Author(s): Doria R. Gordon 1 , * , Crysta A. Gantz 2 , Christopher L. Jerde 2 , W. Lindsay Chadderton 3 , Reuben P. Keller 4 , Paul D. Champion 5
Documented impacts of invasive non-native freshwater aquatic plant species include alterations to water chemistry, hydrologic regimes, temperature and sedimentation rates, and loss of native biodiversity . Additionally, invaders can be expensive for both private and public sectors through the costs incurred by treatment of infestations, increased disease transmission, and lost opportunities for navigation, fisheries, and hydroelectric generation -. As an example, the invasion of water hyacinth ( Eichhornia crassipes ) into Africa's Lake Victoria hampered navigation by growing in thick mats on the surface, which also provided extensive breeding grounds for mosquitoes, resulting in increased transmission of vector-borne diseases . Although full costs are difficult to quantify, invasive aquatic plant species are responsible for an estimated $110 million in annual control costs and damage to navigation, recreation, and agriculture in the US . This number may be a substantial underestimate; the state of Florida (US) alone spends roughly $20 million annually to control just one species, Hydrilla verticillata .
Although the proportion of introduced aquatic plants that cause harmful impacts is small , both the number of species introduced and the frequency of introductions have increased rapidly as global markets have broadened ,  and as interest in water gardening has grown , . In the US, for example, the number of households with water gardens quadrupled between 1998 and 2003, reaching an estimated value of US$1.56 billion , and the global trade in species for aquaria and water gardens is growing by 14% per year . Although some aquatic plant species deemed to be high risk are regulated in some regions, most remain available from stores, and increasingly from on-line commercial and hobbyist sources .
The increased availability of aquatic plants merits particular attention because freshwater aquatic and semi-aquatic plants have a higher probability of becoming invasive than do species from terrestrial plant families . This higher risk means that in many regions most aquatic invaders are derived from intentional imports. For example, 75% of the aquatic invasive plants in New Zealand were imported for horticultural use , as were 76% of all aquatic plants naturalized in the southern New England region of the US , and 85% of aquatic plants naturalized in Australia . Risks of new invasions continue to increase with increasing trade .
The role of intentional imports in producing damaging invasions has motivated efforts to develop risk assessment tools that would allow regulation of potentially harmful species prior to their introduction , -. These tools are developed through identification of consistent patterns in traits of species that have previously become harmful invaders. Historical patterns are assumed to hold into the future, so the tool can be used as a predictive risk assessment for impacts of species not yet introduced . Risk assessment tools with high accuracy give regulators the option to screen species prior...