What If Gone

Species → European Pine Marten

European Pine Marten

Martes martes

no reliable global estimate; as a regional order of magnitude, about…
Extinction risk Least Concern
Biological controlInvasive speciesTemperate forestMesopredator release
European Pine Marten (Martes martes)
Photo: Charles J. Sharp, Wikimedia Commons (CC BY-SA 4.0)

What if it disappeared?

Impact High
In Ireland

Where the marten recovered, the introduced grey squirrel — which had displaced the native red squirrel from entire forests for more than 30 years — collapsed locally, allowing the red squirrel to recolonize those same woods.

Documented
Detail

Sheehy, E. et Lawton, C. (2014). Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland. Biodiversity and Conservation, 23(3), 753–774.

A four-year study in the Irish Midlands documenting a local collapse of grey squirrel (Sciurus carolinensis) populations, a species introduced from North America, in areas where the pine marten (Martes martes) had recovered — accompanied by a recolonization of these same woods by the native red squirrel (Sciurus vulgaris), absent from some of these sites for more than thirty years.

Across the British Isles

The pattern holds at a larger scale: the marten's presence is associated with a marked decline in grey squirrels and a resurgence of red squirrels across the entire study area, not just the original sites of the Irish study.

Documented
Detail

Sheehy, E., Sutton, A., Lawton, C. et al. (2018). The enemy of my enemy is my friend: native pine marten recovery reverses the decline of the red squirrel by suppressing grey squirrel populations. Proceedings of the Royal Society B, 285(1874), 20172603.

By broadening the analysis to the British and Irish scale, the study confirms the relationship between marten presence, the grey squirrel's negative response, and the red squirrel's positive response — ruling out that this was a peculiarity limited to the few sites of the original 2014 study.

By modeling

The effect is not total: the grey squirrel should be able to persist in urban refuges that are inaccessible to or avoided by the marten, even as its decline is confirmed at the regional scale.

Documented
Detail

Twining, J.P., Montgomery, I. et Tosh, D.G. (2021). Declining invasive grey squirrel populations may persist in refugia as native predator recovery reverses squirrel species replacement. Journal of Applied Ecology, 58(2), 248–260.

Modeling of habitat suitable for the marten and both squirrel species shows that despite the strong regional effect of the marten's recovery, the grey squirrel should persist in urban areas that remain out of the predator's reach — a structural limit on the extent of the red squirrel's recovery, not an invalidation of the mechanism.

In Wales

Beyond direct competition, the marten could also protect the red squirrel indirectly by reducing transmission of squirrelpox, a disease that the grey squirrel carries without suffering from it but which is fatal to the red squirrel.

Extrapolated
Detail

Slade, A., White, A., Lurz, P.W.W., Shuttleworth, C., Tosh, D. et Twining, J.P. (2023). Indirect effects of pine marten recovery result in benefits to native prey through suppression of an invasive species and a shared pathogen. Ecological Modelling, 476, 110219.

A modeling study applied to north Wales, combining direct suppression of the grey squirrel by the marten with a second, indirect effect: fewer grey squirrels in contact with red squirrels means less transmission of squirrelpox virus (SQPV), usually fatal to red squirrels but not to grey ones. This is a model result, not a direct field measurement.

What's missing to confirm it: Empirical field confirmation, rather than modeling, of the actual magnitude of this combined benefit (direct competition and disease) remains to be established.

Documentation

Last verified: August 14, 2026