Interactions between native and invasive species: A systematic review of the red squirrel-gray squirrel paradigm

Abstract

The eastern gray squirrel (Sciurus carolinensis) has been labeled as one of the 100 worst invasive alien species by the IUCN. In Europe, the species has been introduced to Britain, Ireland and Italy, and its subsequent spread has resulted in wide-scale extinction of native Eurasian red squirrels (Sciurus vulgaris) from the areas colonized by the gray squirrel. This replacement of a native by an alien competitor is one of the best documented cases of the devastating effects of biological invasions on native fauna. To understand how this replacement occurs, we present a systematic review of the literature on competition and interactions between red and gray squirrels. We describe the patterns of red and gray squirrel distribution in those parts of Europe where gray squirrels occur and summarize the evidence on the different processes and mechanisms determining the outcome of competition between the native and alien species including the influence of predators and pathogens. Some of the drivers behind the demise of the red squirrel have been intensively studied and documented in the past 30 years, but recent field studies and mathematical models revealed that the mechanisms underlying the red-gray paradigm are more complex than previously thought and affected by landscape-level processes. Therefore, we consider habitat type and multi-species interactions, including host-parasite and predator-prey relationships, to determine the outcome of the interaction between the two species and to better address gray squirrel control efforts.

The impact of urbanisation on chipmunks, arboreal and flying squirrels: a global systematic review

“…In Britian and Ireland, the recovery of a native predator, the pine marten ( Martes martes ), influences the outcome of interspecific competition between red and grey squirrels, and in some landscapes can even reverse it in favour of the native species (reviewed in Wauters et al. 2023). Using single‐species and multi‐species occupancy models on 332 sites in Northern Ireland (14130 km 2 ) at a 1 km 2 resolution, based on camera trap data collected by citizen scientists, Twining et al.…”

“…Hence, despite the ongoing recovery of the pine marten and decline of grey squirrels in natural woodlands and forests (Wauters et al. 2023), isolated populations of the alien species are likely to persist in urban areas, making control of these populations a management priority (Twining et al. 2021).…”

Future Range Dynamics Suggest Increasing Threats of Grey Squirrels (Sciurus carolinensis) against Red Squirrels (Sciurus vulgaris) in Europe: A Perspective on Climatic Suitability

“…Under the current conditions and all of the climate change scenarios, Ireland and the UK were consistently in overlapping regions of the grey squirrel and red squirrel ranges in the future. Therefore, these two areas are the focal regions for controlling the grey squirrel threats against red squirrel conservation, and strict control management strategies are already ongoing in the UK and Ireland [43]. Additionally, under the SSP126 scenario, France, Italy, and Germany were in the overlap ranges of grey squirrels and red squirrels but not under the current conditions.…”

“…Many studies have investigated the invasions of grey squirrels and their threats to the survival of red squirrel populations in Europe [34,37,[40][41][42]. Additionally, a variety of studies have focused their attention on the interactions between red squirrels and grey squirrels [43], including the effects of habitat fragmentation on interspecific competition [42], the differentiation of niche use between grey squirrels and red squirrels [28], resource partitioning between grey squirrels and red squirrels [34], and the ranges of grey squirrels in Europe [44,45]. These studies have provided essential information for countering the competition between grey squirrels and red squirrels and developing conservation strategies for red squirrels in Europe.…”

Genomic insights into red squirrels in Scotland reveals loss of heterozygosity associated with extreme founder effects

“…In Britain, the Eurasian red squirrel (Sciurus vulgaris L., 1758) exemplifies a species with a complex demographic history related to changes in land use and cultural attitudes (Harvie-Brown, 1881a, 1881b, 1880Holmes, 2015). Furthermore, it has become a text-book icon of conservation science due to its 20 th century (and continuing) replacement by the introduced North American grey squirrel Sciurus carolinensis (G., 1788 (Gurnell et al, 2004;Gurnell and Pepper, 1993;Rushton et al, 2006)), which has arguably become one of the bestknown examples of a native species being supplanted by an invasive competitor (Gurnell et al, 2004;Wauters et al, 2023). While habitat loss and fragmentation contribute to current population declines, the major driver of contemporary replacement is ecological and diseasemediated competition with the introduced grey squirrel, which can asymptomatically carry a virus novel to red squirrels, squirrelpox virus SQPV (Rushton et al, 2006;Sainsbury et al, 2000;Tompkins et al, 2003).…”