Grizzly Gardeners Foraging Grizzlies Are A Big Help To Tiny Growing Greens

By foraging for certain plants in certain places, animals influence what plants can grow where.

“Microseris” flowers may be scarce this year, because harvester ants ate so many of last year’s seeds. Some areas of northern Michigan have few hardwood trees because moose love their tender seedlings and leaves.

But the indirect affects of animal disturbance eventually can be beneficial.

On the large scale, nothing could rival the roles of temperature and precipitation in shaping plant communities: Palm trees clearly couldn’t survive North Dakota winters. But the distributions of plants on smaller scales, within a forest or on a grassland, may depend strongly on the direct and indirect effects of plant-harvesting bugs and beasts.

“Glacier lilies are candy for bears,” says Sandra Tardiff, a University of Montana graduate student who studies the effects of bear digging on subalpine meadows. Foraging bears churn up the meadow while hunting for that candy, and the exposed soil heats up in the sun. Microbes - tiny bacteria and fungi that decompose dead bits of plants - thrive in the heat. Water moves more easily through the loosened soil, and ensuing wet-dry cycles make microbes even happier and better at decomposing dead plant material.

And decomposition releases nutrients that plants need. Lilies that sprout on bear digs are particularly nutrient-rich. With those extra nutrients, glacier lilies on bear digs produce far more seeds than their ordinary-soil bound neighbors, Tardiff said.

Lilies are also better than many other meadow plants at recolonizing newly exposed soil. So after an area is churned up by bears, lilies sprout back enthusiastically.

High density of nutrient-rich lilies on bear digs, as well as the loosened soil, may help explain why grizzlies frequently target old digs as they forage for bulbs, Tardiff says. It’s almost as if the bears are farming the lily patches over the years.

So although grizzly bears directly reduce numbers of lilies by eating them voraciously, the grizzlies indirectly make it possible for the lilies to spring back in great numbers. Pocket gophers, and even ants, are key components of certain grassland systems, says Richard Hobbs, an ecologist and grassland specialist at Stanford University in California.

Hobbs said that soil disturbances caused by gophers and harvester ants allow certain plants to take root and not others.

“Without continual disturbances, it is likely that the grassland would become much more homogeneous and become dominated by a few relatively aggressive species,” says Hobbs.

Many conservation biologists are concerned about how human-caused climatic changes may effect plant communities. But because human activities affect animal populations more quickly and more clearly than they affect climate, conservation biologists are also concerned about understanding the effects of animal grazing.

For example, if grizzly numbers decline, explains Tardiff, there may be consequences for subalpine meadows. “Bears are a part of this ecosystem,” she explained. “Remove the bears, and the system just won’t look the same.”

No one knows exactly how the bear populations in her sites are faring, but she has some evidence that bears avoid areas of subalpine meadow where they might run into hikers.

In order to manage ecosystems threatened by human activities, scientists need to understand how all bits of an ecosystem fit together, Hobbs says.

Says Stanford: “Once you change the complexity of a system, you may have had it. It is very difficult to restore an ecosystem once you’ve destroyed it.”