Science matters: Grey squirrels, damaged woods and pine martens

BASC’s Julia Newman reviews the research around bark stripping activity in grey squirrels and discusses the role of pine martens as part, rather than all, of the solution.

The grey squirrel (Sciurus carolinensis) is a widespread, non native invasive species in the UK. As well as contributing to the decline of the native red squirrel, it presents a serious challenge for the sustainable management of British woodlands(1).

Grey squirrels cause significant damage by stripping bark from trees, which affects timber quality, woodland health, and long term forest resilience. The total cost of grey squirrels to the UK economy has been estimated at more than £40 million every year (2).

Why do squirrels strip bark?

You might assume scientists already know why squirrels strip bark from trees, but surprisingly this is not the case!

A recent paper by Ash et al. (2026) (3) has reviewed published studies from around the world that investigated bark stripping in squirrel species. Their aim was to identify common patterns and explanations for why squirrels strip bark from trees.

Turns out, very few studies have directly tested the causes of bark stripping, and no single explanation has become clear over another.

Why do we need to understand their behaviour?

A lack of understanding means it is difficult to develop effective solutions. Historically, scientists believed sciurid species (tree squirrel family) stripped bark to access the nutritious inner layer of the tree, particularly during times of food shortage.

However, in Britain, bark stripping often occurs in spring and early summer when food is plentiful, suggesting that hunger alone does not explain the behaviour.

The paper did however uncover several common possible explanations. These include squirrels seeking extra nutrients such as sugars or minerals, meeting the demands of breeding, using bark stripping as part of aggressive or territorial behaviour, or even self-medicating against parasites. However, most of these ideas are supported mainly by observation rather than experimentation.

While there is good evidence that certain tree species and woodland types are more vulnerable to damage, this tells us more about where damage happens than why. Overall, it highlighted our current lack of understanding and further targeted experiments would be required to try uncover the true cause. Without understanding, it is harder to develop effective management tools to add to our toolbox to aid in the control of this non-native species.

How do we reduce the impact of squirrels?

Although it is interesting to find out the reasons behind the behaviour, in the meantime there is ongoing research into reducing the problems too.

Another recent paper written by Watkinson (2026) (5) has explored whether pine martens (Martes martes) might offer a natural solution. There is growing evidence from Ireland, Scotland and parts of Wales that pine martens can influence grey squirrel populations. This happens both through direct predation and by altering squirrel behaviour, such as how and where they forage.

Grey squirrels appear less able than red squirrels to recognise pine martens as a threat, which may make them more vulnerable in areas where martens are established.

However, it’s not that simple. Pine martens are adaptable, opportunistic predators that tend to focus on the most abundant prey in their environment, often small mammals such as voles. While they will take grey squirrels, squirrels are unlikely to become a main food source. When squirrel numbers fall, martens typically switch to alternative prey, allowing squirrel populations to recover.

Pine martens are only part of the solution

Pine martens also breed slowly, producing just one small litter per year, with high mortality among young. This limits their ability to control fast breeding species like grey squirrels. In addition, pine martens tend to avoid areas of human habitation, giving grey squirrels safe areas where they can persist and spread back into surrounding woodland.

Also, across much of England and Wales, suitable habitat for pine martens is limited, making it unlikely that the same effects seen in Ireland and Scotland could be replicated at a national scale.

Overall, the evidence suggests that pine martens are not a standalone solution to the grey squirrel problem. However, they could play an important supporting role as part of a wider, integrated management strategy.

Combined with targeted control, potential fertility management, and woodland habitat improvements, pine marten recovery could still help reduce the grey squirrel population and strengthen broader conservation efforts where conditions are suitable.

Saving the red squirrel

Along with the bark stripping we also know that grey squirrels are bad news for our native reds. Natural England’s recent Red Squirrel Recovery Strategy makes it clear that doing nothing, or relying only on non lethal approaches, is very likely to lead to further red squirrel losses in England over the next 25 years. These approaches also fail to prevent ongoing damage to trees.

Suppression of grey squirrel populations, including methods such as shooting and trapping, is an essential component of the best strategies the report highlights for red squirrel recovery. This highlights the role that land managers and the shooting community must continue to play in protecting woodlands and supporting red squirrel recovery. BASC engaged with the development of this, by contributing to the evidence gathering and participating in the stakeholder workshops, as a member of the UK Squirrel Accord (UKSA).

Interestingly, pine martens were not included as a control measure in England’s strategy, partly because they have their own conservation challenges and partly because the science does not yet show they can reliably control grey squirrels at the scale required.

The control of grey squirrels is a positive conservation action you can get involved in in your local area.

Guidance on how to do so can be found here or by contacting your local BASC regional team. BASC will continue to keep up to date on the latest research and actively contribute to UKSA partnership.

References

1 Department for Environment, Food & Rural Affairs, & Forestry Commission. (2026). Grey squirrel policy statement: Managing the impact of grey squirrels. UK Government

2 Eschen, R., Kadzamira, M., Stutz, S., et al. (2023). An updated assessment of the direct costs of invasive non-native species to the United Kingdom. Biological Invasions, 25(10), 3265–3276. https://doi.org/10.1007/s10530-023-03107-2

3 Ash, A. K., Spake, R., Zhao, Y., Piña Covarrubias, E., Gill, R. M. A., Nichols, C. P., & Doncaster, C. P. (2026). Understanding why grey squirrels (Sciurus carolinensis) bark strip in British woodlands: A systematic map. Mammal Review. https://doi.org/10.1111/mam.70027

4 Kenward, R. E. (1983). The causes of damage by red and grey squirrels. Mammal Review, 13(2–4), 159–166. https://doi.org/10.1111/j.1365-2907.1983.tb00277.

5 Watkinson, R. (2026). Are pine martens a panacea for Britain’s squirrel problems? BioShorts, 1(3), 50–59. https://doi.org/10.70145/BiSh0010

Julia Newman

Julia is BASC’s game and wildlife advisor. She enjoys game shooting, beating, clay shooting and being outdoors exploring the countryside with her dog.