Predator is to prey as owl is to

Predator/Prey Interactions Investigation Manual

ENVIRONMENTAL SCIENCE

PREDATOR/PREY INTERACTIONS

Overview Owls disgorge pellets containing the indigestible parts of their prey. Wildlife biologists routinely collect and examine owl pellets to determine the species, populations, proportions, and seasonal variations of the owls’ prey. Moreover, they can even get a quick assessment of the small mammal population residing in a given area. This activity is intended to take two weeks and requires students to share and aggregate data collected by others in their class.

Outcomes •    Identify bones of an owl’s prey using a dichotomous key. •   Communicate data about the number and types of prey found. •   Compare the diets of owls in two regions from different biomes. •   Infer the impact of an individual species on a community. •    Define the different trophic levels, and assign predators and prey 

to the appropriate levels. •    Describe how an owl pellet is produced and what its contents

reveal about the predator and its prey.

Time Requirements Preparation ........................................................ 5 minutes Activity 1: Owl Pellet Investigation .................... 60 minutes Activity 2: Owl Pellet Investigation Results ....... 30 minutes

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Table of Contents

2 Overview 2 Outcomes 2 Time Requirements 3 Background 8 Materials 8 Safety 9 Activity 1 10 Activity 2 10 Disposal and Cleanup 12 Data Tables

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photograph results and

submit

stopwatch required

warning corrosion flammable  toxic environment health hazard

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Background One of the most important interactions within a community is predator-prey interaction in which one organism kills and consumes the other. The effects on the prey are immediate and lasting,  killing both the individual and its potential future contribution to the population in the form of offspring. The impact on the prey’s population,  however, is potentially beneficial. Predation is  rarely completely random. Frequently the indi- viduals that are preyed upon are those that are easier to catch because they are inherently weaker and slower, lack behaviors that increase their chance of survival, or are weakened by disease or hunger. Thus, not only are slower, weaker individuals more likely to be removed from the population, but those that are less adept at finding food or more susceptible to  disease are also more likely to be eliminated.

Trophic Transfer Scientists group organisms into trophic levels according to their primary source of energy. There are three basic types of organisms: producers, consumers, and decomposers.

•    Producers convert nonorganic energy (usually from the sun) into chemical energy.

•    Consumers are broken down into primary (1°) consumers and secondary (2°) consumers based on whether their main source of energy comes from producers (primary consumers) or other consumers (secondary consumers).

•    Decomposers, or detritovores, feed upon the dead matter of producers, consumers, and other decomposers.

No energy transfer is 100% efficient. When a  consumer ingests an organism, only a fraction of the energy is incorporated into the tissue, or assimilated into the consumer. Assimilation is

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defined as the difference between the amount  ingested and the amount of energy excreted in the form of urine and feces.

Some of the assimilated energy is used for metabolic processes and is ultimately lost to the organism in the form of heat. The rest of the assimilated energy becomes the biomass of the consumer. The production efficiency is the percentage of the assimilated energy that becomes new biomass. Biomass is the amount of organic matter in a system.

This loss of energy results in a corresponding decrease in biomass at each trophic level. Ecologists frequently use 10% production efficiency between trophic levels as a “rule of thumb.” In most terrestrial environments this creates a trophic pyramid (see Figure 1), where the amount of biomass is greatest in the lower trophic levels and each subsequent trophic level has substantially less biomass than the previous one.

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Figure 1.

PREDATOR/PREY INTERACTIONS

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populations within the chain. A complex food web (see Figure 2), where a disruption in only one or two populations can be compensated for by several other populations, results in a more stable community. In the illustrated food web, algae, diatoms, and water plants feed tadpoles, water snails, mayfly nymphs, daphnia, hydra,  annelid worms, and flatworms, which in turn,  feed diving beetles, small fish, bladderwort,  dragon fly nymphs, and water boatman. Large  fish eat the small fish and the dragonfly nymphs.  Consider what happens when the mayfly nymph  is removed from the food web. The algae upon which they feed may experience an increase in growth, but that will quickly be transferred

Food Webs So far we have talked about energy transfer as if it is a linear transition from one group of organisms to another; however, reality is much more complex. The diet of the barn owl is made up primarily of voles and mice (1° consumers), shrews (2° consumers), and rats (2° consumers and detritivores). Predators often exhibit preferences for certain types of prey that are usually determined by their ease of capture and nutritive value. These factors change in different  locations and at different times of the year.

Linear food chains are inherently unstable because a disruption to any member of the community has a profound effect on all 

Background continued

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Figure 2.

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and rats that feed on their harvests.

Owls usually swallow their prey whole. However, owls differ from other species of birds because  they do not have a crop, the baglike organ used to store food after it has been swallowed so that it can be digested later. In owls, food passes directly from the mouth to the gizzard. The gizzard is an organ that uses digestive fluids  and bits of sand and gravel to grind and dissolve all of the usable tissue from the prey.

