Biocontrol News and Information 39(2), 9N–16N www.cabi.org/BNI General News Conservation Scores a Biological Control land and Scotland. These coincided with reports of Success pine marten range expansion, including sightings in areas where the predator had not been recorded for A recent study in Scotland provides robust evidence decades. Results of surveys in the Irish midlands for an unusual case of biological control, involving showed that by the early 2010s, the pine marten, the recovery of two threatened species.1 The results which had been restricted to west of the River reinforce findings of a previous study in Ireland.2 Shannon, now extended throughout the region as far There is now clear evidence from the British Isles as the east coast.2 that the recovery and range expansion of the pine marten (Martes martes), a native mustelid predator, This study also found that grey squirrel populations is skewing competition between the invasive North had crashed in some 9000 km2 of its previous range American grey squirrel (Sciurus carolinensis) and in the Irish midlands and the invasion front had the native red squirrel (Sciurus vulgaris), and facili- retreated 100 km to the east, leaving it a rare species tating recovery of the native squirrel species from a in much of the central midlands. In contrast, the red precipitous decline. squirrel had recolonized 6500 km2 of its historic range, from which it had been absent for up to 30 The pine marten was exterminated in large parts of years, and it is now common and widespread there. the British Isles in the 19th century through hunting The areas lost by the grey squirrel and regained by for fur, predator control by gamekeepers and habitat the red squirrel lie entirely within the expanded loss. Populations became restricted largely to range of the pine marten. Overlap between pine northern Scotland and western Ireland. Although it marten and remaining abundant grey squirrel popu- remains the second-rarest mammal in the British lations was found in the eastern part of the region, Isles, reforestation, legislation and conservation where the recently arrived pine marten is at lower activities have facilitated a comeback, and since the density. The study found pairwise correlations 1990s its range has been slowly expanding. Pine between detection of pine marten and red squirrel on marten is now naturally distributed throughout a landscape level (strongly positive) and pine marten much of Scotland and the Irish midlands, with small and grey squirrel on a woodland level (negative). The populations in parts of Wales and England. methodology did not allow conclusions about cau- sality, although factors such as culling and resource The native red squirrel suffered a steep decline in the limitation were ruled out, and live trapping sug- 20th century initially through resource competition gested that the overall status and health of grey from the larger North American grey squirrel, which squirrels were poorer in the central midlands where was introduced to Great Britain several times in the pine martens are well established. The authors sug- 1870s–1920s and to Ireland in 1911. The grey gested that pine marten density might be the key squirrel has replaced the native species through factor. much of its former range: only isolated red squirrel populations remained in England and Wales, and its The research in Scotland1 built on the Irish study. range otherwise shrank to central and northern Scot- Multi-species sampling by different methods (hair land and the west of Ireland. The threat to red samples from feeders ± trail cameras) allowed differ- squirrel from grey squirrel has been greatly exacer- ences between species in detectability to be identified bated in recent decades by squirrel pox virus (SQPV): and taken into account. The results were used to grey squirrel is an asymptomatic vector but the dis- model red and grey squirrel occupancy for various ease is fatal in red squirrel. First reported in Great factors (e.g. region, attributes of habitats, pine Britain in the 1980s, SQPV incidence in grey squir- marten presence). A key part of the study was iden- rels was 42–100% by the mid-2000s in England and tifying individual pine martens from hair samples to Wales, although the first record in Scotland was not provide spatial capture–recapture information, until 2005, and it was first detected in Ireland as late which was used to estimate not only pine marten as 2011.3 Grey squirrel is not only a threat to red density but also each individual’s use of space (‘den- squirrel, but by stripping bark of young trees is also sity-weighted connectivity’), i.e. how ‘pine marten a forestry pest. Its invasive range also includes Italy, rich’ a neighbourhood is. This latter estimate allows from where it could potentially invade a vast region the likelihood of encounters between pine marten of Eurasia. and