Never underestimate a squirrel. Growing up near the Atlantic coast of North America, I was familiar with the native eastern gray squirrel (Sciurus carolinensis) as a familiar feature of forest and urban environments. It seemed to coexist in both habitats well enough, and I never heard any stories of it causing ecological damage beyond the occasional occupation of a house.

A few weeks ago, I was on a tour of the Forest of Dean in England, led by managers from the government’s Forestry Commission. I knew the eastern gray squirrel was invasive in Europe, displacing the native red squirrel, but was surprised by the level of anger directed at this species by the land managers.

I learned that it causes dieback of many native species as well. Apparently it strips bark from trees like oak and beech. Because the bark conducts water and sugars to different parts of the trunk, bark loss can easily lead to the death of the whole tree. The downstream consequence is shifts in the dynamics of forest succession and the loss of trees that would otherwise be sold for timber. And there was evidence of these losses throughout the forest.

For a North American, this was an incredible surprise – to see a species shift so dramatically in its impact in what appeared to be very similar habitat. I don’t know what exactly causes this behavioral shift, and no one else among this group of ecologists seemed to either. Maybe they do actually strip bark in their native range but no one notices, or minds. Regardless, the land managers were certain about the damages caused by these squirrels and the need to control them, and equally surprised to hear that they caused no such problems in North America.

This experience made me reflect on the difficulty of predicting how species interactions will change in novel climates and biotic contexts. And it gave me a new respect for this species. The eastern gray squirrel is full of surprises.