OBJECTIVES: Food Chains and Webs Owl pellet study 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 1
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 = Bird = Shrew = Other = Mole = 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? In a month? In a year? 2
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? 3
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. 4
DATA TABLE for ENERGY CONTENT of OWL PREY PREY TYPE Number in your pellet Total number in class Average mass of prey type, grams (g) Kcal/g body mass Rodent 60 g 1.5 90 Average Kilocalories per animal TOTAL kilocalories for all class animals Shrew 7 g 1.5 10.5 Mole 80 g 1.5 120 Bird 65 g 1.5 97.5 CALCULATIONS AND QUESTIONS Sum total number of calories in all class owl pellets = Total number of owl pellets analyzed by the class = 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. 5
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. 6