Biology Comicks

Wolf Cascade


The last wolf in Yellowstone National Park was killed in 1926. Two young wolves stepped simultaneously into nearby traps. Wolves were killed to protect elk.

The elk enjoyed the wolves being gone. They soon increased in number.

Coyotes also increased and ate lots of mice, voles and rabbits. They also killed many foxes and skunks and learned to kill newborn antelope. Fewer foxes and skunks meant more ground nesting birds.

Elk eat grass but they also browse twigs of aspen and willow, especially in winter. With wolves gone they spent more time browsing near streams. Soon no tender young trees were left.

Beaver had dams along the streams. Fat trout ate tadpoles in their ponds. Green-winged teal dabbled for insect larvae and small plants. Birds sang from the trees.

As fewer and fewer young trees grew and only big old trees were left, beaver had less and less to eat. Slowly, they starved to death or left. Their dams washed out.

The elk missed the young trees too and looked for them. They found every one. Elk numbers had increased until they were eating all the available food. In hard winters, many elk died.

The females were in poor condition in spring when they bore their fawns. During a hard winter the females absorbed the embryonic fawns into their bodies. But when the weather relented elk numbers would increase once again.

Without beaver ponds and young trees to hold their banks, the streams straightened out and cut deeper into their beds. Water tables near the streams fell.

Fewer birds returned to the old aspens on the streamside. Trout were smaller and fewer mink and otters came to hunt them.

In 1960, after 34 years of no wolves, some scientists had the bright idea that fierce predatory animals, by keeping animals like elk from eating up all the grass and trees, kept the world green. This may have not been a new idea but as a general principle it was revolutionary. Ecosystem control was from the top down, through predators, rather than from the bottom up, through photosynthesizers. (Actually both processes are key.)

By the 1990s the idea had taken hold and the National Park Service decided to reintroduce wolves to Yellowstone. Wolves were trapped in northern Alberta in the winter of 1994-95 and kept in cages in Yellowstone’s Lamar Valley for a few months to acclimatize them to their surroundings.

When the wolves were released it didn’t take them long to figure out what to do with the elk. (These wolves were used to eating moose.) The elk had forgotten about wolves over the last 70 years (many many elk generations).

Soon things were back to normal. When pursued, healthy elk, generally slightly faster than wolves, would tell the wolves to get lost.

Unhealthy or old slow elk did less well. Wolves ate unhealthy, sick and old elk.

Wolves also ate young elk. By eating elk calves, wolves kept numbers of elk somewhat below the carrying capacity of their range, though overall, weather still had the greatest influence on the elk population. Fewer elk starved to death in bad winters and the animals were in better condition.

Wolves also made elk cautious. Elk realized certain places were dangerous because of ambush by wolves. Many of these places were near streams.

As elk spent less time near streams, the streamside forest started to recover. As it got thicker, it provided more cover for the wolves and became more dangerous.

Soon willows and aspen, shrubs and wildflowers were again growing along the streams.

As the trees recovered, beaver started to move back in. Wolves patrolled the valleys too, hoping to surprise a beaver on land. Beaver is a common food of wolves. Soon only very hungry elk would venture near the tasty willows and aspen of the streambanks.

As the beaver dams grew in number, water tables near the streams rose, benefiting the trees and the grasses that grew under them. Downcutting ceased and stream bottoms filled in. Trout grew more plentiful and plump. Mink hunted the trout and the frogs that lived in the ponds.

The wolves killed half the coyotes in the Lamar valley, their competition (as foxes and skunks were for the coyotes), which meant more foxes and skunks, and a recovering population of pronghorn antelope, whose numbers had been decimated by the coyotes. Ground nesting birds probably didn’t enjoy greater numbers of foxes and skunks but neither did they enjoy coyotes.

The wolves left behind a windfall of carrion for foxes, crows, grizzly bears, magpies, golden eagles and early returning mountain bluebirds, who ate the flies hatching out of the carcasses.

Along the streams, the birds returned and sang from the trees.