Internal borders for managing invasive marine species.
This study discusses theoretical and practical issues around the definition of internal borders for the management of marine pests, drawing on principles from freshwater and terrestrial pest management. Internal borders are defined as post-border intervention points around which vector management and related activities, such as pest surveillance and incursion response, can be undertaken. Internal borders can be identified in marine systems based on knowledge of natural barriers to the dispersal or establishment of pests, including planktonic life stages. We highlight opportunities to define internal borders at broad spatial scales according to oceanographic features or environmental conditions. At smaller spatial scales, habitat barriers based on relatively permanent features (e.g. substratum) could be defined for target organisms (or suites of similar organisms), especially those having both restricted habitat requirements and a limited planktonic duration. Where internal borders are identified, risk-based approaches can be used to determine vector management priorities, incorporating knowledge of the connectivity and strength of interactions between hubs of vector activity. However, greater biosecurity and biodiversity benefits may arise from approaches that combine risk-based measures for target pests, with generic measures (e.g. vessel hull anti-fouling) that are applied equally across all human-mediated invasion pathways. Case studies of high-profile marine pests in New Zealand illustrate situations where the benefits of internal border management have been realized. The challenge now is to identify how practical it is to widely apply internal borders in a marine biosecurity context, and to identify new internal borders around which management can practically be undertaken. Synthesis and applications. Effective control of pest species in marine environments is possible even when they become relatively widespread and established. The definition of internal borders provides an approach to marine biosecurity that assists in elucidating opportunities and priorities for management. Although marine systems are regarded as relatively open, the principles and approaches necessary for the successful management of marine pests are conceptually the same as those applied in freshwater systems and on land.