Ian Popay, Peter Williams: Few ways to identify plants with dark side

Strict modern biosecurity regulations may have stopped the kiwifruit industry in its tracks, as Jim Douglas suggested in his recent Perspectives article. But they may also have helped to avoid many agricultural and environmental scourges.

The Environmental Risk Management Authority, had it existed in 1905, might have prevented the introduction of kiwifruit, because it could be described as "a rampant growing vine, a perceived weed, and a likely environmental threat to our native forests".

Had it existed earlier, it might also have stopped the importation of possums, gorse, rabbits, stoats and old man's beard. We say "might" because good cases could have been made to support their introduction:

* Possums would be the beginning of a billion-dollar fur trade.
* Gorse fences fields, brings winter colour, feeds horses, and enriches soil.
* Rabbits provide food, fur, and fun for new migrants.
* Clematis beautifies garden fences in the spring.

Possums had not been seriously invasive anywhere else, and neither gorse nor old man's beard were problems back home. It took some time before gorse was recognised as a serious problem. Rabbits, well you know what they did.

Biosecurity is a mixed blessing, bringing problems for some and benefits for others. The balance, long term, for possums and gorse is clearly in the red. For kiwifruit, at present, it's in the black. Who can tell, though, how that balance sheet might look in another 100 years?

By then, kiwifruit might have spread widely, damaging native forest and bush. Nobody knows.

Biosecurity is a difficult business. Many of our environmental weeds didn't display their dark side until they started to run amok here. That's why Mr Douglas' simple solution of listing known undesirable plants would not work. Pre-border risk assessment is not yet reliable enough.

How many New Zealanders realise that tradescantia fluminensis, wandering Jew or wandering Willy - a wonderful ground cover plant or pot plant - has escaped to invade forest floors where it inhibits the establishment of native trees? This plant would have been hard to classify as high-risk - it is low-growing, does not produce seed, and has no invasive relatives.

As Mr Douglas says, referring to the potential value of exotic plant species, "you don't know until you try". Regrettably, the same thing applies to prediction of weediness - you do not know until you try.

Our native bush is unique, it is what many tourists pay to come to see. Survival of many native plant species is threatened by grazing animals and by competition from invasive weeds. Once lost, a plant species is gone forever.

Mr Douglas was wrong to say, "Sure, some serious weed species have been introduced in the past but they constitute a very low percentage of our flora, and most were introduced unwittingly long ago, before seed importation standards were set to minimise weed-seed introduction".

Introduced weedy plant species now outnumber native species. Some of those have gone wild only recently, and a proportion will, in the future.

Regrettably, too, most of our significant weeds were introduced deliberately, often as garden ornamentals.

About three-quarters of the 300 weeds actively controlled by the Department of Conservation were originally introduced as garden or plantation plants - old man's beard, tradescantia, wild ginger, crack and grey willows, privet, and the rest.

The Environmental Risk Management Authority makes its decisions on the importation of new organisms by evaluating all potential costs and benefits. Plants were included in the legislation as a deliberate attempt to prevent the mistakes of the past. We do not need open slather, unrestricted access of any species.

We agree, though, in one respect with Mr Douglas. The cost of evaluating new species is high, and is a disincentive to exploring new agricultural possibilities. It may also encourage people to smuggle seeds or cuttings past those beagles.

It would be in New Zealand's interest to make evaluation cheaper, so all plant imports are fairly and fully evaluated before they get into our very special country.

* Ian Popay and Peter Williams are plant scientists with, respectively, the Department of Conservation and Landcare Research. They are responding to Jim Douglas, a plant scientist with Crop and Food research, who wrote that our biosecurity regulations are blocking economic development.