Transitional Rainfall Zone

The TRZ is recognised as having a much higher diversity of rare and endemic plant species than the other Zones.

The Zones were first defined by Stephen Hopper in his 1979 paper Biogeographical aspects of speciation in the southwest Australian flora.

Thus the TRZ is now defined as the aggregate of the Geraldton Sandplains, Avon Wheatbelt and Mallee IBRA regions.

The TRZ is widely recognised as having very high species diversity and endemism compared to the other zones.

Other botanists have refuted this, however, claiming that the entire TRZ is species rich compared to the other zones.