Despite almost two decades of heightened visibility of invaders within the “100 of the world’s worst” invasive species list (Lowe et al. 2000; Luque et al. 2014), over one third lacked robust economic costs. Nonetheless, average economic impacts of the 60 worst invasive species significantly exceeded that of all other reported invasive species. Admittedly, inclusion of invasive species on this list may have contributed to increasing the economic costs reported for these taxa, with worst costs increasing 11-fold and others seven-fold after the list was published; but they seem more economically damaging based on the available data. Nonetheless, management spending only increased towards the worst species slightly more (three-fold) than others (two-fold) after 2000, and thus management investments have been outweighed by increasing damage costs from invasion for both groups.
The nature of costs of the worst and unlisted other may be explained by several factors. First, the lack of robust invasion costs, especially for 40% of the ‘flagship’ 100 species listed, illustrates a broader, pervasive issue surrounding robust invasion cost estimations. The vast majority of invasive species have not been examined for economic costs (e.g., Gren et al. 2009; Haubrock et al., 2021b; Liu et al. 2021), with cost quantifications remaining lackluster relative to ecological impact appraisals (Crystal-Ornelas and Lockwood 2020). Therefore, improved invasion cost estimation is required for invasive species more generally, with no published estimates of economic impact for the majority of known aliens (ca. 14,000 species; Cuthbert et al. 2021). Second, the differences in cost structuring of worst and other species costs may be further attributed to underlying criteria for species being listed. Impacts on biodiversity were a key criterion for inclusion on the list (Lowe et al. 2000), with such impacts often non-market in nature and thus challenging to quantify in monetary terms (but see Hanley and Roberts 2019). Indeed, impacts to the environment sector were particularly marked in the worst compared to other taxa (31% vs. < 1%). Similarly, whilst impacts on human activities were also considered when selecting species, such impacts may also frequently be non-market in nature (e.g., certain recreational activities) and thus equally challenging to quantify holistically. Nonetheless, management investments, which should comprise a high proportion of costs for species with few market impacts, were very low overall (worst: 6%; other: 15%).
Third, differences in costs may be an artefact of the filtering and averaging strategies employed in the present study. Indeed, as we solely considered species-specific, observed, highly reliable costs, to minimise irrelevant cost estimates considering cost data associated with non-robust estimation methods and/or not actually observed. However, when the reliability and implementation filters are removed, the number of worst species increases to just 68. Several of the most economically damaging species were not considered for the IUCN worst species list, given the necessity of a broad taxonomic range for the list. Indeed, 90 of the other species exceeded the median cost of the worst (US$ 43 million). This particularly negated inclusion of several known economically-damaging congenerics, such as the yellow fever mosquito Aedes aegypti and brown rat Rattus norvegicus, which imparted marked costs but were already represented at the genus level by another species (Aedes albopictus and Rattus rattus) and therefore were not listed in the worst 100 as a rule. Importantly, this also reflects the caveat that the enlisting of a given species does not imply that it is any more damaging than others (Lowe et al. 2000; Luque et al. 2014). Furthermore, it indicates that inclusion of species on such a list is not a prerequisite for management expenditure.
Data gaps mean that our results should be cautioned in terms of species comparisons and that they are likely underestimates for both worst and other groups. A lack of costs, or complete absence, for certain taxa in the InvaCost database does not equate to a lack of impact. It may be that these impacts are more difficult to quantify in monetary terms in certain sectors (e.g. environmental), located in countries with a lower capacity to study invasions, or in habitats that are more difficult to monitor. As such, even species with the lowest reported economic costs on the worst list (e.g. red-vented bulbul Pycnonotus cafer and chytrid fungus Batrachochytrium dendrobatidis) or those without costs at all (e.g. common wasp Vespula vulgaris, fire tree Morella faya and common malaria mosquito Anopheles quadrimaculatus) could have substantial costs that are as of yet undocumented. Likewise, this applies to the other taxa, whereby the vast majority of known invasive species lack economic cost studies. As such, low costs likely reflect knowledge gaps rather than a lack of impact for many taxa.
Whilst efforts were made to provide a broad taxonomic breadth in the 100 worst list, taxonomic unevenness was also found considering available worst and other species in respect to economic costs, with both being dominated by animals and from terrestrial environments. We found that costs of groups such as invasive plants are lacking. Nevertheless, the much higher damages and losses incurred overall suggests that greater management investments are required to offset costs from all invasive taxonomic groups, particularly at early invasion stages (Leung et al. 2002). Indeed, management spending increased at a much lower rate than damage costs for both worst and other invaders. Geographic gaps accompanied taxonomic unevenness, with the highest costs of the worst species occurring by far in North America, even when considering GDP, whilst other species were more balanced regionally. For example, costs in Asia were five-fold higher proportionally for other taxa compared to the worst. Overall, these taxonomic and geographic results might reflect wider biases in invasion impact research (Crystal-Ornelas and Lockwood 2020). On the other hand, the diversity of sectors impacted indicates that the listing of the worst species was broad in scope.
Temporally, costs generally increased over time per species for both species groups. However, costs of the worst species grew more than the others before and after the list was published in the year 2000 (11-fold vs. seven-fold), but only slightly so for management investment (three-fold vs. two-fold). As one of the aims of inclusion of species on the worst species list was to increase societal awareness, such an increase in communications does not appear to have succeeded to substantially boost management investments for the worst species compared to other invaders. Conversely, damage costs have increased at a much greater rate for both the worst and other species than management. As rates of invasion continue to increase worldwide (Seebens et al. 2017, 2021), it is expected that such costs will continue to rise, and perhaps especially for other species as novel invasions might be accompanied with novel economic impacts. Indeed, the fact that only one of the previously listed worst species has been successfully eradicated illustrates the challenges and shortcomings of invader management more broadly (Luque et al. 2013). Given the cost effectiveness of early-stage invasion management compared to long-term control (Leung et al. 2002), increased investments should be made to prevent introduction of invasive species—both inside and outside of the list. Such interventions could take several forms depending on the pathway of introduction, such as the implementation of airport checks for alien taxa in transit, more efficacious ballast water regulations or tighter restrictions on the trade of exotic pets.
Overall, whilst the present study compiled available information on economic costs of species included in and excluded from the IUCN “100 of the world’s worst” invasive species list, a large share (40%) of worst species lacked robust economic cost appraisals. This reflects a wider absence of cost estimation in invasion science—while we acknowledge that the InvaCost data here are not exhaustive. Nonetheless, the proportionate extent of cost reporting (60 of 100 with robust data) for listed species is far higher considering the very low cost reporting for other, unlisted species (463 overall, relative to all known invaders worldwide, of ca. 14,000 aliens; Cuthbert et al. 2021). Accordingly, our findings suggest that the list effectively increased cost reporting for the worst species, or that they are generally better-studied. We again stress that many of those still lack robust monetary appraisals. Moreover, we note that listed worst species were often selected on the basis of their economic impacts, and so they may simply be more likely to have reported costs. Despite differences identified here, the “true” economic impact of the 100 worst invasive species thus remains unknown, as well as the cost of invasions more broadly. We therefore encourage more resolute cost reporting to quantify the global extent of invasion costs for all invasive taxa.