Invasiveness Ranking System for Non-Native Plants of Alaska

Hydrilla verticillata (L. f.) RoyleRanking SummaryEcoregion known or expected to occur inSouth CoastalInterior BorealArctic AlpineYesYesNoPotential Max. ScoreEcological Impact 40 38Biological Characteristics and Dispersal 22 17Amplitude and Distribution 25 14Feasibility of Control 10 9Relative Maximum 78Climatic ComparisonCollected inAlaska regions?CLIMEXsimilarity?South Coastal No YesInterior Boreal No YesArctic Alpine No NoHydrilla verticillata has not been documented in Alaska (Hultén1968, Pfauth and Sytsma 2005, UAM 2004). The CLIMEXclimate matching program indicates a climatic similaritybetween south coastal region of Alaska and areas of the speciesdocumented occurrence are high. The native range of hydrillaincludes Akita, Japan and Thredbo, Australia (Cook and Lüönd1982) which have 55% and 53% of climate similarity with Juneau.The distribution range of hydrilla also includes Minsk, Belarusand Semipalatinsk, Russia (Cook and Lüönd 1982) which havea 62% and 61% climate similarity with Anchorage, respectively.Climatic similarities between Fairbanks and Semipalatinsk andBlagoveshchensk, Russia and Qiqihar, China have a 64%, 61%,and 50% similarity with Fairbanks respectively. Hydrilla is notknown from arctic regions. In general, aquatic species are lessimpacted by variation in terrestrial climates. Hydrilla verticillatais likely to become established in the south coastal and interiorboreal regions of Alaska.Ecological ImpactScoreImpact on Ecosystem Processes (0–10) 8Hydrilla infestations slow the movement of water, causingflooding. Slow waterflow can also increase the sedimentationrates, water temperature, and pH level (Estes et al. 1990, Joyce etal. 1992) and decrease dissolved oxygen (Bossard et al. 2000). Italso affects water nutrient turnover (Bole and Allan 1978, Sinhaet al. 2000).Impact on Natural Community Structure (0–10) 10Hydrilla forms a dense mat of vegetation at the water surface andlimits light penetration degrading or eliminating all layers below(Bossard et al. 2000). Haller and Sutton (1975) reported thatlight penetration is reduced by at least 95% at 1 foot of depth. Anaquatic vegetation study in Florida found that area coverage ofsubmersed aquatic macrophytes increased from 8% in 1987 to90% in 1989 and 1990 due to the expansion of hydrilla (Estes etal. 1990).common names: hydrillaImpact on Natural Community Composition (0–10) 10Hydrilla infestations can cause a reduction or the extirpationof populations of native aquatic species (Bossard et al. 2000).Hydrilla may also shift the phytoplankton composition (Canfieldet al. 1984). Infestations also adversely affect fish populations.Hydrilla may reduce seed production of native species, resultingeventually in a reducing of a number of native species in thecommunity (de Winton and Clayton 1996). A study in Floridafound that the frequency of occurrence for the most abundantnative submersed plants, coontail and southern naiad decreasedfrom 11% to 4% and 56% to 4%, respectively, from 1987 to 1990(Ester et al. 1990).Impact on Higher Trophic Levels (0–10) 10Hydrilla is eaten by waterfowl and fish. Some studies supportthe view that hydrilla is beneficial as a fishfood and cover (Esteset al. 1990); other researches suggest that fish populations areadversely affected when hydrilla coverage exceeds 30% (Colle andShireman 1980). Hydrilla appears to be an important habitat for anumber of mosquito species (Hearnden and Kay 1997).Total for Ecological Impact 38/40Biological Characteristics and Dispersal ScoreMode of Reproduction (0–3) 3Hydrilla reproduces by seeds, but seed production has minorimportance. Vegetative reproduction is very efficient and occursby fragmentation of the stem, or by the production of axillarybuds (turions) and below-ground tubers. One plant can producean average of 6,046 tubers per season (Sutton et al. 1992). Anexperiment by Thullen (1990) showed that hydrilla can producedup to 46 axillary turions per 1.0 g dry weight (estimated of2803 turions per m³). About 50% of the fragments with a singlewhorl can sprout and form new plant, and more than 50% of thefragments with three whorls can sprout (Langeland and Sutton1980).Long-distance dispersal (0–3) 2Tubers, turions, and stem fragments disperse with flooding.Tubers survive ingestion by waterfowl and might be transportedfrom one water body to another (Joyce et al. 1980). Theimportance of tubers dispersal, therefore, is unknown.Spread by humans (0–3) 2Hydrilla was first introduced into North America as an aquariumplant. Turions or small pieces of hydrilla stems can travel on boattrailers or planes. Accidental introductions with planted waterlilyhave been reported (Washington State Department of Ecology2004). Hydrilla twigs survive 16 hours of desiccation (Basiouny etal. 1978, Kar and Choudhuri 1982). Tubers can remain viable forseveral days out of water (Basiouny et al. 1978).Allelopathic (0–2) 2In experiments by Elakovich and Wooten (1989) extracts ofhydrilla exhibit high allelopathy potential and inhibited thegrowth of lettuce seedling and duckweed.B-53