Invasive alien species — those introduced into regions where they are not native and in which they cause negative impacts to nature and/or society — are among the main drivers of global biodiversity decline (Bellard et al., 2016; Pyšek et al., 2020), and a growing threat with multiple feedbacks to ecosystem services and human well-being (Vilà & Hulme, 2017; Shackleton et al., 2019a, b; Sinclair et al., 2020). The ecological and socio-cultural impacts of invasive alien species are substantial and expected to continue increasing due to climate (Seebens et al., 2021a), geopolitical (Ribeiro et al., 2022), and economic changes (Essl et al., 2020a).
Most assessments of the impacts of invasive alien species have relied on scoring systems, such as that proposed by Panetta (1993) and subsequently updated by Kumschick et al. (2017) and Vilà et al. (2019). However, such systems are often context-dependent, with some scoring criteria lacking objectivity or requiring high assessor expertise (González-Moreno et al., 2019), rendering them unsuitable for broader generalisation or prediction. There is accordingly a need to improve the reliability of impact assessments by considering monetary costs arising from the loss and damage of resources as well as management actions. Attempts to quantify the economic costs of invasive alien species at broad spatial scales have occurred in recent decades (Pimentel, 2005; Kettunen et al., 2009), although early attempts had considerable shortcomings (Hensley, 2012) or were limited to economic inventories of specific sectors (Lovell et al., 2006; Paini et al., 2016).
Most recently, the InvaCost project and the structured and publicly available database it has produced (Diagne et al., 2020; Leroy et al., 2022), have provided an unprecedented opportunity to investigate taxonomic and geographic trends in the monetary costs of invasive alien species worldwide (e.g., Cuthbert et al., 2021a; Haubrock et al., 2021). Studies with a national (e.g., Renault et al., 2021), regional (e.g., Kourantidou et al., 2021), continental (e.g., Crystal-Ornelas et al., 2021), taxonomic (e.g., Kouba et al., 2022), or pathway (e.g., Balzani et al., 2022; Turbelin et al., 2022) focus have, however, all identified difficulties with making comprehensive monetary cost estimates owing to spatial and taxonomic gaps. These data gaps include: (i) costs for many known, established invasive alien species that have not been assessed, (ii) cost information that is often inaccessible or otherwise not available publicly, or (iii) many cost estimates that do not fulfil quality criteria (i.e., peer-reviewed and/or transparent calculations).
In Europe, the monetary costs incurred by invasive alien species from 1960–2020 were recently estimated to be US$140.2 billion (Haubrock et al., 2021), although persistent data gaps suggest that this is an underestimate (Zenni et al., 2021). This inferred cost might appear low relative to the annual gross domestic product of some European countries (e.g., 3.3% when compared to Germany's 2021 gross domestic product (GDP) of US$4.3 trillion; data.worldbank.org). However, it is a more substantial quantity when considering countries with smaller economic turnover such as Hungary (US$182.3 billion) or Slovakia (US$115.0 billion). The lack of comprehensive and reliable cost estimates undervalues the economic threat biological invasions represent, and hinders national policies, research, and educational efforts, along with multinational initiatives to combat alien species (Cuthbert et al., 2022a).
The combined economic turn-over of European Union member states makes it the third largest economy worldwide (data.worldbank.org), and Europe as a whole has been a historic centre of trade, human migration, and tourism. As such, both continental Europe and the European Union (with its 27 member states) have been particularly vulnerable to biological invasions, despite recent transboundary legislation to tackle invasive alien species (Genovesi et al., 2015; Turbelin et al., 2017; Carboneras et al., 2018; Capinha et al., 2023). The European Union Invasive Alien Species Regulation 1143/2014 directs efforts to tackle the threats presented by invasive alien species to the European Union. However, many such species, including the 88 species of concern (of which 12 are thought not to be present yet in the Union’s territory), lack any evidence of economic costs. despite the European Union being an economic and monetary union with coordinated economic and fiscal policies (e.g., a common monetary policy and currency. The problem is compounded because there are already several thousand established alien species in the European Union (Dawson et al., 2017), with large discrepancies between the total number of established invasive alien species and those with assessed costs (for instance, France has a conservative minimum of 2,621 invasive alien species, but only 98 species [< 4%] with reported costs; Renault et al., 2021).
An absence of information on economic impacts risks downplaying the threats posed by invasive alien species (including costs and other impacts), further reducing the likelihood of cooperation among and within countries to implement effective risk assessments and management measures. The lack of comprehensive and reliable cost estimates for national economies or political entities is also problematic because it underestimates the economic threat biological invasions represent and hinders national policies, governance, research, and educational efforts along with broad-scale multinational initiatives to combat the problem (Sharp et al., 2011; Keller et al., 2011; Caffrey et al., 2014). Improved quantification of these species' impacts are also essential for prioritisation, mitigation, and eradication that underlie the Convention on Biological Diversity 2020 (Essl et al., 2020b), the 2022 Kunming-Montreal Global Biodiversity Framework, and the European Union Invasive Species Regulation 1143/2014 (EU, 2014). However, even when implemented, such actions are most often locally focused, and are frequently of insufficient length or magnitude to address the scale of the problems because of budget limitations. This lack of allocated funding compromises outcomes while simultaneously increasing both long-term management and damage costs (Sutcliffe et al., 2018; Pergl et al., 2019). Thus, the feasibility of management remains impaired by the political choice not to ensure adequate resources (Heink et al., 2018).
Because cost information for many invasive alien species is missing, national and transnational estimates would benefit from a more complete accounting. Given that invasion history is one of the best predictors of eventual impacts (Kulhanek et al., 2011), using existing cost estimates of a measured species would seem the most appropriate and straightforward approach to interpolate missing costs in other invaded countries. However, context is also important, and we would not necessarily expect impacts to be identical across countries (Sinclair et al., 2020). For instance, macroeconomic differences could influence the cost of labour (and management) or the per-unit currency value of a sector, and hence, the magnitude of damages. Alternatively, wealthier or larger countries could be better prepared to respond to invasions, resulting in inverse relationships between costs and macroeconomic indicators (Early et al., 2016). Given the challenges triggered by a lack of detailed cost information, we projected the available data to fill both spatial and taxonomic gaps to assess the extent to which we have so far underestimated the costs of invasive alien species to the economies of European Union member states. We also forecast how baseline invasion costs and factors correlated with costs (i.e., number of costs reported in the literature and number of species with reported costs) will develop up to 2040 using several temporal modelling approaches. As a result, we develop a more comprehensive accounting of costs of alien species to the European Union using invasion history and fitted macroeconomic scaling factors.
Our analyses are therefore important methodological and applied advances towards improving spatial and temporal estimates of costs. We expected that (i) the projected amount of costs of invasive alien species will be considerably higher than those reported in the InvaCost (v4.1) database due to (ii) mismatches between known invaders and those with recorded costs (Angulo et al., 2021; Renault et al., 2021; Cuthbert et al., 2021b), and (iii) expected increasing future trajectories of costs of invasions (Diagne et al., 2020; Seebens et al., 2017, 2020). Our study makes a critical step towards improving cost predictions of biological invasions for the European Union and elsewhere and represents a substantial development beyond existing representations of their monetary impacts.