ENV101 Lab 1
Name: __________________________
Food Chains and Webs – Owl pellet study
OBJECTIVES:
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How energy moves through living organisms is best understood by analyzing food chains
which will help a student understand why there are so few predators in comparison to the
number of prey.
Use data gathered from owl pellets to understand relationships between trophic levels, to
learn about energy pyramids and to introduce the laws of thermodynamics.
Draw bar graphs to visually represent data, and use data collected in lab to calculate number
of prey needed through time and to determine energy transfer between trophic levels.
General Procedures -Work in groups of two.
General Introduction: Trophic levels describe the feeding relationships within a community. A food
chain is a diagram that shows the transfer of food energy up the trophic levels. There are several
different trophic levels, starting with primary producers as the bottom of all food chains:
Primary producers (Autotrophs): - Use energy from the sun to make chemical energy (in the form of
sugars). Primary productivity is the amount of light energy converted to chemical energy (organic
compounds or biomass) by autotrophs during a given period of time.
Consumers (Heterotrophs) - Feed on other organisms for energy and nutrients
• Primary consumers (herbivores) - eat primary producers
• Secondary consumers (carnivores) - eat herbivores
• Tertiary consumers (carnivores) - eat other carnivores
• Quaternary consumers (top carnivores) - eat other carnivores
Detritivores and decomposers- eat dead organic material
In reality, food chains are more often found as food webs, with most interactions showing a web of
connectivity rather than a direct chain. An important aspect of trophic level interactions produces a
trophic (energy) pyramid. These pyramids represent the ecological efficiency within a food chain.
The figure below shows a trophic pyramid. It shows that a top carnivore (fox) needs to eat many
more primary carnivores to survive over its lifetime. Each of the primary carnivores it eats needs to eat
many more herbivores, and the herbivores need to eat many more producers. This is because, for
every meal eaten by the fox, only a portion of
that food goes to producing energy for the fox
and only a portion goes to growth. The rest of
it gets lost as heat in the transfer of energy and
much also gets passed as waste products
(feces), or is not digested at all (bones, etc).
The same is true for every organism down the
trophic energy pyramid.
We will examine food chains (webs) by
determining the prey of owls. Owls typically
swallow their prey whole or in large pieces.
Bird stomachs include an anterior pouch called the proventriculus and a muscular posterior portion
called the gizzard. In owls, the gizzard compresses indigestible parts of their prey (hair, bones, teeth,
and feathers) into matted pellets that pass into the proventriculus where they remain until something
Rev. 9/2013
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ENV101 Lab 1
Name:
stimulates the owl to spit them out (regurgitate). Pellets are not exclusive to owls. Hawks and related
birds of prey also regurgitate undigested remains, and so do some gulls and herons. Even some robins
have been seen ejecting parts of their food.
Scientists study pellets to discover any regional, seasonal, and
habitat differences in owl prey. Pellets also reveal information
about the relative numbers of small animals found in an owl’s
feeding area. The owl pellets used in our study will be from Barn
owls, Great Horned owls, or Great Gray owls. The majority of
the pellets are collected from the wild. These pellets have been
autoclaved and are, therefore, sterile.
MATERIALS:
• Owl pellets (sterilized)
• Owl pellet bone charts for identification.
• Gloves, Dissecting tools
CAUTION: Please let me know if you suffer from asthma or have strong allergies to animal hair. The
animal hair found in many owl pellets could aggravate your condition.
PROCEDURE:
1. Find a lab partner.
2. Obtain an owl pellet from your instructor.
3. Select some dissection tools and gloves.
4. Carefully dissect your pellet.
5. Identify the bones and sort into different species, using the bone identification charts.
Part 1 Questions:
1. Using the number and type of skulls for your data, record the number of each of the animals
that your owl ate in forming your pellet NOTE that you want to count the number of organisms, not
the number of bones! Sort the different bone sets that would be found in a single organism and count
as one:
Rodent =
Shrew =
Mole =
Bird =
Other =
2. Did you find a complete skeleton?
3. What other undigested material (if any) did you find? Describe:
4. Assume that an owl forms on average 2 pellets each day and that your pellet is an average pellet.
How many animals would an owl eat (show calculations):
• In a week?
2
• In a month?
• In a year?
ENV101 Lab 1
Name:
5. Record the class findings on the
chart at right and use it to construct a bar
graph below to describe the types of owl
prey in our area. Be sure to label each bar
in your graph with its prey type!
6. Looking at the bar graph, what inferences might you draw about the mammal population in our
area, assuming that all prey types are equally available to the owls?
7. Sketch a food chain in the space below, with an owl at the top. Assume that the owl’s prey are
herbivores. Label each trophic level in the chain (primary producer, etc.).
How does a food web differ from a food chain? Which is a more accurate representation of
community feeding interactions?
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ENV101 Lab 1
Name:
Calculating energy transfer in a simple food chain.
