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The gray squirrels that you see in city parks and around neighborhoods are actually forest dwellers. But many of their natural, wooded habitats have been converted into farms, so they've sought refuge in places where they don't have too many predators—like in your backyard. In this Science Update, you'll learn how the loss of gray squirrels is affecting the forest.

Transcript

How squirrels change a forest. I'm Bob Hirshon and this is Science Update.



Squirrels collect and store nuts so they'll have food to last through winter. That thriftiness benefits more than just the squirrel—it helps the trees, too. That's according to Rob Swihart, a wildlife ecologist at Purdue University.



Swihart:

Nuts clearly are dependent on either gravity or animals for dispersal. Squirrels are one of the most important species in this regard.



But not all squirrels provide this service. Swihart and his colleagues have found that gray squirrels bury nuts all over the place, and often forget them. That results in trees growing in new areas.



But red squirrels store nuts in piles on the ground. Those piled-up nuts tend to dry out and don't take root. And because red squirrels adapt better to changes in the landscape, Swihart says the squirrel population might be shifting toward that species.



Swihart:

We are predicting that that will have a significant impact on the nut-producing trees that depend on—at least to a fair extent—gray squirrels for dispersing their offspring.



As a result, forests of oak, walnut, and other hardwoods could shrink, because of the nut-gathering strategy of the wrong kind of squirrel. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Just from looking around, you might guess that squirrels are one of the few animals that have actually benefited from man-made environmental changes. Walk around a suburban neighborhood in many parts of the country, and you'll see dozens of them. Even New York City has plenty of squirrels.



But as you heard in the report, appearances can be deceiving. First of all, not all squirrels adapt equally well to environmental change. And that, in turn, could affect the forest.



Swihart conducted his research in west central Indiana, where only about 16 percent of the land is still forested. His only technological tool was a pair of binoculars. For many animal behavior studies, careful observation is the only technique that works.



He says that each type of squirrel's nut-storing strategies has to do with its original habitat. Red squirrels evolved in damp evergreen forests, and stake out clearly defined territories that are relatively free from interference by other squirrels. Their main challenge is to keep their cones dry through the winter, so they make big piles in the center of their territories and live off them all winter long. Gray squirrels, on the other hand, live in deciduous forests, where they face stiff competition from other squirrels, birds, and animals for the nuts they gather. So they bury their nuts in a bunch of different places—and those they leave behind grow into trees.



Over the past century and a half, however, many of these squirrels' original habitats have been converted into farmland. And it turns out that red squirrels have an easier time finding a new place to live. As a result, the remaining deciduous forests in Indiana are being taken over by red squirrels. But old habits die hard, and these squirrels are making piles of nuts much like the piles of cones they made in their old homes. Because nuts in a pile dry out and die, there are fewer and fewer new nut-producing trees.



So what does that mean? Well, it turns out that hardwood nut trees, like the black walnut, oak, and hickory, provide food and shelter for a number of other forest species that can't depend on other types of trees. In other words, this shift in the squirrel population could start a chain reaction that could radically alter what's left of the forest ecosystem.



But there is hope. It turns out that just as humans can easily alter an ecosystem, there are some simple things we can do that could help undo some of the damage. For example, gray squirrels are known to use wooden fences as guides to find their way to new territories. By strategically placing fences in between wooded areas, communities could create a kind of squirrel highway system that would help keep the forest in balance.



Now try and answer these questions:

What are the differences between red squirrels and black squirrels? How do those differences affect the composition of a forest? In what ways are gray squirrels and nut trees interdependent? Can you think of other species that have interdependent relationships? What do you think would happen if gray squirrels invaded a forest that was populated by red squirrels (assuming they could eat the same foods)?

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