TITLE: IT’S LONELY AT THE TOP
AUTHOR: Kasey Yturralde
DATE: Spring 2005
BACKGROUND
This lesson plan emphasizes the relationships between organisms that are characterized
by ecologists as food chains and food webs. In food webs, organisms are divided into
groups according to the manner in which they acquire energy. The main source of energy
is the sun (this should be stressed during mobile construction.) Producers harness light
energy from the sun and combine this energy with carbon dioxide to produce sugars.
Examples of producers are plants and photosynthetic bacteria and protists. These
photosynthetic organisms are consumed by a variety of organisms called consumers.
Consumers describe any organism that cannot make its own energy, as plants do.
Consumers get their energy from ingesting producers and other consumers. Consumers
that only eat producers are also referred to as primary consumers, herbivores, or
vegetarians. Consumers that only eat other consumers are known as secondary
consumers, or carnivores. Animals in the next trophic level that eat secondary consumers
are known as tertiary consumers. Some organisms get their energy from both plants and
animals. These organisms are called omnivores and some examples are bears and
humans. When all of these consumers and producers die, the energy left in their bodies is
broken down and redistributed by organisms called decomposers.
All of these organisms occupy a distinct trophic level in this hierarchy as energy is
transformed from light energy to plant matter and into animal matter. Organisms are
connected to other trophic levels by their interactions with other organisms. These
interactions consist of being eaten, eating another organism or decay and redistribution
by decomposers. A linear chain from the producers on up to the top consumer is called a
food chain. Combining food chains together with lateral connections forms the complex
food web.
Food chain showing
trophic levels.
Food web example.
http://www.sparknotes.com/testprep/books/sat2/biology/chapter10section3.rhtml
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Trophic levels also describe the transfer of energy through a food web. As energy moves
from one trophic level to the next, much energy is lost through respiration and heat loss.
As a result, only about 10% of the energy in a given trophic level is transferred to the
next. Along with this loss of energy, there is a reduction in the number of organisms and
biomass at each higher trophic level. It is more “expensive”, energy-wise, to produce
large consumers compared to the amount of energy needed to grow producers.
It is important to consider food webs because they illustrate how organisms affect each
other through direct and indirect interactions. Food webs can be disturbed by the
introduction of new species, elimination of species, or pollutants. As pollutants move
through food webs they may increase in concentration, a process called biomagnification.
In biomagnification, pollutants may start out at relatively harmless levels. However,
because biomass decreases at each trophic level, pollutants become increasingly
concentrated toward the top of the food chain or food web. An example of this is the
high concentration of mercury in fish that occupy high trophic levels. Mercury from the
atmosphere settles onto the ocean surface and may enter the food chain through bacteria
or plankton (free floating micro-organisms, some photosynthetic). These are consumed
by fish (primary consumers), who themselves become food for other fish (secondary
consumers or carnivorous fish). These fish may be consumed by tertiary consumers such
as bears, alligators, birds or humans.
OBJECTIVES
Student will be able to
1. Construct a food web diagram
2. Model a food web using a mobile
3. Apply their knowledge of food webs to new ecosystems
a. identify trophic levels
b. describe how organisms fit into their food web
GOALS
Students will learn
1. How energy flows through natural systems
2. How we characterize organisms according to their energy source
3. How organisms affect one another through different trophic interactions
4. Learn how an addition, elimination or disturbance has far-reaching effects within
food webs
STANDARDS
ARIZONA
Grades 6-8
™ Strand 1: Inquiry process
¾ Concept 1: Observation, Questions and Hypothesis
ƒ Formulate predictions, questions or hypotheses based on observations. Locate
appropriate sources.
¾ Concept 2: Scientific testing (investigating and modeling)
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ƒ Design and conduct controlled investigations.
¾ Concept 3: Analysis and Conclusions
ƒ Analyze and interpret data to explain correlations and results; formulate new
questions.
¾ Concept 4: Communication
ƒ Communicate results of investigation.
™ Strand 3: Science in Personal and Social Perspectives
¾ Concept 1: Changes in Environments
Grade 6
™ Strand 4: Life Science
¾ Concept 1: Structure and Function in Living Systems
¾ Concept 3: Populations of Organisms in an Ecosystem
Grade 7
™ Strand 4: Life Science
¾ Concept 3: Populations of Organisms in an Ecosystem
NATIONAL
Content Standard A: Science as inquiry
™ Abilities necessary to do scientific inquiry
™ Understanding about scientific inquiry
Content Standard C: Life Science
™ Populations and ecosystems
MATERIALS
Florida Everglades video
(Any video that describes ecosystems and interactions between trophic levels.)
