The distribution of Carrichtera annua in Australia: Introduction, spread and probable limits

Carrichtera annua (L.) DC. (Brassicaceae) or Ward's Weed, a major weed of semi-arid rangelands of southern Australia, has been collected widely since its introduction early in the 20th century. Collated records were used to suggest a single site of accidental introduction in South Australia, evidence of a lag phase of ∼30 years (probably due to edaphic restrictions) before rapid spread, involving infrequent long-distance human-aided dispersal across southern Australia and a relatively stable range since the 1960s. Climate and soil analyses suggest that abiotic factors limit the distribution of C. annua, with the species being restricted to areas with winter-dominated rainfall and calcareous soils. Documentation of the history of a successful invasion, including the spread and probable limits of the current distribution of a species, is important for managing invasions. This study also highlights that a single, accidental introduction can result in a long-lasting, widespread problematic weed.