Invaders in our waters: at what cost?

Likewise, rates of new species introductions and biological invasions are higher than ever before, and show no sign of slowing down over at least the next thirty years, write Phillip J. Haubrock, Ross N. Cuthbert and Melina Kourantidou.

Once introduced, non-native species which spread and successfully establish in their novel range are labelled as invasive and bring along a broad range of potential impacts to ecosystems, societies, and economies. For example, invasions are one of the leading causes of native species extinction, can reduce public health and social welfare through vectoring of disease, and lead to substantial damage to critical human infrastructures. Nowadays, invasive species occur on virtually all continents and habitats, meaning there are very few ‘pristine’ environments that are free from invasive species.

While many introduced species have a benign impact, some of them evolve into notorious invaders such as for example the gypsy moth (Lymantria dispar),a forest pest, or the black rat (Rattus rattus), which diminished native biodiversity especially on islands and spread disease to humans. On the island of Guam, where there were previously no snakes, the brown tree snake (Boiga irregularis) caused the extinction of half the island’s bird and lizard species, and two thirds of bat species, many of which were not found anywhere else in the world.

Aquatic invasions

While there are many examples of marine invaders that have a global distribution and substantial impacts in their introduced ranges (such as the green crab Carcinus maenas, the red king crab, Paralithodes camtschaticus and the snow crab Chionoecetes opilio), inland waters are often the most vulnerable to invasion. This is largely because inland waters have been subject to intense anthropogenic alterations, including channeling or habitat restructuring, and have been affected by various intentional species introductions in the past. This anthropogenic pressure has made it easier for non-native species to disperse across newly interconnected aquatic ecosystems that were previously isolated.

These problems are exacerbated by a lack of biosecurity and the challenges of monitoring submerged environments for invasions, with non-native species often already established and spreading well before receiving attention by researchers and managers. Examples include, among others, the introduction of common carp Cyprinus carpio in North America or the escapes from aquaculture facilities of rainbow trout Oncorhynchus mykiss in Europe – two species that, spurred by the enthusiasm of anglers, have spread aggressively.

Many submerged aquatic plants have also been introduced from elsewhere as oxygenators to Europe, such as the curly waterweed Lagarosiphon major from southern Africa, which spreads rapidly through vegetative fragments. Other more recent examples showcase how invasions and spread have been facilitated through artificial European waterways, such as the Rhine-Main-Danube, Volga-Don and Volga-Baltic canals, which link the North and Baltic Seas with the Black and Caspian Seas, as well as to other European inland waters. These have led to the introduction of multiple Ponto-Caspian species such as the zebra mussel Dreissena polymorpha or the killer shrimp Dikerogammarus villosus — with all their disastrous impacts in their invaded ranges.

Meanwhile invasions in marine ecosystems are poorly understood with pressing questions about their impacts persisting. In the marine environment specifically, invasions are harder to detect and monitor, therefore often coming to the attention of researchers and resource managers at a later stage when impacts may already be prohibitively severe and tough to mitigate. The increase in vessel traffic globally, with ballast water and hull-fouling as leading vectors, is a key driver of the increase in marine invasions. The challenge is exacerbated in remote marine ecosystems, such as those in the Arctic, where resource use and research has been limited historically. With icy barriers receding, rapidly increasing anthropogenic activities in the region such as shipping, and climate warming facilitating shifts in species’ ranges, the risks of impactful invasions are amplified.

Across all aquatic ecosystems, a persistent lack of biosecurity has thus far largely failed to prevent the growing introduction of aquatic species through pathways such as shipping, intentional introductions and the pet trade. For example, ships’ ballast water has resulted in the introduction of a large number of aquatic invaders that were able to survive and transit across entire oceans within ballast tanks, before being released in their destination/novel range. Indicatively, this has led to a large number of high-risk and high-impact invasions from European waters to the Great Lakes in North America due to a previous lack of effective ballast water management or treatment.

Costs from aquatic invaders

Aside from being one of the leading drivers of extinction globally, threatening the integrity of ecosystems and associated ecosystem services, invasions are also a growing strain on the global economy, across multiple socio-economic sectors. At present, most countries have limited capacity to effectively manage invasions and face limited budgets for conservation. As a result, in making decisions regarding investments in biosecurity, control and management they are forced to balance and justify the trade-offs between the benefits or avoided costs from such measures versus other societal needs. A significant challenge in understanding these trade-offs and therefore informing these decisions is the lack of understanding of the costs incurred due to invasive species. It is well known for example that while biosecurity, applied at early stages of a species introduction, can be relatively cheap compared to long-term management expenditure for invasive populations, a limited understanding of the costs from invasions has hindered investments so far.

Recently, the InvaCost project, an initiative led by Dr. Franck Courchamp and Dr. Christophe Diagne from the University of Paris Saclay, as well as a consortium of international researchers, has led to important advances in better understanding the costs of invasive species. It is the first effort to systematically compile and synthesize the monetary costs of invasive species globally and is a living database, meant to be updated on an ongoing basis in the future. The researchers involved found that costs from aquatic invasive species reached US$345 billion, and that there has been a substantial increase in costs in recent decades. Moreover, the costs of only invasive bivalves to freshwater environments were estimated at US$ 63.6 billion – underlining the imminent economic threat this category alone poses. Further, the majority of reported costs from aquatic invaders were because of damages to resources, with over $50 billion in impacts to fisheries.

Very costly species included the zebra mussel Dreissena polymorpha, the sea lamprey Petromyzon marinus, and the American bullfrog Lithobates catesbeianus — another species cultured for human consumption. While these species directly affected fisheries, prominent and costly aquaculture-driven invasions included also invasive crayfish. Introduced almost globally, the InvaCost database currently lists costs reflecting US$1.3 billion in impacts from crayfish, resulting from aquaculture- and human-facilitated introductions, mostly in Europe, Asia and Africa.

However, because costs were reported for only a few species, and many countries had no reported aquatic invasion costs, these figures are undoubtedly conservative and the total cost of aquatic invasions likely highly underestimated. The largest knowledge gaps in costs of invasive species were from marine ecosystems. Considering these unknowns and knowledge gaps, the ‘true’ costs of aquatic invasions could therefore be several magnitudes higher.

Future outlook

While some non-native species currently support and add value to industries such as aquaculture and agriculture, the potential economic threats they pose might be staggering, and in several cases, outweigh the benefits to society as a whole. Given the many indicators suggesting tremendously high and continuously increasing monetary impacts of introduced species, policy makers and stakeholders within aquaculture and agriculture should be wary of the risks and costs associated with newly introduced species. This is particularly important considering that while specific actors may reap the benefits from introduced species, society as a whole likely has to bear the costs and impacts borne by invasive species.