Life Comes To The Island
At the time of Isle Royale's birth, many forms of
life were pressing northward as the warming climate rapidly destroyed
the ice and exposed the land. Which ones first crossed the water to
those smooth gray rocks, pounded by frigid waves, and to those bare,
sandy hills?
Then as now, the wind and water carried spores and
fragments of algae and lichens, and some of these settled on the new
surface. Algae gained a foothold in wet places, and lichens pioneered
mostly on rock, wet or dry. The air, the lake waves, and perhaps also
wandering birds carried seeds of higher plants from the forests and
grasslands south of the ice border. Some of these seedsthose from
plants best adapted to cold, sterile conditionsgerminated and
grew on thin glacial deposits or in cracks in the rock.
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Spiders, by virtue of their abundance and variety, have important roles
as predators in most terrestrial communities. (Photo by Robt. G. Johnsson)
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Quite likely, many of the early colonists were tundra
plants, of species now found only in the Arctic or on high mountains.
Gradually these grasses, sedges, dwarf birches and willows, and other
low-growing plants wove a green carpet over the gray-brown hills, except
in the many places where rock still resisted. Perhaps here and there, in
sheltered spots, spruce trees grew.
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Lichens, the first plants to become established in a new, rocky
environment, help prepare the way for mosses, ferns, and eventually
flowering plants such as blue bell or three-toothed saxifrage. (Top photo by Wm. Dunmire)
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Where there is plant food, animals can follow, and
no doubt the vanguard was not long in coming. Airborne insects and
microscopic animals drifting down upon the greening landscape now could
survive. Birds could begin nesting here. A few caribou may have arrived
across the lake's winter ice. Close on the heels of the plant eaters
came animals that preyed upon them, including insects, spiders, hawks
and owls. If there were caribou, wolves may well have braved the 15-mile
ice crossing to hunt them.
On the Isle Royale of 10,500 years ago, we can
imagine a scene much like that of the subarctic today. Mats of grasses
and bright flowering little plants cover the ridge-tops; shrubby
birches and willows fill the draws; and down in the wet valleys
scattered spruces raise their dark spires. Small bands of caribou,
pausing frequently to scan the landscape for wolves, move up the
hillside, browsing shrubs and nipping ground plants. But this is a
transitory scene in the story of Isle Royale; for the forest is
coming.
As the ice retreated, the land, relieved of its great
burden, slowly rose. The basin's waters, also lifted somewhat by the
rising land beneath, flowed out through the lowest available outlet.
Over the centuries, the trend was for land areas to rise higher above
the water level, as downcutting and escape through successively lower
outlets generally resulted in lowering the elevation of the water
surface.
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(click on image for an enlargement in a
new window)
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By about 10,500 B.P. (before present), the glacial
ice had receded to the northern edge of the Superior Basin, which was
then occupied by Lake Minong. This lake, which emptied by way of the Au
Train-Whitefish strait (Munising, Michigan, area) and St. Marys Strait
(Sault Ste. Marie area), remained stable for many years, thus building
well-defined shorelines which are easily traced today. On Isle Royale,
Lake Minong shore features are found at elevations of about 680 to 765
feet. (Subsequent uneven uplift accounts for the variation.) They
include beach ridges (just west of Siskiwit Bay one is followed by the
Feldtmann Ridge Trail), sea cliffs, and sea stacks. Of the latter,
Monument Rock, on the Lookout Louise Trail, is the most spectacular.
As North American climates continued to warm and the
continental ice sheets melted further, vegetation changed accordingly.
On Isle Royale, spruce gradually took over, first forming scattered open
groves and later, dense forests. Following soon came smaller amounts of
pine, fir, aspen, and paper birch. With the establishment of these
species, the forest must have looked much like the dominant forest on
Isle Royale today.
And what of its animals? Many of these, too, were
species present now. In the Great Lakes forests 9,000 years ago roamed
marten, fisher, wolverine, lynx, snowshoe hare, beaver, muskrat,
porcupine, wolf, woodland caribou, and moose, to name some of the larger
ones. But which of these swam, rafted, or crossed the ice to Isle
Royale we have no fossil record to tell us. It is tempting to imagine
mastodons here, but these great beasts apparently preferred open rather
than closed forests, and by 9,000 B.P. they were nearing extinction. The
island's complement of insects and birds probably included many of
today's. In the shore waters swam trout, herring, and whitefish,
cold-water species that had been able to survive near the glacial
front.
