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

Invasive species pose one 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 populations across the wider landscapes of Ireland, Scotland and Northern England seems probable, whilst in Southern England and Wales, although possible, chances are more remote. Increasing native woodland cover on a landscape scale may assist in realising this scenario. Despite the recovery of the pine marten, isolated populations of grey squirrels are likely to persist. 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.

外来入侵物种是全球生物多样性面临的最严峻威胁之一。当前针对本土与非本土物种间相互作用的研究，多聚焦于单一物种的影响，却忽略了这些相互作用实则嵌入在多物种及其环境构成的直接与间接相互作用网络之中。本研究针对覆盖北爱尔兰14130平方千米的区域开展调查，依托公民科学家在332个监测点采集的定量红外相机监测（camera trap）数据，构建了分辨率为1平方千米的单一物种种群占用模型（occupancy model）与多物种种群占用模型。已有研究显示，英国与爱尔兰地区的本土红松鼠与入侵性灰松鼠之间，存在由资源竞争与疾病传播介导的种间竞争，二者共享天敌——欧洲松貂（European pine marten）。本研究证实，在区域尺度上，欧洲松貂的存在会逆转灰松鼠取代红松鼠的进程：红松鼠的种群占用率会因接触松貂而显著提升；与之相反，灰松鼠的种群占用率则会因松貂的存在而大幅下降。通过对欧洲松貂与两种松鼠的栖息地适宜性进行建模分析，本研究发现：尽管恢复中的松貂种群对灰松鼠的分布具有较强的抑制作用，但灰松鼠仍可能在松貂难以抵达或主动规避的城市庇护所中持续存活。综合与应用：在爱尔兰、苏格兰与英格兰北部的广阔景观范围内，红松鼠与欧洲松貂种群实现可持续恢复的可能性较高；而在英格兰南部与威尔士，尽管存在恢复的可能，但达成这一目标的难度更大。在景观尺度上提升本土林地覆盖率，或有助于实现这一恢复愿景。即便欧洲松貂种群得以恢复，灰松鼠的孤立种群仍可能持续存续。针对城市庇护所内灰松鼠种群的人工管控工作亟需资金支持与落地实施，以避免灰松鼠源种群演化出能够更好规避松貂捕食的新型基因型。