Veronica Price-Jones, Visitor Engagement volunteer, writes:
Eastern grey squirrels (Sciurus carolinensis) are a common sight both in Cambridge and in my hometown of Caledon, Ontario, Canada. I can see them outside the window while I type, running across the lawn and jumping onto the birdfeeders. (My friend once called them “land dolphins,” for how they leap through the grass, and I will never forget it.) However, their status in the two places is very different: in Ontario, they are native and coexist fairly peacefully with American red squirrels (Tamiasciurus hudsonicus), whereas in the UK, they are introduced and outcompete Eurasian red squirrels (Sciurus vulgaris).
I have often wondered why red squirrels and grey squirrels can live together in North America, but not in the UK, and recently I decided it was time to find out. It turns out that grey squirrels and Eurasian red squirrels fill the same ecological niche: this means that they consume the same foods and use the same habitat. This creates a problem for Eurasian red squirrels because the introduced grey squirrels are bigger, socially dominant, and can put more energy into finding food. Eurasian red squirrels haven’t been able to adapt to avoid competition when resources become tight, so they’ve been crowded out of their original habitat.
In contrast, in North America, American red squirrels have different habitat preferences and foraging habits. For example, grey squirrels are more common around soybean fields and only like deciduous forest, whereas red squirrels are more common around corn fields and are very at home in coniferous forest. Thus, instead of fighting over the same resources, they use slightly different ones and can live side by side.
That’s one mystery solved, but it leaves another that organisations like the RSPB are tackling: how to conserve the Eurasian red squirrel, whose UK range is now restricted to Scotland, Northern Ireland and small pockets of England and Wales.