New Zealand is a very weedy country. Indigenous plant species are matched in number by naturalized, exotic species, and about 20 new invaders are discovered each year. Thus, a weed eradication program has been under way for the past 10 years, but completely eradicating an unwanted plant species is much more difficult than it might seem.
The current issue of the journal Invasive Plant Science and Management assesses the progress of 111 weed eradication programs carried out by New Zealand's Department of Conservation. Only four of these programs have met with success, while 21 have been discontinued, and the rest remain an ongoing challenge.
Several factors can predict the success of weed eradication, primarily preventing further entry of the species. The extent of the invasion, ease of access to infestations, and longevity of the species' seedbanks also influence the effectiveness of suppression. It is essential to start eradication efforts quickly after an infestation is discovered. Once a species increases exponentially, the effort becomes much more difficult.
Correctly assessing the extent of infestations was a problem for the discontinued New Zealand eradication programs. Inconsistent visitation of the infestation sites also has hindered progress. Longer and more frequent visits to infestation sites may improve the success of weed elimination. Although some programs are progressing toward eradication, it has taken longer than expected.
Eradication can be an economically efficient method of handling invasive plant species. However, the success rate of such attempts should be considered when assessing whether to undertake this environmental weed management strategy. After a decade, New Zealand's weed eradication strategy has yet to yield significant results.
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