(chris) 21. Alien plant invasions continued

Question 1

Criteria for choosing agents if biological control?

Answer 1

Must be a specialist

(must be able to hit the plant where it hurts)

• Host specificity
• Potential to damage the target under NZ conditions
• Complementarity of damage with other high-priority potential agents
• Susceptibility to predation in New Zealand
• Climatic match to New Zealand

Question 2

Case study

Biological control of Tradescantia

-Flowers, but not known to seed in New Zealand

(fragment/reproduction by cloneing only)

-Solely dependent on vegetative reproduction here

(Limited genetic diversity, potentially easier to eradicate)

-So low diversity of nz populations might limit Tradescantias ability to evolve ways of defending itself against natural enemies

Question 3

Buddleja davidii - biological control effort 2006

• B. davidii a threat to native plants and birds in riverine habitats.
• Leaf weevil from Asia introduced 2006

Answer 3

• Effective but very slow to disperse (300m in 2.5 yeara)
• Many introductions needed throughout nz to make successful

Question 4

Alien conifers affect much bigger areas

• At least ten conifer species are invasive in NZ, all having spread from plantation origins
• Now occupy c. 1.8 million hectares
• Spreading at c. 5% per year

Answer 4

Worst offender is: Pinus contorta

-Most aggressive species

(youngest coning age 6-7 years)

-Widespread invasion of tussock grasslands above the treeline

(Central North Island Volcanoes)

(Eastern South Island high county)

-In 2001 declared an unwanted organism under the biosecurity Act 1993:

(Illegal to propagate, breed, distribute or sell P. contorta in New Zealand)

Question 5

Pinus contorta is a transformer weed in NZ

(will turn apline grassland into forest)

• Widely planted at high altitudes in the 1970’s, to control soil erosion.
• P. contorta and some other alien conifers are more cold-resistant than any native trees
• P. contorta resists temps as low as -70*
• Most cold resiliant native tree Halocarpus bidwilli -25oc*

Question 6

Why is the NZ alpine zone so invasible?

Answer 6

NZ lacks trees that are as cold tollerant to those in the Northern Hemisphere.

• So some alien conifers could potentially form new forests above present native timberlines
• Cold high-mountain environments relatively recent in New Zealand

(Southern Alps only got high during Pilocene)

Question 7

NZ a isolated archipelago

• Evolved from very cold and harsh > modern day temperate climate
• Because so isolated limited scope for immigration of plants that can take advantage of a new habitat and then destroyed during climatic flucuations
• Local flora has had to evolve to cope with rapidly changing circumstances

Therefore…

Answer 7

NZ flora selected against selective specialisation

(extreme cold tolerance specialisation has a cost as it could be incompatible with shade cover for example)

-When warm conditions return the cold specialists die out and visa versa

NZ FLORA IS GENERALIST - A RESPONSE OF ISOLATED FLORA TO RAPID CLIMATE/ENVIRONMENT FLUCTUATIONS

Question 8

Controlling invasive conifers strongly opposed by forestry industry, why?

Answer 8

-Risk of control agents attacking other pines, eps P. radiata & Douglas fir

(back bone of timber export economy)

-Suggested that eed predators would be a good idea as they would just kill the seeds and not the timber. Won’t happen as forestry lobby still worried about damage to seed orchards and possible roll of beetles as vectors of pathogens.

Question 9

Alien connifer invasions are one of the most difficult environmental problems facing NZ

Why?

Answer 9

• Biological control problematic
• Aerial application of herbicides feasible only for dense stands

(Must be applied manually to scattered individual plants to minimize by- kill (labour intensive)

-Formidable difficulties in finding all seedlings

A 12 yr old pinus contorta produces 12,000 viable seeds per year

Currently winning in North Island, but not in the south.