Tomorrow's weeds
We can predict three phases of problem weeds emerging in New Zealand.
The first phase will arise from the over 2000 plant species alien to New Zealand that have 'escaped', or become naturalised in the wild. Some of these have already become important weed species, and others will do so. Among these are the weeds that will become the new problems of the next 20 or so years.
The second phase will emerge from the 17,000 plant species in cultivation in New Zealand. Four thousand of these have been shown to be invasive overseas, and the chances are high that, given the right conditions, these same species, and others, could become naturalised and weedy in this country. These will probably the second phase of problem weeds in the next 50 to 100 years.
The third phase will arise from plants not yet found in New Zealand but which could, if imported and established, become the major weeds 100 years or so into the future. Warm-zone plants from Australia and Asia will increasingly feature in this group65.
These time periods are only approximate, of course, and a newly imported serious weed could, given the right conditions spread very much more quickly.
Let us look at weeds in the different categories.
- Plants already present in hill country pastures as major weed problems
Weeds like gorse and manuka are already common and widespread in hill country in many parts of New Zealand. Changing conditions in hill country farm management (reduced fertiliser inputs, reduced stocking rates) combined with a changing climate probably means that scrub weeds will increasingly encroach on hill country pastures. The respondents to our survey strongly suggest this is already happening. Existing scrub weeds will tend to become even more widespread. Some of these 'weeds', like gorse, could be beneficial to the environment if they are left undisturbed to revert to native bush and manuka is often the first step in such reversion.
Climate change, favouring more droughts and floods, means that annual or biennial weeds like thistles and barley grass will come and go with greater regularity and infestations will affect a greater proportion of individual farms and of regions. Rising levels of atmospheric carbon dioxide may promote the growth of broad-leaf species rather than grasses. Thistles may be promoted, for example, at the expense of barley grass.
- Plants already present in hill country pastures but of limited distribution and identified as potential weeds
These include plant species currently identified as potential threats and already appearing on lists of pest plants or as surveillance species. The difficulty, given the large number of factors involved, is knowing which have already reached their potential distribution and populations, and which are going to become even bigger problems in the future.
Some of these species, like barberry, boxthorn and hawthorn, have long been regarded as noxious plants, and have probably extended their ranges or their importance, but little is known of the details of such spread. We therefore have to rely on the perceptions of farmers and Biosecurity Officers for information on increasing spread of such plants.
Many of these weeds have the potential to create problems on forestry, conservation and farm land, and for cross-infestation between these land uses. Some, like Darwin's barberry or Himalayan honeysuckle, have only a marginal impact on the productivity of pastoral land, but may be important in conservation or forestry areas.
Some of the rarer thistles, like saffron thistle, could become more widespread, and current infestations could become denser and harder to control. Several Regional Councils are now targeting weeds like saffron thistle.
The species included in this category can readily be seen by examining lists of past and present noxious plants (Appendix A). All of these species have demonstrated their ability to become invasive here or overseas. Some of these species are still of limited geographical distribution and official control efforts should concentrate on such species rather than on those that are already widely distributed.
Attention should be paid to those weeds noted as 'emerging' by the respondents to our survey
- Plants already present in hill country environments but not yet considered dangerous
There are 17,000 plant species under cultivation in New Zealand. If 2% of these become naturalised, and weedy45, we can look forward to 340 new weed species. However, 4000 of these species under cultivation have demonstrated invasive characteristics in countries overseas44. Furthermore, Williams et al.44 have shown that some of the species now known to be invasive in New Zealand had no previous history of invasiveness elsewhere, which suggests that the proportion of species that naturalise and become weedy could be substantially higher than 2%.
The greatest dangers to New Zealand come from plants that are common and widespread in parks or gardens throughout the country, or that have already been extensively planted on or near farms. An example not strictly relevant to pastoral farming is the common rhododendron which has become a major environmental weed in parts of Britain.
