Invasiveness Ranking System for Non-Native Plants of Alaska

Competitive Ability (0–3) 3Hydrilla is highly adaptive to the environment and competitivewith most other aquatic plants (Haller and Sutton 1975). It is ableto outcompete native submerged plants for light and nutrients.The growth habit of hydrilla enables it to compete effectively forsunlight. It can elongate up to 1 inch per day, and produces themajority of the stems in the upper 2–3 feet of water (Haller andSutton 1975). This mat of vegetation intercepts sunlight and leadsto exclusion of other aquatic plants. Hydrilla is also adapted to uselow light levels for photosynthesis (Barko and Smart 1981, Vanet al. 1976). Hydrilla efficiently uses a limited supply of nutrientssuch as carbon, nitrogen, and phosphorus.Thicket-forming/Smothering growth form (0–2) 2Hydrilla can form a dense mat near the water surface (Bossard etal. 2000).Germination requirements (0–3)N/AGermination of seeds is not a significant factor in reproduction.(Bossard et al. 2000).Other invasive species in the genus (0–3) 0NoneAquatic, wetland or riparian species (0–3) 3Hydrilla is a submerged aquatic perennial. Typical habitats ofhydrilla include ditches, canals, ponds, reservoirs. It can be foundin fresh and brackish, flowing, and still waters (Bossard et al.2000, Thorne 1993).Total for Biological Characteristics and Dispersal 17/22Ecological Amplitude and Distribution ScoreHighly domesticated or a weed of agriculture (0–4) 0Hydrilla is not an agricultural weed.Known level of impact in natural areas (0–6) 1Hydrilla causes severe alterations of plant communitycomposition, community structure, and ecosystem processesin water bodies in California (Bossard et al. 2000). This aquaticweed displaces native plants and adversely impacts freshwaterhabitats in Florida (Langeland 1996). Hydrilla is reportedfrom one lake system in Washington. This is the only knownoccurrence of hydrilla in the Pacific Northwest and impact onnative aquatic ecosystem has not been recorded (WashingtonState Department of Ecology 2004).Role of anthropogenic and natural disturbance in5establishment (0–5)Hydrilla can be readily established in undisturbed aquaticecosystem (Bossard et al. 2000).Current global distribution (0–5) 3Hydrilla is probably native to the warmer regions of Asia (Cookand Lüönd 1982). It is a cosmopolitan species that occurs inEurope, Asia, Australia, New Zealand the Pacific Islands, Africa,and North and South America.Extent of the species U.S. range and/or occurrence offormal state or provincial listing (0–5)In the United States hydrilla populations occur in allSoutheastern states and in Arizona, California, and Washington(USDA, NRCS 2006). Hydrilla verticillata is declared a federalnoxious weed in U.S. It also is listed noxious in 17 American states(Rice 2006, USDA, NRCS 2006).Total for Ecological Amplitude and Distribution 14/25Feasibility of ControlScoreSeed banks (0–3) 2Seed production and seed viability is probably low. However,propagules of hydrilla tubers survived in undisturbed sedimentfor a period of over 4 years. Axillary turions usually do not remainviable for more than 1 year (Van and Steward 1990).Vegetative regeneration (0–3) 3Hydrilla can regenerate from stem fragments, tubers, and turions(Basiouny et al. 1978, Spencer and Rejmanek 1989, Steward 1992,Sutton et al. 1992). About 50% of the fragments with a singlewhorl can sprout and form new plants. More than 50% of thefragments with three whorls can sprout (Langeland and Sutton1980).Level of effort required (0–4) 4The cost of hydrilla management is extremely high. Managementmethods currently include mechanical removal, herbicidesapplications, and biological control. Hydrilla is fragmented easilyand damaged plants that are not removed by mechanical controlmethods can act as a source of reestablishment. Several speciesof weevils, leaf-mining flies, and moth have been introduced tocontrol hydrilla (Bossard et al. 2000, Langeland 1996).Total for Feasibility of Control 9/10Total score for 4 sections 78/97§5B-54