Our planet is a highly connected place. Despite the many beneficial aspects of this connectivity with regard to, for example, economic development, there is an externality which ironically can damage the economy and to an often-larger extent the environment; invasive species.

Invasive species are one of the primary drivers of environmental change. In some contexts, they are responsible for the extinction of numerous species, particularly in island ecosystems. Unfortunately, the number of invasive species continues to rise, a rate at which we can expect to increase in the coming decades. With so many species requiring our attention, we need to be able to prioritise which species are of most concern.

Impact assessments, like the IUCN’s Environmental Impact Classification for Alien Taxa (EICAT) framework, are a useful tool for multiple reasons. One example is that they assist in the identification of those invasive species that are causing the most harm in their introduced environments. However, performing these assessments requires the collation of available evidence from various sources (e.g. scientific literature, government reports, etc) and subjecting this evidence to the impact classification framework. These assessments are, therefore, also subject to the vagaries of human decisions. If people differ in how they interpret both the impact assessment framework, and the evidence used for assessments, then the basis of species priority lists may be flawed. As such, it is important to understand how and why people performing independent assessments may differ in their final conclusions.

In a study recently published in the open access journal Ecosphere, we sought to identify and classify the sources and types of uncertainty that may arise when performing impact assessments using the EICAT framework. Additionally, this was an opportunity to apply EICAT to a previously unassessed taxonomic group, the insects. We found that even though those performing assessments had jointly gone through a familiarity session with the EICAT framework, there were considerable levels of disagreement between independent assessors across the EICAT components (mechanism of impact, severity of impact, and confidence in the evidence) in the first round of assessments. Upon examination of the first-round results, and the proposed reasons for disagreement, the types and sources of uncertainty were identified. Although seeking agreement between the assessors was not the aim of the study, a second round of assessments did see a marked improvement in the levels of agreement, across the EICAT components.

Discovering how and why people disagree when using the same assessment framework, on the same species, provides important information not only for improving the framework itself, but can also help reduce the prevalence of uncertainty in the final outcome. As such, we proposed a set of recommendations that, if followed, should lead to a more complete and accurate assessment of the environmental impacts of invasive species. Being confident that the assessment outcome is indicative of the best available evidence is especially important given the large number of potential invasive species and the need for prioritising the species of most concern.