Predicting invasive success of Hydrilla verticillata (L.f.) Royle in flowing water

Abstract

Alien aquatic plant species have had spectacular success in invading New Zealand’s freshwaters. Their continued introduction has resulted in few water bodies retaining their natural or original indigenous aquatic vegetation, and associated adverse changes, such as reduced biodiversity and amenity values are well known. This highlights the need to accurately assess aquatic plant species that are already in the country and those outside for their potential to have a negative impact on New Zealand’s lakes and waterways. The study presented in this paper describes an approach to evaluating the invasive weed potential of Hydrilla verticillata (L.f.) Royle compared with Elodea canadensis Michx. under flowing water conditions. The two species were planted in combination and alone, and in a sheltered or exposed position relative to the direction of the water flow. Over the 2-year study, two different waterflow rates were used (0.1 and 0.2 m/s) which represented the gauged flows from a stream of interest that contains E. canadensis and flows from a H. verticillata infested lake. H. verticillata was competitive compared with E. canadensis and able to persist and thrive in the presence of E. canadensis in both exposed and sheltered habitats. These results corroborate observations from other countries of H. verticillata growing in flowing waters and highlight the significant threat posed by H. verticillata to streams and waterways in New Zealand.