High potential for Brassica tournefortii spread in North American introduced range, despite highly conserved niche

Identifying areas vulnerable to the spread of invasive species is critical for designing effective management plans. Species distribution modeling is commonly used to predict species suitable habitat. However, invasive species may not be in equilibrium with environmental conditions within the introduced range, leading to inaccurate predictions. We focused on Brassica tournefortii to (1) assess the role of niche dynamics, (2) identify environmental drivers facilitating spread, and (3) identify suitable habitat within currently un-invaded areas in North America (introduced range) by quantifying the realized niche in its home range (Mediterranean basin and Middle East) and introduced range. Brassica tournefortii continues to expand its distribution within North America, threatening native species and altering ecosystem dynamics through invasion. We found that Brassica tournefortii has a highly conserved environmental niche, driven primarily by warm and dry climates with resource-limited soils. Despite high niche stability, we detected environmental non-equilibrium between the ranges, with 55% niche unfilling and 1% expansion into more arid and resource-limited environments within the introduced range. Our distribution models also highlighted invadable regions in the introduced range where this species is not found currently, indicating opportunities for this species to continue to spread within North America, and suggesting that previous estimates may underestimate areas at risk for invasion. To improve spatial predictions of the potential spread of invasive species, we must consider the role that niche dynamics and environmental equilibrium play during biological invasions.