ThoughtLab 11-B INVESTIGATION Skill Check Sampling Hare Populations Initiating and Planning Performing and Recording ✓ Analyzing and Interpreting ✓ Communicating The snowshoe hare (Lepus americanus) is a North American mammal, residing predominantly in the boreal forests of Canada but also extending into the tundra. It eats various plants, depending on what grows where it lives, and in the winter survives on a diet of buds, twigs, bark and even meat under severe conditions. Predators include the lynx (Lynx canadensis), red fox (Vulpes vulpes), arctic fox (Vulpes lagopus), great horned owls (Bubo virginianus), and various hawks, ravens, and crows. One of the longest running studies of hare populations began in 1976 in a boreal forest habitat in the Kluane Region of the Yukon Territory. The goal of the Kluane Ecological Monitoring Project was to measure long-term changes in the communities in this region. As shown by the food web diagram below, the researchers thus monitored not only hares, but several other species, all of which affected or were affected by the hares, either directly or indirectly. Materials • notebook paper • graphing paper • calculator (optional) • spreadsheet or graphing software (optional) Pre-Lab Questions 1. How can a change in the dominant species of primary producers affect the rest of the food web? A simplified food web (only the major feeding links for boreal forest of the Kluane Region, Yukon Territory). The shaded boxes indicate which species were studied as part of the Kluane Ecological Monitoring Project. 2. If a population of about 200 000 herbivores supports about 200 predators of those herbivores for long periods of time, what would the effect be of a flash flood causing 1000 herbivores to die? 3. If the same flood killed off 100 of the predators, what might happen? golden eagle hawk owl greathorned owl lynx coyote goshawk red fox kestrel wolverine red-tailed hawk wolf northern harrier moose small rodents red squirrel ground squirrel snowshoe hare willow ptarmigan spruce grouse fungi forbs passerine birds insects grasses bog birch 536 MHR • Unit 5 Population Dynamics grey willow soapberry white spruce balsam poplar aspen number of days in both spring and fall, and the same number of traps was used in each area. Traps were opened in the evening, and were checked again in the early morning. When a hare was caught for the first time it was tagged and various variables were recorded (such as sex, weight, grid location where caught) before it was released. If a hare was recaptured its tag number was recorded and the same measurements were taken again. On the basis of these captures and recaptures an estimate of the number of animals in each study area was calculated. These estimates from the fall of 1976 to the spring of 2001 are shown in the table. Question How can data be found and used to estimate population numbers and densities in a given year, and how can the fluctuation pattern of the population be determined? Organize the Data 1. Study the data in the table. Although these data have been simulated, they are representative of the densities observed in each year of the Kluane study. In this hypothetical study, traps were laid out at regular intervals in a grid pattern in four study areas each 60 hectares in size, as shown in the figure on the next page. In each area, trapping was done for the same Data Table Estimated Number of Hares in Each Study Area in Fall Year of Study (fall/ spring) Area A Area B Area C Estimated Number of Hares in Each Study Area the Next Spring Area D Area A Area B Area C Area D 1976/77 1.2 3.4 2 3 3 2 5 4 1977/78 11 16.6 15.2 19.5 18 17 19 18 1978/79 68 59.75 65 78 50 42 39 44 1979/80 191.33 202.5 226.25 245.55 117 112 125 117 1980/81 202 190 214 204 172 182 162 172 1981/82 252 242 288 280 40 50 45 45 1982/83 22 20.33 9 20 8 9 8 9 1983/84 15.33 21.25 11.5 18.66 11 10 9 9 1984/85 11 16.6 15.2 19.5 9 9 10 11 1985/86 14 17.2 17.66 19.5 7 8 6 7 1986/87 1.2 3.33 4.66 3 7 8 9 10 1987/88 52 52 62 70 60 50 70 50 1988/89 130 137.5 122.66 157.33 45 55 35 45 1989/90 142 121 152 144 88 98 68 88 1990/91 87 110 86 103 50 40 60 50 1991/92 58 59.75 75.5 68 17 12 20 19 1992/93 10 5.5 7.25 8.75 5 4 6 5 1993/94 11 14 15.2 9 4.5 5.5 3 5 1994/95 32 36 35 28 11 6 17 10 1995/96 70 52 64 67 30 40 20 30 1996/97 100 99 86 111 45 65 35 45 1997/98 122 132 116 132 120 130 110 120 1998/99 149 166 184 157.33 80 90 70 80 1999/2000 66 63 72 76 14 16 6 9 2000/01 3.75 5.5 8.75 3 Chapter 11 Describing Populations and Their Growth • MHR 537 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 7. In the Kluane Region, the snowshoe hare is what many ecologists refer to as a keystone species: one whose disappearance would have catastrophic effects on many other species in the community in which it lives. Consider the food web shown on the previous page. How do you think removal of all hares from this area would affect the structure of the community? Conclude and Communicate 8. What might cause the difference between the spring and fall cycles? 9. What might cause the multiyear cycle? A B C D E F G H I J K L M N O P Q R S T Each dot on this grid represents a trap. There are approximately 30 m between each point on the grid. The study area considered to be “covered” by these traps is larger than the grid itself, being about 60 ha in size. 2. Calculate the average number of hares captured per 60-hectare area in the spring of each year, then do the same calculation for fall. You might decide to work in groups, with some members of a group doing the spring data and others doing calculations for fall. Using this average figure, determine the density of hares per hectare in each season. 3. Make a graph of your data—for spring or fall densities or both—showing how the population density changes over time. Year (this is the manipulated or independent variable) should be plotted on the x-axis and hare density (this is the responding or dependent variable) on the y-axis. Analyze and Interpret 4. For step 1, what is the advantage of sampling the number of hares in relatively small areas rather than trapping over the entire area? Why was the trapping done the same way in each area, or why was each area the same size? 5. For step 2, why were four areas studied instead of just one? Why is the density you calculated really an estimate of the density in this area, rather than a measure of the actual density? 6. Examine your graph from step 3. Describe the period of the hare population cycle in the Kluane Region. That is, how many years are there between successive density peaks or lows? Was there a difference between the periods of the spring and fall cycles? Suggest a reason why or why not. 538 MHR • Unit 5 Population Dynamics 10. How would you expect the population of predators to vary over the same time frames? Explain a possible means for the predator population to undergo a cyclic change similar to the hares. Which predators might do this? 11. Explain a possible means for the predator population to remain constant throughout the year. Which predators might do this? 12. Would the predator population show the same population curve on the multi-year cycle as the hares do? Explain your answer. 13. In this lab, the method used for gathering data was “mark and release”. If you were to modify this study to gather data about lynx in the same region, what would the best method be to estimate their population, and why? Propose a Course of Action Research the rationale for introducing rabbits to Australia, their effects on the marsupial population, and the effects of introducing other species to try to control rabbit populations there. What suggestions do you have to deal with this situation? Extend Further 14. INQUIRY This is a “textbook” example of population curves, meaning that it seems simple and the real world data match what the model would predict. Research a “chaotic population bifurcation” and write a paragraph to explain what you found. 15. RESEARCH The snowshoe hare was introduced to Newfoundland in the mid 19th century. What effects did that introduction have on Newfoundland’s indigenous arctic hare population? Why were the effects so much less than the introduction of rabbits to Australia?