Declining invasive grey squirrel populations may persist in refugia as native predator recovery reverses squirrel species replacement

Abstract

Invasive species pose one of the most serious global threats to biodiversity. Investigations into the interactions of native and non‐native species focus on the impacts of single species, despite being embedded in a network of direct and indirect interactions between multiple species and their environments. We developed 1‐km2 resolution, single‐species and multi‐species occupancy models using quantitative camera trap data collected by citizen scientists at 332 sites in a regional survey comprising the 14,130 km2 of Northern Ireland. Recent research suggests that native red and invasive grey squirrels in Britain and Ireland are linked by resource and disease‐mediated competition, and by a shared enemy, the European pine marten. We demonstrate that the presence of the pine marten reverses red squirrel replacement by grey squirrels on a regional basis, with red squirrel occupancy positively affected by exposure to pine marten. In contrast, the grey squirrel has a strongly negative response to the presence of pine marten. Modelling habitat suitability of pine marten and both squirrel species suggests that, despite the potentially strong effect of a recovering population of pine martens in controlling grey squirrel distribution, the latter is likely to persist in urban refugia that are either inaccessible or avoided by the pine marten. Synthesis and applications. Sustainable recovery of both the red squirrel and the pine marten across the wider landscapes of Ireland, Scotland and Northern England seems probable, while in Southern England and Wales, although possible, chances are more remote. Increasing native woodland cover on a landscape scale to facilitate pine marten recovery may assist in realizing this scenario. Despite the ongoing recovery of the pine marten, and resultant declines in grey squirrels, isolated populations are likely to persist in urban areas. Human‐lead control of grey squirrel populations in urban refugia requires urgent funding and implementation to avoid the development of novel genotypes in grey squirrel source populations that enable the species to better avoid predation by the pine marten.

All forests are not equal: population demographics and denning behaviour of a recovering small carnivore in human modified landscapes

“…The European pine marten Martes martes is a small carnivore currently recovering in Britain and Ireland following severe historical decline driven by habitat loss and persecution (Langley and Yalden 1977, O'Mahony et al 2017, Sainsbury et al 2019. Recent research demonstrates the species may be limited by human disturbance to the more remote, forested parts of Europe (Balestrieri et al 2019, Twining et al 2020. There is some evidence that it is the structural elements of ancient forest that are required, not the presence of old-growth forest per se (Brainerd et al 1995, Caryl et al 2012.…”

Natural strongholds for red squirrel conservation in Scotland

“…This study used data collected by citizen science to show that red squirrel occupancy was positively correlated with coniferous woodland and negatively correlated with urban habitat. Likewise, grey squirrels were positively correlated with broadleaved forests and urban habitat and negatively correlated with coniferous habitat (Twining et al 2020). The study also found a negative correlation between grey squirrels and pine marten (Martes martes), a native mustelid whose numbers are increasing, and a positive correlation between pine marten and red squirrels.…”

“…Theoretical studies using a two prey-one predator ecoepidemic model have also shown that the predator (pine marten) can destabilise the previous grey squirrel dominant equilibria, favouring the native red squirrel (Travaglia et al 2020). Thus, the impact of a native predator could aid red squirrel survival in the United Kingdom and Ireland via a reduction in grey squirrel density and geographic spread (Twining et al 2020).…”

Towards ecological science for all by all

“…This is because traps can be left in fixed locations on site, and over more defined survey periods. Twining et al (2021) use such data in a study of red (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis), and pine marten (Martes martes) distribution at 332 sites distributed across Northern Ireland.…”

“…species presence) in a more planned and systematic way. This is because traps can be left in fixed locations on site, and over more defined survey periods Twining et al (2021). use such data in a study of red (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis), and pine marten (Martes martes) distribution at 332 sites distributed across Northern Ireland.Camera trap data were used to build species occupancy models and demonstrated that the recovery of pine marten was strongly, and positively associated with red squirrel populations, with grey squirrel having the opposite relation.…”