Just as it takes a village to raise a child, it takes a squirrel community to raise a forest. Some tree species such as black walnut or oaks can't disperse seeds widely enough for forest regeneration. Squirrels burying seeds hither and yon can do the job. Seeds they don't eat can sprout to ensure a continuing supply of young trees. Why, then, are Indiana's squirrel-rich forests threatened?
When Prof. Rob Swihart and Jake Goheen at Purdue University in West Lafayette, Ind., looked into it, they found the culprits also are squirrels - red ones. They're alien to the area, so they don't cooperate in the seed-dispersal game. They hoard seeds in heaps on the ground or in trees where few will germinate. Populations of alien reds are on the rise. Native seed-scattering gray and fox squirrels are declining. That's bad news for forest regeneration.
This good-guy, bad-guy scenario typifies a general problem in forest management. Human actions and alien species invasions can combine in unexpected ways to subtly damage a woodland.
Professor Swihart explains that factors other than seed dispersal drive hardwood regeneration. Fire has been especially important in clearing the way for new trees to grow. Now that such disturbances are suppressed, Swihart says seed-scattering squirrels "provide the only mechanism by which acorns and other nuts can get far enough away from the shade of the parent tree to have a chance of succeeding."
Mr. Goheen - now a PhD student at the University of New Mexico in Albuquerque - says that, with grays declining, red squirrels won't provide that regenerative service even though they compete for the same food supply.
Given the ubiquity of the bird-feeder bandits and picnic-bench beggars, it may be hard to imagine that gray squirrels are vulnerable. Swihart blames landscape change. He says grays are forest creatures inept at dodging predators in the open. They thrive in urban settings where predators are few. But they're dead meat when they dash across open areas in fragmented forests.
Swihart suspects that landscape changes have encouraged red squirrels to migrate from their native boreal forests. In those forests, conifers, whose seeds spread via the wind, dominate. Regeneration is less dependent on squirrels. But in Indiana's hardwood forests, the red invasion coupled with the gray decline is bad news.
The computer simulations that reveal this scenario are specific to Indiana's woods. Yet they illustrate how subtle and dangerous human impacts on forests can be. Goheen considers that to be the take-away message from this research.
He notes that the earthworm plague in northern forests makes the same point. Worms enrich garden soil. But, in forests, they gobble up the humus on which many plants, birds, and other animals depend. They move into forest areas from stream and lake shores where fishers dump excess bait.
"We need to recognize that the extent to which we've altered the landscape so far has really had an effect," Swihart says, adding, "by doing little things," we can mitigate some of the bad effects.
That could mean developing an ethic of not dumping earthworms where they can infect forests. In Indiana, Swihart says that having more protected connections between forest patches would help gray squirrels survive.