Download Food Webs Logistics Module Overview

Food Webs
Logistics
Number of Volunteers: 1 volunteer for every 10 students
Amount of Set-up Time: 2 minutes (prepare ball of yarn and notecards)
Amount of Running Time: 20-45 minutes, depending on the length of each of
the pre-, during-, and post-activity discussions.
Module Overview
Trophic cascades, food web theory, and the keystone species concept are
illustrated by connecting the students themselves into a food web. Each student
represents a species in a given trophic level, and a ball of yarn is passed through the
levels to represent the flow of energy through that system. System stability and the role
of specific species are also explored. We use the local, though under-studied, Missouri
glade habitat as our example ecosystem, which also introduces the students to the
species that exist, and are restricted to, the threatened prairie glades. There is ample
time for discussion pre- and post-food web activity about related topics.
Note: there is no student worksheet accompanying this activity. It is purely
discussion-based.
Curriculum Links
Missouri Science Standards
4.2.A-a. Illustrate and describe the flow of energy within a food web
4.2.A-b. Explain why there are generally more producers than consumers in an
energy pyramid
4.2.A-c. Predict how the use and flow of energy will be altered due to changes in
a food web
Next Generation Science Standards
HS-LS2-6. Evaluate the claims, evidence, and reasoning that the complex
interactions in ecosystems maintain relatively consistent numbers and types of
organisms in stable conditions, but changing conditions may result in a new ecosystem.
[Clarification Statement: Examples of changes in ecosystem conditions could include
modest biological or physical changes, such as moderate hunting or a seasonal flood;
and, extreme changes, such as volcanic eruption or sea level rise.]
Goals
1. Describe the flow of energy within a food web
2. Explain the difference between producers and consumers
3. Predict how changes in the environment or species composition will affect the rest of
the members of the food web
4. Learn about the glade ecosystems of Missouri
Materials
-ball of string or yarn that can be passed
-note cards (with photos of different organisms in a glade ecosystem food web)
What to Expect
Based on past experience running this activity, you can expect…
-most students WILL know food webs basics, and the difference between a
producer and a consumer ahead of time; however, students may be confused about
primary produces and where they get their energy (you may need to discuss
photosynthesis and how energy comes from the sun, depending on the age group)
-students may come in with the preconceived notion that you can remove one
organism from a food web and not affect the other organisms
-for middle and high school students, it will be important to extend the activity to
incorporate food web theory, to make the discussion interesting; for example, talk about
the roles of the “10% rule”, bioaccumulation, overpopulation of any one species, and the
beneficial outcomes of ie. deer hunting, given the removal of top predators. See the
discussion questions key.
Protocol
1. Introduce or review food web basics, following the “before/yard notecard
activity” discussion questions below. Draw food webs from both land and ocean
ecosystems on the board to address the answers to the discussion questions, having
students help you fill in the food web.
2. Talk about the glade ecosystem of Missouri (see questions about glades
below) to define glades and bring context to them. Go through the notecards, which are
examples of organisms in a glade food web (sun is included, since it will later provide
energy in a web); briefly discuss the animals with the students. Explain that glades are
good systems to study food webs because they are similar to closed off “islands” or
naturally isolated laboratories, versus forests which are more continuous. Therefore,
glades may be less affected by nearby food webs than other ecosystems.
3. Let students choose notecards with the animals on them. Make sure the sun
and at least one decomposer, one producer, and one consumer are selected. The
student with the sun will be given a ball of yarn, because the sun is “the creator of the
ball of energy.” That student will hold on to one end of the string, while throwing the ball
of yarn to a student holding a card of a producer. The student representing the
producer will throw the ball of yarn to a consumer, still holding onto the part of the string
between the sun and the consumer. The ball of yarn will then be passed to a
decomposer, etc., and this will be repeated (another food chain will be made and the
yarn passed among students) until all students are holding a piece of string.
