Integrating Prevention and Control of Invasive Species: Lessons from Hawaii

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Integrating Prevention and Control of Invasive
Species Lessons from Hawaii

• Basharat A. Pitafi
• (University of Hawaii)
• Brooks Kaiser
• (Gettysburg College)
• James Roumasset
• (University of Hawaii)

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Overview

• Prevention and control as integrated effort
• Problem characterization
• Analysis
• Algorithm
• Optimal policy minimizes expected damages and
  costs of control or eradication
• Two cases Miconia calvescens and Boiga
  irregularis (Brown tree snake)

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Roles of Policy

• Prevention
• Reduce entrance vectors
• Interdiction at source
• Interdiction at destination

• Control
• Eradication
• Control (including pulsed control)
• Spatial Containment
• Adaptation

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Strategies

• Control Only (species already established, e.g.,
  Miconia calvescens)
• Prevent and Control (new species, e.g., Boiga
  irregularis)
• Prevent and Eradicate (new species)

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Control

• Minimize damage (D) and control cost (c) by
  removal (x) of the species

Green Golden-Rule
High D, low c, low c gt low N Denote the
minimized value of V by V
______________________________ Special case of
Heal (1998, p. 48, eq. 4.4)
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Prevention and Control
Probability of introduction (p) is a function of
prevention (y). If introduced, optimal control
costs V.
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Prevention and Control
Choose y (prevention) to minimize the present
value of the total expected costs (including
control). The required optimizing condition is
Thus, high V, high p, low r gt high prevention
(y) Denote the minimized value of the total
expected costs (including control) by W
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Prevention and Eradication
Probability of introduction (p) is a function of
prevention (y). If introduced, eradication
costs E.
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Prevention and Eradication

• Choose y (prevention) to minimize the present
  value of the total expected cost (including
  eradication). The required optimizing condition
  is

Thus, high E, high p, and/or low r gt high
prevention (y) Denote the minimized value of
the total expected cost (including eradication)
by Z
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Control or Eradicate?

• Choose lower of W and Z
• Low E, high c, high D, and/or low r
• gteradicate

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Lessons from theory

• prevention vs. control
• eradication vs. prevention

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Miconia calvescens

• Existing invader
• Aggressive and quick shading canopy
• Shallow roots
• Rapid spread
• Monotypic stands cover approximately 70 of Tahiti

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Miconia costs net damages

• Costs
• Costs of removing Miconia (manual uprooting,
  spraying, helicopters)
• Decreasing in population n
• Non-decreasing in removals x

• Benefits
• Estimated from cost of trees and visitor payments
  to botanical gardens
• Value as ornamental 0.0075n1/2
• Damages
• Destruction of endangered species habitat
• Change in the water balance (increased runoff,
  decreased groundwater recharge)
• Per unit damages 3.77n
• n number of trees

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Est. Total Cost by Pest Popn and Removal Policy
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Lessons from Miconia

• Stabilizing pest population at high level may be
  inferior to no control
• Optimal policy (control, eradicate or
  accommodate) dependent on initial population, as
  well as biological growth rates, habitat, and
  expected economic and ecological damages
• Pulsed-harvest steady state may be cost effective
  if economies of scale in harvesting

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Boiga irregularis

• Eradicable/ controllable potential invader
• Several specimens have arrived alive in HI
• Arrived in Guam in the 1950s
• 50 snake/hectare densities
• 1 hour long power outages every 4 days
• Poisonous bites treated in hospitals
• Loss of 9 native bird species

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BTS costs damages

• Unit Costs
• Costs of catching snakes
• Decreasing in n, linear in x
• Based on trap efficacy in Guam

• Damages
• Extrapolated from Guam experience, based on power
  outages, snakebites, and loss of biodiversity

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BTS Results
Estimated minimum losses for one eradication
Estimated minimum losses for optimal control
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A comparison
Sample comparison of eradication investment vs.
controlled population, C(n) 7,360/ n 0.73
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Lessons from BTS

• Comparing eradication vs. annual control for a
  population close to 300 snakes, the pulsed
  eradication option frees up over 3 million
  dollars for prevention activities.
• While today's expenditures for control may be low
  compared to pulsed eradication, potential long
  term benefits of eradication are great.
• The long term benefits of eradication depend on
  the efficacy of prevention
• Adding prevention decision to optimal control
  problem does not destabilize control decision
  results

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To do

• Solve iterated eradication-prevention problem
• Analytics for model
• Analytical results (steady state on parameters)
• Role of functional forms for costs, damages etc.
• Parameterization,role of biology etc.
• Local/Global Optima
• Econ of scale in harvesting
• Interspecies comparisons