Ask the Naturalist

Are Fox Squirrels Replacing Gray Squirrels in California?

May 28, 2019

I live in rural Solano County and have always enjoyed watching the antics of our native western gray squirrel. During the past three years, however, they’ve almost disappeared from this area and seem to have been replaced by the introduced eastern fox squirrel. I’ve noticed this same phenomenon elsewhere. Is there any hope for our beautiful silver native? -Les Barclay, Fairfield

In short, yes! And there are things that we can do that can help them. Let’s first go through a quick rundown on California’s tree squirrel species.  

For a majority of Californians, the tree squirrels in our neighborhoods will be one or more of three species: the native western gray squirrel (Sciurus griseus), the introduced eastern gray squirrel (Sciurus carolinensis), and the introduced eastern fox squirrel (Scirus niger). A good way to tell western gray from the others is to look at the color of the fur on the face. Western grays are predominantly cool gray and white and don’t have brown on their faces, in contrast to the two introduced species who have a lot of orangey-brown all over their faces. The black squirrels that are common in some areas are simply a melanistic (hyper-pigmented) version of a regular eastern gray squirrel. Both eastern gray and eastern fox were brought from the other side of the United States in the early 1900s and have been increasing their range and population ever since, both on their own and from humans deliberately spreading them through the state, unaware of the consequent damage to environment, agriculture, and property that would cause. Meanwhile, the western gray has decreased in range and abundance.

squirrel comparison chart
From left to right, the western gray squirrel, fox squirrel, and eastern gray squirrel. (Photos, left to right, by alanmuchlinski, sambiology, CA Clark, iNaturalist CC)

Why is this? It’s not due to the invaders being more aggressive—in fact, a researcher studying western gray versus eastern fox found that 75 percent of aggressive interspecies encounters were started and won by the western grays, and that young eastern fox squirrels wouldn’t leave their nests if they saw a western gray out foraging nearby. Western grays have a slight size advantage, and apparently a fighting spirit, and are capable of holding their own on an individual level. However, they have a big disadvantage at the population level due to their low tolerance for human development.

Western grays are true woodland creatures and need a continuous stretch of mature oak trees to provide their food and shelter. They’re highly arboreal, preferring to be up a tree or near a tree, and won’t use fragmented habitat with lots of exposed spaces and few trees. The bulk of their diet is acorns, pine nuts, and truffle-like underground fungi, all of which come from trees. Acorns and pine cones are produced by mature trees, and the truffle-y fungi are species that live in symbiotic relationships with the roots of trees, helping them to take up nutrients and resist disease (western grays themselves are a big positive for forest health since they spread the spores of the truffles around when they eat them). While more aggressive with other squirrel species, they’re much more timid with humans, preferring to make their nests away from busy human-created areas. They have only one litter a year.

In contrast, the introduced eastern gray and eastern fox squirrel thrive in human-dominated landscapes like city parks, suburban backyards and attics, and nut orchards, and are very familiar sights (and often, pests) to most Californians. They’re nearly as ecologically flexible as rats and are perfectly happy to nest close to humans and to eat the same native plants as western grays, plus human garbage and non-native plants that the westerns won’t touch. They’re not much troubled by habitat fragmentation (eastern fox squirrels actually seem to prefer it) and will readily travel across open fields, roads, and power lines to get to isolated trees or food sources. They’ll commonly have two litters a year, which means that they quickly make up for very high mortality rates. However, they have not been able gain much of a foothold in undeveloped native forests like Los Padres National Forest or Mount Tamalpais.

So, while having to fight with the eastern squirrels for territory and acorns definitely doesn’t help, the real problem is that western grays only like native forest, which unfortunately has been decreasing in size and quality due to residential development, drought, more and hotter wildfires, some land management practices, and invasive pests and disease. This means fewer places to live and less food to find, and more stress and mortality.  But it also means that anything we do to help protect our trees and forests—such as habitat restoration and voting for preservation and healthy land management—will also help our native grays, as well as a whole lot of other animals and plants.

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Also, despite western grays being more human-shy, they’re fine with living alongside us as long as there are enough large trees to provide food and cover. Western grays can still be found in towns like Santa Rosa that have lots of large trees along the streets and wooded parks nearby, along forested creek corridors like Stevens Creek in Mountain View/Cupertino, or even in Sacramento, where there are enough trees still lining the banks of the American River to support western grays. In our own neighborhoods, we can help our native squirrels (and other wildlife) and begin to de-fragment habitat by planting food and shelter-providing plants like native oaks, pines, California bay laurel, manzanita, hazel, or flannelbush in our yards, or by asking our city government to save water and landscape with any of those instead of more oleanders.

Ask the Naturalist is a reader-funded bimonthly column with the California Center for Natural History that answers your questions about the natural world of the San Francisco Bay Area. Have a question for the naturalist? Fill out our question form or email us at atn at baynature.org!

About the Author

C.A. Clark is a biological illustrator with a particular fondness for mammals.

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