New Zealand Journal of Ecology (2011) 35(2): 163- 173

Quantifying the success of feral cat eradication, San Nicolas Island, California

Research Article
David S. L. Ramsey 1,*
John P. Parkes 2
David Will 3
Chad C. Hanson 3
Karl J. Campbell 3
  1. Arthur Rylah Institute, PO Box 137, Heidelberg, Victoria, Australia 3084
  2. Landcare Research, PO Box 40, Lincoln 7640, New Zealand
  3. Island Conservation, 100 Shaffer Rd, Santa Cruz, California 95060, U.S.A.
*  Corresponding author

It is usually uncertain when to declare success and stop control in pest eradication operations that rely on successive reductions of the population. We used the data collected during a project to eradicate feral cats from San Nicolas Island, California to estimate both the number of cats remaining towards the end of the project, and the amount and type of surveillance effort required to declare successful eradication after the last known cat was removed. Fifty seven cats were removed between June 2009 and April 2010 and our model estimated that there was a 95% chance that a further 1 to 4 cats remained, with 1 cat being the most likely number. After this time a further two cats were detected and removed and the model predicted this outcome with a probability of 0.25. If managers wished to confirm eradication success at this point, we estimated that 55 km of effort searching for recent evidence of cats over the whole island without detecting any would provide 99% certainty that no cats remained (stopping rule 1). Alternatively, the optimal amount of search effort for evidence that minimized the joint cost of searching and the cost of wrongly declaring eradication was 75 km (stopping rule 2). The equivalent amount of camera-nights (26 cameras were available) required to declare successful eradication were 416 (stopping rule 1) and 1196 camera nights (stopping rule 2). During the confirmation phase, 270 km of sign search effort and 3294 camera-nights surveillance were used from late June 2010, when the last cat was removed, through August 2010, without detecting signs of survivors. Managers can be very confident that eradication has been successful.