A model of ruddy duck Oxyura jamaicensis eradication for the UK

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Summary Non‐native species are the second most important threat to global biodiversity after habitat loss. The North American ruddy duck Oxyura jamaicensis established a feral population in the UK more than 40 years ago, and it is now threatening the endangered white‐headed duck Oxyura leucocephala through hybridization. We utilized a simple generic model, which includes uncertainty in parameter estimates and can be used for any invasive species, to assess the time frame for successful control. A simple spreadsheet‐based model was produced to provide the UK government with an assessment of whether sufficient ruddy ducks could be culled to allow the UK population to be reduced to fewer than 175 individuals (> 97% population reduction) within 10 years. Data on changes in numbers of ruddy duck at the main UK wintering sites were collected during a regional control trial, and a stochastic Monte Carlo simulation model was constructed to project the UK population under a variety of control strategies. Three variables were included in the model: cull rate per person (i.e. by how much each control officer could reduce the national population per year), number of control officers, and changes in the ruddy duck population growth rate as the population was reduced. Parameter estimation was kept to a minimum by using historical annual population growth rates as a basis for future growth rates. The historical data show a reduction in both the mean growth rate and in its variability when the population is in excess of 2000 birds. Density‐dependent and density‐independent models were produced. These all gave similar times to reach the target population, with some difference in the variance estimates between models. Simulations were run with a maximum annual population reduction of between 0·50 and 0·80, and between 12 and 20 control officers. A total of 8000 simulations was performed for each of the 63 different scenarios for each of the models. The mean time to reduce the UK ruddy duck population by 97% was predicted to be between 3 and 5 years, with 14 or 15 control officers reducing the population by between 65% and 70% per year. There was an 80% certainty that the population could be reduced to this level by 16 control officers within 4–6 years if annual reductions of more than 60% were achieved. Synthesis and applications. The novel spreadsheet model presented here can easily be applied to a range of eradication schemes where limited data are available on population size and culling efficacy. This would allow funding bodies to have a greater degree of certainty about the cost and outcome of a trial eradication programme.