A Big Year for Acorns, A Big Year for Mice
—Courtney Rothaus, Environmental Educator
photo: natgeocreative.com
As many of us have noticed, 2015 has been a big year for acorns. In fact, I'm surprised that I haven't yet twisted an ankle on a hike or gotten pelted in the head by one of these falling oak seeds! When we have a year with ridiculously heavy seed production like this one, it is referred to as a "mast year," mast referring to the fruit of forest trees. It is a trait of oak trees to let loose a bumper crop of acorns one year, while some years will see relatively little seed production. Oaks are not the only trees that behave this way, with variable seed production from year to year; beeches, hickories, and maples also have mast years.
Mast production years mean happy squirrels, chipmunks, and mice. For these tiny herbivores, a forest floor covered in acorns means that there's less need to go out foraging widely and, consequently, less exposure to predators like hawks. Acorns are nutritional powerhouses - high in protein, lipids, and micronutrient vitamins - and due to the tannins inside, they store well throughout the winter.
Due to the abundance of nutrient-rich, easily-stored food, a mast year of acorns is generally followed by a spike in the populations of squirrels, chipmunks, and, especially, white-footed mice. Mice store acorns over the winter and will sometimes even breed in the middle of winter if they have a particularly good supply.
The rapid growth in mouse population has been known to have some pretty interesting consequences. By the time that the following summer comes around and the mouse population is at its peak, new baby ticks (uninfected with tick-borne pathogens at this point) hatch out of their eggs and feed on mice where they get infected with the Lyme-disease agent and other tick-borne pathogens. However, it is not until the following year that these ticks have matured enough to transmit these diseases to humans.
If you're ready to chop down all of your oak trees-stop right there! The connection to ticks and Lyme disease is only one of many ripples of consequences caused by a mast year of acorns. A high white-footed mouse population also controls gypsy moths, an imported insect pest that attacks oak trees and can kill millions of trees if left unchecked. So while white-footed mice spread Lyme disease, they also play an integral role in protecting oak trees, which are one of the forest's most valuable food resources. In fact, acorns are eaten by over 100 vertebrate species in the U.S.
Nothing in nature occurs in a vacuum and the success of one organism can oftentimes occur to the detriment of another. In the acorn-mouse-tick scenario, we may get the short end of the stick, but at least we know to use DEET in 2017.
