A Study of the Predator-Prey Relationship

Transcription

1 Name: Block: v3/06 A Study of the Predator-Prey Relationship The predator-prey relationship is important to ecosystems. How can we measure that relationship? It would be difficult to follow a predator such as a wolf for a day, a week or longer to record exactly what and how much it ate. Owls, however, provide a convenient means of studying their prey consumption. They eat their entire prey, including the fur and bones, digest the muscle and other soft tissues, and form the indigestible parts into a pellet, which they regurgitate. The common barn owl, Tyto alba, weighs an average of 340 g, is about 30 cm long, and has a wingspan of about 85 cm. It produces one or two pellets per day, depending on the abundance of food. In this investigation, you will examine these pellets, identify the bones present by comparing them with drawings of various animal skeletons or using a key, and thus determine what the owl has eaten. By extrapolating measurements of lower jaws to body mass, you also will determine the approximate food energy in the prey. Materials (per team of 2-3) forceps 2 self-closing plastic bags 1 dissecting needle sheet of white paper cm ruler owl pellet balance Procedure 1. On a sheet of white paper, carefully unwrap the owl pellet, which has been treated so it contains no living organisms. Measure its length (in mm), its width (in mm), and its mass in grams. Record this information below, and also in Figure 3 in your lab and on the board in the classroom. Once everyone in class has recorded their data on the board, you will need to record the class data to complete your laboratory calculations. Take note of the range of mass of the owl pellets in your class. Your pellet data: Length: Width: Mass: 2. Using the dissecting needle and forceps, carefully pick apart the pellet. It is essential to work deliberately, there are fragile skulls in your pellet, which will be destroyed if you rush. Look carefully for the bones, many of which are very tiny. Separate the bones from the other materials and place them in one of the plastic bags. Pick all the bones free of hair. Place the hair and other remains in the other plastic bag; you may need to examine these again. 3. Examine the bones. Look for skulls, skull bones, or lower jaw bones (the jaw bones are the most likely to remain intact). The lower jaws contain teeth or pockets where teeth were. Each jaw typically has a large front tooth, almost like a claw. Use the diagrams in the classroom to identify the rodent prey. Try also to identify remains of any non-mammal prey, such as birds, snakes, or insects. How many different species of prey are present in your pellet? How many individuals of each prey species are present? NOTE: be careful not to count any prey twice. The jaws come in pairs, a pair of jaws represents a single prey.

2 4. Most pellets contain bones of a small, mouse-like rodent called a meadow mouse or vole (Microtus). Pair into right and left halves any vole jaw bones in your pellet. Figure 1: Measuring jaw length. 5. Measure, in millimeters, the length of each vole jaw, as shown in Figure 1. Be sure to measure each pair of jaws only once. Record these measurements in Figure 4 and in Figure 5 in your lab and on the board. When all teams have reported to Figure 5 on the board, record the class data in Figure 5, showing the total number of jaws for each length. 6. The pellet is waste material from live prey that does not pass completely through the digestive tract. To determine how much food energy the pellet represents, you will relate jaw length to live mass of vole. This relationship is an estimate, because the condition of each vole differs, depending on its age, health, nutritional state, the season, and other factors. A large vole weighs about 60 g and a small vole that is out of the nest weighs about 20 g. If the longest jaw is about 30 mm and the shortest about 10 mm, the ratio of mass per mm is 2 g/mm. Figure 2 is a graph of this ratio. Use this graph to complete the table in Figure 4 & 5. For example, if your pellet contained a 15-mm vole jaw, find 15 mm on the x axis in Figure 2 and move up from that point until you intersect the line. Then move left from the intersection to the y axis and read the mass. Repeat for each jaw, recording the mass in your table. 7. If there are remains of other animals in your pellet, treat them as if they were voles, using Figure 2 to estimate their mass. If necessary, extend the graph in Figure 2, using the same ratio of 2 g/mm for jaws longer than 30 mm and shorter than 10 mm. Record these data in your table. 8. Wash your hands thoroughly before leaving the laboratory. Figure 2: Ratio of body mass to jaw length

3 Name: Block: Data Sheet Figure 3: Class Pellet Data A Study of the Predator-Prey Relationship Group Name Length (cm.) Width (cm.) Mass (g.) Total pellet data Average pellet data Figure 4: Individual Group Prey Data Prey Species Jaw Length (mm) Estimated "Live" Mass (g) Total Average Figure 5: Class Prey Data On a separate piece of paper, create a table (called Figure 5: Class Prey Data) to record the class prey data (Group name, Jaw lengths and Estimated live masses). Keep track of which group is reporting each piece of data.

