Relative abundance in an alien weed flora.

Both relative abundance and absolute abundance of alien weed species in a large geographic region increased from the year 1900 to 1980. The increase in relative abundance may have been due to competitive displacement among the weeds, patterns of landscape disturbance, or simply time. The increase in absolute abundance indicates the ineffectiveness of past weed control policies in stemming weed migration. Combined, the two scales of alien weed abundance suggest that in the future we can expect an increasing diversity of increasingly common weeds.