Squirrel Interaction with Humans - Woodland Regeneration

Squirrels eat a substantial number of seeds and nuts, but also bury a great many, aiding woodland regeneration. - Credit: Marc Baldwin

Invariably some caches are forgotten about, or the owner doesn’t survive to retrieve the contents. Coupled with the fungal spores distributed by Reds in their droppings, this makes squirrels an important ecosystem engineer. They take seeds and plant them elsewhere and, assuming they’re not recovered, these may germinate into new hazel, oak and beech trees. We call this zoochory and it has been reasonably well studied in squirrels. Interestingly, the data we have suggest that Greys may be a more powerful force in woodland regeneration than Reds. Working in the Nopporo Forest Park in Japan, for example, Tsung Hung Lee at the SHU-TE University in Taiwan found that Red squirrels tended to eat more seeds than they buried, although this varied with tree species.

In a 2002 paper to the journal Acta Theriologica, Lee presented his data showing that of 32 see species included in the diet of his study population, 12 were both eaten and hoarded and the remaining 20 were eaten and never cached. Cached seeds were the ones with highest energy content, which was expected given that this resource must see the squirrel through winter. Seeds of species such as some conifers, yews, acers and magnolias were either rarely or never cached, being eaten upon discovery, while acorns and some fir seeds were more often cached than eaten.  Chestnuts were almost always cached and walnuts were always cached. Overall, Lee described these Reds as more seed predator than seed disperser. A more recent study in southwest Spain, by a team led by José Gómez at the Universidad de Granada, found that about 1.3% of the experimental acorns provided to their Red test subjects were found alive in caches the following spring. Gómez and his colleagues described the Red squirrel as “moderately effective dispersers” of oak.

A Red squirrel (Sciurus vulgaris) caching a hazel nut. - Credit: Marc Baldwin

By contrast, we have more data suggesting that Greys play a significant role in woodland regeneration. Modelling caching behaviour and recovery by both Greys and North American red squirrels (Tamiasciurus hudsonicus) in Indiana, Jacob Goheen and Robert Swihart found that walnuts cached by Greys were significantly more likely to survive and germinate the following year than those cached by Reds. Greys are declining in this part of North America and the authors suggest that even if the Red were able to colonise these areas they’re unlikely to be able to compensate for the gap in forest regeneration left by the Greys. In 1977 Raymond Barnett published a paper in the American Midland Naturalist presenting data implying that Greys were the primary agents of oak dispersal at his study on the Duke University campus in North Carolina owing to their habit of scatter-hoarding large numbers of acorns to suitable germination sites.

More circumstantially, observations by Christine Hanrahan and Annie Bélair in the Fletcher Wildlife Garden (FWG), a long-term project of the Ottawa Field-Naturalists' Club, suggest a significant increase in the number of certain trees in areas inhabited by Greys. On their website, Hanrahan writes:

We certainly benefit from [grey squirrel] tree planting activities at FWG. Many new red oaks are popping up all over the place and, thanks to squirrels, our walnut trees have substantially increased in number. Between 2002 and 2004, an inventory of walnut and butternut trees at the FWG revealed 43 trees - 11 butternut and 32 walnut. In the summer of 2006, another inventory was done. While 5 new butternut trees were found, an astonishing 33 additional walnut trees were located, doubling the number since the last inventory.”

Hanrahan’s observations tie with those of Lee’s, in which Reds always cached walnuts when they came across them.

Perhaps more interestingly, Greys appear to preferentially eat seeds infected with insect larvae and may, therefore, help improve the dispersal of trees by planting only healthy seeds that may be more likely to germinate. In a fascinating series of experiments conducted in Kirby Park in Pennsylvania in the early 1990s, Michael Steele and colleagues at Wilkes University found that Grey squirrels were better than they were at identifying acorns infested with weevil larvae. Over all of their trials, squirrels were observed to eat the weevils in just over 76% of cases, selectively caching the uninfected acorns. In their 1996 paper to the Journal of Mammalogy, Steele and his colleagues concluded:

Results herein further suggest that by selectively caching sound acorns, gray squirrels may exert a greater effect than previously realized.”

A Grey squirrel (Sciurus carolinensis) about to cache an acorn. Some may survive to sprout, but equally the squirrels will often bite the apical tip before burying, preventing germination. - Credit: Marc Baldwin

Despite the foregoing, there are also observations of Grey squirrels puncturing the apical root of acorns before burying them – this prevents the seed germinating and prolongs the life of the cache.