Pencils
Paper
Branches/Twigs
Twine
Pre-cut pressed wood
Wood paint
Drill
Old newspaper (to place over table while students are painting)
TIMELINE
Day 1 Pre-lab discussion
Florida Everglades video
Day 2 Student groups choose and research system
(Students investigate systems using prepared information packets)
Day 3 Groups being mobile construction: painting
Day 4 Continue mobile construction: assembly
Day 5 Mobile presentation and discussion
Day 6 Assessment essay
Extension
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Day 1 ZOO trip
GRADE LEVEL
This lesson plan is designed for middle school grades 6-8. However, it could certainly be
adapted for high school grades.
PROCEDURE
Day 1
Pre-lab discussion
o What do you know about food webs?
o What do you think you know about food webs?
o What about food webs in unclear?
o What do you want to know about food webs?
o What does it mean to be “at the bottom of the food chain”?
o Why are people warned not to eat too much fish?
Florida Everglades Video
Wrap –up discussion
Day 2
Mobile Project
o Students form groups and together choose a system: Sonoran Desert, California
Chaparral, Redwood Forest, Antarctic, Coral Reef,
o Students read about their system and fill out worksheet that helps them identify
trophic levels and respective organisms
Day 3
Mobile construction
o Draw out schematic of food web
o Draw organisms and prepare each piece for the mobile
Day 4
Mobile construction
o Place pieces on table and organize them before adding twine and connecting to
mobile
o Connect pieces at low trophic levels first
o These pieces should form 2-4 food chains
o Attach food chains to form food web
Day 5
Mobile Discussion
o Student groups present their food webs to the class
o Class discussion
o Where does energy ultimately come from?
o Where would the sun go in your food web mobile?
o Where does energy go?
o Are all systems dependent on sun as primary energy source?
o How do omnivores fit into food web model? Do they?
o Discuss example of what happens when food webs are disturbed
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Day 5
Summative Assessment
o Students read short article about the elimination or addition of organism to a
trophic level
o Use real life example such as invasive species or extinctions
o Students write news article for local community describing the issue and give
suggestions for local citizens
Extension
ZOO trip
Day 1
o Take students on trip to ZOO
o Speak with ZOO staff to determine which exhibits would be best suited for
discussion of food web
o Present students with description of ecosystems at ZOO
o Students complete scavenger hunt worksheets for producers and producers in
African savannah, Sonoran desert and tropical rainforest
GLOSSARY
Trophic level: organisms in an ecosystem that obtain their energy from a common source
Biomagnification: increasing concentration of a substance in successive trophic levels of
a food chain
Biomass: multiply the number of individuals in a population by the weight of an average
individual in it, gives an estimate of the weight of the population; biomass decreases
through successive trophic levels
Consumers: group of organisms that get their energy by eating other consumers or
producers
Producers: organisms (green plants, photosynthetic bacteria and protists) that
produce energy rich organic molecules upon which nearly all other organisms feed
Food chain: describes path of energy as it moves from one organism to another
Food web: complex series of interconnected food chains
EVALUATION
Formative assessment
Students will participate in class discussions before lab activity to highlight their
knowledge and concepts they have yet to grasp. Throughout the lesson students will join
in discussions of ecological concepts.
Summative assessment
Students will read about an ecosystem in which a pesticide is being used and is suspected
of having detrimental effects. Students will respond through a news article that
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highlights characteristics of this food web and how they think pesticide is affecting plants
and animals. Students will make recommendations to local citizens.
EXTENSION
Strange Days on Planet Earth video and curriculum
o This is a very recent nature series by National Geographic that outlines how we
have changed out environment on earth and the ultimate results.
o The relevant volume in this series is the “Predator” video.
o This video related to this lab because it describes how removing the species from
the highest trophic levels has far-reaching effects.
o There is accompanying curriculum that is very well done and all available to be
downloaded from National Geographic’s website.
o These lesson plans include background information and hands-on activities.
WEBSITE RESOURCES
http://www.nhptv.org/natureworks/nwstmich.htm
Has multiple links that explain trophic levels.
http://www.gould.edu.au/foodwebs/kids_web.htm
Website with food web game.
http://www.marietta.edu/~biol/102/ecosystem.html#FoodChainsandWebs4
Info about food webs and biomagnification. At good level for middle school.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FoodChains.html#Food_Webs
Info at higher level than middle school.
http://www.ericdigests.org/2004-1/ecosystems.htm
Discussion of eco-literacy and the standards.
http://www.seattleaudubon.org/education.cfm?id=58
This website has one example of a food web lesson plan for 6th graders.
http://www.sitesalive.com/oil/tg/private/oiltgfoodwebs.html
This has a great idea of using a mobile to model a food chain.
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