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Last week I found a walnut on my windowsill, a big
fat one with an unblemished bright green husk. Soon after, another appeared
stuck in the wheel of my car. Now they’re cached everywhere around the outside
of our house, from the shelf on the grill to the flowerpots on the deck.
Acorns are everywhere now, too, as anyone who
bicycles where there are oaks can attest. A person’s got to keep both hands on
the handlebars to avoid having them wrenched sideways.
While these seeds may be present me with minor
annoyances, they’re much more interesting and important from an ecological
perspective.
Scientists group walnuts and acorns together with hickory
nuts and beechnuts in the category of hard mast. This they distinguish from
soft mast, which generally refers to fruits like crabapples and blueberries but
can also apply to other parts of plants that serve as food for wildlife.
[Photo by author. Gray squirrel eating an acorn in a Chinkapin oak.]
According to Ed Heske, a mammal ecologist with the
Illinois Natural History Survey at the U of I Prairie Research Institute, “The
most important thing about hard mast from the perspective of wild animals is
that it’s storable. Without hard mast many mammals that don’t hibernate in
winter would have little to eat.”
Of course, while it’s a good thing for squirrels
that acorns can be stored for eating over the winter, it is not in the interest
of oak trees to expend all of the resources needed to produce such wonderful
seeds if all of them wind up as squirrel food.
Evolution has provided oaks with a clever
reproductive strategy to avert that outcome, referred to as masting cycles.
In most
years, oaks produce a sort of baseline quantity of acorns, and populations of
animals that depend on them become calibrated to that. But every few years or
so, depending on weather and other factors, the oaks of a local area
synchronize their energy and produce a bumper crop—up to a hundred times the baseline
quantity of seeds in some species. With populations of acorn eaters limited by
the leaner years, chances are that some portion of acorns from the bumper crop
will go uneaten and grow into the next generation of oaks.
There is another wrinkle to this story, though. Some
years back Heske and a colleague conducted a study that found acorns would
result in new oak seedlings only if some of them were buried by squirrels and
then never recovered, a situation expected primarily when acorns are
superabundant in mast years. Otherwise something—whether it was a deer, turkey,
mouse or weevil—always ate them up from the soil surface before they had a
chance to germinate.
In addition to promoting new generations of oaks, Heske
explained to me, bumper crops of acorns initiate a cascade of other ecosystem
effects. Extra acorns, for example, enable forest-dwelling mice to reproduce
especially well; during mast years they can add an extra litter or two, and add
to the size of their litters as well.
Good for the mice, right?
But what’s good for mice is, in turn, good for
great horned owls and the other predators that eat mice. They generally
experience a bump in reproductive success in the year following a mast year.
