Assessing invasion impact: Survey design considerations and implications for management of an invasive marine plant

We use a three-year study of sheltered low shore assemblages colonised by the non-indigenous Asian kelp Undaria pinnatifida to explore survey design issues for assessing the ecological impacts of invasive species. The weight of evidence overall suggested little impact from Undaria on low shore assemblages, with control-impact contrasts that could plausibly be interpreted as impacts probably reflecting natural causes. We demonstrate that the potential for reaching incorrect conclusions regarding the impacts of invasive species using control-impact designs is greater than when such designs are used to assess traditional forms of anthropogenic impact. We suggest that a before-after control-impact framework is essential, but recognise that such an approach has a number of limitations. In particular, there is no assurance that the before-after impact site will be invaded at all, or to the extent that provides worst-case impact information for coastal managers. We discuss possible ways of assessing invasive species impacts, but suggest that the uncertainty inherent in extrapolating impact information to other places and times means that the precautionary principle should be applied, and 'worst-case' impacts assumed, until the level of scientific uncertainty is reduced. Such an approach should only be applied, however, after an evaluation of the feasibility, costs and benefits of managing a particular pest in relation to other priorities for invasive species.