Acorns? Are there enough?
When a tree produces a ridiculously abundant crop of nuts, it is called a "mast" year. Particularly in Europe, people have been keeping track of the years when trees produce mast crops. Pines, oaks, chestnuts, spruce, and many other species of trees have been tracked for hundreds of years, allowing us to look for clues in the environment that might predict the impressive mast years. No fool-proof predictors have been discovered so far.
Why would any tree produce so many fruits on some years? Various suggestions abound, including "predator saturation". Most years, only a few acorns are produced, and most animals glean them rapidly. Generally, only a few acorns hit the ground under a tree. Most acorns are snatched from the tree by birds before they fall. At Hastings and elsewhere, people have put out "acorn traps" (bags or boxes or trash cans) under oaks. In most years, only a few acorns end up in the traps, even though the trees have an obvious crop. Many animals act as "predators" on the fruits, eating them before they have a chance to germinate. These include deer, mice, woodrats, pigs, jays and woodpeckers. Populations of these "seed predators" are more or less in balance with an average year of nuts. When faced with acorns ankle deep under most trees in a mast year, there are just not enough of these animals to eat all the acorns, thus ensuring that many acorns get planted and have a chance to get started as seedlings. For an engaging, easy-to-read, well-illustrated article on "Masting" in trees, and our oaks in particular see the 2005 article in American Scientist by Walt Koenig and Jean Knops.
Acorns are not exactly without any defense. Plants often have chemical compounds that are repulsive to animals that might try to eat them. Acorns have a relatively high content of tannin, a bitter compound we use to tan hides. Tanning a hide means soaking a soft, flexible, tender skin in tannins to produce shoe leather, among other things. Mammals and birds have a very soft, almost transparent lining of the intestines, where dissolved food is absorbed. Carbohydrates and other valuable compounds in food are not absorbed if a diet is high in tannins. Makes sense; if your gut lining was tanned into shoe leather, it would be relatively resistant to nutrient transfer.
A few animals have digestive systems that can tolerate tannins, or break them down. Unfortunately, we are not among these. If you want to eat a diet heavy in acorns, you too will need to crack open the acorns and soak them for a while to rinse out the tannins. Native people in California were very good at this. They used rock grinders to break open the acorns, and they would soak the acorns in the streams for many days. Later, they would haul the well-rinsed acorns meats to large woven storage bins or "granaries" where acorns were stored above ground during the winter.
Many animals just go ahead and eat the acorns when they are plentiful. Any loss of digestive absorption due to tannins may be overcome by a simple increase in the volume of acorns eaten. So, deer, pigs, squirrels, quail, jays and many other native animals eat as many acorns as they can, often leaving them for a few weeks on the ground before they eat them. Each fall, the clackety clatter of Band-tail pigeon wings through yellow sycamore leaves against the cerulean morning sky means these wandering birds have returned to gorge on acorns once again, before they continue to Mexico for the winter.
Lives of some animals revolve around acorns. Acorn woodpeckers are such specialists and require acorns to support a stable, resident population. There are wide areas where only one species of oak occurs. Acorn woodpeckers rarely occur in such "one-oak" areas. But, where two or more species of oak occur, acorn woodpeckers can thrive. You see, the chances of a complete failure of the acorn crop in only one species of oak is high enough to occur in the average lifetime of an acorn woodpecker. If all the acorns failed, the woodpeckers would have to migrate out of the area in search of food. However, by living where two or more oak also live, the acorn woodpeckers almost always find food. If one species of oak has a dismal acorn year, it is highly unlikely that the other oak species there would also have a complete crop failure that year. It takes many generations of woodpeckers to build a granary capable of supporting a stable population, and these are often focused on large, old oaks with plenty of dead branches for storing acorns. Their lives and social systems revolved around these granaries.
Acorn woodpeckers have a very complex social system. If you compared it to that of humans, it appears to include group sex, infanticide, communal living, and incest avoidance.
Acorn woodpeckers depend on acorns for survival. So, the variation in the abundance of acorns has occupied Dr. Koenig for the last 20 years as well. Acorn crops appear to be synchronous over wide areas of California. If it is a mast year for the valley oak in your back yard, it is probably a mast year for valley oaks at Hastings. Conversely, if there are very few acorns on your blue oak, there are probably very few acorns on blue oaks at Hastings as well. Walt and his colleague Jean Knops tell the story of how oaks, across long distances, have highly synchronous years of abundance.
Woodpeckers are not the only animal that depends on the acorns. Over 1,000 species of insects live in oak trees. Some wasps are specialized predators on them! It is a myth that the acorn woodpecker is choosing acorns occupied by larval insects. Acorn woodpeckers would eat those insects as a little bonus, if they found them, but woodpeckers depend on acorns for carbohydrates- "junk food" for fast energy.