4. Have a discussion with the students during this activity (see below).
5. Have a discussion with students after the activity, going back to the board to
draw/etc. (see below).
Discussion Questions—AND ANSWER KEY
Before yarn/notecard activity, for review:
1) What is a food chain? What is a food web? A food chain shows the feeding
connections (who eats whom) in an ecological community. The food web is
represented by nodes of different species (usually drawn in circles) and connected by
connections or lines. There are trophic levels in a food chain, which is made up of
producers and consumers. Consumers (including the sun and decomposers). A food
web explains how food chains are connected to one another.
2) What is an example of a food web that humans are involved in? (Draw
terrestrial food web of for example Sun  plants  deer  humans 
bacteria/fungi/worms)
3) What is an example of a food web not on the land? (Draw an aquatic food
web or for example Sun Algae or phytoplankton (aquatic producers)  small fish 
large fish  sea lion  shark  bacteria)
4) What is a producer? Also called an autotroph, a producer is an organism that
makes its own food (like carbohydrates, fats, and proteins) from its environment, usually
by using energy from light (via photosynthesis) or inorganic chemical reactions (via
chemosynthesis). Examples are plants and algae.
5) What is a consumer, a primary consumer, and a secondary consumer? Also
called a heterotroph, a consumer is an organism that cannot fix carbon on its own, so it
consumes other organisms to grow. Examples of consumers include animals and fungi.
A primary consumer eats the producers and the secondary consumer eats the primary
consumers.
6) What is a decomposer? Decomposers (or saprotrophs) are organisms that
break down dead or decaying organisms, and in doing so carry out the natural process
decomposition. Like herbivores and predators, decomposers are heterotrophic (or also
consumers), meaning that they use organic substrates to get their energy, carbon, and
other nutrients for growth and development. Decomposers can break down cells of
other organisms using biochemical reactions that convert the prey tissue into
metabolically useful chemical products. Decomposers use dead organisms and nonliving organic compounds as their food source. They include bacteria, fungi, and
worms.
7) Why are there generally more producers than consumers? As energy is
passed on from one organism to the next (like when one organism eats another
organism), most of the energy is actually lost (around 90% gets lost with each
transition). Thus, to get enough energy to survive, a consumer must eat a lot of
producers.
While going through notecards of species; discuss glades, their species, and
some Missouri natural history:
8) What is a glade? Glades are very hot and dry environments found in the
middle of more moist (mesic) forests. They have many desert-adapted species,
including cacti, scorpions, tarantellas, and lizards, and there are species that occur
nowhere else in the world – these species are endemic to these glades. Glades are hot
and dry because they have thin soils, exposed rock that absorbs sunlight and heat, and
they tend to be on west-facing slopes, which are also more exposed to the sun for a
longer portion of the day.
9) Why do you think we may have these dessert species in Missouri? In a sense
these species and other glade inhabitants of Missouri can be considered relicts of the
Xerothermic Period, 8,000 to 5,000 years ago, when the state’s climate was hotter and
drier than today. Many inhabitants of these glades such as the greater roadrunner and
tarantula are at the northeast edge of their natural range. During the Xerothermic period
many southwestern and great plains species expanded their ranges into Missouri.
Since then the climate has ameliorated and these desert species have found suitable
habitats on Missouri’s mini-deserts, the glades, which are like dry islands surrounded by
mesic forest.
After creating a food web with students connected to each other through yarn:
10) What is a keystone species? A keystone species is the species that has a
bigger impact on the food web than any other species. The student holding the most
pieces of yarn or most “connected” in the food web and therefore has the greatest
potential influence on the food web if, for example, its species numbers increase or
decrease.
11) Which of these species did you not know occurred in Missouri? Students
may say tarantula, roadrunner, etc.
