Climate change will move weeds
Scholars have long suspected that many invasive plants species will benefit and suffer from climate change in New Zealand.
It’s thought that some species will extend their range to the south, while simultaneously losing range in the north as the climate heats up.
But by themselves, predictions of a southward march and northern retreat aren’t particularly useful and more fine grained work is needed, especially on individual species.
In an article published late last month, three Unitec scholars did that work with an invasive weed called mexican daisy. The daisy, Erigeron karvinskianus, produces masses of pretty white and pink flowers. It was introduced to New Zealand from Mexico as a garden flower in the 1940s and it is now common in Auckland, Wellington, Christchurch and has even spread to the Chatham Islands. There are also current pockets in Dunedin and Invercargill.
It grows in dense matts that choke other plants, including natives. It has been declared unwanted and it’s illegal to propagate, sell or knowingly spread the species.
In the wild, the daisy prefers disturbed areas, including rock walls, roadside banks and wastelands. It is also increasingly invading native ecosystems including native and replanted forests, wetlands and riparian zones.
It already has ‘‘serious impacts’’ on revegetation or replanting projects and ‘‘may require significant resources for clearing, preparation and maintenance to ensure the growth and survival of replanted species,’’ wrote Lauren Hannah, Glenn Aguilar and Dan Blanchon in the journal Climate.
The trio mapped the likely spread of the daisy under various climate change scenarios and using several different models.
They found the South Island becomes much more suitable for the daisy as the Earth warms, while the North Island becomes less suitable.
One model, for example, assumed the Earth will warm by about 2 degrees Celsius by 2100. As that happens, it will likely spread down both coasts of the South Island, probably reaching it’s broadest extent about 2050. The trio called it an ‘‘obvious range expansion’’ to the south.
Another model assumed the Earth will warm by about 4C by 2100. If that happens, the daisy will have abandoned much of the North Island, except around Wellington, by about 2050. Similar patterns were evident for the year 2070.
The research tells officials in the South Island that the daisy is coming, so they can prepare and find suitable approaches to minimising the species’ impact, said Aguilar in an interview.
Conversely, officials in Auckland and the northern reaches of the North Island could perhaps devote scarce resources to other species.
Auckland Council funded the work, which was largely carried out by Hannah, an undergraduate at the time. She was also first author on the journal article, a rare honour.
Aguilar and colleagues got broadly similar results when they mapped suitability for the Chinese windmill palm under various climate change scenarios.
Using similar techniques, they showed that climate change will make life better for the Queensland fruit fly and the brown marmorated stink bug, both considered major threats to New Zealand and not yet established here.
Stray cats will also enjoy climate change, although they will still prefer the North Island over the South as the Earth warms, his research showed.
Aguilar expected the Earth will warm by between 2C and 4C by the end of the century, meaning the targets of the Paris Agreement will not be achieved.