Squirrels and forest ecology

Tim Iverson

(10/2017) With the onset of fall comes cooler temperatures, changing leaves, and acorns by the dozens. Acorns can make a hazardous nightmare for a hiker or anyone just enjoying the outdoors. The forest floor becomes one giant tripping hazard covered in a blanket of marbles and dodging these falling projectiles is nearly an olympic feat of agility and acrobatics. Acorns are much more than bombs and slip hazards. Our native oak trees prepare for the future by producing and dispersing these seed packages.

Acorns are the nut produced by oak trees. An oak tree can produce up to 10,000 acorns in a single year. These durable casings contain a single seed, which under the right conditions can germinate and eventually grow into a tree. Typically oaks will produce acorns in two to five year cycles. These boom and bust cycles are called mast years, the last of which occurred regionally in 2015 and 2016. Scientists aren’t sure about how or why this occurs. Research shows it could be triggered by chemical signalling between the trees or environmental conditions. However, there doesn’t seem to be a strong connection between weather patterns and production.

Evolutionary biologists suggest that it could be the best way to ensure the overall success of seed distribution and growth. By staggering production cycles it allows the predator population (chipmunks, squirrels, mice, deer, jays, etc.) to thin out. Then all at once the trees collectively produce millions of acorns that blanket the forest floor. The numbers produced are simply too overwhelming and abundant to be eaten by the diminished animal populations. Another reason could be that it’s taxing on the trees to make the acorns, so by alternating production years it allows them to recover for the next round of production cycles. Either way, the animal populations that do rely on these calorie packed snacks do very well those years. Typically, those dependent animals see population numbers explode the following year.

Most trees and plants rely on the wind to spread seeds, but acorns are just too heavy to get around this way. This is where biological vectors come in - in other words, animals. Acorns are a favorite meal of many different kinds of animals. They are a great nutritious high calorie snack to fatten up animals before a long migration or a long cold winter. Some animals, like squirrels, will cache, or hide, thousands of acorns for later use. Initially, squirrels don’t seem all that intelligent. Frequently they’re spotted darting across roads meeting untimely ends. However, they are the unsung heroes of forest growth. When it comes to forest ecology they are the primary agent in acorn dispersal.

In forests with oak trees there can be hundreds of acorns in a square foot during mast years. Squirrels can differentiate between species of oak acorns often preferring to eat acorns produced by White Oaks immediately and storing acorns produced by Red Oaks for later. This is because the White Oak acorns spoil faster, usually within a few months, while the Red Oak acorns can last up to 16 months in storage. These discerning rodents are also deceptive.

Concerned by opportunistic spies, squirrels will trick others by creating fake caches. If a squirrel thinks it is being watched it will dig a hole, pretend to place the acorn in it (while actually leaving the acorn in its mouth), cover the hole back up, then scurry off elsewhere to actually hide its treasure. A single squirrel is estimated to create hundreds, possibly up to a thousand, of caches each season.

Remarkably they can remember where each and everyone is located. Researchers have demonstrated that some of this is done by memory, but as they close in on the specific location scent can help them hone in on the specific spot. For the acorns that don’t get eaten that season they can germinate and sprout. By carrying the acorns further than the tree would be able to disperse them squirrels help expand the boundary and the genetic diversity of the forest.

The eastern gray squirrel, while native, at this point is considered an invasive pest. People with bird feeders would’ve agreed with this sentiment long ago, but it’s their ingenuity and evolutionary adaptations that have given them this edge. Their tails are one of the remarkable features that have secured their niche. Squirrels dart, dash, and dive throughout the forest canopy.

Like a child walking astride a curbside with their arms extended for balance the tail of the squirrel keeps them balanced through all their aerial acrobatics up in the branches. Additionally, their tails are basically a thermostat helping to regulate their core body temperature. During the cold winter months it directs blood flow back into the core of their body, while during the summer body heat is radiated away as the blood circulates through the tail. Squirrels are one of the only mammals that can climb down a tree face first. Incredibly their wrists allow their hands to rotate a full 180 degrees backwards. This ability allows their claws to securely grip tree bark making their descents rapid and smooth.

Primary predators include hawks, owls, and foxes. Squirrels are incredibly agile and have excellent peripheral vision that makes it difficult for predators to sneak up on them. The eastern gray squirrel is crepuscular, meaning they are most active during dawn and dusk allowing them to avoid the heat of midday. Maryland is also home to flying squirrels too. Flying squirrels are rarely seen though, as they are nocturnal. The flying squirrel nests in tree cavities making them more difficult to spot, whereas gray squirrels create visible nests high in the tree tops. Flying squirrels, despite their name, can not actually fly. They can glide up to 300 feet from tree to tree using folds of skin between their front and hind legs.

To homeowners and drivers squirrels can be bothersome. They are, however, a remarkably adaptive species that is truly beneficial for the overall health of a forest ecosystem. While we may occasionally have to swerve around them or sacrifice some seed from our feeders we need them for the services they provide for forest growth.