The potential distribution of yellow bristle grass (Setaria pumila) in New Zealand.

Yellow bristle grass (Setaria pumila), an invasive annual grass weed in North America, Africa, Australia and New Zealand, has become a problem on dairy farms in the upper North Island. To define its potential distribution in New Zealand, an eco-climatic model was constructed using CLIMEX. The model was parameterised using the known distribution of the species in its native range in Eurasia and validated against its invaded range in North America. The model predicted all known occurrences in New Zealand and revealed extensive tracts of land in both the North and South Islands that are currently climatically suitable yet, according to current records, unoccupied by the weed. Under climate change, this potential distribution increases substantially. These results imply that yellow bristle grass could become a much wider problem on dairy farms throughout New Zealand and that management to limit its spread is justified.