Many creatures are partial to acorns. Pooh and Piglet sang songs in their praise. Squirrels squirrel them away as winter snacks; many other animals, such as American blue jays, follow their example. These animals get through a lot of acorns – in parts of America jays may eat a fifth of the crop straight away, and bury a further half of it – but in their way they are helping the oaks. Not all of the buried acorns are recovered. Those that survive ungnawed through the winter are in a good position from which to grow into mighty oaks.
Some acorn-loving species seem to bring no such benefit. Acorn weevils regularly bore their way through a sizeable fraction of the American crop and are regarded by the people charged with worrying about such things as a pest without redeeming features. But ecology moves in mysterious ways. With his colleagues Carter Johnson of South Dakota State University has been studying the relationship between the trees, the jays and the weevils. They have found that the apparently pesky beetles may be as important as the jays in sustaining healthy oak forests.
The jays begin stocking their underground larders with acorns in September, and labour busily until November. It is hard work, and uses a lot of energy – which people used to think, came from the acorns eaten, not buried. But acorns are a relatively poor source of food. They are worth putting by for lean times, but hardly the sort of slap-up feed that hard digging requires. Like the rest of the oak, they contain tannin, a chemical that dissuades hungry herbivores by doing to the proteins they eat what it would do to a cow’s hide in the hands of a skilled tanner. Oaks were the main source of tanning chemicals until the late 19th century.
Though jays may be poor at digesting acorns, weevils do it happily, undaunted by tannin. And jays can eat weevils. Dr. Johnson’s team has found that jays fed on a diet of healthy acorns lost around six grams of weight a day. Jays fed on a mixture of acorns and weevil larvae, however, did much better. Well enough, in fact, to have the energy to stock up their winter larders. With no weevils, the jays might not plant the acorns; with no oaks, there would be nothing for the weevils to eat.
The Economist October 2nd 1993
Tree Life 170 April 1994