12) How would changes in the environment or species composition affect the rest
of the members of the food web? (Have student volunteer to discuss what would
happen if the environment changed and it affected the numbers of their species.) If one
species goes extinct, the species who eat it will suffer a loss of their food supply. And
any species that used to get eaten by the now extinct species will grow in population
size—this could lead them to overexploit the organisms they eat, causing more
extinctions.
13) In a food chain, what happens to the abundance of herbivores when a top
predator is added? (Have student volunteer to discuss what would happen if the
species they represent increased or decreased)? Herbivores decline. What happens to
the abundance of producers? Producers increase in number. (Think about increase in
wolves causes a decrease in deer, which in turn causes an increase in the number of
herbs generally eaten by dear).
14) What do you think happens to the amount of the sun’s energy originally
captured by plants, as it passes through herbivores and predators? Energy is lost to
carry out metabolic processes as it passes through herbivores and predators, and the
energy is used for motion or lost as heat.
After activity discussion:
15) Why might humans want to “eat lower on the food chain,” and what does this
have to do with the “10% rule?” (Draw squares on board, taking 10% of the previous
square’s size each time, to explain the 10% rule.) It is technically more energy efficient
to eat lower on the food chain. The 10% rule stands for how only 10% of energy is
maintained as it moves up the food chain. Pretend a primary producer gets, for
example, 100,000 calories of energy from the sun. If an herbivore (such as a cow) ate
the plant, the cow would get 10% of the 100,000 calories. So, the cow will only gain
10,000 calories from eating the plant. If a human or carnivore/omnivore eats the cow
(beef/steak), the human will gain only 10% of the 10,000 calories that the cow gained.
So, the human will get 1,000 calories from the steak. So, energy is lost as it moves up
a food chain, which it is technically more efficient to eat lower on the food chain.
16) Would a species’ extinction have a greater impact on a big food web or a
small food web? (Return to board with food webs on it and cross out species to show
what would happen.) A small food web; less replacement producers and consumers to
keep the trophic levels in check.
17) Is it important to protect species that are NOT keystone species or top
predators? What reasons might you give for protecting these species? Yes, it is
important to protect all species; if a disease wipes out one species, for example, then it
will be important to have other species fulfilling similar producer or consumer positions
in the food web.
18) Bioaccumulants are substances that increase in concentration in either a
single trophic level or across many trophic levels, as the organisms breathe or ingest
contaminated air, water, or food. Bioaccumulants are barely (or are not) excreted or are
very slowly metabolized. Have you ever heard of any bioaccumulants in nature?
Example: Fish carrying heavy metals which are toxic to humans?
19) What happens to organisms within a trophic level that ingest many
bioaccumulants? Bioaccumulation occurs within a trophic level, and is the increase in
concentration of a substance in certain tissues of organisms' bodies due to absorption
from food and the environment.
20) If the bioaccumulants are not easily excreted or metabolized, then what
happens to the animals higher on the food chain (i.e. secondary consumers) that eat
many organisms having bioaccumulants (i.e. primary consumers)? Biomagnification,
also known as bioamplification, is the increase in concentration of a substance
ACROSS THE FOOD CHAIN OR FOOD WEB, involving multiple trophic levels. The
following is an example showing how bio-magnification takes place in nature: An
anchovy eats zoo-plankton that have tiny amounts of mercury that the zoo-plankton has
picked up from the water throughout the anchovies lifespan. A tuna eats many of these
anchovies over its life, accumulating the mercury in each of those anchovies into its
body. If the mercury stunts the growth of the anchovies, that tuna is required to eat
more little fish to stay alive. Because there are more little fish being eaten, the mercury
content is magnified. Biological magnification often refers to the process whereby
certain substances such as pesticides or heavy metals move up the food chain, work
their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in
turn are eaten by large birds, animals or humans.
21) Why do you think that deer hunting is encouraged by the Missouri
Department of Conservation? The hunting of deer, and other animals, keeps the food
web in check. Since we no longer have wolves to kill the deer in Missouri, we have to
lower deer populations in order to protect the herbs that they eat.