Spatial ecology of grey squirrels (Sciurus carolinensis) in an urban environment
The North American eastern grey squirrel (Sciurus carolinensis) is a highly invasive species that has replaced the sympatric native Eurasian red squirrel (Sciurus vulgaris) across much of the United Kingdom through exploitative competition and disease spread. The city of Aberdeen remains a stronghold for the grey squirrel in Scotland, and Saving Scotland’s Red Squirrels (SSRS) has identified lack of access to perform control in private gardens as a potential barrier to eradication of the species from this urban area. Successful eradication requires (i) all individuals of a population to be vulnerable to control efforts, (ii) the rate of removal of individuals to exceed the rate of population growth, and (iii) immigration into the population to be zero. Development of effective management and control strategies for grey squirrels meeting these criteria is currently hindered by significant gaps in knowledge of urban grey squirrel spatial ecology, as highlighted by a systematic literature review, with no previous studies estimating home ranges in residential areas or quantifying space use and habitat preference at the individual level. This thesis aims to address this firstly by estimating the home ranges of 10 individual grey squirrels, tracked using GPS telemetry for up to five months in Aberdeen, and assessing habitat selection within these home ranges. This information is used to determine the distribution of traps required to put each individual of the target population at risk of coming into contact with a trap within various habitat types. Secondly, environmental factors and trapping procedures, such as pre-baiting, which are likely to influence trapping success for grey squirrels are, assessed through analysis of 12 years of historical trapping data from Aberdeen provided by SSRS. Results of this research suggest that trapping in private gardens is not necessary to the eradication of the species from Aberdeen; not one of the tracked grey squirrels’ home ranges falls entirely within residential habitat and the individuals exhibit a preference for woodland habitat. SSRS resources should be allocated to concentrating grey squirrel trapping in broadleaved and mixed woodland compartments throughout the city, with traps spaced no more than 150 metres and 190 metres apart in these habitat types, respectively. Traps should be pre-baited for two weeks and trapping effort should be increased in late spring and early summer, to ensure rate of grey squirrel removal exceeds rate of population growth following the winter breeding season, and in early winter, when availability of natural food resources is low.
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