Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland

Abstract

In Ireland, the UK and Italy, the invasive North American grey squirrel, Sciurus carolinensis, threatens the survival of the Eurasian red squirrel, Sciurus vulgaris, as the effects of competition and disease almost inevitably lead to total replacement of red squirrel populations. However the results of a recent national squirrel survey suggested that the normally invasive grey squirrel had gone into decline in the Irish midlands, which was anecdotally attributed to an increase in European pine marten, Martes martes, range and numbers. This study aimed to quantify changes in squirrel distribution in Ireland and to investigate the role, if any, of the pine marten in red and grey squirrel population dynamics. A distribution survey of the midlands was carried out which confirmed the grey squirrel population has crashed in approximately 9,000 km2 of its former range and the red squirrel is common after an absence of up to 30 years. At landscape level, pine marten and red squirrel abundance were positively correlated, whereas a strong negative correlation between pine marten and grey squirrel presence at woodland level was found to exist. Squirrel demographics were determined by means of live trapping programs which confirmed that the red squirrel in the midlands is now in competitive release and the grey squirrel is present at unusually low density. This study provides the first evidence of a regional grey squirrel population crash and suggests that European pine marten abundance may be a critical factor in the American grey squirrel’s success or failure as an invasive species.

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Acknowledgments

Author one was funded by the Irish Research Council under the ‘Embark’ initiative. We would like to thank the woodland owners and managers in particular David Hutton Bury, Eamonn Doran, National Parks and Wildlife Service (Department of Arts, Heritage and the Gaeltacht) and Coillte for allowing access to the study sites. Thanks to all who participated in the sightings survey in particular the National Association of Regional Game Councils. Thanks to Michael Carey and Huw Denman for advice in the early stages of the project and to Alan Murphy for help with the website.

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Correspondence to Emma Sheehy.

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Communicated by David L. Hawksworth.

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Sheehy, E., Lawton, C. Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland. Biodivers Conserv 23, 753–774 (2014). https://doi.org/10.1007/s10531-014-0632-7

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Keywords

  • Competitive release
  • Grey squirrel
  • Hairtube survey
  • Invasive prey
  • Live squirrel trapping
  • Native predator
  • Pine marten
  • Population crash
  • Red squirrel
  • Sightings survey