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Photographs of the two diurnally active tree squirrels that are present in Southern California: (a) the native Western Gray Squirrel, Sciurus griseus and (b) the non‐native Fox Squirrel, Sciurus niger. The Fox Squirrel has replaced the Western Gray Squirrel in some habitats, while the two species coexist in other habitats. Photographs by Alan Muchlinski
Source publication
Competition from invasive species is an increasing threat to biodiversity. In Southern California, the western gray squirrel (Sciurus griseus, WGS) is facing competition from the fox squirrel (Sciurus niger, FS), an invasive congener.
We used spectral methods to analyze 140 consecutive monthly censuses of WGS and FS within a 11.3 ha section of the...
Citations
... Although quantitative data are lacking, available evidence suggests that populations of western gray squirrels have declined in human-dominated areas (Muchlinski et al. 2009;Cooper and Muchlinski 2015). This decline is generally attributed to competition with introduced congeneric species native to eastern North America, eastern gray squirrels (Sciurus carolinensis) and eastern fox squirrels (Sciurus niger;Jessen et al. 2018; Desharnais et al. 2022;Tran et al. 2022). At the northern edge of their range in Washington state, for instance, Western . ...
... Furthermore, this range expansion coincided with an increase in climatic suitability, as measured by the climate present in the species historical range, in the area of expansion along the eastern Sierra Nevada and western Great Basin. Colonization of a tree squirrel into a community that previously lacked arboreal species exemplifies earlier predictions regarding the formation of nonanalog ecological communities in response to climate change (Williams and Jackson 2007) and represents a possible refugium for the species that faces pressures from introduced species elsewhere in its range (Desharnais et al. 2022;Tran et al. 2022). ...
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.
