Invasive mammal impacts on forest tree weta

Wendy Ruscoe

Wildlife Ecology & Epidemiology Team

Landcare Research, Lincoln

Weta

Tree weta

Ground weta Giant weta

Tusked weta Cave weta

>70 species

Weta in the environment

• Consumers of invertebrates, fruit.

• Seed dispersal

• Food: tuatara, kiwi, weka, saddleback and laughing owl are native predators

• 16 species considered ‘threatened’

Threats

1. Predation by invasive mammals(rodents, cats, mustelids, hedgehogs)

2. Habitat loss.

3. Herbivory (invasive herbivores) removing/changing habitat structure.

Rats – the primary culprit!?• Some species confined to rat-free islands.• Effects of rat eradications

Rat diet

• Predominantly seeds, fruit, and invertebrates (up to 50% of diet).

• Tree weta and stick insects predominate.

Sweetapple & Nugent 2007, P. Sweetapple pers. comm.

Study: Impacts of Rats on Weta

• Part of a larger experiment

• Removed rats (and other pests) and monitored tree weta.

• Monitored rats

1080 Aerial Control

1080 Aerial + Diphacinone

(2yrs)

Stoat Trapping

No Treatment

x 2

Experimental Setup–manipulating rat populations.

900 ha900 ha

900 ha 900 ha

1080 Aerial Control

1080 Aerial + Diphacinone (2yrs)

Stoat Trapping

No Treatment

Effect on rat populations – 3 years Post Treatment:

x 2

• Stoat Trapping and No Treatment: ‘average’ rat numbers in both areas.

• 1080 operation: 1 year of really Low rats and then 1-2 years of High rat numbers.

• 1080+continuous diphacinone poisoning:

Low rat numbers for 3 years.

?

Treatment effect on rat populations – 3 years Post Treatment:

Changes in tree weta occupancyNon treatment sites – average rat numbers

Treatments

Changes in tree weta occupancy1080 + diphacinone – Low rat numbers

Treatments

Treatments

Changes in tree weta occupancy1080 only – Low then high rat numbers

Effects of Rat Control (1080 + ongoing control)

- Higher numbers of tree weta maintained

Effects of a 1080 operation on weta:

- Higher numbers of tree weta in the mid-term <3 years. ‘Window of relieve from predation’

**** No negative impact within 3 years ***But after that???

Biodiversity ‘outcome’ monitoring

• Provide a low-tech alternative to bird monitoring to show benefits of pest control.

• Tree weta respond reasonably quickly to pest control operations that impact rats.

• They are easy to monitor and can engage the general public.

Thanks

• Funding provided by Foundation for Research, Science & Technology

• Department of Conservation

• Field staff.