ENVIRONMENT IN-DEPTH Red Squirrel Decline
Cracking the mystery of red squirrel decline
Red squirrel numbers in the UK have declined dramatically over the last 60 years. Once widespread across England, Wales and much of Scotland, the species has suffered a rapid contraction in both range and abundance. Current estimates suggest that fewer than 287,000 red squirrels remain in the UK, with approximately 75% of the population now found in Scotland. This long‑term decline is driven primarily by competition for resources and the spread of disease from the invasive grey squirrel, which was introduced to the UK from North America in the late 19th century.
Grey squirrels are larger, more adaptable, and better able to exploit a wide range of food sources than red squirrels, giving them a competitive advantage. However, the most devastating impact of grey squirrels on red squirrel populations comes from their role as carriers of squirrelpox virus. While grey squirrels typically show few or no symptoms, the disease is often fatal to red squirrels, leading to mortality rates of up to 90% in infected populations. In areas where the two species coexist, squirrelpox outbreaks can cause local red squirrel populations to collapse in a matter of months.
There may be fewer than
red squirrels left in the UK, most of which are found in Scotland
To address this complex conservation challenge, mathematical modelling research has been used to inform policy and practice aimed at protecting red squirrels, a protected species in the UK, from squirrelpox-carrying grey squirrels. Mathematical modelling involves translating real‑world problems into mathematical terms - using equations, graphs, and computer algorithms - to analyse systems, understand underlying mechanisms, and predict future outcomes under different scenarios. In this context, modelling was used to explore how squirrelpox spreads across landscapes, how red and grey squirrel populations interact, and how different management strategies might affect both disease outcomes and population viability over time.
By turning a complex conservation challenge into data-driven scenarios, researchers can see how red and grey squirrel populations interact — and which interventions are most likely to work.
Red squirrel numbers have dramatically declined over the last 60 years, with approximately 75% of the population now found in Scotland.
Prior to this research, red squirrel conservation policy primarily focused on reactive control measures in areas where grey squirrels tested positive for squirrelpox. These interventions temporarily reduced grey squirrel numbers at local scales and were often resource intensive. However, while this approach slowed disease transmission in some cases, it did not prevent squirrelpox from spreading more widely, nor did it lead to sustained increases in red squirrel populations. In many regions, grey squirrels rapidly recolonised controlled areas, reintroducing the disease and undermining long-term conservation efforts.
The modelling research demonstrated that the red squirrel conservation policy in place up to 2015 would not be sufficient to contain squirrelpox across the UK. The findings showed that controlling the disease is extremely challenging in areas where grey squirrels are already well established and widely distributed. Once squirrelpox becomes endemic across a landscape, eradication is highly unlikely, even with sustained management effort. Importantly, however, the research also revealed that squirrelpox would not persist in areas where red squirrels are present but grey squirrels are absent. This highlighted the critical importance of maintaining and protecting grey squirrel‑free refuge areas where red squirrel populations still survive.
These findings triggered a radical shift in red squirrel conservation policy towards prevention, prioritisation, and landscape‑scale planning. The research directly informed the Scottish Strategy for Red Squirrel Conservation (2015), which identified priority conservation areas and placed a strong emphasis on preventing grey squirrel incursions rather than attempting disease control after establishment. The modelling evidence also supported the policy adopted by the Red Squirrel Survival Trust on Anglesey, where proactive grey squirrel eradication has successfully protected a key red squirrel population and kept squirrelpox at bay.
Heat map showing squirrel concentrations.
Beyond disease management, the research also evaluated red squirrel population viability under different forest management scenarios. Woodland composition and structure have a significant influence on squirrel populations, with conifer and mixed forests generally favouring red squirrels, while broadleaved woodlands tend to benefit grey squirrels. This work underpinned commissioned reports for Forestry and Land Scotland, helping to guide forest design and management practices that balance timber production with red squirrel conservation. As a result, the research has had a direct impact on current and future forest management decisions across the UK.
The mathematical techniques developed for red squirrel conservation have wider applicability to other environmental and biosecurity challenges. The same modelling approaches are currently being adapted to guide control measures for African swine fever in wild boar populations following a recent outbreak in the province of Barcelona. If the disease spreads from wild boar to domestic pigs, it could have devastating consequences for Spain’s pork industry, valued at approximately €25 billion per year. This demonstrates how mathematical modelling can play a crucial role in informing evidence‑based decision‑making across conservation, agriculture, and animal health policy.
Using equations, algorithms and real-world data, researchers are mapping the future of red squirrel conservation.
Mathematical Modelling of the UK Squirrel System Directs Conservation and Forest Management Policy and Practice to Protect Endangered Red Squirrels.