Potential problems in pastures in New Zealand, especially in hill country include:
- Plants now being planted as crops, like olives, a noxious weed in parts of Australia and already naturalised in New Zealand9.
- Garden, farm or plantation trees like Gleditisia triacanthos (honey locust), a pasture and fencerow weed in Iowa, and bamboos, known to be invasive here and overseas.
- Plants common in hill country farm gardens, like arum lily, already invasive in some parts of New Zealand
- Plants grown as house plants, like cacti, bryophyllum, Mimosa pudica, Jerusalem cherry, all known to be invasive overseas and, in some cases, already invasive in New Zealand, and which have the potential to survive in sheltered spots and, with increased climate warming, to spread.
The Department of Conservation has developed a surveillance plan for early detection of new invasive weeds68. Perhaps the Ministry of Agriculture and Forestry could play its part by adding potential agricultural weeds to that plan.
- Weeds not yet present in hill country but found in lowland habitats
Many species so far only present in flat, lowland habitats have the potential to become increasing problems in hill country pastures and other environments, especially in the face of climate change, which will see cold-sensitive species able to move into higher hill country. Spiny broom, a weed of very limited distribution is a good example. Decorative grasses like species of Pennisetum and warmer-zone scrub weeds like hakea and woolly nightshade may spread further into hill country pastures.
- Weeds not yet present in NZ
Both Williams et al.44 and Green65 highlight the dangers of increased passenger and container traffic bringing new pests into New Zealand. Williams notes that 34% of the plant items seized at the border because they do not comply with import regulations are undeclared. Green points out that since the passage of the HSNO Act and the establishment of ERMA there have been only three applications to bring in new plant species in two years, a dramatic change from the previously large annual inflow of new species. He goes on to speculate that introductions are still being made, but by smuggling. Green also stresses that plant imports from east Asia or south-east Asia pose considerable threats at present, but that increasing trade with South America will need to be monitored carefully. Australia, as our major trading partner, is also seen as a source of future weeds.
For importing new plant species, New Zealand uses a weed risk assessment model based on the Australian model69. Further modifications are being made to include potential impacts on indigenous biodiversity. The model is intended to be used with the ERMA process for handling applications for import.
- Naturally selected herbicide resistant plants
Five species of common New Zealand weeds (fathen, nodding thistle, Chilean needle grass, giant buttercup and winged thistle) have developed resistance to herbicides. The spread of these resistant biotypes to new areas poses a threat to traditional farming practices, as does the continued development of resistance as a result of repeated application of the same or similar herbicides.
- Genetically engineered plants
The first practical uses of gene technology have been in the production of crops resistant to commonly used herbicides. In the United States, crops like soya bean, cotton and cereals resistant to glyphosate (Roundup) have been developed. In New Zealand, companies have plans to introduce similar herbicide-resistant crops, glufosinate-resistant pine trees are approaching field testing, and plans are in place for the production of glyphosate-resistant pinus radiata.
Gene flow can easily take place from herbicide-resistant plantings to wild populations of the same or related species by wind-dispersed seeds or pollen. 'Wilding pines' are already of concern in parts of the South Island, and the threat of herbicide-resistant wilding pines is frightening. Kareiva et al.70 (1996) cast doubts on the idea that small-scale, short-term ecological experiments can offer accurate predictions of the invasiveness of genetic constructs.
Possible Impact of future technologies
As we have just seen, new technologies may help create new weeds, but they could also improve surveillance and control of existing problems. Some examples include:
- New and better biocontrol agents for specific problem weeds
- Genetically-engineered pasture species better adapted to insect attack, drought and aluminium toxicity would also be better suited to resisting weed invasion, but could also become weeds of other environments
- new, better targeted weed control measures that could include new, more selective herbicides and better application systems
- better surveillance technology to follow the progress of new weeds and the expansion of established ones
- modelling techniques to help in prediction and monitoring of weed infestations.
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