An American Invasion of Great Britain: The Case of the Native and Alien Squirrel (Sciurus) Species

Abstract

The invasion of Great Britain by the gray squirrel from America has posed many interesting problems in ecology and conservation (interactions with the native red squirrel) and in forest management (the gray squirrel severely damages young stands of deciduous trees). Since 1973, mapped data on the annual distribution of the two species in state‐managed forests have been published by the Forestry Commission. Using the established methodology of annual ornithological index numbers, series of index numbers have been calculated to show the distributional dynamics of the two species. In Great Britain as a whole there has been rather limited change over the 15‐year period from 1973. However, this masks (1) the dramatic distributional decline in the red squirrel in Wales balanced by a modest expansion in Scotland, and (2) the substantial distributional increase in the gray squirrel in the small number of grid squares in Scotland in comparison with the relatively steady distribution in the large number of grid squares in England. A new index, measuring the annual replacement of one species by the other species, indicates little overall change in England and Scotland, but a net advance of the gray squirrel in Wales. A series of Markovian matrices predicted that the red squirrel would persist in forests in Scotland and in some areas of England, but would be rare in Wales. Over Great Britain as a whole, the gray squirrel is likely to occur in about twice as many forest areas as the red squirrel.

Interference competition and species coexistence

“…While such situations may reflect ongoing, but slow, competitive exclusion, it is also possible that invaders coexist with native species by virtue of superiority in beneficial interference (e.g. Usher et al 1992;Dick 1996). One would expect such coexistence to be more prevalent in animal rather than plant communities because beneficial interference is more likely to operate in animals than plants.…”

Effects of the invasive Barbary ground squirrel (Atlantoxerus getulus) on seed dispersal systems of insular xeric environments

“…Squirrels have been introduced to more than 60 islands worldwide (Lo´pez-Darias, in preparation), and except for one well-studied and managed case (Sciurus carolinensis in Great Britain;Tompkins et al, 2003;Usher et al, 1992) and one island eradication programme (Stanton, 1994), the status and impacts of the majority of these squirrel invasions have been scarcely evaluated. However, their impacts are neither scant nor irrelevant.…”

Epidemiological and postmortem findings in 262 red squirrels (Sciurus vulgaris) in Scotland, 2005 to 2009

“…The protection of this last remaining substantial red squirrel population is a priority if the species is not to become extinct on the UK mainland. The red squirrel's demise has been attributed mainly to the presence of the American grey squirrel (Sciurus carolinensis), a non-native species first introduced into the UK in the late 19th century (Usher andothers 1992, Kenward andHolm 1993). Direct competition between red and grey squirrels for resources may partly account for the decline in the red squirrel population, with grey squirrels tending to outcompete red squirrels in some types of habitat Holm 1993, Kenward andothers 1998).…”