Niche conservatism among non‐native vertebrates in Europe and North America

Niche conservatism, the hypothesis that niches remain constant through time and space, is crucial for the study of biological invasions as it underlies native‐range based predictions of invasion risk. Niche changes between native and non‐native populations are increasingly reported. However, it has been argued that these changes arise mainly because in their novel range, species occupy only a subset of the environments they inhabit in their native range, and not because they expand into environments entirely novel to them. Here, using occurrences of 29 vertebrate species native to either Europe or North America and introduced into the other continent, we assess the prevalence of niche changes between native and non‐native populations and assess whether the changes detected are caused primarily by native niche unfilling in the non‐native range rather than by expansion into novel environments. We show that niche overlap between native and non‐native populations is generally low because of a large degree of niche unfilling in the non‐native range. This most probably reflects an ongoing colonization of the novel range, as niche changes were smaller for species that were introduced longer ago and into a larger number of locations. Niche expansion was rare, and for the few species exhibiting larger amounts of niche overlap, an unfilling of the niche in the native range (e.g. through competition or dispersal limitations) is the most probable explanation. The fact that for most species, the realized non‐native niche is a subset of the realized native niche allows native‐range based niche models to generate accurate predictions of invasion risk. These results suggest that niche changes arising during biological invasions are strongly influenced by propagule pressure and colonization processes, and we argue that introduction history should be taken into account when evaluating niche conservatism in the context of biological invasions.