Species on the move: Stowaways and contaminants cause the greatest economic impacts

Abstract Introduction pathways play a pivotal role in the success of Invasive Alien Species (IAS) – the subset of alien species that have a negative environmental and/or socio-economic impact. Pathways refer to the fundamental mechanism that leads to the introduction of a species outside of its native range – marking the beginning of all alien species invasions. Increased knowledge of pathways is essential to help reduce the flow and impacts of IAS and ultimately improve their management. Here we use the InvaCost database, a comprehensive repository on the global monetary impacts of invasive alien species, combined with the CBD hierarchical classification of introduction pathways to address four key questions: (i) Are particular IAS introduction pathways economically impactful? (ii) How are costs taxonomically and spatially distributed across pathways? (iii) Are there differences in costs between species introduced intentionally and unintentionally? and (iv) is there a relationship between the number of possible introduction pathways of IAS and their costs? We found first that both the total and average cost of species introduced through ‘Stowaways’ (US$144.9bn; US$89.4m) and ‘Contaminants’ pathways (US$99.3bn; US$158.0m) were more costly than species introduced primarily through ‘Escape’ (US$87.4bn; US$25.4m) and ‘Release’ (US$64.2bn; US$16.4m). Second, insects drove the costs of unintentional introductions whilst mammals drove the costs of intentional introductions; ‘Stowaways’ had the highest costs in Asia, Central America, North America and Diverse/Unspecified regions, whilst Antarctic-Subantarctic and Oceania incurred the greatest costs from species introduced through ‘Release’. Third, the cost of species introduced unintentionally is more than double the cost of species introduced intentionally ($192bn vs. $90bn). Equally, species introduced unintentionally cost more on average than species introduced intentionally in terms of damage, management, and mixed costs. Finally, the total and average cost of IAS was not related to their number of introduction pathways. Our findings provide important material for the targeting of priority pathways - something that will be critical in prioritising limited management budgets to combat the current acceleration of species invasions.