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Determining if a newly detected marine species is introduced to an area is an important first step when considering if control or eradication should be attempted. This step is often challenging, especially when data and introduced species expertise is limited: yet decisions about responding to a new invasion needs to occur in a timely manner. The crux is that biosecurity crisis decisions are often made in a vacuum. To improve this process, we consider expanded criteria to determine if a species is native, cryptogenic or introduced and outline application in a rapid response approach that uses a non-probabilistic decision tree to support decision makers. Effective use of the rapid response decision-tree and species criteria requires a multi-disciplinary approach drawing upon biology (taxonomy, phylogeny, genetics, ecology, biogeography) and monitoring. We assessed the expanded criteria against 213 bryozoan species present in Australian waters. A multivariate evaluation highlighted that a weight of evidence approach using the expanded criteria was successful in differentiating between native and introduced status. Our assessment highlighted that five criteria provide a high level of congruence with heuristic assignments, and provide a precautionary assignment of species' status by reducing mis-classifications of introduced species as native species (Type I error) in comparison to the original criteria. However, differentiating between introduced and cryptogenic species remains problematic, especially when using the original criteria. We highlight the critical need for taxonomic identification, appropriate application of assigning cryptogenic status, and monitoring requirements to enable use of the criteria in a rapid response context. Using both the rapid response decision tree and the criteria provides a quantifiable mechanism to aid decision-makers in deciding whether to respond to a marine species introduction.

Reduced competition is a frequent explanation for the success of many introduced species. In benthic marine biofouling communities, space limitation leads to high rates of overgrowth competition. Some species can utilise other living organisms as substrate (epibiosis), proffering a competitive advantage for the epibiont. Additionally, some species can prevent or reduce epibiotic settlement on their surfaces and avoid being basibionts. To test whether epibiotic pressure differs between native and introduced species, we undertook ex situ experiments comparing bryozoan larval settlement to determine if introduced species demonstrate a greater propensity to settle as epibionts, and a reduced propensity to be basibionts, than native species. Here we report that introduced species opportunistically settle on any space (bare, native, or introduced), whereas native species exhibit a strong tendency to settle on and near other natives, but avoid settling on or near introduced basibionts. In addition, larvae of native species experience greater larval wastage (mortality) than introduced species, both in the presence and absence of living substrates. Introduced species' ability to settle on natives as epibionts, and in turn avoid epibiosis as basibionts, combined with significantly enhanced native larval wastage, provides a comprehensive suite of competitive advantages contributing to the invasion success of these biofouling species.