A global invader's niche dynamics with intercontinental introduction, novel habitats, and climate change

Species niches have been defined in different ways, variably encompassing abiotic and biotic parameters limiting an organism's spatial distribution. Climate is often the primary component of the abiotic (fundamental) niche, especially among terrestrial plants. In invasion biology, there is an ongoing debate on the prevalence of niche shifts, which may be linked to divergent traits in a species' native and invaded ranges, as well as dispersal limitations and invasion lag phases. Using a global dataset, we tested whether shifts occurred in the invader Johnsongrass (Sorghum hale-pense), which has undergone intercontinental spread as well as habitat-switching within its invaded range in North America. Climate space ordination showed that North America is, on average, colder than Afro-Eurasia, and North American Johnsongrass occupies wetter environ-ments than in its native range. Within North America, both agricultural and non-agricultural ecotypes shifted slightly toward colder climates. Notably, while non-agricultural populations far outnumber agricultural ones, the former occupy a narrower niche (DM = 42.7 +/- 0.8) than the latter (DM = 59.3 +/- 1.1), implying agriculture's role in providing suitable environments in otherwise suboptimal climates. Maximum entropy models of agricultural and non-agricultural North American populations showed limited ecotypic differences in current suitability and range expansion under climate change through the rest of this century. We also predicted climatic suitability for Johnsongrass to increase most in the Upper Midwest and Great Plains by 2100. Our results help contextualize the empirical evidence for ecotypic differentiation in Johnsongrass, as well predict future range expansion and damage niches.