Introduction to Environmental Science and Lab ENV105
Owl Pellet Survey: Energy Transfer from Prey to Predator
Adapted from Biological Science Initiative (BSI)
Name: ____________________________ Partner(s): ____________________________
INTRODUCTION
Energy pyramids are used to represent important aspects of trophic level interactions. They represent the ecological efficiency within a food chain, which is the amount of usable energy transferred from one trophic level to the next. The figure below shows an energy pyramid in which the top carnivore (Sparrowhawk) needs to eat many more carnivores to survive over its lifetime. Each prey 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 sparrowhawk, only a portion of that food goes to producing energy for the sparrowhawk (cellular respiration) and only a portion goes to growth. The rest of the energy is lost as heat in the transfer of energy and much also gets passed as waste products. The same is true for every organism down the energy pyramid.
Owl pellets provide a nice hands-on tool for teaching the concept of food chains. By examining the contents of the barn owl (Tytoalba) pellets we can gain information about the owls’ seasonal and regional diet and habitat differences. In addition, the pellet contents can effectively illustrate the nature of the food chain and give us a sense of the energy transfer from the prey to predator.
Owls prey on animals from small insects to small mammals such as voles, mice, rabbits and sometimes fish, frogs and small birds. When an owl ingests a prey only the soft tissues of the prey are digested. The indigestible remains (hair, bones, teeth, claws and fur), are evacuated through the mouth in the form of a pellet. It is the undigested matter that provides us the information to construct the owl’s food web.
Part 1: Identification of Small Mammal Prey Items
Objectives:
• Sort skulls and jaw bones.
• Identify skulls using the “Key to Skulls and Jaws Found in Owl Pellets” and other visuals provided.
1. Visually sort skull and jaw bones for the barn owl pellet collection.
2. Using the keys, drawings, and photos provided, identify the skulls and jawbones.
Helpful information:
• Three main types of small mammalian prey:two types of rodents (voles and mice), and insectivorous prey (almost all are shrews)are found in owl pellets.
• Details of the tooth and root patterns allow identification to species.
• There are large differences between the molar teeth, tooth and root patterns of the two rodent subfamilies.
• The teeth of all three mainland shrew species have red tips.
3. Arrange specimens as seen in the figure below (skull, left and right jaw). TAKE A PICTURE FOR YOUR REPORT!
Part 2: Results by Number of Prey Items
Objective:
• Use skull data to determine the total number and percentage of each prey type represented in the pellet collection.
1. Once identification has been made, record the number of each prey type found (based on the number of skulls or paired jaw bones) onTable 1A
2. Determine the Percent of Diet (i.e., availability of the prey species to the owl)by dividing the number or prey Frequency Found(n1)for each species by the Totalnumber of preyfound. Record your values on the column labeled Percent of Diet.
Part 3: Results by Food Value
Objective:
• Determine the biomass that each prey represents in the diet of the barn owl.
Ecologists often want to quantify the relationships in a food web and determine, for example, how much each prey group contributes to the total intake of a top predator (e.g., Barn Owl). The importance of a given prey species to the owl diet depends not only on the relative number of individuals of the species consumed, but also on the typical body weight of the species. That is to say there is a difference between the “food value” (the energy) derived from a shrew (average mass 2 grams) as opposed to a vole (average mass 40 grams).
We can determine how much each prey group contributes to the total intake of the owl food web by taking the average mass of each prey group and expressing it in terms of biomass of prey consumed. This calculation is done by taking the number of individuals in each prey group (n1) and multiplying it by the biomass conversion factor (b). In this activity the conversion factor is a standardized unit based on the assumption that a 20 gram mammal has a biomass conversion factor of 1.0.After calculating the biomass, divide the biomassby the total biomass to determine the Percent Biomass(B1/TB1).
Table 1. Owl Pellet Data Sheet Northeast
See Table from lab activity
QUESTIONS
1. Describe the diet of barn owls based on the pellet sample.
2. Which animal provided the highest percent of biomass to the owl’s diet?
3. Which animal did the owls eat most (the highest number found in pellets)?
4. Are the answers to #2 and #3 the same? Explain why or why not. (Examine # found, biomass, and average mass for the prey that was eaten).
5. The energy transferred between the trophic levels (prey to predator) was not 100% efficient. How was energy lost in this system, what did it leave as?
6. Construct a bar graph illustrating the quantity (Frequency Found) of each prey group represented in the diet of the barn owl. Above each bar write in the percent of biomass for each prey group.
Bar Graph: Quantity (Frequency Found) of Prey Consumed with Percentage of Biomass Indicated for Each Prey Species in the Pellet Collection
7. If an owl needs 100g of food per day, how many shrews will it need to capture?
8. Based on your results do you predict that a crash in the vole population would seriously affect owls? Explain your answer.
9. Based on your results do you predict that a crash in the shrew population would seriously affect owls? Explain your answer.
10. In Part 3 you used a conversion factor to calculate the percentage of biomass of each prey represented in the collection. The conversion factor is based on an average body mass for a population. Would the average body mass for a prey type be affected by the breeding season in the organism? Explain your answer.
11. Use the data obtained from this activity to construct a food web that contains the Barn Owl at the highest trophic level, and grass and seeds at the lowest level.
12. What is biological magnification?
13. Which animal would you expect to have more toxins in its system, an owl or a mouse? Explain why.
Animals Found in Barn Owl Pellets
With Skull Diagrams