In a typical food chain, no more than 10 percent of high quality energy is transferred from one
trophic level to another. In other words, energy transfer between trophic levels are only about 10%
efficient. The remainder is waste heat. This rough “10 percent rule” is in accordance with the Second
Law of Thermodynamics, which states there is an increase in entropy, or disorder, when energy is
transformed. Note that the energy is conserved, not destroyed, according to the First Law of
Thermodynamics, but a tremendous amount of low temperature waste heat (which is a form of
energy) is produced, which radiates out into space.
Example of the Second Law of Thermodynamics. For every 1,000 sun energy units sent to plants, the
producers, 90 percent is converted to waste heat during photosynthesis and other metabolic
activities. This waste heat radiates out into space. The plant keeps and stores 100 energy
units. Consumers, which eat the vegetation will convert 10 heat units to rat stored energy and 90
heat units to metabolic waste heat. The African wild cat (right) eats the rat and converts 9 rat energy
units to waste metabolic heat, which radiates out into space. That leaves only 1 sun energy unit
stored in the form of cat tissue. In summation, of the 1,000 units of sun energy, 999 units degraded to
heat in only three steps of this food chain. In summation, of the 1,000 units of sun energy, 999 units
degraded to heat in only three steps of this food chain.
Often these energy relationships are shown as
trophic energy pyramids, as on page 134 in your
textbook, or in the model at right. Note how this
pyramid differs from the trophic pyramid shown
on page 1 of this lab, showing energy units
rather than number of organisms.
A good estimate of the caloric needs of an owl
may be obtained by identifying the number
and kinds of organisms eaten and knowing the
caloric content of each prey animal. Energy of
food is most often measured in kilocalories
(Calorie, as opposed to calorie). The total
kilocalorie (kcal) content in a small mammal varies with time of year and is primarily due to
differences in the amount of fat tissue. It is usually highest in the summer at 1.7 kcal/gram body
weight, and drops to a low of 1.3 kcal/gram in winter. For this lab, an average value of 1.5 kcal/gram
will be used as the calorie content for small mammals eaten by owls.
Using class data, calculate the total number of kilocalories provided to owls by different prey types to
complete the data table below.
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ENV101 Lab 1
Name:
DATA TABLE for ENERGY CONTENT of OWL PREY
PREY TYPE
Number in
Total number Average
your pellet in class
mass of
prey type,
grams (g)
Rodent
60 g
Kcal/g
body
mass
Average
Kilocalories per
animal
1.5
90
Shrew
7g
1.5
10.5
Mole
80 g
1.5
120
Bird
65 g
1.5
97.5
TOTAL
kilocalories
for all class
animals
Sum total number of calories in all class owl pellets =
Total number of owl pellets analyzed by the class =
CALCULATIONS AND QUESTIONS
1. Calculate the number of kilocalories one barn owl needs per 24 hours.
2. Assume that only 10% of the energy is transferred from the grass and seeds the prey animals eat,
and only 10% of the energy stored in the prey animals goes to the owl. How many kilocalories of
grass and seeds does it take to feed the prey species per day, and how many kilocalories of prey
species is needed to feed the owl?
Example calculation: Suppose your class averaged 165 kcalories/pellet, so 330 kcalories is needed
by a barn owl every 24 hours.
10% × (#calories of seeds consumed by the prey animals) = # calories in barn owl prey/day
0.1 × (Energy Consumed by Prey) = Energy Consumed by owl/day
0.1 × (EPrey) = EOwl
Example: (EPrey) = EOwl/0.1 = 330kcal/0.1= 3300 kcalories/day
Show your calculation of (EPrey) = EOwl/0.1. Do not just give the answer.
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ENV101 Lab 1
Name:
3. Calculate the calories of plant material needed to produce the calories of prey animals in an
average
pellet, assuming a 10% efficiency and using the equation:
0.1 × (Eplants) = Eprey
EPlants = Eprey /0.1
Example: EPlants = Eprey /0.1= 3300 kcalories/day/0.1 = 33,000 kcalories/day
Show your calculation of EPlants = Eprey/0.1
4. Calculate the calories of sunlight needed to produce the plants. Assume that plants are not very
efficient in converting sunlight into stored chemical potential energy, such as cellulose and sugars.
This efficiency varies from 1.5% for most crop plants to sugarcane’s 8%, which is exceptionally
high. Assume a 1.5% efficiency for converting sunlight into plant material energy, the grass and seed
energy that is consumed by the prey:
EPlants =0.015 × (ESunlight)
(ESunlight) = EPlants/0.015
Example: (ESunlight) = 33,000 kcalories/day / 0.015 = 2,200,000 kcalories/day
Show your calculation of (ESunlight) = EPlants/0.015
5. Why do food chains tend to be short? Why are there no “superpredators” feeding on animals like
owls, wolves, or killer whales?
6. Based on what you’ve learned in this lab, explain why some people concerned about the
environment are strong proponents of vegetarian diets for humans.
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