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Optimal eradication: when to stop looking for an invasive plant

Authors

  • Tracey J. Regan,

    Corresponding author
    1. The Ecology Centre, School of Integrative Biology, The University of Queensland, St Lucia, QLD 4072, Australia
    2. CRC for Australian Weed Management, PMB 1, Waite Campus, Glen Osmond, SA 5064, Australia
      * E-mail: t.regan@uq.edu.au
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  • Michael A. McCarthy,

    1. Australian Research Centre for Urban Ecology, Royal Botanic Gardens Melbourne c/- School of Botany, University of Melbourne, Victoria 3010, Australia
    2. School of Botany, University of Melbourne, Victoria 3010, Australia
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  • Peter W. J. Baxter,

    1. The Ecology Centre, School of Integrative Biology, The University of Queensland, St Lucia, QLD 4072, Australia
    2. Australian Research Centre for Urban Ecology, Royal Botanic Gardens Melbourne c/- School of Botany, University of Melbourne, Victoria 3010, Australia
    3. School of Botany, University of Melbourne, Victoria 3010, Australia
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  • F. Dane Panetta,

    1. Department of Natural Resources and Mines, PO Box 36, Sherwood, QLD 4075, Australia
    2. CRC for Australian Weed Management, PMB 1, Waite Campus, Glen Osmond, SA 5064, Australia
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  • Hugh P. Possingham

    1. The Ecology Centre, School of Integrative Biology, The University of Queensland, St Lucia, QLD 4072, Australia
    2. CRC for Australian Weed Management, PMB 1, Waite Campus, Glen Osmond, SA 5064, Australia
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* E-mail: t.regan@uq.edu.au

Abstract

The notion of being sure that you have completely eradicated an invasive species is fanciful because of imperfect detection and persistent seed banks. Eradication is commonly declared either on an ad hoc basis, on notions of seed bank longevity, or on setting arbitrary thresholds of 1% or 5% confidence that the species is not present. Rather than declaring eradication at some arbitrary level of confidence, we take an economic approach in which we stop looking when the expected costs outweigh the expected benefits. We develop theory that determines the number of years of absent surveys required to minimize the net expected cost. Given detection of a species is imperfect, the optimal stopping time is a trade-off between the cost of continued surveying and the cost of escape and damage if eradication is declared too soon. A simple rule of thumb compares well to the exact optimal solution using stochastic dynamic programming. Application of the approach to the eradication programme of Helenium amarum reveals that the actual stopping time was a precautionary one given the ranges for each parameter.

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