Andrew Bishop: Invasive plants biding their time

The debate about whether some palms are invasive, environmentally damaging pests or desirable garden specimens seems to have reached fever pitch.

Palm growers are up in arms about what they see as unjustified vilification of the palms they propagate and sell [ see Palm Policemen's Exotic Hit List ], while others support the moves by Auckland Regional Council to consider these species for inclusion in their revised regional pest management strategy.

The main complaint from the growers seems to be that there is not enough proof about palm invasiveness for these species to be put on the hit list. Others argue that the fact these palms are starting to naturalise within the Auckland region in urban bush areas - the ecological equivalent of the canary in the coalmine for larger natural areas further afield - means that the precautionary principle must be applied.

The growers complain that describing their palms as invasive is harming their businesses. Those left to deal with the problems of garden species that have "jumped the fence" and are damaging natural areas would reply that, if left to spread, the clean-up costs of invasive palms would come out of the pockets of ratepayers and taxpayers, not of the commercial growers who benefited from selling these species.

The cost of dealing with invasive plant species in New Zealand is estimated at $100 million a year, already a major drain on public funds desperately needed for other things.

So the debate is really about being proactive versus being reactive. Being proactive means catching invasive species early, just as they start to move into areas such as urban bush, before they spread into other natural areas. This is the time at which the cost of control is small and something can realistically be done. Being reactive is waiting until the problem is so obvious and widespread that it is too expensive and unrealistic to tackle, and the damage is already done.

The argument that is often used is that the plant has been here for so long that if it was going to spread, it will have done so already.

The reality is that plants' lag phases - the time between a plant being brought into a country and the time when it starts to spread and cause a problem, if it is going to - can be significant; 100 years is a conservative lag phase. In one area of Germany, the mean time between the introduction of a new shrub to the region and the start of its spread was 131 years. For a tree it was 170 years.

Lag phases may be shorter in the tropics and, possibly, in New Zealand. An as-yet-unpublished study in Australia estimates the mean time between introduction and naturalisation there at 141 years for woody plants. Most of our future invasive plant problems are sitting in our gardens biding their time.

The science of invasive plants is not exact. Many people around the world are researching methods of improving our ability to predict which of the tens of thousands of plants introduced to gardens and plantations in most countries are likely to become invasive and achieve weed status.

Two of the best characteristics used to predict invasiveness and impact are the behaviour of a plant in other countries and the behaviour of closely related species at either family or genus level - the sort of assessment that was carried out by the ARC in the case of the palms in question.

Since this assessment was done, results of a further weed-risk-assessment system designed for New Zealand found that the bangalow palm had one of the highest risk factors among newly naturalised species.

New Zealand, isolated from the rest of the world for 80 million years, developed a unique ecosystem dominated by birds. Many of the plants that grow there are found nowhere else, and many are threatened by grazing animals and invasive weeds.

New Zealand already has as many introduced plants as native ones. It's important that newly naturalised weeds are stopped dead in their tracks, before they become serious problems.

The science dealing with predictions of effects is often controversial - the climate-change debate is a good illustration of that - but the public must get both sides of the story. In this case, we are concerned that the interests of the wider community and of future generations of New Zealanders are heard.

The Council of Australasian Weed Societies is an organisation that represents weed societies in New Zealand and Australia, and promotes the responsible management of weeds or invasive plants and assists people who study weeds and their management.

* Andrew Bishop is the president of the Council of Australasian Weed Societies.


Readers' Views

The bangalow palm has been here for 115 years or more. Despite displaying some key characteristics which should have made it a highly invasive weed decades ago - it has completely failed to do so. Lag phases are shorter in warm climates. Bangalow palms were, of necessity, in the 19th century planted close to native bush and have completely failed to invade any undisturbed bush. In fact attempts to establish these plants in native bush have met with complete failure. Clearly the weed assessment points system created by the experts contains some fundamental flaws. The disciplined medical practitioner changes the theory to fit the facts rather than persist with a logically untenable position. I invite those involved in the management of our ecology to do the same - or at least consider the possibility that somebody outside of their field has an important contribution to make to the understanding of their discipline. Over the years patients have contributed enormously to my understanding of medicine and psychology - because I listen to them.
- - - posted 11.17pm Oct 24, 2006 by Andrew Montogmery BSc MBChB