It’s happening everywhere right now. In the woods, you can hear them falling around you as squirrels work hard to bury them and deer try to get all they can before the squirrels have a chance. At home, the sound of them hitting the roof of a garage or awning always seems to surprise even though it has been happening regularly for weeks. They litter the tops of our cars while postal employees and pedestrians try to navigate sidewalks that are sometimes like walking on ice. We are experiencing a major burst of oak reproduction, and the acorns are everywhere.
Acorns are the fruits of oak trees. Yes, we call them nuts, but they are also fruits, as they are the result of successful pollination of the oak’s flowers, which although small are quite productive. Right now, we are experiencing a major masting event.
Mast is a generic term for the fruits of many forest trees. Masting occurs when trees across a forest or region all reproduce at the same time. Although the mechanisms by which trees are able to do this are still being explored, scientists believe it is a combination of spring temperatures and rainfall that set mass flowering into motion, and more flowers means more acorns hitting the ground in the fall. Although we don’t completely understand how, we have a much better idea why they do it - safety in numbers.
Masting creates a refuge. Refuges are situations where prey are protected from predation. Many species use burrows, tree cavities, thick brush, or other hiding spots as physical refuges, places they can go for safety when pursued by a predator. Other species use size as a refuge. For example, elephants and whales have very few predators because they are simply too big to kill. Masting uses a third type of refuge: numbers.
Species live in large groups for many reasons, but one reason we see large flocks of birds and schools of fish is because they are safer in large groups. In animals, more eyes can warn of approaching predators. There are also advantages of just being one of a thousand. From a probability point of view, each individual is less likely to be chosen by a predator - sort of the reverse of winning the lottery. Additionally, large flocks of birds, schools of fish, or herds of elk that are moving together, particularly when moving away from a threat, make it tough for predators to focus on one potential victim.
There are other numerical advantages to prey, however, with numbers, the prey can overwhelm the predator. Although some predators can consume thousands of prey at once - like the large gaping mouths that baleen whales use to catch millions of plankton at a time - most predators are quickly overwhelmed by too many prey. They simply cannot handle them all, eat them all, or even digest them all, at one time. Even if they were more efficient at handling prey, at some point there aren’t enough predators to consume all of them.
Think about your own situation. Could you eat an entire bag of M&Ms at one time? Of course you could. Could you eat two? Or three? Even the hungriest candy-lover would have issues with that. And it would be more difficult if we dropped the M&Ms around your front yard, like acorns, and you had to find them all, one by one. What if you had five family members or friends over (post-COVID) to help? Even with these extra candy predators, there is some number of M&Ms that will leave everyone laying around on couches, grasping their stomachs. At some point, we are overwhelmed.
That is where masting comes in. When oaks produce thousands of acorns, all at once, they overwhelm the predators. Deer, squirrels, blue jays, and other species that eat acorns - as much as they can - simply cannot consume them all. Because of this, some are not eaten, and get covered with falling leaves, and eventually sprout, producing the next generation of oaks. Additionally, squirrels bury so many acorns - as a way to store food for the winter - that they cannot remember all of them. During masting years, this problem becomes much worse, and more acorns are forgotten, having been buried into soil where conditions are even better for the success of a young oak. Natural selection has favored oaks to synchronize their reproduction, because masting leads to greater offspring success.
There are other species that mast as well, in their own way. One of these you likely heard all summer - cicadas. Cicadas and other insects like mayflies and grasshoppers also use safety in numbers, by emerging all at once and overwhelming their predators. There are simply not enough birds to handle them all, and so most survive long enough to mate and produce the next generation - even if their offspring won’t emerge themselves for years.
Predators also benefit from masting. Whether it is forest species feeding on acorns, fish on mayflies, or birds on cicadas, masting means that predators are getting their fill, and that aids their own survival and reproduction. But it is short lived, as masting only occurs every few years, and so even if food is abundant now, predators will have to find other food sources next year. This is another advantage of masting - predator populations cannot grow quickly enough to take advantage of the extra food, so there are never enough predators to consume all of the mast. They have one good year to grow and reproduce, and then it is gone.
Although masting can be a pain for some homeowners and pedestrians, it is a boon for hunters. When a mast year occurs, you know where the deer are in the fall, and sometimes one good white oak will attract deer night after night at just the right time.
The next time you sweep the acorns from your porch, or your car, or try to navigate an acorn-filled sidewalk, don’t blame the trees. Like many of us, they are just trying to be good parents, by giving their young safety in numbers.