Predators and Prey

Isle Royale is an island in Lake Superior where wolves hunt moose. In the winter of 2016, scientists counted only 2 wolves on the whole island, and they estimated 1,300 moose.

Wolves eat moose. Moose eat young trees and other plants. Scientists have studied the wolves and moose on this island for decades. So why did the wolves not eat every last moose, and why were there so few wolves? First, decide what you think now.

Explore: run an island with wolves and moose

In this simulation you watch an island with two populations, moose and wolves. Bars show how many of each there are, and a line graph tracks them over 40 years. You set the starting number of wolves, how fast moose have calves, and whether a harsh winter strikes. A food meter shows how much plant food is left for the moose.

Wolves at the start (relative): 20

How fast moose have calves

Low Medium High

Run 40 years Harsh winter next year Reset Year 0 of 40

Press Run. Bars show moose, wolves and plant food at the start.

In this sketch: all sizes are relative units, not real counts, and the wolf count can be a fraction. The starting moose, how fast plants grow back, how many moose each wolf catches, how fast wolves gain and lose numbers (which sets how long they lag behind the moose), the calf rates, and a harsh winter taking half the moose in one year are invented. Moose eat the plants. There is no disease and no age groups, and the numbers are smooth averages, not individual animals.

Pattern scientists often see: when prey are plentiful, more predators tend to survive. When prey become few, predators can run short of food and fewer may survive, so the prey can recover.

Try this.

1. Set the starting wolves to 20 (a relative number in the model, not a count). Predict which line will reach its high point first, the moose or the wolves. Then run it and watch the two lines.
2. Set the wolves to 0. Predict what will happen to the moose and to the food meter over 40 years. Then test it.
3. Press the harsh winter button once. Predict how the moose line and then the wolf line will change in the next few years. Then check.

Explain: populations, communities and food webs

A population is all the members of one species living in one place at the same time. All the moose on Isle Royale make up one population. All the wolves make up another.

A community is all the populations that live together in one place. On Isle Royale, the community includes the wolves, the moose, the fir trees, beavers, birds and many more. The populations in a community depend on one another.

A predator is an animal that catches and eats other animals. The animal it eats is its prey. Moose are the main prey of the island’s wolves.

You met food chains in Food Webs and Energy. A first-order consumer eats producers, and a second-order consumer eats first-order consumers. A moose that eats fir trees is a first-order consumer. A wolf that eats the moose is a second-order consumer.

balsam fir → moose → wolf

A food web links many food chains to show who eats whom in a community. Each arrow points from the food to the eater. It shows which way energy moves. When a wolf eats a moose, some of the energy stored in the moose passes to the wolf. Every animal gives off some of its energy as heat.

Virginia has predators and prey too. In the Chesapeake Bay, ospreys dive for menhaden, a small fish.

Explain: how predators and prey change each other’s numbers

Predators and prey affect each other’s numbers. Here is the pattern scientists often see:

• When prey are plentiful, predators find food easily. More predators survive and raise young, so the predator population grows.
• More predators catch more prey, so the prey population falls.
• With less prey to eat, predators run short of food. Fewer survive, and the predator population falls.
• With fewer predators hunting, more prey survive. The prey population grows again.

So the predator numbers often rise and fall a little after the prey numbers. The predators usually run short of food before the last prey animal is caught.

Real populations are messier than this pattern. On Isle Royale, harsh winters, ticks, disease and the moose’s plant food also change the counts. Scientists report that the island’s wolf and moose numbers do not follow one simple pattern. Predators are one cause of change, not the only one.

Other limits on populations, such as competition for food and space, come in Competition and Cooperation.

Explain: herds and packs

Many animals live in groups. These social behaviors help them get what they need.

A herd is a group of plant-eating animals that live and move together. More eyes can spot danger, and adults can guard young. Muskoxen, large shaggy plant eaters of the far north, give one example. When several predators come near, adults can form a circle with their young inside. Moose mostly live alone, so they do not do this.

A pack is a group of predators, such as wolves, that live and hunt together. On Isle Royale, wolves often hunt moose as a pack. Pack members share the prey they catch. They also help care for the pups, and, once the pups stop drinking their mother’s milk, they feed them meat that they bring up from their own stomachs.

Group	Who	How the group helps
Herd	Muskoxen and other plant eaters	Spot danger and guard the young
Pack	Wolves	Hunt large prey, share it, raise pups

Elaborate: read the Isle Royale counts

Here are selected winter counts from Isle Royale. Wolves are counted from the air and are close to a full count. Moose are estimates from aerial surveys of sample plots and other methods.

Winter	Wolves (animals)	Moose (estimated animals)
1980	50	664
1990	15	1,216
1995	16	2,400
1997	24	500
1998	14	700
2010	19	510
2016	2	1,300
2017	2	1,600
2019	15	2,060
2024	30	840
2026	37	524

Source: National Park Service, Isle Royale National Park, “Wolf and Moose Populations from 1980 through 2026” (downloaded 2026), winter counts for the whole island. In 2018 and 2019, the Park Service brought new wolves to the island, so the later rise in wolves is partly from people. Some years have no count.

1. Find the year with the most moose in the table. What happened to the wolves in the next row?
2. Compare 2017 and 2026. About how many moose were there for each wolf in each year?
3. From 2019 to 2026, describe how each population changed.

Design problem

Does living in a group protect prey? You can test this with a model. Spread 30 paper squares, each 2 centimeters on a side, on a square of floor 1 meter by 1 meter. They are the prey. Drop a 10 centimeter cardboard square from 1 meter high. It is the predator. Any prey it touches is caught. Plan a fair test that compares prey in one tight group with prey spread out.

One way to do it

• Question: Are prey in a tight group caught on fewer drops than prey spread out?
• Independent variable: how the prey are placed, in one tight group or spread out evenly.
• Dependent variables: the number of drops that catch any prey, and the number of prey caught on each drop.
• Constants: 30 prey squares, the same field, the same predator card, the same drop height of 1 meter, the same person dropping with eyes closed.
• Control: the spread-out field, which you compare the group against.
• Trials: 20 drops for each setup. Put caught prey back after each drop. Repeat the whole test three times and find the mean.
• Safety: drop only paper and cardboard, keep the area clear, and pick up all pieces.

A tight group is likely to be caught on fewer drops, but some drops may catch many prey at once. This model has limits. Real predators aim, and real herds watch and move.

Evaluate: check your thinking

The wolves on Isle Royale have not eaten every last moose. When moose become scarce, wolves can run short of food, so fewer may survive and the moose can recover. When moose are plentiful, wolves can increase. Weather, food, disease, and people moving wolves also changed the numbers.

Science practices you used: testing predictions with a model, reading and comparing data in a table, making a claim from evidence, and planning a fair test.