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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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

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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

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