A rapid technique for assessing the suitability of areas for invasive species applied to New Zealand's rivers

Early responses to incursions of non‐indigenous species (NIS) into new areas include modelling and surveillance to define the organisms’ potential and actual distributions. For well‐studied invasive species, predictive models can be developed based on quantitative data describing environmental tolerances. In late 2004, an invasive freshwater diatom Didymosphenia geminata, an NIS for which we had no such quantitative data, was detected in a New Zealand river. We describe a procedure used to rapidly develop a classification of suitability for all New Zealand's rivers, based on two sources of information. First, from a review of the limited available literature and unpublished data, we determined that temperature, hydrological and substrate stability, light availability, and water pH were the most important environmental gradients determining D. geminata's broad‐scale distribution and capacity for establishing and forming blooms in rivers. The second information source was a GIS‐based river network developed for a national classification of New Zealand's rivers, with associated data describing environmental characteristics of each section of the network. We used six variables that were available for every section of the network as surrogates for the environmental gradients that determine suitability. We then determined the environmental distance of all the river sections in the network from our assessment of the optimal conditions conducive to D. geminata blooms. The analysis suggested that > 70% of New Zealand's river sections (stream order > 3) fell into the two highest suitability categories (on a five‐point scale). At the time of writing, D. geminata had spread to 12 catchments, all of which were within these two categories. The technique is applicable in initial responses to incursions of NIS where quantitative information is limited, and makes optimal use of available qualitative information. Our assessment contributed to evaluations of the potential ecological, social, and economic impacts of D. geminata and is currently being used to stratify site selection for ongoing surveillance.