prey to be assessed as a measure of overall expo- sure to predation. Conservation measures for red squirrel remnant populations and at the edge of the grey squirrel inva- Field work was carried out in January–May 2016 in sion rely on heavy (therefore expensive and also not three regions: the northern Scottish highlands, universally supported) culling of grey squirrel popu- which pine marten recolonized some 45 years ago lations to below competitive levels. But even this and grey squirrel has not reached; central Scotland, level of culling was insufficient to break transmission which pine marten has recently recolonized (8–14 of SQPV, and the outlook for the red squirrel seemed years) and where grey squirrel has been present for bleak. From the 1990s onwards, however, anecdotal some 70 years; and the southern Scottish borders, reports of red squirrel recovery were emerging in Ire- which pine marten began to recolonize in the past Are we on your mailing list? Biocontrol News and Information is always pleased to receive news of research, conferences, new products or patents, changes in personnel, collaborative agreements or any other information of interest to other readers. If your organization sends out press releases or newsletters, please let us have a copy. In addition, the editors welcome proposals for review topics. 10N Biocontrol News and Information 39(2) few years while grey squirrel has been present for at have used in future research in this area. Such least 30 years. SQPV is now endemic in central Scot- research could also help inform a related conflict of land, but was not detected in the borders until 2017. interest: the impact of pine marten recovery on the In the regions where the squirrel species coexist (cen- game bird industry. The firm evidence in this paper tral and borders), study sites were selected to favour about the negative impact of the pine marten on grey grey squirrel (>30% mixed broadleaf woodland). squirrel may help influence land managers with Although pine marten densities were lower in the interests in both commercial forestry and rearing borders than in the other two regions, and grey game birds that gains to the former from pine squirrel is not found in the highlands, the authors marten presence could outweigh potential losses to argue their analysis is valid as it is based on large the latter. intra- as well as inter-regional variation. 1 Sheehy, E., Sutherland, C., O’Reilly, C. and Results showed a strong relationship between Lambin, X. (2018) The enemy of my enemy is my squirrel occupancy and likelihood of encounter with friend: native pine marten recovery reverses the pine marten (density-weighted connectivity). The decline of the red squirrel by suppressing grey correlation was negative for grey squirrel and posi- squirrel populations. Proceedings of the Royal Society tive for red squirrel. The negative impact of pine B 285, 20172603. marten on grey squirrel occupancy was greater in central Scotland where the species have coexisted for 2 Sheehy, E. and Lawton, C. (2014) Population crash longer than in the borders. The positive relationship in an invasive species following the recovery of a between likelihood of encounter with pine marten native predator: the case of the American grey and red squirrel occupancy included the highlands squirrel and the European pine marten in Ireland. where there are no grey squirrels. Biodiversity and Conservation 23, 753–774. The authors suggest how behavioural differences 3 Rushton, S., Lurz, P., Gurnell, J., Nettleton, P., may influence squirrel competitiveness in the pres- Bruemmer, C., Shirley, M. and Sainsbury, A. (2006) ence of pine marten. Camera data showed that red Disease threats posed by alien species: the role of a squirrels are less likely than grey squirrels to use poxvirus in the decline of the native red squirrel in feeders. The Irish study reported a difference of Britain. Epidemiology & Infection 134, 521–533. 15.6% and 2.5% in frequency of grey and red squirrel in pine marten scats, respectively. As a coevolved species, red squirrel may be more wary of feeders Contact: Xavier Lambin, Professor of Ecology, (but not otherwise deterred from suitable habitat) if University of Aberdeen, UK. pine marten is in the area, compared with grey Email: [email protected] squirrel, which has no coevolutionary history with pine marten or similar species, and is thus more prone to direct encounter including predation. Chris Prior: Good Company and an Important Scientist The authors conclude that the results provide “une- quivocal support” for the role of the recovering pine Dr Chris Prior, born on 27 June 1949, was an inter- marten in modifying competition between the two nationally renowned biologist: perhaps best known squirrel species, and skewing it heavily in favour of for a discovery leading to the development of effective the native species.