Who Eats Whom?
By Andy Boyles • Illustrated by Tom Powers
On Michigan’s Isle Royale,
scientists have a rare chance to
study a simple food chain at work.
At the bottom of this food chain,
there are several kinds of plants.
They store energy from sunlight as
food.
Herbivores (plant eaters) are in the
middle of the chain. They get their
energy by eating the plants. These
herbivores include one thousand to
two thousand moose, which eat
most of the plants.
At the top of the food chain are the
carnivores, which get their energy
by eating meat. On Isle Royale, the only carnivores are twenty-five to fifty wolves. Their
diet is mainly moose.
In most wild habitats, the food chains are hard to
understand. Usually, several chains are tangled
together. In these food “webs,” two or three kinds of
meat eaters may live off several types of plant eaters,
which may feed on many kinds of plants. Compared to
food webs, Isle Royale is a simple laboratory set up by
nature.
For twenty-five years, Dr. Rolf Peterson has been
studying this habitat. He is a wildlife ecologist at
Michigan Tech University. By flying over the island in
an airplane, he and other scientists watch and count the
animals.
A Discovery
In the 1980s, the scientists discovered that the number
of wolves and the number of old moose change from
year to year in an interesting way. When the island has
more old moose, its wolf packs grow larger. When there
are fewer old moose, the wolf packs become smaller.
The scientists think these changes mean that old moose
are the most important source of food for the wolves.
Wolves hunt both moose calves and old moose. But
most calves have strong mothers to protect them, so old
moose are easier to catch and kill. The number of
wolves increases when there are plenty of old moose to eat because more wolf pups can
survive in a well-fed pack.
After these pups become adults, the wolf packs need even more meat. They eat more
moose, including old moose and calves. The number of moose goes down.
When the old moose are gone, wolf packs begin to starve, and the number of wolves goes
down. Then more moose can grow old, and the cycle begins again.
The Trees’ Story
This discovery included only two levels in the Isle Royale food chain: wolves and moose.
Dr. Peterson and another scientist, Brian McLaren, also wanted to know how this cycle
might affect the plants.
They wondered: When there are few wolves and many moose, do the moose “overeat”
the trees? In other words, does the number of wolves have an effect on the trees, two
levels down the food chain?
To study the plants, the scientists chose balsam fir trees. Firs are an important source of
food for moose in winter, after most other trees have lost their leaves.
They cut down a few of the trees to study them. As in most trees, each year in the life of a
fir tree is marked by a ring of light and dark wood that shows in the cut end of the trunk.
Each dark line of hard wood was made at the beginning of winter. In between the dark
rings are softer, lighter- colored rings that are made during the growing season. A good
growing season normally leaves a thicker “growth” ring than a poor one does.
The scientists thought they could also learn something about the island’s moose from
these rings. When moose strip the needles from a tree, the tree cannot grow well, even in
good weather. That year’s growth ring would be thin.
They studied the rings of trees that grow on the east end of the island to learn which years
had good or bad weather for tree growth. These trees experience the same weather as the
island’s other trees, but they grow where moose don’t feed.
On the west end of the island, where moose live and feed, Peterson and McLaren cut
down more sample trees. In the rings, they saw that the trees sometimes grew very
little—even in years when the weather had been good for growth. Their records showed
that in those years, there had been few wolves and many moose. In other years, when
there were many wolves and few moose, the trees grew thicker rings.
A Big Cycle
Peterson and McLaren seem to have discovered how all three links in this food chain
affect one another. When there are many wolves, there are few moose, and the trees are
free to grow.
At these times, the wolves begin to starve. Later, when there are few wolves, more and
more moose survive until there are so many that they strip the trees.
When more moose survive, more of them grow old. Then hunting becomes easier, and
the wolf packs grow larger. They eat more moose and . . . the cycle repeats over and over.
Scientists hope to understand the complex systems of food chains called food webs. But
first they have to learn more about simple food chains, partly by studying the wolves,
moose, and trees that live on Isle Royale.