The types of tissue that can be dissolved by an owl’s digestive system include muscle, fat, skin, and internal organs. These tissues are broken down into a variety of nutritional substances

to the tadpoles and daphnia. More daphnia in the system will replace the need for the mayfly  nymph in the diet of the small fish. In a more  linear food chain that consists of only algae, mayflies, and small fish, for example, removing  the mayflies would have a catastrophic impact  on the small fish population since their only food  source would be eliminated.

Barn Owls Barn owls prefer quiet, secluded forest edges near open grasslands where they can feed on rodents, small birds, and other small mammals, such as rabbits. Such a diet compels them to settle into barns and silos, where farmers welcome their assistance in controlling the mice

Figure 3.

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are not actually broken during the attack and the subsequent digestion process, they can be readily identified in the pellet. Most pellets  include a skull(s), which can be used to identify the prey. It can be challenging to distinguish all the features and characteristics of a type of organism; however, a dichotomous key can be used to facilitate this process. A dichotomous key is a tool for identifying items based on a series of choices between pairs of descriptors that refer to the organism under investigation. With each choice, the user is directed to another choice, ultimately leading to one that definitively  identifies the organism. 

In this lab, you will examine the contents of two owl pellets from very different regions: the  Pacific Northwest and the Southeast US. These  different regions occupy different biomes (see Figure 3). Biomes are defined primarily by their  temperatures and humidity. Regions of similar temperature and humidity tend to support similar ecosystems (see Table 1). You will share your data with your classmates and use it to extrapolate the likely numbers and types of prey consumed by an individual owl in each of the regions. You will hypothesize a potential food web and use it to discuss differences in the  communities represented.

by the owl’s gizzard and intestines. Some of these tissues (e.g., fur and bones) cannot be digested. This material, along with other waste collected throughout the body, is ejected from the vent, which is a combination of reproductive opening and excretory opening in birds. The pasty white excrement is known as urea. It is very rich in nitrogen and similar to urine in mammals, only thicker. Indigestible material left in the gizzard—such as teeth, skulls, claws, and feathers—are too dangerous to pass through the rest of the owl’s digestive tract. To safely excrete this material, the owl’s gizzard compacts it into a tight pellet that the owl regurgitates. The regurgitated pellets are known as owl pellets.

An owl pellet generally reaches its final form  a few hours after the owl has eaten. However, the pellet is not usually ejected immediately after it is formed. Owls can store a pellet in a structure known as the proventriculus for as long as 20 hours before disgorging it. Since the stored pellet partially blocks the entrance to the digestive system, it must be ejected before the owl can eat again. Owls typically produce 1 to 3 pellets per day.

The actual process of regurgitating a pellet lasts from a few seconds to several minutes. The pellet is forced out by spasms of the owl’s esophagus that make the owl look like it is coughing painfully. However, it is not hurt by the process because the pellet remains soft and moist until it leaves the owl’s body.

Through careful examination of the pellet’s contents, researchers can discover quite a bit about an owl’s lifestyle and the community in which it lives. Since most of the prey’s bones

Background continued

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Table 1.

Biome Average Annual Temperature Annual Precipitation

Primary Vegetation

Tropical Forest 20–35° C >200 cm/year Deciduous Trees

Eastern (Temperate) Deciduous Forest 5–20° C 50–225 cm/year

Deciduous Trees

Grassland -5–20° C 50–90 cm/year Grasses

Desert 20–30° C <50 cm/year Cacti and Short Grass

Chaparral 10–40° C 25–40 cm/year Scrub Grass and Trees

Northern Coniferous Forest -40–20° C 35–90 cm/year

Coniferous Trees

Moist Coniferous Forest 5–20° C >200 cm/year

Coniferous Trees

Tundra -40–18° C <50 cm/year Lichens, Moss, and Grasses

Safety Wear your safety goggles, lab apron, and gloves at all times while conducting this investigation.

Read all instructions for this laboratory activity before beginning. Follow the instructions closely and observe established laboratory safety practices, including the use of appropriate personal protective equipment (PPE).

Owl pellets contain the remains of small animals that the owl has ingested and can be a source of bacterial contamination. Carolina’s individually wrapped owl pellets are heat sterilized at 250 º F for 4 hours to eliminate most bacteria, including salmonella strains. We do not treat them with chemicals. Keep them wrapped until use to prevent insect infestations or contamination.

Complete dissection activities in one day. Afterwards promptly dispose of the owl pellets and all disposable materials used in the activity.

Handle owl pellets, even sterilized ones, as though they could be a source of bacterial or viral contamination.

