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.