THE ECOLOGICAL PROBLEMS
AND NEED FOR THE PROJECT
(Operations Manual Section 7)
The discovery of sulphide
copper ore on Kawau Island in 1844 led to the establishment of a
successful underground mine and a small village. After the mine closed
the Island was purchased in 1862 by former Governor of New Zealand
Sir George Grey.
By building onto the
coppermine manager’s house, Grey produced the famous Mansion House
which is without doubt one of the Auckland Region’s best known
heritage buildings. In the Mansion House valley setting it is
Grey also introduced many
of exotic plants and animals to Kawau Island, including pinus species,
and Australian marsupials the possum and five species of wallaby.
The pines have spread
widely from Grey’s planting. Seeds transported from west to east by
the prevailing wind have now spread wilding pines all the way to the
east coast. The pine trees have become Kawau Island’s greatest weed
species although there are many serious weeds on the island.
The possums had almost
destroyed Kawau Island’s pohutukawa trees
when the community successfully intervened in 1985. The intervention
following a period of original work studying damage and possum habits
before this was common knowledge.
There was nobody with the
knowledge at the time and the community did all the work.
The four remaining
species of wallaby destroy all emerging indigenous tree seedlings with
very few exceptions, the most notable exception being kanuka. For this
reason there is now no regeneration of native trees on Kawau, the
present mature specimens being essentially the last generation for
Large areas of the Island
have an almost monoculture cover of
kanuka. These represent areas that were once in pasture and
are the result of wallabies having driven farmers off the land by
1970, and the pasture reverting to bush under the influence of the
intense wallaby browsing. Much of this land had reverted long before
1970 and had already been cut over for firewood more than once by
There is no understorey
below the canopy, because the wallabies graze almost all vegetation,
and it is common for the ground to be bare except for invasion in
patches of locally significant weed species unpalatable to the
As well as disrupting the
cycle of organic matter and plant nutrients on the land the absence of
a normal forest understorey leads to a short residence time for
stormwater and a fast run-off during heavy rain, stripping the
skeletal soils and carrying significant
sedimentation to the sea, where
it smothers marine organisms. The sediments also compromise fresh
water and habitat quality in the island’s small streams and wetlands.
The loss of diversity of
flora has destroyed habitat and disrupted continuity of food supply
for birds through the seasons, resulting in numbers plummeting.
Browsing vertebrates including the animal pests starve in dry years
due to insufficient vegetation to sustain their numbers.
The present mature native
trees are the last generation. They produce seed but browsing
wallabies depend on the seedlings for their food supply.
Possums destroy mature trees and when
they fall there are no young trees to replace them because wallabies
have eaten all the seedlings.
Kanuka usually functions
as a nursery crop, protecting a diverse range of canopy species as
they grow, until eventually the canopy species grow above the kanuka
which then dies. On Kawau the canopy species germinate but are eaten
by wallabies, leaving only the kanuka. Once the kanuka canopy closes
there is insufficient light reaching
the ground for most seedlings to germinate and grow.
(Operations Manual Section 10, May 1996)
The vertibrate pests
Feral cat Mouse*
The marsupials possum
and wallaby were introduced to the Island about 1868-69 from Australia
by Sir George Grey.
The ship rat is the
result of European settlement. Kawau has a history of many centuries
of human activity, with the Pacific rat kiore once present but being
displaced progressively by the more aggressive ship rat from when the
present settlement commenced in 1840.
The stoat arrived in
about 1988/89 by sea, being first seen in the North of the Island
(Vivian Bay) and spreading to all locations within 5 years.
The feral cat is
particularly common in the vicinity of the Schoolhouse Bay ridge but
is found throughout the Island, even in areas remote from settlement.
The mouse is very
uncommon but has been seen rarely in Schoolhouse Bay only. The mouse
is believed to arrive with equipment and supplies but does not
establish due to the presence of aggressive predators. Mice have never
been a problem in dwellings on Kawau.
Magpie and Mynah numbers
are not high but they are pests noted for the record.
ANIMAL PEST STRATEGY
The possum destroys
pohutukawa, ponga, and other species on Kawau.
The first priority, to
apply the results of investigations into the rapid decline of Kawau
Island pohutukawa has been completed using Timms Kill Traps,
distributed in the community in 1986 to cost effectively save the
pohutukawa. This is believed to be the first large scale use of these
traps in New Zealand.
The strategy is to
encourage the community to be vigilant and to continue to use traps
when possum signs are found. This is to hold the gains at minimum cost
with minimum damage with possum to be eradicated in a later phase.
of possum control enabled the next phase of raising awareness of the
significance of wallaby damage and a survey last year (1995) to
mandate eradication of feral wallaby.
The survey yielded 98%
support from respondents for eradication.
Wallaby habits have been
studied and a basic plan for eradication has been developed.
Wallaby eradication is
the top priority on the critical path.
Impediments to progress
on the critical path to be resolved are:
Seek a commitment to action from the Department of Conservation to
complete the undertaking to build the escape-proof wallaby fence, or
alternatively to abandon keeping wallabies on the Historic Reserve.
