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Weed Risk Assessment for non-botanists
Peter A.Williams
Landcare ResearchNelson, New Zealand
Definitions
• Weed – a plant species controlled ( or potential to be so) because of its impact
• Impact – interference with human health, economy, or environment etc.
• Weed risk – probability x intensity of weed impact
• Invasiveness – spatial spread
Invasion is crossing barriers
• Human responses: love it or hate it
• Spatial: continents to farm-scale
• Biological: pollinators, dispersers etc.
• Environmental: temperature, nutrients etc.
Four Elements of Risk
• Probability - likelihood, chance
• Uncertainty - the unknowable
• Consequences - impact in space and time
• Manageability - resources, timing
Constraints to weed prediction
• Uniqueness of species x environment matches
• Non-reoccurring events
• Not measurable directly prior to invasion
• Continuous change through time
Science, sociology,economics
People Pathways
Weed-riskassessment
PlantsHabitats
< 1 spp.per year p.a.
16 spp.per year
WRA opportunities
Risk component
Pre entry
Post entry
Probability Low High
Uncertainty High Low
Consequences
Estimate Measurable
Manageability
Estimate Measurable
Elements of a Pre Border WRA
• Origin of the plant and climate match
• History of the species
• Pathways of introduction
• Establishment and spread likelihood
• Impacts features
WRA outcome thresholds and examples
Score range
Outcome
Example species
Actualscore
< 1 Accept Camelia -14
1-6 Evaluate Cineraria +1
1-6 Reject Gorse +22
Table 1. Accuracy rates for weeds and non-weeds using the WRA system from various studies outside Australia (with locally-adapted questions but without the decision tree of Daehler et al., 2004). Sensitivity is the percentage of known weed species which had a ‘reject’ outcome. Specificity is the percentage of known non-weed species which had an ‘accept’ outcome.
Study
Sensitivity(% true weeds
predicted)
Specificity(% true non-
weeds predicted)
Pheloung et al. 1999 69% 92%
Daehler and Carino 2000
90% 54%
Daehler et al. 2004 ?% 66%
Jefferson et al. 2004 100% 40%
Krivanek and Pysek 2006
73% 71%
Mean ?? 65%
Elements of a Post Border WRA
• Evaluate weed risksInvasiveness criteria
Impacts criteriaPotential distribution
• Feasibility of controlCurrent distributionCosts and duration
• Weed management prioritiesWeed risks versus feasibility of control
Mathematics and Post Border WRA
• Evaluate weed risksmodelling traits
modelling spread environment matching
• Feasibility of controloptimum search strategiesdetermining real costs
• Weed management prioritiescost benefit analysis
