Aquatic invasive alien species (IAS) are a major threat to biodiversity and ecosystem functioning (Ricciardi & Rasmussen, 1999; Molnar et al., 2008; Strayer, 2010), as well as human health (Galil, 2018; Souty-Grosset et al., 2018). New alien species continue to be introduced at increasing rates (Seebens et al., 2017), and the portion of them that become invasive bring considerable and increasing economic costs (Diagne et al., 2021). Recently, an open database which compiled the economic costs of biological invasions (Diagne et al., 2020) has allowed quantification across a variety of geographical regions (e.g. Haubrock et al., 2022a), ecosystems (e.g. Cuthbert et al., 2021), and taxa (e.g. Angulo et al., 2021; Kouba et al., 2022).
Aquatic IAS spread through multiple vectors and pathways, intentionally or unintentionally, either actively (i.e. by migrating) or passively (i.e. by passive transport). For example, they can escape from confinement (Lockwood et al., 2013), be unintentionally translocated as contaminants or parasites of a certain good (e.g. food, plants, timber; Lockwood et al., 2013), through hull fouling (Sylvester & MacIsaac, 2010; Sylvester et al., 2011) or ballast waters of ships (Briski et al., 2012, 2013). Also, removing biogeographical barriers allows new species invasions to previously ‘pristine’ ecosystems (Gollasch et al., 2006; Kaiser & Kourantidou, 2021; Kourantidou et al., 2015) or the further spread of IAS to secondary invaded areas from primary ‘stepping stones’ (Bertelsmeier & Keller, 2018). For example, roads and railways represent important corridors for IAS (Hulme, 2009), also increasing their propagule pressure (Woodford et al., 2013).
One of the most important pathways that allow the spread of IAS are canals connecting geographically-isolated aquatic systems (e.g. Asth et al., 2021), such as the trans-isthmian Suez and Panama Canals, and the cross continental North Sea-to-Black Sea Rhine-Main-Danube Canal. These highly trafficked strategic canals connect transport networks of critical economic and socio-political value, mainstays of global trade and globalisation (Amato, 2020). They considerably reduce travel time and operating costs to shippers and consumers, thus increasing commerce and economic growth (e.g. Lloyd, 2018; Park et al., 2020; Cordoba, 2022). Nonetheless, these economic benefits are counterbalanced by facilitated introduction and spread of IAS in goods, vessels and in water, due to the increased connectivity. If established, IAS can have detrimental ecological consequences, as well as negative economic impacts (Bij de Vaate, 2002; Leuven et al., 2009; Galil et al., 2017; Turbelin et al., 2021).
Most canals directly connect two seas by crossing the smallest distance between those, as in the case of the Kiel, the Corinth, the Suez, and the Panama Canals. While the former two can be considered of regional importance (in Germany and Greece, respectively), the latter two can be regarded as the most important at a global level, as they connect completely separated areas of much larger size. Indeed, the Suez Canal connects the Red Sea with the Mediterranean Sea, while the Panama Canal connects the Atlantic Ocean with the Pacific Ocean, allowing ships to avoid circumnavigating Africa and South America, respectively, and reducing travel by thousands nautical miles. The Suez Canal was officially opened as early as 1869 and was recently doubled by creating a new lane (the New Suez Canal, functionally opened in 2016; Bereza et al., 2020), but also by widening and deepening the old canal, to increase its traffic capacity. The Panama Canal was opened in 1914 and due to the rising in traffic, as for the Suez Canal, it was recently expanded and a new set of larger locks were created and opened in 2016, doubling its capacity (Wang, 2017). Also, the Suez Canal Authority accelerated $10 billion project plans to further extend and enlarge the canal after the accident of Ever Given container ship that blocked the waterway in 2021 ( https://www.maritime-executive.com/article/suez-canal-sets-new-record-for-traffic-volume ; https://www.reuters.com/business/suez-canal-expansion-due-finish-july-2023-sca-chairman-2022-01-16/ ).
In Europe, the situation is more complicated, with canals connecting multiple water bodies, thus forming a dendritic inland network system of connected major European rivers that ultimately link northern and southern European seas. The major and longest canal-river connections are the Rhine–Main–Danube, Volga-Don and Volga-Baltic canals, which together with other minor systems form the European Inland Canals connecting the North and Baltic Seas with the Black, Azov and Caspian Seas, crossing all over Europe (Jażdżewski, 1980; Bij de Vaate, 2002). Among these, the Rhine–Main–Danube canal, completed in 1992, is the southernmost and longest one and has a particularly high economic importance (Bij de Vaate, 2002; Leuven et al., 2009). Nevertheless, despite the known risks and potential consequences from invasions facilitated by canals, there is no systematic review of the economic costs associated with those.
This paper aims to quantify available economic costs associated with IAS considered to have been facilitated by canals, enabling active (i.e. self-moving species) or passive (i.e. hitchhiker species) spread of these species. For this study, we focused on three major canal systems: Suez, Panama, and European Inland Canals, as these represent major circumventions of important biogeographical barriers, and for which greater information is available. In particular, we hypothesised that (i) the Suez Canal majorly contributed to IAS economic costs, that (ii) these costs are not evenly distributed among countries of the same canal system, and that (iii) these costs are attributed to different taxa in different canal systems.