How much do out-of-the-native range occurrences of invasive alien species reflect their native niche?

Invasive alien species (IAS), meaning the fraction of non-native species that cause harm to ecological processes and/or economy and/or public health, present a major threat to biodiversity and ecosystem services. Managing and controlling them is of paramount importance. To guide conservation efforts, detailed maps of potential distributions of IAS, as for instance produced by species distribution models (SDMs), are very useful, but lacking in the dimension of probability of invasion success. An important barrier to the successful establishment of IAS is climate. The Niche Margin Index (NMI) provides a measure of (dis)similarity between the native climatic niche of IAS and the environmental conditions in another site (which, for these purposes, would be a (potential) invasion site). This measure was shown to be positively correlated with invasion success in invasive vertebrates. However, special caution is advised to pick the method to define the native climatic niche, as Species Distribution Modelling outputs have been advised over expert-based maps to support conservation decisions. This study therefore examines the effect on NMI of using three different methods to define the native range: the full native range polygons, the geographical occurrences of the species inside the native range, and a binarized SDM of the native range. The last method occupies the middle ground between the first and second, which present the highest and lowest NMI, respectively (i.e. occurrences < SDMs < range polygons). This has implications for nature conservation and invasion science, as it points out new considerations that must be made before the probability of invasion success is assessed. These include decisions on when (at what dispersal distance) the climatic barriers become important, and whether different barriers must be considered when an invasion has moved beyond the first stage. Finding the right questions to answer is a critical prerequisite for finding the right answers, which in turn allows us to determine the right areas to target for conservation.