Globally, animals that are range-restricted are frequently becoming species of conservation concern, in part due to competitive exclusion by phylogenetically and ecologically similar species that are more tolerant of human disturbance. However, climate and land use changes to natural landscapes can create pockets of refugia for range-restricted species. Western gray squirrels (Sciurus griseus) are native to the west coast of North America, principally California and western Oregon. Over the past several decades, Western Gray Squirrel populations have declined in human-dominated areas, with increased competition from introduced congeneric species native to eastern North America cited as a primary driver. Despite declines in their established range west of the Pacific Crest in western North America, western gray squirrels are extending their range into the Great Basin, where they were not historically found. Using a network of remote camera traps deployed across the Sierra Nevada–Great Basin ecotone in northwestern Nevada, we detected western gray squirrels across 16 of 100 camera-trapping sites. The majority of detections were located in piñon–juniper woodland, a land cover type not previously occupied by this species. Occupancy modeling revealed that western gray squirrels were equally likely to occur in piñon–juniper woodland compared to mature pine forest that they occupy elsewhere in their range. A species distribution model parameterized with historical gray squirrel observations (pre-1950), indicated increased climatic suitability for the species on the eastern side of the Sierra Nevada in recent decades, which may have facilitated this range expansion. Our findings reveal the potential for species declining in their historical range to colonize novel habitats that become increasingly suitable as a result of human-driven changes to ecosystems.

中文翻译：

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正在衰退的森林物种西部灰松鼠 (Sciurus griseus) 的活动范围扩大到半干旱林地

在全球范围内，活动范围受限的动物经常成为受保护的物种，部分原因是系统发育和生态上相似的物种对人类干扰的耐受性更强，从而导致竞争性排斥。然而，气候和土地利用对自然景观的变化可能会为范围受限的物种创造一些保护区。西部灰松鼠（Sciurus griseus）原产于北美西海岸，主要是加利福尼亚州和俄勒冈州西部。在过去的几十年里，在人类主导的地区，西部灰松鼠的数量有所下降，主要原因是北美东部引进的同类物种的竞争加剧。尽管西部灰松鼠在北美西部太平洋脊以西的既定活动范围有所缩小，但它们正在将活动范围扩大到历史上从未发现过的大盆地。利用部署在内华达州西北部内华达山脉-大盆地交错带的远程相机陷阱网络，我们在 100 个相机陷阱点中的 16 个发现了西部灰松鼠。大部分检测结果位于松柏林地中，这是一种以前未被该物种占据的土地覆盖类型。占用模型显示，西部灰松鼠出现在松柏林地的可能性与它们在其活动范围内其他地方占据的成熟松树林的可能性相同。根据历史灰松鼠观测（1950 年之前）参数化的物种分布模型表明，近几十年来内华达山脉东侧的物种气候适宜性有所增加，这可能促进了这种范围的扩大。我们的研究结果揭示了历史范围内数量减少的物种有可能在新的栖息地中定居，而这些栖息地由于人类驱动的生态系统变化而变得越来越适合。