Border control for potential aquatic weeds, stage 1 : weed risk model / P.D. Champion and J.S. Clayton

Abstract

This report is the first stage in a three-stage development of a Border Control Programme for aquatic plants that have the potential to become ecological weeds in New Zealand. A large number of freshwater aquatic plants have already been introduced and are naturalised in New Zealand, impacting on most waterbodies within this country. There are many additional potential weed species reported as present in New Zealand, but not naturalised, and an even greater number not recorded as introduced here. Some of these species could pose an even greater threat to our aquatic environment than those weeds currently naturalised. A range of tables is presented to illustrate the array of new aquatic species that are already believed to be in New Zealand or that could enter and become established. Possible entry pathways identified in this report include natural spread from wind- and bird-dispersed seed, introduction of ornamental, culinary and medicinal herbs, contaminants in other plants and produce, mislabelled plants, and various types of illegal imports. Existing weed risk assessment models fail to adequately separate aquatic plants with different levels of impact. A new model is presented, tailored to the impacts of aquatic species. Tables are presented to demonstrate the improved system of ranking risks for aquatic plant species. A combination of assessments for weediness and the risk of entry into New Zealand will determine the potential threat of each species, allowing a comparison with existing weed species and other species not yet naturalised or introduced here. The greatest risk is perceived to be posed by some species reported to be present, but not yet naturalised in New Zealand, followed by species not reported here, but traded overseas with the potential to be brought here illegally.