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The spring peeper, which may have reached the island on driftwood,
breeds in water but lives as an adult in shrubby vegetation. (Photo by Robt. G. Johnsson)
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The warming trend that forced the retreat and
eventual disappearance of the continental ice sheets reached a climax
some 6,000 years ago. The climate then was warmer and drier than now,
and prairie grasslands displaced forests as far east as Ohio, Indiana,
and southern Michigan. On Isle Royale this climatic change resulted in
the decline of spruce and an increase of pine, oak, maple, and yellow
birch. The pollen record in bogs suggests that toward the end of this
warm period hemlock, basswood, elm, walnut, and hickory also may have
grown on the island, though none of these is present today. Spruce-fir
forests probably survived only in cool, wet areas near the shores, in
swamps, and on some north-facing slopes, while the deciduous hardwoods
covered most of the uplands.
Animal life probably changed in a similar way, with
increase of the more southerly species and decrease of northerly ones.
Reptiles and amphibians, being cold-blooded and therefore requiring
fairly warm climates, quite possibly survived on the island for the
first time. Some aquatic species, such as newts and painted turtles, may
have been able to swim through the temperate water. Other turtles,
frogs, snakes, and salamanders, as well as their eggs, may have been
carried on or in driftwood to these shores. Most probably they came from
the Ontario shore to the north and east, since the prevailing currents
come from that direction.
Warmer water during this period no doubt also aided
fish in crossing the big lake and becoming established in Isle Royale
waters. Some larger species may have found a new home throughout the
lake, while some smaller ones accompanied "rafts" of flotsam to
sheltered water around the island, and then traveled upstream to
interior lakes. At the same time, the cold-water fishes such as whitefish
and trout, which probably had become established in Isle Royale lakes
during colder times, now could not tolerate the warmer inland waters and
died off in all but the deepest lakes. Lake trout, for instance, now
live only in Siskiwit Lake, while whitefish survive only in Desor and
Siskiwit.
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Monument Rock, a sea stack at an elevation of about 700 feet above sea
level, is a remnant of the shoreline of ancient Lake Minong. (Photo by Robt. G. Johnsson)
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About 5,000 B.P., near the end of the warm "pine
period," the level of the three upper Great Lakes, now all at the same
elevation, stabilized long enough to form another prominent shoreline.
This gigantic body of water, known as Lake Nipissing, formed beach
ridges, sea arches, and other features on Isle Royale at elevations now
from 640 to 660 feet above sea level. (Lake Superior is about 600 feet
above sea level.) During this stage a bar forming across a cove mouth
created Lake Halloran, a sea arch was cut on Amygdaloid Island, and
Suzy's Cave, facing Rock Harbor, was carved out by the waves. By this
time, with lake levels only 40 to 60 feet higher than now, Isle Royale
had nearly reached its present configuration. Siskiwit Lake had been cut
off from the big lake by a low ridge, and most of the other inland lakes
had also been formed. (In another 3,000 years or so, the present Great
Lakes would be formed and Isle Royale would have emerged to its present
extent.)
Isle Royale's biologic story since that time has been
shaped by increasing coolness. Spruce-fir forests have spread to all but
the highest, driest areas, tightening a noose of competition around the
remaining stands of sugar maple, yellow birch, and pine. Some
"southerly" animals probably have been eliminated. Whether this trend
will continue into another ice age or will shift toward greater warmth,
we can only guess.
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Gravel beaches have formed at some indentations in the rocky shoreline. (Photo by R. Janke)
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What life inhabits the island at this particular
point in its long-short history? Many species for which the climate is
suitable either could not make the trip across or, once arrived, could
not become established. Most of the plants and birds of the nearby
Canadian mainland have succeeded on the island, but many other forms of
life have not. Among the missing vertebrates are black bear,
white-tailed deer, raccoon, striped skunk, porcupine, eastern
cottontail, and a number of small rodents, as well as snapping turtle,
spotted and red backed salamanders, and leopard frog. The island's
wildlife drama today has only a few principal mammal actors: wolf,
moose, beaver, snowshoe hare, red squirrel and deer mouse; the few other
mammals are uncommon, rare, or ecologically unimportant. Perhaps the
small size of the mammal cast heightens our interest in it; certainly it
focuses the action of the play.
Many processes of creation and destruction continue
today as they have since rotten ice first melted off this piece of land.
Ever so slowly, rock weathers and helps to form soil. Still responding
to the removal of its tremendous burden of ice, the northern part of the
Superior Basin rises a foot or more per century. Waves carve into the
rock shores, make beaches, and build bars underwater across coves.
Lichens and the plants that follow create forest on rock. Creeping mats
of vegetation form "cataracts" over the eyes of lakes and eventually
fill them. We witness on youthful Isle Royale the earth's primordial
work.
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