Developing optimal strategies for limiting the spread of grey squirrels (Sciurus carolinensis) and conservation of reds (Sciurus vulgaris) in Ireland

Abstract

Ireland has two species of squirrel, the native red squirrel (Sciurus vulgaris) and the non-native and invasive grey squirrel (Sciurus carolinensis). Since the initial introduction of the grey squirrel to a single site in the Irish midlands in 1911, it has spread to cover the eastern half of Ireland. Furthermore, red squirrels had disappeared from the areas in which the grey squirrels were longest established. This phenomenon has also occurred in Britain and Italy, and is the result of competition between the two species. In Britain the replacement of red squirrel populations has also been mediated by the squirrelpox virus (SQPV), a disease asymptotic in grey squirrels but lethal to reds. In Ireland, the continued displacement and disappearance of red squirrels has been attributed to the effects of competition only, but SQPV has recently been discovered in red squirrels in the east and north of Ireland.

The continuing expansion of grey squirrel populations and displacement of red squirrels was documented through a series of distribution surveys throughout the decades. Despite some infrequent records, grey squirrels have failed to spread to the west of Ireland, and the river Shannon has traditionally delineated the western boundary of the species’ distribution. However, in the 2007 survey (Carey et al., 2007) a number of grey squirrel sightings were received to the west of the river. It was believed that the barrier to grey squirrel spread may have been breached, compromising the potential of the west of Ireland to act as a refuge for red squirrels in Ireland. However, a notable shift in grey squirrel distribution was observed in the 2007 squirrel survey, with the apparent disappearance of grey squirrels from part of their former range in the Irish midlands. Anecdotal evidence suggested that this occurred concurrent to the return of a native woodland predator. Following protective legislation in 1976, pine marten (Martes martes) populations have been increasing and expanding in the Irish midlands, from their stronghold in the west of Ireland. Recent research has now shown a negative correlation between high density pine marten populations and grey squirrels.

This main aim of the project was to establish the current status of grey squirrels in the area around the river Shannon, and to develop a strategy to prevent grey squirrels colonising the west of Ireland. This was to be achieved by pinpointing key woodlands along the river Shannon, that may be support large grey squirrel populations, and by identifying landscape corridors that could facilitate grey squirrel dispersal into the west of Ireland. Protecting the west of Ireland against grey squirrel incursion would allow for the development of a management strategy for the long term conservation of red squirrels in the west of Ireland, to ensure conservation of the species in Ireland in the long term.

The current distribution of both red and grey squirrels, and pine marten was established in the area surrounding and to the west of the river Shannon, using a public sightings survey. Hair tube studies and live trapping was used to validate the results of public sightings. The number and extent of red squirrel and pine marten sightings have increased since 2007, and both species were widespread in the study area. Red squirrels were recorded in 10 of the 14 sites investigated during a hair tube survey; grey squirrel were not recorded using this technique. A 12-month live trapping study highlighted a thriving red squirrel population in the study area, which was attributed to the absence of grey squirrels and increased habitat quality of a mixed forest arising from the benefits of Continuous Cover Forestry as a management technique. The occurrence of grey squirrels was infrequent from the majority of the study area, apart from the south/southeast where they were continuing to spread. Grey squirrel sightings were also reported from west of the river Shannon, but no established populations were detected. Grey squirrels had retracted from a much larger expanse of the Irish midlands than was previously believed. This was confirmed using live trapping programmes in the area where the greys were longest established. Pine marten occurrence was found to be significantly different in discriminant analysis of two regions where the fate of grey squirrel differed i.e. pine marten was a significant predictor for the midlands region, from where grey squirrels have retracted. Squirrel sensitivity to marten odour was examined using a giving-up density experiment. Although experimental re-design is required, a reduced feeding rate in grey squirrels in response to marten odour observed in the study could explain the mechanism behind the negative correlation with pine marten.

A species distribution model (SDM) highlighted areas of high habitat suitability for grey squirrels in the study area. Woodlands vulnerable to invasion have been selected based on the SDM results. The least cost pathway (LCP) modelling identified “pinch points” i.e. bottlenecks in dispersal corridors. Monitoring and control (if necessary) of grey squirrels in both vulnerable woodlands and at pinch points will be the key to excluding grey squirrels from the west of Ireland. A species distribution model was also used to highlight areas of suitable habitat for red squirrels in the west of Ireland. Habitat quality and landscape connectivity analysis were used to objectively select the most strategic sites in which to invest conservation efforts. This will ensure the maintenance of viable red squirrel population and thus long term conservation of the species. Recommendations were made regarding forest management to favour red squirrel populations. The use of Continuous Cover Forestry (CCF), as a forestry management technique, was demonstrated as being extremely advantageous to red squirrel populations. The current study showed that red squirrels were thriving, with good fitness and breeding rates, despite a poor seed crop; which was attributed to the good mix of trees and absence of grey squirrels.