Yellowstone moose decline due to hunger, not predators

The number of young moose may be limited by the availability of willows, their major winter food.

October 6, 1999
Web posted at: 1:29 p.m. EDT (1729 GMT)

It's the amount — or lack of — food, not increased predators that is limiting moose populations in the Yellowstone area, says a University of Nevada researcher.

When wolf and grizzly populations were reduced in the greater Yellowstone ecosystem in the 1930s, moose populations reached peak levels. But since the reintroduction of the two predators into the Yellowstone area after a 60-year absence, the number of moose offspring being counted has fallen. Scientific opinion has been divided over the extent to which predation, nutrition and other factors may limit large herbivore populations.

Using an innovative technology, biologist Joel Berger has demonstrated conclusively that fewer moose are getting pregnant, suggesting that the real reason for reproduction declines is that there simply isn't enough food for more moose.

Moose pregnancy rates in the Yellowstone area have fallen from 90 percent in 1966 to 75 percent today. To determine the current moose pregnancy rate, Berger and his colleagues analyzed progesterone levels in moose droppings. The researchers found that in protected areas of the greater Yellowstone ecosystem, moose pregnancy rates were extremely low relative to pregnancy rates of other populations in North America.

This suggests that the drop in young moose is not due to wolves and grizzly bears. Rather, the researchers suspect that the number of calves is limited by the availability of willows, their major winter food.

"Because moose have expanded so greatly and they are now at such high densities, it is possible that they are over-eating their food sources," Berger said.

"We are pushing the envelope here and suggesting that knowing pregnancy rates can tell us whether populations are at or above the food ceiling of their ecosystem," says Berger.

The researchers' fecal pregnancy test will be used in the future to monitor the effects of large carnivores as they are restored to reserves in North America, Europe and Africa. The fact that the test is non-invasive makes it particularly useful in national parks, where handling animals is generally discouraged, and in remote areas, where reaching the animals is difficult.

The study was published in the October issue of Conservation Biology.

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