its meaning as we tried to manage our
personal space. Now, I just assign each organism to one
student and have the remaining students without roles
sit in chairs surrounding the “ecosystem.”
Beginning with the salmon, I give each student a piece
of string and ask them to name one other organism in the
Julie LaConte ([email protected]) is a classroom teacher
at Hoech Middle School in St. Ann, Missouri.
S C I E N C E
sampler
ecosystem with which they have a relationship. Students
typically identify predator-prey relationships very easily
at this point, and those organisms are connected by a
string. For example, the salmon would hold one end of a
piece of string and the bear would hold the other end. As
you can imagine, this food web can get pretty complex.
Once the connections are made, students must not let
go of their strings until the activity is completed. I have
lots of extra pieces of string on hand as students realize
that they are often connected to multiple organisms,
requiring them to hold more than one piece of string.
There are often students holding three or four pieces of
string at a time.
I keep students observing the formation engaged by
having them identify the relationships between the
members of the food web. Because I am usually physically
entangled in the midst of the food web, we identify the
relationships by calling them out as they are established.
The students outside the web also serve an important
function by helping to manage the physical challenge
of passing the ends of string from one “organism” to
another to create the connections. After our food web
is built, I ask those students who are not part of the food
web to identify abiotic parts of this ecosystem, which
include elements such as water, air, rocks, soil, and the
Sun. Again, students call out the relationships as they
identify them. I then assign these abiotic roles to the
student observers, and ask them to link themselves with
string to the organisms that depend on them. At this
point, anyone walking into my classroom might only see
a web of confusion and wonder why my students appear to
be tied up, but it is clear to students how intricately the
organisms in the food web are connected to each other
and the abiotic parts of the environment.
Once all of the connections have been established,
I tell students that, due to overfishing, the salmon
population has drastically decreased this year and I
ask the salmon to gently tug on any strings he or she is
holding. I then ask students who felt the pull to call out
so everyone knows who is being affected. We then explore
the organisms that are indirectly affected by pointing
out that the bear will have to compensate for the lack of
salmon by eating other fish. The class begins to realize
that a ripple effect takes place throughout the ecosystem
and that all organisms are affected by the overfishing of
the salmon, just to varying degrees.
During the discussion, other students who felt the tug
speak up, even if they’re not directly connected to the
organism doing the pulling, and we discuss how it’s possible
that they felt the pull if they weren’t directly connected
to that organism. We then verbally trace the pathway of
the pull. For example, students might explain that “The
salmon pulled on the string, which made the bear feel it,
which made the berries feel it because the bear also eats
berries.” Then we’ll discuss that relationship through
questions such as, “If the salmon population decreases,
and the bear eats salmon, what will happen to the berry
population that is also food for the bear? Why?”
FIGURE 1
Interdependence scenarios
Directions
Read each scenario. Think about how each of the
populations of organisms identified in the scenario would
be affected in the situation.
Scenario 1
There is a population of rabbits living in a large meadow
and feeding on leafy plants. Foxes often hunt these
rabbits. Last year, there was plenty of rain and the leafy
plants flourished in the meadow. Describe how the
populations of plants, rabbits, and foxes are affected in
this situation.
Scenario 2
On a plateau, deer feed on a limited supply of grass.
Coyotes roam the area, feeding on the deer. Lately,
farmers have been killing the coyotes in the area because
they feel the coyotes are a threat to their farm animals.
Describe how the farmers’ actions affect the populations
of deer and coyotes and the condition of the grass.
Scenario 3
Field mice eat the grains of wheat in a field. Snakes prey
on those field mice. Last summer, there was a drought
in the region, limiting the wheat crop. Describe the effect
the drought has on the populations of mice and snakes
and the condition of the wheat.
Scenario 4
In the North Pacific Ocean, sharks feed on the seal
population. The seals survive by eating smaller fish in the
area. An oil tanker hit an iceberg and spilled a large amount
of oil into this ecosystem. Describe how the oil spill affects
the populations of the sharks, seals, and small fish.
