Evaluating Risk to Native Plant Communities from Selected Exotic Plant Species

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Evaluating Risk to Native Plant Communities from
Selected Exotic Plant Species

• Cohesive Strategy Team, Region One - USFS The
  Nature Conservancy of Montana

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Weed Risk Assessment

• The need
• Other approaches
• Our approach
• Risk Vs. Inventory

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Study Area

• Region One - USFS Idaho north of the Salmon
  River, east through central MT
• 225 m to 3901 m
• 258,721 km2 (64 million acres)

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Definitions

• Exotic Species
• Invasive
• Disturbance
• Susceptibility
• Threat
• Probability of Exposure
• Risk

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Methods

• Determine species to evaluate
• Determine susceptibility of native plant
  communities to these species
• Determine level of threat to susceptible
  communities
• Determine the probability of weed seed exposure
  to specific sites in the study area

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Species Analyzed

• Began with all county listed noxious weeds in
  study area
• Added unlisted species that were considered high
  threat by expert panel
• Eliminated native species
• Highly threatening to at least one native
  vegetation type
• Known from the study area, and enough data to
  model risk

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Methods

• Determine species to evaluate
• Determine susceptibility of native plant
  communities to these species
• Determine level of threat to susceptible
  communities
• Determine the probability of weed seed exposure
  to specific sites in the study area

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Potential Natural Vegetation (PNV)
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Disturbance
A ground disturbance caused by wild animals B
burned areas F recent flooding G grazed
by domestic livestock M mechanical disturbance
of the soil surface
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PNV Susceptibility(same codes used by Peter Rice)

• U - Unknown
• C - Closed
• I - Invasive
• D - Disturbance

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Susceptibility Matrix
37 Species
34 PNVs
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Potential Natural Vegetation (PNV)
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Disturbance
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Susceptibility
PNV


Disturbance
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Leafy Spurge Susceptibility
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Methods

• Determine species to evaluate
• Determine susceptibility of native plant
  communities to these species
• Determine level of threat to susceptible
  communities
• Determine the probability of weed seed exposure
  to specific sites in the study area

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Threat to PNV
U - Unknown Threat N - No Threat L Low Threat H
High Threat
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Threat Matrix
37 Species
34 PNVs
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Leafy Spurge - Threat
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Methods

• Determine species to evaluate
• Determine susceptibility of native plant
  communities to these species
• Determine level of threat to susceptible
  communities
• Determine the probability of weed seed exposure
  to specific sites in the study area

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Probability of Exposure

• Calculate distance to nearest county with known
  species occurrence
• Assign road density value to each pixel
• Code high traffic area pixels
• Assign grazing allotment value for each pixel
• Calculate Probability of Exposure Factor by
  summing above values

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Leafy Spurge - Probability of Exposure
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Results

• For each species
• A map displaying risk
• Quantitative summary of area at risk
• Aggregate Risk Map
• Risk by major vegetation groups

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Leafy Spurge - Existing Condition
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Orange Hawkweed - Existing Condition
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Canada Thistle - Existing Condition
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Aggregate Weed Risk
0 1-5 species 6-10 species 11-15 species
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Risk Summary by Vegetation Type

• Grouped PNVs into 8 groups
• alpine/subalpine
• mesic/cold coniferous forest
• mesic/warm coniferous forest
• dry forest/woodland
• grasslands
• mesic shrub
• dry shrub
• riparian deciduous forest

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Leafy Spurge
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Orange Hawkweed
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Canada Thistle
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Plant Communities at Risk

• 1. Grasslands
• 2. Dry Shrublands
• 3. Riparian Deciduous Forest
• 4. Mesic Shrublands
• 5. Dry Forest
• 6. Mesic/warm forest
• 7. Mesic/cold forest
• 8. Alpine/subalpine

Decreasing Risk
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• How can I get my hands on this information?
• Cohesive Strategy Team Website
• http//www.fs.fed.us/r1/cohesive_strategy/
• on-line viewing of maps
• downloadable .jpgs
• For those more serious about conducting further
  GIS analysis
• You can order CD with all GIS grids (free of
  course!)

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