Twelve Years at Hollow Creek: Graphing Voles and Owls · Chapter 21

Learning target: I can graph predator and prey counts, calculate lags, percent changes and prey-per-predator ratios, and use the results to explain a repeating pattern and the factors that bend it.

Directions

The dataset below is a teaching dataset: the counts were invented to be realistic for a meadow in the western United States, not collected by a real survey. Plot both graphs so that the year axes line up one above the other; you will compare peaks by looking straight down from one graph to the other. Show every calculation and write units on every number.

You will need: This worksheet, Pencil, Ruler, Calculator (optional)

Each spring, a field crew counts meadow voles (small, fast-breeding rodents) on a marked 10-hectare patch of Hollow Creek Meadow, and counts the breeding pairs of barn owls nesting in the valley around it. Voles are the owls' main prey. The crew also notes anything unusual about the year.

Simulated data for teaching. Realistic for a vole population that cycles every few years and a small owl population that depends on it.

Part A: Find the pattern

Hollow Creek Meadow spring counts, years 1-12 (teaching dataset)
Simulated teaching dataset: the numbers are invented to be realistic, not measured.
Year	Voles counted on the 10 ha patch	Barn owl breeding pairs in the valley	Crew notes
1	120	4
2	260	5
3	410	9
4	380	14
5	190	13
6	90	8
7	140	4
8	300	6
9	450	11
10	330	15	Dry summer; grass set very little seed
11	150	12
12	110	7

1.Complete the table from your graphs.

First peak (year, count)	Second peak (year, count)	Lowest point (year, count)
Voles
Owl pairs

2.Calculate the lag between each vole peak and the owl peak that follows it. Then calculate the length of one full cycle for voles (first peak to second peak) and for owls. Show the subtractions.

3.Calculate the percent change in the vole count from its first peak (year 3) to its lowest point (year 6). Use: (new − old) ÷ old × 100. Round to the nearest whole percent.

4.Calculate the number of voles per owl pair in year 3, year 6 and year 9. Round to one decimal place. In which year did each owl pair have the least prey available?

Part B: Explain the pattern

6.Why does the owl peak come after the vole peak instead of in the same year? Explain in terms of what has to happen inside the owl population.

7.Look at year 10. Voles dropped from 450 to 330 while owl pairs were still rising to their peak. Using the crew notes, give a cause of the vole drop other than owls, and explain what this year shows about predicting population changes.

8.On the Arctic tundra, lemmings are the main prey of Arctic foxes. Neither species lives at Hollow Creek. Predict whether a 12-year dataset of lemmings and Arctic foxes would show the same pattern as your graphs. State what you expect to be the same, what might be different (for example, the cycle length or the size of the peaks), and why the pattern is likely to repeat even though the species are different.

9.Suppose a second vole predator, the long-tailed weasel, moves into the valley in year 13 and becomes common. Predict the effect on the owl graph over the following years and explain which interaction between owls and weasels you are describing.

Percent change in three lines

Write the new value minus the old value on line 1 (the answer is negative if the count fell). Divide by the old value on line 2. Multiply by 100 on line 3 and add the percent sign. Example with made-up numbers: 50 − 200 = −150; −150 ÷ 200 = −0.75; −0.75 × 100 = −75 percent. For the voles-per-owl-pair question, you are dividing voles by pairs, so the unit of your answer is 'voles per pair'.

The owls' point of view

Barn owls lay more eggs when prey is abundant and may skip breeding when it is scarce. Using the dataset, estimate how many voles the whole owl population needed in year 4 if each breeding pair plus its chicks eats about 1,500 voles in a year, and compare that number with the 380 voles counted on the 10-hectare patch. Explain why the comparison does not mean the owls ate every vole in the meadow, and what it tells you about how large the owls' hunting area must be compared with the counting patch.