Rodents, fungi and other small wonders play big role in forest chain

Posted: Sunday, January 07, 2007

Southeast Alaska forests are home to several kinds of small mammals, including red squirrels, flying squirrels, red-backed voles, deer mice, shrews and a few others.

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These species are much more than prey for hawks, owls, ravens, and weasels. They are also more than consumers of seeds, pollen, leaves, insects, lichens, baby mice and birds, though all their ecological roles are important.

Small mammals constitute a critical link in a chain of interactions that involves fungi and plants, a chain that is essential to the ecological functioning of the forest.

In a nutshell: Most plants depend on certain soil fungi for nutrients, and the fungi obtain essential energy from the plants. The fungi are essential dietary items for many small mammals, and they depend on small mammals for spore dispersal, a critical component of fungal reproduction.

And the mammals depend on trees and other plants for habitat, nest sites and material, and food. In a sense, by providing an essential service for the fungi on which the plants depend, the mammals help maintain their own necessary resources provided by plants.

Here's how it works (in brief): Previous columns in this space have mentioned the mutualistic relationship between many kinds of fungi and most of the trees, shrubs, and herbs that grow here.

This relationship is called "mycorrhizal" (meaning 'fungus-root"). The fungi exist chiefly as networks of filaments that spread through the soil. The filaments surround and, in some cases, penetrate the roots of plants.

They pass soil nutrients to the plant and, in return, use some of the carbohydrates synthesized by the plant. The fungi use the energy from the carbohydrates for growth and reproduction, and the plants grow better if they have their mycorrhizal associates. Both partners benefit.

When these fungi reproduce, some of them produce typical mushrooms, such as chanterelles, whose above-ground fruiting bodies shed spores that are carried around by air currents.

Small mammals such as red squirrels harvest certain mushrooms, often storing them in trees, perhaps facilitating spore dispersal by increasing exposure to wind. And deer sometimes eat these fungi, dispersing the spores in their scat.

More interesting from an ecological perspective are the fungi such as truffles and false truffles that produce subterranean fruiting bodies in the organic layer of the soil.

These are usually very aromatic, and they are avidly sought by many small mammals. At some times of year, these fungi comprise the main dietary items for these consumers. In contrast to the above-ground mushrooms, however, these fungi rely on small-mammal consumers for dispersal of the spores, which pass unharmed through the digestive tract and are deposited in the scats.

After dispersal and deposition, the spores germinate and produce new filament networks, which establish new mycorrhizal associations with a new set of plants.

The seedlings of trees and other plants usually acquire their mycorrhizal associates during their first year, and the plants grow better after this acquisition. Orchid seeds don't even germinate unless a suitable mycorrhizal fungus is available, and the fungus helps the seedling grow. However, most adult orchid plants apparently don't have these fungal associates.

Although many of the small mammals that dig up and eat the subterranean fungi prefer to live in mature forests, they may venture into adjacent habitats and carry the spores with them.

Sometimes spores are also carried in soil moved by animals or on fur or skin of animals that explore new habitats. If they deposit spores in a clear-cut, for example, the animals aid in the establishment of new forests on that site.

Animals carried mycorrhizal spores several kilometers onto lands disturbed by volcanic eruption, such as Mount St. Helens. In our area, they might carry spores onto recently deglaciated lands and onto land rebounding above sea level after glacial retreat.

Even within the same habitat, they spread the fungal spores around and increase the encounters between fungi and plants.

Some of the mycorrhizal fungi are very specific to only one kind of plant and, conversely, some plants associate with only one kind of mycorrhizal fungus. In other cases the mutualistic partners are less fussy.

Similarly, the small mammals that are the essential link in the chain of interaction are sometimes quite selective for certain kinds of fungi, while others are more generalized fungus-eaters. The jumping mouse, a rare beastie in Southeast Alaska, prefers a very different kind of fungus than do squirrels or voles, and the preferences of squirrels tend to differ from those of mice and voles.

So there's complex web of reciprocal dependencies. The loss of any element in this network could have reverberating effects on many other organisms.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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