January 2007
science scope
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S C I E N C E
sampler
We use the same string-pulling technique to explore how
the food web is affected by
•
•
•
•
a drought that reduces the number of available plants,
an oil spill that fouls the water,
a disease that decimates the bear population, and
any other event that affects a population or abiotic element
of the web.
These events demonstrate to students that no organism
exists in isolation, and that each organism is affected
and will have to react to changes in their environment.
Creating the human food web and debriefing
When we debrief at the end of this lesson, I collect the strings
and ask students to review the connections and conditions
that created the tugs on the strings. This is usually as a class
discussion, but sometimes I will record the connections
and student comments on the board or chart paper. The
discussion is never the same twice and I have never had a
problem with students not being engaged. The activity and
discussion usually take up an entire class period.
During the next class, we begin to talk about the
factors that challenge and affect the size of populations
in an ecosystem, which we define as limiting factors. I ask
students to identify things that humans do that affect an
ecosystem or another species in some way, either positive
or negative. I record their comments on a chart at the
front of the class. Under the heading “human-caused
limiting factors,” students will name things such as
pollution, habitat destruction, and hunting. Then, we
create a list of “natural limiting factors” that include
items such as disease, severe weather (freezing, droughts,
floods, fires, and so on), and predation. Again, we reflect
on the concept of interdependence and discuss how a
limiting factor may affect one population directly, but
many other populations indirectly.
To further illustrate the fact that population size does
not remain stable, but changes in response to limiting
factors in an ecosystem, we play a game called Meeting
Animals’ Needs (Allen 2002). In this game, a group of
students chooses an organism, such as polar bears, and
tracks a small population (15 to 20) over time. Students
take turns rolling one die five times to generate the basic
needs of food, water, and shelter for their organisms.
Rolling a 1 or 2 gives their animal food, a 3 or 4 gives
their animal water, and 5 or 6 gives their animal shelter.
If the five die rolls result in all of the animal’s needs being
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met, then its population increases by 1. If all the animal’s
needs have not been met, then its population decreases
by one.
The game continues for 10 rounds (five die rolls equal
one round). However, I often extend the game and add
limiting factors that affect the population’s ability to
reproduce. For instance, I might tell students that there was
a forest fire and shelter is in short supply, so they can only
secure shelter by rolling a 6 instead of a 5 or 6. Or, I might
say there is a drought and water is scarce, so only rolling
a 3 gets their animal water. This makes the game not only
more fun for the kids, but more ecologically challenging as
students begin to see how limiting factors often determine
their animals’ population size and other populations that
depend on them.
After completing both the salmon food web activity
and the Meeting Animals’ Needs game, I assess students’
understanding of the concepts of limiting factors and
interdependence by presenting them with the scenarios
in Figure 1. Students read each scenario and explain
how the populations in the scenario would be affected by
the described limiting factor. These scenarios are often
surprisingly easy for students after having modeled a
similar scenario with the salmon food web and playing the
game. A number of students will often develop their own
interdependence scenarios to challenge their classmates.
Upon realizing that interdependence was not a topic
to which middle school students could easily relate, I
branched out and tried an extremely hands-on, interactive,
and exciting activity to illustrate this difficult concept.
Regardless of the advance preparation, the physical
challenge, and the tangled students that are an inevitable
part of the activity, the resulting conceptual understanding
is substantial. The follow-up limiting factors discussion and
game build on the initial ideas demonstrated by the food
web activity in a fun way. Overall, students are excited
and engaged by these activities while able to reach a deep
understanding about the interconnectedness of all organisms
and abiotic factors in an ecosystem.
References
Allen, K.Z. 2002. Living things: ScienceWorks for kids series. Monterey,
CA: Evan-Moor Corp.
National Research Council (NRC). 1996. National science education
standards. Washington, DC: National Academy Press.
The PLNet’s Salmon Tales Field Trip. 2000. Provincial Learning
Network, Vancouver Aquarium Marine Science Centre, and
British Columbia Ministry of Education—www.vanaqua.org/
salmontales/english/learningcentre/salmonfoodweb.php.