Squirrels, Advance!

The rapid growth of human population over the last century or so has led to a decline in many species. As I talked about last month, however, some animals can live alongside us even in urban environments, and there are many more than can tolerate us in rural - yet not truly wild - habitats, such as cropland or pasture. Any species that can do this clearly has an advantage, in many cases being able to move into new parts of the world previously inhabited by some similar, but less human-tolerant species. Thus, we can see some native species replaced by foreign invaders, as has happened, for example, with mink in continental Europe and jackrabbits in the American southwest.

In Britain, the most familiar example of this is probably the replacement of our native red squirrels (Sciurus vulgaris) by invasive eastern grey squirrels (Sciurus carolinensis). Red squirrels were once common across the British Isles, but have now vanished from most of England and Wales, surviving in the far north of England and a few pockets elsewhere, but otherwise replaced by the greys. In large part this is due to the greys carrying a virus to which they are immune but the reds are not, but simple competition is another factor.

But this is not the only place where eastern grey squirrels are pushing out local, but very closely related, species. The eastern grey is a widespread and adaptable species; its native range is essentially the entire eastern half of the US, reaching as far as a line that extends from roughly the middle of Texas to the Minnesota/South Dakota border, with additional populations further west in North Dakota and Saskatchewan. But cross the Rocky Mountains and keep going, and you will eventually reach the homelands of the western grey squirrel (Sciurus griseus).

Physically, the two do look rather similar, although the western species is slightly larger on average and has darker fur without the hints of brown often seen in its eastern counterpart. While their native range is reasonably large, it's still nothing like as extensive as that of the eastern grey: they inhabit areas west of the mountains through California, Oregon, and up into southern Washington. They are also less adaptable, living primarily in forests of pine and oak, while the eastern grey can live almost anywhere in the temperate zone that has trees. 

As a result, while they are common overall, and the Pacific Coast of the coterminous US isn't exactly a small area to inhabit, their population density is relatively low. Eastern greys can reach population densities of 20 per hectare (8 per acre) in parkland with plenty of food, while the western species reaches only 2.8 per hectare (1 per acre) even in the very best forests. And, while they're hardly endangered, their overall population is declining, to the point that they are listed as a "threatened species" in Washington state.

This is partly due to the usual issues of urban expansion, logging, and so on that affect many other forest-dwelling mammal species. But a major factor is the introduction to the Pacific West of not only the eastern grey squirrel but the fox squirrel (Sciurus niger) as well, both of which manage to outcompete them. (The latter, incidentally, is native to much the same area as the eastern grey, but is absent from New England). If you live in, say, San Francisco, and have seen a grey squirrel there, it probably won't have been the native Californian sort. Not because the western grey wouldn't win in a fight (it probably would; it's bigger) but just because they don't like living close to humans.

But the story is not necessarily one-sided because, in recent decades, western grey squirrels have been reported in places that they were not previously thought to live at all. For example, older sources state that the species was restricted to the three coastal states, but, since at least the 1980s, there has been recognition that some are found in Nevada, having been seen there since around the middle of the 20th century. True, they are restricted to the mixed pine-fir forests in the mountainous far west of the state - basically, a small section of the corner of the state around Reno and Carson City - but it's a start.

This could be due, in part, to global warming making mountain-dwelling a more attractive proposition for the squirrels than had been previously. Other species, such as black bears, have been reported to be moving further into the Sierra Nevada in recent decades, so this is a possibility for squirrels, too. In a similar vein, they may also have recently expanded into ponderosa pine forests in northeastern California. However, to travel any further east, they would have to leave the pine forests, since the vegetation type changes on the eastern side of the mountains, heading down into the Great Basin.

On the other hand, a 2011 report suggests that western grey squirrels may also extend further south than previously thought. While they have always been known to live in the far north of Baja California in Mexico, this report was from Cataviña, about 200 km (125 miles) beyond their previously known limit. It's possible that nobody had really looked for them there before, but it at least suggests that the squirrels may be more adaptable than they had been given credit for.

Between 2018 and 2020, researchers conducted a camera trap study in western Nevada, primarily to look for black bears, but in a way that would inevitably capture other animals that happened to live in the same neighbourhoods. Over the two-year period, they captured 656 photos of western grey squirrels, at sixteen different sites. Four of these were where you'd expect to find them, in that small region of pine forest around Carson City. The other twelve were not, including one about 120 km (75 miles) away from what had previously been thought to be their maximum extent in the region. 

That's not a small distance in itself, but, more importantly, it's well beyond any place we'd find the sort of oak and pine forests that the animals normally inhabit.

Instead, the region is dominated by pinyon-juniper woodland, a transition zone between the mixed pine forests of the mountains and the sagebrush scrub of the drier parts of Nevada. While western grey squirrels have not previously been recorded here, pinyon is a type of pine tree and as such does produce the same sorts of cones that the animals would normally eat. However, they are much smaller than the pines the squirrels would be familiar with, being 10 to 20 metres (35 to 65 feet) high when fully grown rather than 25 to 40 metres (80 to 130 feet) and with cones that are also correspondingly smaller. 

One might question whether the squirrels had always been there, but had simply been missed by previous surveys. The researchers argue that this is unlikely, partly because surveys of the area in the early 20th century are regarded as having been especially thorough, but also because the squirrels are active during the day, can't be confused with anything else living in the area and really shouldn't be that hard to spot. After all, they found plenty of them, and they were looking for bears. 

Another reason why we wouldn't have expected to find western grey squirrels here is that the climate just isn't suitable. Unlike the eastern grey and many of their other relatives, this particular species doesn't like deep or long-lasting snow cover, something thought to keep it to regions on the western side of the Sierra Nevada where winters are comparatively mild - as well as preventing them from heading into northern Washington or British Columbia. And winter temperatures in the open pinyon-juniper woodlands of western Nevada regularly plunge below zero.

Or, well, they used to. Because, since those thorough wildlife surveys of the early 20th century minimum winter temperatures have risen by an average of 2.4°C (4.3°F) across the region. Trees may be too long-lived for the forests to have dramatically changed in form yet, but those milder winters may have made the region more tolerable to the squirrels and it probably helps that one thing you still don't find in pinyon-juniper woodlands are... eastern grey squirrels. 

There are reasons why these woodlands may create problems for the squirrels in future years. They take longer to recover from forest fires than mixed pine woods, and, while they are long-lived, the trees are struggling to reproduce as temperatures rise.

But, for the time being, climate change is actually helping one species of animal colonise new lands it could never have reached before. Every cloud, and all that...

[Photo by Alan Schmierer, from Wikimedia Commons.]