Invasiveness Ranking System for Non-Native Plants of Alaska

Capsella bursa-pastoris (L.) Medik. L.Ranking SummaryEcoregion known or expected to occur inSouth CoastalInterior BorealArctic AlpineYesYesYesPotential Max. ScoreEcological Impact 40 7Biological Characteristics and Dispersal 25 11Amplitude and Distribution 25 18Feasibility of Control 10 4Relative Maximum 40Climatic ComparisonCollected inAlaska regions?CLIMEXsimilarity?South Coastal Yes –Interior Boreal Yes –Arctic Alpine Yes –Capsella bursa-pastoris has been documented in all ecogeographicregions of Alaska (AKEPIC 2005, Hultén 1968, UAM 2004).Ecological ImpactScoreImpact on Ecosystem Processes (0–10) 1Shepherd’s purse colonizes open ground and may inhibit theestablishment of native species (Rutledge and McLendon 1996).Though this plant is only found in highly disturbed environments(Densmore et al. 2001, Welsh 1974) it has the potential to retardsuccession after sites have been disturbed.Impact on Natural Community Structure (0–10) 3Shephard’s purse is a pioneer of disturbed ground. It tends tohave a high percentage of cover initially. However, after perennialgrasses enter the area, it declines in abundance and soondisappears (Aksoy et al. 1998, I. Lapina pers. obs.).Impact on Natural Community Composition (0–10) 0Shepherd’s purse has not been observed in undisturbed areas inAlaska and no perceived impacts on native populations have beendocumented (Densmore et al. 2001).Impact on Higher Trophic Levels (0–10) 3Shepherd’s purse is grazed by cattle, horses, sheep, and rabbits(Crawley 1990). Its leaves are also eaten by insects and slugs(Aksoy et al. 1998, Dirzo and Harper 1980, Cook et al. 1996).Flowers are usually self-pollinated; however, small insects,particularly flies and small bees, visit the flowers (Aksoy et al.1998). Shepherd’s purse is a host for a number of nematodes andviruses (Royer and Dickinson 1999).Total for Ecological Impact 7/40Biological Characteristics and Dispersal ScoreMode of Reproduction (0–3) 3Shepherd’s purse reproduces entirely by seeds. Stevens (1932)recorded 38,500 seeds per plant. Hurka and Haase (1982)conducted experiment in which they recorded a minimum of 500seeds and a maximum of 90,000 seeds per plant. The number ofseeds per plant varies mainly depending on habitat.Long-distance dispersal (0–3) 3Seeds are small and light, and carried by wind or rainwash. Seedsbecome sticky when moistened and can be dispersed on the feetof birds and mammals (Aksoy et al. 1998, Hurka and Haase1982).common names: shepherd’s purseSpread by humans (0–3) 3Seeds may be transported in mud sticking to human feet andto car tires (Aksoy et al. 1998, Densmore et al. 2001, Hurkaand Haase 1982). Hodkinson and Thompson (1997) notethat horticultural stock carried Shepherd’s purse seeds as acontaminant.Allelopathic (0–2) 0Shepherd’s purse is not known to be allelopathic.Competitive Ability (0–3) 1Shephard’s purse is a serious competitor with annual crops;however, it cannot compete with perennial grasses (Aksoy et al.1998).Thicket-forming/Smothering growth form (0–2) 0Shepherd’s purse is capable of creating a dense stand of up to 300plants per m² (Harker et al. 2000); however, plants are small,up to 18 inches tall, and do not posses a climbing or smotheringgrowth habit (Douglas and Meidinger 1998, Royer andDickinson 1999, Whitson at al. 2000).Germination requirements (0–3) 0Shepherd’s purse requires open soil and disturbance togerminate. Plants may appear on sites that have been redisturbedseveral decades after the last human disturbance (Densmore etal. 2001).Other invasive species in the genus (0–3) 0The genus Capsella is monotypic (USDA, NRCS 2006).Aquatic, wetland or riparian species (0–3) 1In its native and introduced range, Shepherd’s purse is a weedof cultivated crops, gardens, and waste areas (Alex and Switzer1976, Aksoy et al. 1998, Royer and Dickinson 1999, Rutledge andMcLendon 1996, Welsh 1974, Whitson at al. 2000). However,this weed has been observed invading gravel bars at BrooksCamp, Katmai National Park and Preserve (J. Heys pers. obs.).Total for Biological Characteristics and Dispersal 11/25Ecological Amplitude and Distribution ScoreHighly domesticated or a weed of agriculture (0–4) 4Shepherd’s purse is considered one of the dominant species in theweed flora (Aksoy et al. 1998).Known level of impact in natural areas (0–6) 1Shepherd’s purse is established in Rocky Mountain NationalPark, Colorado, where it may inhibit the establishment of nativespecies (Rutledge and McLendon 1996). Shepherd’s purse is notknown to impact natural areas in Alaskan National Park Units(Densmore et al. 2001), with the exception of open gravel bars atBrooks Camp, Katmai National Park and Preserve (J. Heys pers.obs.).Role of anthropogenic and natural disturbance inestablishment (0–5)Shepherd’s purse usually requires open soil and disturbancefor establishment (Densmore et al. 2001). However, Jeff Heysobserved infestation of Shepherd’s purse on river erosion sites atBrooks Camp, Katmai National Park and Preserve.3B-12