Abstract
<jats:p>Invasive alien species (IAS) present a major threat to biodiversity and ecosystem services. Specialized ecological tools, such as niche-based climate-matching approaches, can be used to anticipate potential distributions of IAS, but when based on the native range, their use may be limited because they can only quantify conditions within the native climatic niche (NCN); i.e. niche “innerness”. The recently developed Niche Margin Index (NMI) provides a complementary measure that allows situating any colonized site inside (positive), at the margin (zero) or outside (negative) the NCN; i.e., it allows measuring niche “outerness” as well. This measure depends on a multivariate quantification of the NCN and was for instance shown to be positively correlated with establishment success of invasive mammals. How the species data (e.g. occurrences or range maps) used to quantify the NCN influence the NMI was, however, never assessed so far, although of crucial importance for understanding biological invasions. Here, we use two types of species data – occurrence-based (pixels in the native range occupied by the species) and range map-based (polygons of the native range) – as input to calculate the NCN of 46 IAS and compare the NMI values obtained for occurrences in the alien range. Results show that the occurrence-based approach yields an average NMI of just above zero and shows many negative outliers, which could be interpreted as niche expansion, whilst the average range map-based NMI was much higher, which could be interpreted as niche conservatism. Whether a colonized site in the alien range is within or outside the NCN thus depends to some extent on the data used to calculate it as well as on the individual species. We suggest future avenues of research to assess methods of quantifying the NMI that would approximate reality most, as well as investigate the ecological mechanisms of invasion.</jats:p>