Beyond tracking climate: Niche shifts during native range expansion and their implications for novel invasions

Aim Although ecological niche models have been instrumental in understanding the widespread species distribution shifts under global change, rapid niche shifts limit model transferability to novel locations or time periods. Niche shifts during range expansion have been studied extensively in invasive species, but may also occur in native populations tracking climate change. We compared niche shifts during both types of range expansion in a Mediterranean annual plant to ask (i) whether the species' native range expansion tracked climate change, (ii) whether further range expansion was promoted by niche expansion, and (iii) how these results changed forecasts of two ongoing invasions in Australia and California. Location Eurasian Holarctic, California and Australia. Taxon Dittrichia graveolens (L.) Greuter (Asteraceae). Methods Niche shifts were quantified in both environmental and geographic space, using the framework of niche centroid shift, overlap, unfilling and expansion (COUE) as well as maximum entropy modelling. We used the historic native distribution and climate data (1901–1930) to project the expected distribution in the present climate (1990–2019) and compared it to the observed current distribution of D. graveolens. Finally, we compared invasion forecasts based on the historic and present native niches. Results Dittrichia graveolens expanded its native range well beyond expectations based on the observed climate change, associated with a 5.5% niche expansion towards more temperate climates. In contrast, both invasions showed niche conservatism and were (still) constrained to climatic areas matching the historic native niche. Main conclusions Contrary to hypotheses in the literature, niche shifts are not necessarily more rapid in invasions than in native range expansions. We conclude that niche expansion during the process of climate tracking may cause further range expansion than expected based on climate change alone.