4 Discussion Questions / Analysis of Data (answer on a separate sheet, show all calculations, units, etc ) 1. Use the class data recorded in Figure 5 to prepare a frequency distribution graph of jaw lengths. Label the x axis Vole jaw length (mm) and the y axis Frequency. Create a bar graph by marking the number of jaws for each range of jaw length on the graph (suggested range/bar: 3 mm). What is the general shape of the frequency distribution of vole jaw length? 2. What happens to the numbers of vole jaws present in a pellet as the voles become larger in size? 3. What is the shortest vole jaw on the graph? Voles are smaller than this when born. Why is there no data for these smaller voles? What might have happened to the small voles, which surely were consumed by the owl? 4. From Figure 4, determine the total estimated live mass of prey eaten by the owl that made your pellet. How much of the live mass was made of indigestible material? (For this and all following calculations: show your work/calculations, always include units and box your answer) 5. Assuming that an owl, on average, produces 1.5 pellets per day, how many grams of food does the owl that produced your pellet eat per day? 6. A single pellet may not be from a "typical" day. The average of the data from all the pellets examined in your class provides a better estimate. From the class data recorded in Figure 5, determine the average number of prey eaten by a barn owl per pellet and per day. 7. What is the average mass of prey eaten by a barn owl per pellet? per day? (Determine the average jaw length from the class data and use that number to find the average mass from Figure 2.) 8. How many voles and other rodents does an average barn owl eat in a year (365 days)? 9. What is the total estimated mass of prey eaten by an average barn owl in a year? 10. Most food chains can be thought of as pyramids, with just a few predators being supported by many, many prey individuals. A typical barn owl weighs about 340 g. (see introduction to laboratory). How many times its own mass will an average barn owl eat in a year s time? (use your total from question 9 above). 11. Not all of the prey mass calculated above provides Calories to the barn owl. Why not? What components of a prey do not supply Calories to the owl? 12. Based on the mass of an average owl pellet and the fact that most rodents have a body mass which is 65% water, what is the actual mass of prey per year that provides Calories to your owl. Explain your calculations. 13. The soft tissue of prey consists primarily of muscle and fat. Fat contains approximately 9 Calories per gram while protein and carbohydrates (the principle components of muscle) contain only 4 Calories per gram ( Mice

5 tend to have 4.9% body fat. Red-backed voles average 2.1% body fat. Meadow voles average 1.7% body fat. ( If we assume that the average owl eats only mice, red-backed voles and meadow voles, in equal proportions, then how many Calories did the average owl eat in a year? 14. Let s make the incorrect assumption that all 340 g of a barn owl is available Calories (how many g of owl would you guess is actually available Calories?). How many Calories does the average barn owl consist of if it has 10% body fat ( Based on this, does the barn owl appear to follow the ten percent rule? What percentage of Calories actually makes it to your owl? 15. Assuming the 10% rule applies, how many Calories of producers, grasses, grains, seeds, etc will it take to sustain one owl? Draw a pyramid of energy for the barn owl, its prey and the producers that feed its prey. 16. The food pyramid you drew for #15 is a simplification. Some of the prey skeletons in the owl pellets are those of shrews, which eat other animals. How does this information affect the trophic level of the owls? What does this mean for the estimated Calories of producers needed to support an owl? 17. Another simplification made on #15 is assuming that 10% of the Calories of producers become Calories of prey. The 10% rule is an average, the Calories may be more or less. In fact, in terrestrial mammals, only about 5% of consumed Calories become Calories of mammal flesh. Why might this be so? Revise your energy pyramid from #15 based on this new information. 18. In no more than three sentences, what is the basic point of this investigation?

6 Discussion Questions / Analysis of Data 1. Use the class data recorded in Step 5 to prepare a frequency distribution graph of jaw lengths. Label the x axis Vole jaw length (mm) and the y axis Frequency. Create a bar graph by marking the number of jaws for each length on the graph. What is the general shape of the frequency distribution of vole jaw length? Bio2 2. What happens to the numbers of vole jaws as the voles become larger in size? 3. From your table, determine the total estimated live mass of prey eaten by the owl that made your pellet. How much of the live mass was made of indigestible material? (For this and all following calculations: show your work/calculations, always include units and box your answer) 4. Assuming an average of 1.5 pellets per day, how much food does the owl that produced your pellet eat per day? 5. A single pellet may not be from a "typical" day. The average of the data from all the pellets examined in your class provides a better estimate. From the class data recorded in Step 5 (see table 3), determine the average mass of prey eaten by a barn owl per pellet? (Determine the average jaw length from the class data and use that number to find the average mass from Figure 2.) 6. Now, what is the average mass of prey eaten per day? (Remember that owls produce 1.5 pellets per day.) 7. What is the total estimated mass of prey eaten by a barn owl in a year? 8. Most food chains can be thought of as pyramids, with just a few predators being supported by many, many prey individuals. A typical barn owl weighs about 340 g. (see introduction to laboratory). How many times its own mass will a barn owl eat in a year s time? (use your total from question 7 above). 9. Using the multiplier that you calculated in question 8, estimate the amount of plant material eaten in a year by the herbivores found in the owl s diet. (Assume all the prey are herbivores.) 10. Draw a biomass pyramid of the owls, the prey they eat, and the producers. Include the units (in g.) for each level. How would a pyramid of numbers differ from the pyramid of mass?