All reintroductions and translocations should follow IUCN guidelines, include post-release monitoring, and be undertaken as part of a national strategy and not ad hoc. All translocated animals should be tested for relevant diseases, which is particularly important for red squirrels.
The native red squirrel has declined precipitously due almost wholly to the introduction of the North American grey squirrel. Following multiple deliberate introductions, the invasive grey squirrel has expanded and replaced the red squirrel throughout much of its former range, resulting in the red squirrel being listed as Endangered in England and Wales and Near Threatened in Scotland on the IUCN-compliant Regional Red List for British mammals (Mathews & Harrower, 2020). The rapid replacement of red squirrels by grey squirrels was catalysed by the squirrelpox virus (SQPV), a viral pathogen for which the grey squirrel is a reservoir host. SQPV manifests as skin lesions and ulceration in red squirrels, resulting in mortality. Recently, another emerging pathogen has been identified in both wild and captive red squirrels: Adenovirus (ADV) is an enteric virus and, like SQPV, ADV is carried asymptomatically by grey squirrels (Everest et al. 2009;2019), but may cause mortality in red squirrels (Everest et al. 2014). This presents a major challenge to red squirrel conservation, with disease outbreaks adversely affecting isolated and fragmented populations, and conservation translocations (Everest et al. 2014; 2018; Shuttleworth et al. 2014). The majority of captive breeding facilities monitored in recent research were affected by ADV, and it is highly likely that the disease has been transmitted between locations via the exchange of infected animals (Everest et al. 2018).
Over the last twenty years translocations of red squirrels have been used widely as a key conservation tool, with great success when undertaken alongside sustained grey squirrel control to reduce prevalence of SQPV (e.g. Schuchert et al. 2014), for example in Anglesey, which now has the largest red squirrel population in Wales (Shuttleworth et al. 2015). However, the list of failed translocations is far more numerous, with failure often attributed to disease outbreaks (e.g., Prichard, 1995; Lawton et al. 2015). Whilst the debate around the benefits and risks of red squirrel translocations rages on in the scientific literature (Sainsbury et al. 2020; Shuttleworth et al. 2021), translocations have become more common and are undertaken by a wide range of organisations varying from private individuals to zoos. Proposals have also been made to translocate squirrels to places outside their native range (e.g., Isle of Man) and this is also happening in parts of Scotland where squirrels are not known to occur naturally. The international species translocation standards (IUCN, 2013) require translocations to consider all locally prevailing factors, which for red squirrels not only entails consideration of disease risk, e.g., SQPV and ADV (which can be undertaken using non-invasive viral assays ala Everest et al. (2019)), but also consideration and testing of genetic risk factors, such as inbreeding and outbreeding (Simpson et al. 2013). This is particularly relevant to red squirrels, which have been historically moved around so frequently that today there are numerous pockets of genetically distinct red squirrel populations that have their origins in divergent locations (O’Meara et al. 2018).
Whilst red squirrel translocations have been, and continue to be, an integral tool to the species’ survival, it is vital to recognise that translocations are more frequently being used as a publicity tool. The risks to the red squirrel on both local and national scales are dynamic as are the requirements of conservation translocations. Whilst the widening of the geographical distribution of the red squirrel in certain localities (e.g., Wales and England) is still an ecologically defensible position, recent evidence that pine marten recovery results in landscape-scale control of grey squirrels, and recovery of red squirrels (Sheehy & Lawton, 2018; Twining et al. 2020; 2021) may suggest the case for further island population refuges is weaker than it was last century.
All reintroduction and reinforcement projects, whether using captive-bred or wild-caught and translocated animals, must abide by international standards (IUCN, 2013). Translocations should only be considered where a) native populations of red squirrels have previously existed, b) identified extinction causes have been removed, or sufficiently reduced, c) there are clearly defined and quantifiable conservation outcomes and ecological benefits, and d) sufficient funding is available to cover costs of pre-translocation screening, and subsequent long-term monitoring of population survival and health. A combination of viral assays to ascertain disease status and mtDNA testing to ascertain historic origins of both donor and recipient populations should be a prerequisite for the translocation of any individuals.
Translocations and reinforcements should be undertaken as part of a wider national strategy, with regular reviews of successes and failures so that lessons can be learnt.
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