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Fauna Series No. 7
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Methods and Extent of Present Research
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Fauna of the National Parks — No. 7
The Wolves of Isle Royale |
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STUDY AREA
Climate
The climate of Isle Royale is similar to that of the rest of the upper-Great-Lakes region. Some snow may be expected any time from September to May, but it accumulates only from mid-November to April. Temperatures are moderated by Lake Superior, especially on Isle Royale, where daily lows in winter may be 6° warmer than those of the mainland. In summer Isle Royale is much cooler than the mainland. Trees are not fully leaved until about June, and traces of autumn color appear in late August.
Weather records for Isle Royale are incomplete, for in most years no one is there from December to May. Table 1 gives the data recorded at Mott Island, near the northeast end of the park, from 1940 to 1952. Since snowfall records are unavailable for Isle Royale, data are presented from the nearest other U.S. Weather Bureau station, Grand Marais, Minn., approximately 36 miles west of the southwest end of the island (table 2).
TABLE 1.—WEATHER RECORDS FROM MOTT ISLAND, ISLE ROYALE, 1940-52
[U.S. Department of Commerce, 1956a]
| Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sep. | Oct. | Nov. | Dec.a | |
| Mean total precipitation (in.) | 2.31 | 1.86 | 2.12 | 2.27 | 2.19 | 3.58 | 2.81 | 3.34 | 3.99 | 2.35 | 2.68 | 1.52 |
| b(3) | (3) | (3) | (4) | (10) | (12) | (12) | (12) | (11) | (10) | (9) | (4) | |
| Mean snowfall (in.) | .... | .... | .... | .... | .8 | 0 | 0 | 0 | .4 | .9 | 12.2 | .... |
| Mean temperature | .... | .... | .... | .... | 45.2 | 52.2 | 58.3 | 60.5 | 52.9 | 44.6 | 30.9 | .... |
| Mean maximum temperature | .... | .... | .... | .... | 54.4 | 61.1 | 67.5 | 68.1 | ? | 51.4 | 35.8 | .... |
| Mean minimum temperature | .... | .... | .... | .... | 35.8 | 43.0 | 49.0 | 52.8 | 46.1 | 37.8 | 26.0 | .... |
| Maximum temperature | .... | .... | .... | .... | 79 | 86 | 87 | 86 | 81 | 72 | 62 | .... |
| Minimum temperature | .... | .... | .... | .... | 19 | 33 | 37 | 38 | 29 | 12 | 7 | .... |
aAnnual precipitation 1941: 30.69; 1942: 35.68 (only years available) bNumber of years on which mean is based. All other figures based on 10 years. | ||||||||||||
TABLE 2.—WEATHER RECORDS FROM GRAND MARAIS, MINN., 1931-52
[U.S. Department of Commerce, 1956b]
| Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sep. | Oct. | Nov. | Dec. | Annual | |
| Mean totala precipitation (in.) | 1.53 | 1.01 | 1.40 | 1.90 | 2.53 | 3.48 | 2.72 | 3.07 | 3.06 | 2.25 | 2.02 | 1.36 | 26.33 |
| Meanb snowfall (in.) | 14.3 | 12.8 | 9.4 | 5.4 | .1 | Tr. | 0 | 0 | .1 | .9 | 7.9 | 12.9 | 63.8 |
| Meanc temperature | 15.2 | 16.1 | 25.0 | 36.3 | 45.5 | 51.7 | 58.5 | 61.1 | 54.2 | 44.1 | 30.7 | 20.4 | 38.2 |
| Mean maximumc temperature | 24.4 | 25.4 | 33.2 | 44.5 | 55.0 | 61.9 | 69.2 | 9.8 | 62.2 | 52.0 | 37.7 | 28.6 | 47.0 |
| Mean minimumc temperature | 6.0 | 6.8 | 16.9 | 28.2 | 35.9 | 41.7 | 47.9 | 52.7 | 46.2 | 36.2 | 23.7 | 12.1 | 29.5 |
| Maximumd temperature | 48 | 51 | 63 | 70 | 85 | 88 | 94 | 96 | 86 | 72 | 62 | 55 | 94 |
| Minimumb temperature | -34 | -34 | -24 | 5 | 17 | 25 | 28 | 33 | 23 | 6 | -13 | -27 | -34 |
aBased on 21 or 22 years. bBased on 19 years. cBased on 20 years. dBased on 18 years. | |||||||||||||
Figure 6—Greenstone Ridge (in background) which runs
the length of the center of the island, as seen from the north.
Microclimates in the interior of Isle Royale differ significantly from those along the shore. Robert M. Linn (1957:96—97) in a study of the island's climax forests and their microclimatological differences found that ". . . in areas near to Lake Superior, temperatures are lower and have less range, and atmospheric moisture is greater than at the higher elevations in the center of Isle Royale. Here temperatures are highest and atmospheric moisture is lowest. These two extreme habitats possess climatic patterns which differ enough to be expressed by different climax vegetation types."
During the present study, the February—March snow depth on the level in wind-protected areas was 16 to 24 inches in 1959, 12 to 16 inches in 1960, and 20 to 26 inches in 1961. Drifts on the northwest sides of ridges were 3 to 6 feet deep, but exposed southeast slopes and thick swamps often had less than a foot of snow. Hakala (1953) reported snow depths of 18 to 36 inches for a similar period in 1953.
By January, extensive sheets of floating ice surround Isle Royale on calm days; during windstorms these break and wash up on shore. This action keeps the lake open south of the island, but a shelf forms along the shoreline and across the smaller bays. In 1959 and 1961 all the harbors and bays (including Siskiwit Bay) were frozen their entire lengths by February. Similar conditions were not encountered in 1960 until March.
During 1959 and 1960 ice often appeared to connect Isle Royale with Canada, but after each high wind the ice span disappeared. However, in 1961, the "bridge" remained intact from February 15 until at least March 21, despite several windstorms.
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Last Modified: Thurs, Jul 4 2002 10:00:00 pm PDT
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