Beyond species-specific assessments: an analysis and validation of environmental distance metrics for non-indigenous species risk assessment

Abstract

Environmental distance metrics quantify environmental similarity between locations using a number of environmental variables. They are commonly applied in aquatic non-indigenous species risk assessments to assess the relative risk of species transfer between different location pairs. Despite their application in governmental risk assessments globally, these metrics have not yet been empirically validated. We use empirical data for 419 species obtained from the Global Invasive Species Information Network database to perform a validation of environmental distance metrics. We explore the ability of environmental distance to discriminate presences from absences in both aquatic and terrestrial environments. We examine the effect of variable choice (both the number and types of variables included) and different metrics (Euclidean distance, Mahalanobis distance, and weighted versions of each) on metric performance. Environmental distance calculated using unweighted Euclidean distance performed best overall. When applied with appropriate variables, it was able to discriminate between presence and absence distances for up to 93 % of species. Variable choice significantly influenced metric performance, and including fewer, relevant variables outperformed applications where many variables were included. Our results support the use of environmental distance metrics in both aquatic and terrestrial environments.