Introduced, invasive weeds are lying in wait around the country, ready to take advantage of climate change. Elaine Fisher reports.

New Zealand’s future pastoral weeds are already in the country and climate change may enhance their opportunities to spread widely, Crown Research Institute, AgResearch principal scientist Dr Graeme Bourdôt believes.

“New Zealand has strong biosecurity protocols at our international border to stop new pests coming into the country but most weed scientists, including myself, would say most of our future weeds are already here.

“What we have is an internal biosecurity problem around future weeds, but that fact often gets lost amongst those who consider we have biosecurity sorted at our international border.

“Our future weeds are amongst the huge pool of about 21,000 exotic plant species introduced to New Zealand. Of those 1798 are naturalised, amongst which are all our current weeds. Another 1043 species are ‘casuals’ and potentially in the early stages of naturalising.”

AgResearch is using CLIMEX modelling software to create models for many of NZ’s existing agricultural weeds to determine the likely impacts of climate change on their potential distributions.

Among these is Chilean needle grass, an invasive weed able to out-compete productive pasture grasses and take over large areas if uncontrolled, particularly in dry environments such as east coast Hawke’s Bay, Marlborough and Canterbury. It is unpalatable to stock, and when the sharply pointed panicle seed is present (November-January and March-May), it becomes an animal welfare problem. A published AgResearch paper on Chilean Needle Grass, shows the plant could grow on 39% of the country’s land mass but by 2080, through climate warming, this potential distribution will increase by 60%, mainly by range expansion in westward and southerly directions.

Modelling has also been carried out for another invasive plant, Nassella Tussock which is able to spread and dominate pastures. The tussock is unpalatable to stock and has a high reproductive capability with 120,000 seeds being present on one mature plant.

“The modelling shows that the land area in New Zealand with a climatically suitable environment for Nassella, currently 46% of the total landmass, will increase by 14% by 2080 due to climate change.”

The potential distribution of Californian thistle, one of the most common thistle species in NZ, has also been modelled. Once established in pastures, California thistle is difficult to control and forms a patch of shoots connected by a creeping root system.

“We have built a model to show the potential distribution of Californian Thistle under the current climate but have not yet looked at the future areas of potential for this weed,” he says.

While most pest plants have not been subject to climate change modelling, Graeme says it is possible to build climate niche models and produce maps to show where these plants could grow in future.

“For the ones we have modelled so far, and based on several future climate global models, it’s certainly clearly evident that New Zealand becomes climatically more suitable for these weeds.”

NZ is also likely to become more suitable for the spread of plants which are as yet not considered weeds. These are classed as ‘sleeper weeds’.

“These are weeds in waiting if you like and they include many thistles which are enormously serious agricultural weeds in North America. In New Zealand they are currently very localised.”

Modelling to discover what their future ranges could be hasn’t been conducted yet, but Graeme believes they could be among our serious weeds of the future. According to the Ministry for the Environment’s website, NZ has around 2500 named native plant types, including flowering plants, ferns and conifers and more than 80% of them are found nowhere else. However, since the arrival of humans around 1000 years ago about 21,000 plant species have been introduced. As a result, 1798 of these exotic plant species are now established in the wild and considered naturalised.

“We now have an order of magnitude more exotic plant species in New Zealand than natives!” says Graeme who believes many will be more widely spread in natural eco-systems, pastoral and arable lands and orchards in the coming decades.

“Our current and future weeds have got here either by themselves or been imported for agricultural, horticultural or garden use.

“We should worry about unwanted plants arriving unintentionally at our borders in imported grains, in soil on vehicle tyres or through other means, because there are tens of thousands of other potential pest plants in the world which could come into New Zealand.

“However, we have a huge pool of plants already here from which many of our future weeds will arise.”

Understanding which plants will become threats is vital, Graeme believes. “So many weeds have already invaded our natural and agricultural ecosystems that there is no way that we can ever manage them all. There is not enough manpower or funding to do so.

“The important question now for the Department of Conservation, Ministry for Primary Industries, and regional councils, is how to prioritise weeds for management to ensure biosecurity resources are applied to achieve the ‘best bang for the buck’.”

It is likely this will become the subject of a substantial new collaborative research programme, which would take climate change into account, and involve major science providers, including AgResearch, Manaaki Whenua Landcare and universities, and regional councils, DoC and MPI.

“The programme would aim to come up with objective methods to determine the weed potential and prioritise for management, all of our exotic plant species, not just those which are currently invasive, but also those “sleepers” which could invade our natural, agricultural, and urban environments.

“The biggest issue facing decision makers is how they can best decide which species to target now to avoid huge problems in future.”

To help in that decision making, Graeme says new tools are needed to identify “sleeper weeds” before they become a serious problem, and that includes many plants currently grown in home gardens and public parks.

NZ needs to learn from past mistakes in not attacking pest plants early enough. “Imagine how different the Mackenzie Basin would be today if we had controlled Pinus contorta when we first knew it was naturalising.”

Most of NZ’s weeds are spread by humans, or by birds which feed on exotic fruits and spread the seeds in their droppings, and by animals.

“Arguably though, humans are the main vector for spreading weeds, including through garden waste and transporting of livestock or animal feeds. We are the architects of our weed problems. Once we know about these pathways, theoretically we can prevent the spread of weeds.”