New Zealand Biosecurity Legislation and the Naturalization of Exotic Weeds

New Zealand biosecurity legislation is world renowned, yet we still have many naturalized weeds to control. We present an historical outline of the New Zealand plant biosecurity legislation and examine whether it might have had an effect on naturalization rates over the period of European occupation. We used a pool of 669 weeds, that is, species that are controlled in some way, classified as agricultural weeds, conservation weeds, or weeds common to both sectors. The arrival of agricultural weeds peaked before 1900 and most naturalized before 1950 so that the cumulative total has barely increased since the late 1900s. Until the early 1900s there were twice as many agricultural weeds as conservation weeds which were slower to naturalize, with a peak in the period 1941-1980. They are continuing to naturalize, primarily from horticulture, but the rate also appears to be slowing. While it is not the only cause, the legislation has probably contributed to these major differences between agricultural and conservation weeds. The longer the existing regulations governing the entry of new species continue to operate, the more pronounced the flattening of the naturalization curve is likely to be.