Case Study: Moose and Wolves on Isle Royale
In the early 1900s, a small herd of moose wandered across the ice of Lake Superior to Isle Royale, an island about 72 km long by 13 km wide and about 25 to 30 km from the mainland. There were no other large herbivores on the island and no predators, and the food supply was plentiful, so the population exploded. Over a period of a few years, the large population overgrazed the vegetation and exhausted the supply of their preferred food plants. The population crashed but survived.
Sometimes during the 1940s, a pair of timber wolves reached the island as the moose did, by crossing over the ice. The wolf population slowly grew, and wolves have survived on the island ever since, eating moose as their main winter prey. Although wolves do occasionally kill a healthy adult moose, they normally feed on the very old and very young moose, or those that are weakened by disease or injury. The wolf population normally fluctuates between12 and 25 animals, but it reached a high of about 50 in 1980.
Since the wolf is the only predator of the moose on the island, the populations of both animals have varied over the years. Since 1958, wildlife biologists have been tracking the populations of both species.
Table 1 Wolf and Moose Populations on Isle Royale
Year / Wolves / Moose / Year / Wolves / Moose1958-59 / 20 / 563 / 1980-81 / 30 / 863
1959-60 / 22 / 610 / 1981-82 / 14 / 872
1960-61 / 22 / 628 / 1982-83 / 23 / 932
1961-62 / 23 / 639 / 1983-84 / 24 / 1038
1962-63 / 20 / 663 / 1984-85 / 22 / 1115
1963-64 / 26 / 707 / 1985-86 / 20 / 1192
1964-65 / 28 / 733 / 1986-87 / 16 / 1268
1965-66 / 26 / 765 / 1987-88 / 12 / 1335
1966-67 / 22 / 912 / 1988-89 / 12 / 1397
1967-68 / 22 / 1042 / 1989-90 / 15 / 1216
1968-69 / 17 / 1268 / 1990-91 / 12 / 1313
1969-70 / 18 / 1295 / 1991-92 / 12 / 1590
1970-71 / 20 / 1439 / 1992-93 / 13 / 1879
1971-72 / 23 / 1493 / 1993-94 / 17 / 1770
1972-73 / 24 / 1435 / 1994-95 / 16 / 2422
1973-74 / 31 / 1467 / 1995-96 / 22 / 1163
1974-75 / 41 / 1355 / 1996-97 / 24 / 500
1975-76 / 44 / 1282 / 1997-98 / 14 / 699
1976-77 / 34 / 1143 / 1998-99 / 25 / 750
1977-78 / 40 / 1001 / 1999-00 / 29 / 850
1978-79 / 43 / 1028 / 2000-01 / 19 / 900
1979-80 / 50 / 910 / 2001-02 / 17 / 1100
Questions
1. Plot both sets of data from Table 1 on a single graph. Use different scales on two vertical axes. On the left vertical axis plot the moose population and on the right vertical axis plot the wolf population.
2. From the graph, describe any patterns or trends that you observe in the data.
3. Why do you think the wolves did not totally wipe out the moose population on Isle Royale?
4. Biologist discovered that the deadly canine parvovirus was introduced to the island by dogs. From your graph, in what years did the virus affect the wolf population? What effect did this have on the moose population?
5. The moose population crashed a second time in 1995. Propose possible explanations, other than predation, for this dramatic decline in the population.
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