Grey squirrel Sciurus carolinensis management in Italy ‐ squirrel distribution in a highly fragmented landscape

Abstract

American grey squirrels Sciurus carolinensis introduced to northern Italy in 1948 have caused damage to commercial poplar plantations and have replaced the native red squirrel Sciurus vulgaris from most of the 350 km2 of the Piedmont Po‐plain they currently occupy. In order to plan a control programme aiming to stop grey squirrels from further spreading and to decrease their numbers, the current distribution and population size in the highly fragmented landscape of the Po‐plain were studied. The probability of finding grey squirrels in woodland fragments increased with habitat quality (diversity of trees producing large, consumable seeds), woodlot size and the proportion of poplar. Adding isolation variables did not improve the fit of the logistic regression model that predicted squirrel presence. The density of squirrel dreys, an index of population density, in the large Stupinigi forest also increased with tree species diversity. An estimate of the minimum population size for all woodlots assessed for squirrel presence was 1,260 animals in the summer of 1996. This extrapolates to a total of ca 2,500 grey squirrels in Piedmont. Grey squirrels continue to increase their range and are getting close to the continuous mixed forests of the pre‐Alps and to large hazel plantations. Control measures to stop the spread of grey squirrels, and eventually to eradicate them, should be implemented immediately.

Relative use of contiguous and fragmented boreal forest by red squirrels (Tamiasciurus hudsonicus)

“…The mechanisms causing sensitivity to patch area and isolation in this species are unclear, but are thought to be due to problems in dispersal across open agricultural habitats (Wauters et al 1994). Similar patterns have been observed for gray squirrels (Sciurus carolinensis), although they presumably are better at crossing open habitats, given their rapid spread across fragmented landscapes in Europe (Kenward 1985;Fitzgibbon 1993;Wauters et al 1997). In contrast, the fox squirrel (Sciurus niger) seems to be well adapted to fragmented landscapes, as it is more likely to be found in intermediate-sized forest fragments in agricultural landscapes than in large forest blocks (Nixon and Hansen 1987;Sheperd and Swihart 1995).…”

The Distribution and Habitat Selection of Introduced Eastern Grey Squirrels, <em>Sciurus carolinensis</em>, in British Columbia

“…Attempts to eradicate, control, or slow the spread of EGS have been costly failures, in part due to a lack of understanding of the population biology and methods of spread of EGS (Sheail 1999;Gonzales 2000*;Bertolino and Genovesi 2001). Conifer forests and plantations, mountainous terrain, and bodies of water have slowed spread or were barriers to movement, while deciduous forests and poplar plantations have provided suitable habitat for EGS in Europe (Williamson and Brown 1986;Wauters et al 1997;Ó Teangana et al 2000aÓ Teangana et al , 2000b. EGS were found in conifer forests in Great Britain, but the maintenance of those populations required immigration from neighbouring habitat types Bryce et al 2002).…”

A grey future for Europe: Sciurus carolinensis is replacing native red squirrels in Italy

“…According to our results, a stepping-stone mechanism could be proposed to explain the dispersal process. Grey squirrel presence in highly fragmented landscapes is influenced by woodland size, quality and isolation (Fitzgibbon 1993;Wauters et al 1997a). For instance, in Piedmont the mean distance between woodlands in the area colonised by the grey squirrel in the 1990s was about 1 km (Wauters et al 1997a).…”

“…At least 20 separate introductions took place in Italy (Bertolino 2008;Martinoli et al 2010). Presently, the range of introduced grey squirrel populations covers most of England and Wales, part of Scotland, the eastern part of Ireland, as well as many areas in Northwestern Italy and a location in central Italy (Wauters et al 1997a;O'Teangana et al 2000;Bertolino 2008;Gurnell et al 2008;Martinoli et al 2010).…”