Owl pellets should not to be dissected in areas where food is consumed. Cover your work space with the dissection pad provided. Also use the disposable tray provided for dissection of the pellets, and dispose of it promptly upon completion of the activity.

Use gloves and wash hands before and  immediately after the activity and after disposing of gloves. Wash your hands with soap and warm water by rubbing them into a lather for at

Materials Included in the materials kit:

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Wooden probe

Owl pellet from North- western US

Owl pellet mat

Owl pellet from South- eastern US

Dissecting tray

Forceps

Reorder Information: Replacement supplies for Predator/Prey Interactions, item number 580205, can be ordered from Carolina Biological Supply Company.

Call 800-334-5551 to order.

Needed but not supplied: •   Blank white paper •   Soap and water •    70% Ethanol or 10% bleach solution for

sanitizing your work space

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ACTIVITY

least 20 seconds before rinsing. Do not use a waterless hand sanitizer that does not have an alcohol content of at least 70%.

Immediately after the dissection activity, wash and sanitize work surfaces using a cleaning agent that is bactericidal and virucidal. Alternatively, 70% ethanol (Beware: it is flammable)  or a 10% household bleach solution

(Beware: chlorine bleach is corrosive and irritating to the skin and may damage clothing) may be used. Use disposable paper towels and  throw them away. Do not use sponges or rags that might hold and spread bacteria or viruses.

Do not eat, drink, or chew gum while performing this activity. Keep pets and children away from lab materials and equipment.

ACTIVITY 1 A Owl Pellet Investigation

1. Gather your supplies. 2.  Place the owl pellet on the tray. Using your 

fingers or the probes, break off a small chunk  of the pellet. Be gentle and do not push the probes any farther into the pellet than necessary. Remember, the bones you are looking for are small and easily broken.

3. Carefully tear the pellet apart and set aside any bones that you find within it.

4. Compare the bones you found with the ones pictured on the bone charts.

5. Identify as many bones as possible. 6. If your instructor wants you to examine

differences in the prey skeletal anatomy,  organize the bones by placing them according to the corresponding bone chart.

7. Repeat this process until you have completely dissected the pellet and removed all of the bones.

8. The prey eaten are recorded according to the number of skulls found in the pellet. Identify each of the skulls found using the dichotomous key provided (Table 2), and record your findings by completing the first  row in the data tables.

9. To use the key, start at number 1 and decide if the specimen at hand has the first  characteristic or the second. If the skull has a beak, then the specimen is a bird, and classification is complete. If the skull has  teeth, on the other hand, move to the second pair of characteristics. Continue until the skull is identified.

ACTIVITY

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ACTIVITY 2 A Owl Pellet Investigation Results

1. Share your data with the class. Compile everyone’s data to determine the total for each type of organism in the owl pellets from each region and record the results in the data tables.

2. Calculate the percentage each prey contributes to the diet of owls in each region by dividing the total number of each prey counted in your class by the total number of prey organisms counted.

3. Refer to your instructor’s directions for further analysis options.

Disposal and Cleanup 1. Package the remains of the owl pellets,

wooden probe, and dissecting tray into the plastic bag provided and place in the trash.

2. Rinse and dry the lab equipment using a 10% bleach solution, and return the materials to your equipment kit.

3. Wash and sanitize work surfaces immediately after the dissection activity according to the Safety instructions on page 8.

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1. Has beak Bird

1. Has teeth 2

2. Canines present between incisors and molars; little to no gap

3

2. Long pair of incisors on upper and lower jaw; gap between incisors and molars

4

3. Zygomatic arch complete; unpigmented teeth

Mole

3. Zygomatic arch incomplete; tips of teeth reddish

Shrew

4. Skull flat and broad; molars flat  topped and acutely angled; may appear to be a single tooth

Vole

4. Skull rounded; molars have rounded cusps

5

5. Upper incisor notched at tip; skull  less than 32 mm

Mouse

5. No notch in upper incisor; skull more than 32 mm

Rat

Figure 4.

Table 2.

Data Table 1.

Rat Mouse Shrew Vole Bird Total Prey

Northwest Pellet

Class Total Northwest

Pellet

Percentage of Northwest

Owl Diet

Rat Mouse Shrew Vole Bird Total Prey

Southeast Pellet

Class Total Southeast

Pellet

Percentage of Southeast

Owl Diet

Data Table 2.

DATA TABLES

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NOTES

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NOTES

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ENVIRONMENTAL SCIENCE Predator/Prey Interactions

Investigation Manual

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Predator/Prey Interactions
Table of Contents
Overview
Outcomes
Time Requirements
Background
Trophic Transfer
Food Webs
Barn Owls
Materials
Safety
ACTIVITY 1
A Owl Pellet Investigation
ACTIVITY 2
A Owl Pellet Investigation Results
Disposal and Cleanup
DATA TABLES