Integrating Prevention and Control of Invasive Species: The Case of the Brown Treesnake

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Integrating Prevention and Control of Invasive
Species The Case of the Brown Treesnake

• Kimberly Burnett, Brooks Kaiser,
• Basharat A. Pitafi, James Roumasset
• University of Hawaii, Manoa, HI
• Gettysburg College, Gettysburg, PA

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Objectives

• Illustrate dynamic policy options for a highly
  likely invader that has not established in Hawaii
  
• Find optimal mix of prevention and control
  activities to minimize expected impact from snake

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Boiga irregularis
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Methodology

• First consider optimal control given N0
  (minimized PV of costs and damages) gtNc
• We define prevention to be necessary if the
  population falls below Nmin (i.e., Nc lt Nmin)
• Determine optimal prevention expenditures (to
  decrease probability of arrival) conditional on
  the minimized PV from Nc

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N0 Nmin
Nc Best stationary N without prevention
Nc Nmin
Nc lt Nmin
We have a winner! N Nc
Choose y to min cost of removal/prevention cycle
V(Nmin)
Z(Nc)
N Min (Z,V)
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Algorithm to minimize cost damage
gt V gt Nc
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PV costs damage if Nc lt Nmin

• If Nc lt Nmin, we must then consider the costs of
  preventing re-entry.

Z
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Prevention/eradication cycle

• Expected present value of prevention and
  eradication
• p(y) probability of successful introduction with
  prevention expenditures y. Minimizing Z wrt y
  results in the following condition for optimal
  spending y

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N0 Nmin
Nc Best stationary N without prevention
Nc Nmin
Nc lt Nmin
We have a winner! N Nc
Choose y to min cost of removal/prevention cycle
V(Nmin)
Z(Nc)
N Min (Z,V)
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Choose optimal population

• If N Nmin, same as existing invader case
• Control only
• If N lt Nmin,
• Iterative prevention/removal cycle

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Case study Hawaii

• Approximately how many snakes currently reside in
  Hawaii?
• Conversations with expert scientists between
  0-100

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Growth

• Logistic b0.6, K38,850,000

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Damage

• Power outage costs 121.11 /snake
• Snakebite costs 0.07 /snake
• Biodiversity 0.32 1.93 /snake
• Total expected damages

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Biodiversity Losses
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Control cost

• Catching 1 out of 1 1 million
• Catching 1 out of 28 76,000
• Catching 1 out of 39m 7

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Probability of arrival a function of spending
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Results

• Aside from prevention, eradicate to zero and stay
  there.
• Since prevention is costly, reduce population
  from 28 to 1 and maintain at 1

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Snake policy status quo vs. optimal (win-win)
NPV of no further action 147.3 billion
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Summary

• Re-allocation between prevention and control may
  play large role in approaching optimal policy
  even at low populations
• Eradication costs increased by need for
  prevention, which must be considered a priori
• Catastrophic damages from continuation of status
  quo policies can be avoided at costs much lower
  than current spending trajectory

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Uncertainties

• Range of snakes currently present (0-100?)
• 8 captured
• More mayve gotten away
• Not much effort looking
• Probability of reproduction given any popn level
• Dont know, need to look at range of
  possibilities
• Here all control
• If NltNmin, prevention makes sense
• Need to find optimal mix