Offer to assist with
fundraising for a proper facility “wallaby park” if wallabies are
to be retained.
(b) Seek a phase-out of
wallaby live capture for export with support from the main landowners,
to remove this incompatible activity so that eradication technology
can be further developed and applied on the private land.
(c) Demonstrate locally
that wallaby can be eradicated by a process of “attrition” using bait
stations and a first generation anticoagulant, and firearms in the
presence of weka. Demonstrate an acceptable bi-catch level of weka
without threatening the overall weka population.
(d) Complete development
operations using Type 1 (chronic) and Type 2 (acute) toxic baits,
including solving problems relating to high humidity deterioration of
cereal based baits.
(e) Seek comment from
the RSPCA concerning the acceptability of the toxic baits of interest
from an animal welfare viewpoint.
(f) Complete development
of novel cholecalciferol and novel cyanide based systems.
(g) Continue to raise
awareness among landowners of wallaby damage and grow support and
funding for total eradication.
Ship Rats on Kawau are
very common and their pest significance is reinforced by observations
of them running up kanuka and other trees to raid birds nests and to
destroy fruit and seeds.
The priority is to first
reduce wallaby numbers without targeting rats, which may be useful as
a more veracious scavenger than the weka, thus reducing weka exposure
to possible secondary poisoning. During wallaby eradication rats can
be expected to help remove spills on the ground from active bait
stations so that weka do not eat them as well as scavenge toxic
components of carcasses and thereby reducing weka exposure.
Rats can be effectively
controlled and could probably be eradicated with second generation
anticoagulants or novel cholecalciferol baits.
An understanding and
assessment of the feasibility of eradication and avoiding
reinfestation will be developed. Eradication of ship rats could only
be justified if the strategy included long term measures to prevent
reinvasion. This would be quite difficult but not impossible.
Stoats have infested
Kawau Island recently and surprisingly quickly, and sighting records
now include a progressive spread to Sandy Bay, Swansea Bay,
Schoolhouse Bay and on tracks in the Historic Reserve. As an
aggressive predator they need to be targeted as part of the integrated
pest management and eradication program.
The aim is to raise
awareness of the presence of stoats on Kawau and to initially
encourage control by individuals, and to assess the possibility of
secondary poisoning of stoats using second generation anticoagulants
concurrent with rat control or eradication.
Feral cats identified
are an obvious and serious predator pest. Some have been effectively
eradicated by targeting areas where they are located, using
conventional cat food with addition of cholecalciferol and pival as
The aim is to target
feral cats, eradicate them, and explore community acceptable
restrictions to prevent reinfestation.
colonies are unknown on Kawau, and this is interpreted as due to their
position at the bottom of the pest hierarchy. The aim is to repeat a
mouse survey at intervals to monitor for any change. Some cats have
escaped from cruising vessels and gone wild.
Small groups of these
birds exist. In some areas they have existed for years with no
apparent change in numbers. They have been seen to chase and attack
The aim is to monitor
numbers and to encourage control and eradication by individuals.
(Operations Manual Section 11, May 1996)
THE GENERAL WEED
The marsupials possum and
wallaby depend upon almost every species of native plant on Kawau
Island as a food supply. Browsing marsupial numbers are dependent upon
plant food availability and seedlings are destroyed as they emerge.
Mature native plants are therefore not replaced and the seed bank is
declining. Rats eat a proportion of the seeds produced, further
reducing seedling numbers. The result is areas of bare ground.
The outcome of this
scenario is enhanced colonisation of the bare ground space by plant
species unpalatable to possum and wallaby, which without competition
for space become invasive weeds.
A further transient event
will occur when browsing animals are eradicated, because weed species
presently suppressed by browsing animals will then increase. Many of
these will not become a serious problem and a general suppression tool
will be to plant coprosma species to occupy potential weed sites as
soon as wallaby browsing is sufficiently reduced to enable survival.
Birds will aid dispersal by feeding on berries.
The most significant
weeds on Kawau Island at present are the pinus species, mostly
P radiata. Although the
transition would take several hundred years, with no other changes
pinus species would eventually invade the whole island if left
unmanaged, crowding out and replacing the majority of native plant
species remaining with a great loss of both plant and animal
Some of the other main
weeds (local site infestations) and their common names are:
Example. In sector between Lookout, Cemetery,
Schoolhouse Bay Road.
Bay Valley. Little Vivian Bay to Vivian Bay.
Example. Common on any cleared or open land. Power line routes.
Wild iris “Stinking iris”
Both yellow and rust
Example. Clumps on sides of tracks. Open areas under pohutukawa.
Example. Large numbers
centred on flanks of Mansion House Valley. Isolated tree privets
elsewhere, where birds have spread seeds. (Birds feed on
Example. Widespread with
some dense infestations. Infestations on East Coast cliff areas.
A serious and invasive
Example. Inland from
Pembles Bay. (Birds feed on fruit.)
Example. Not common but
dispersed in occasional clumps. (Birds feed on fruit.)
seedlings from sources at Smelting House Bay and Sunny Bay.
Example. Various remnant
hedge plantings at Swansea Bay, Kidds Cove and other dwelling sites.
(declining) inland from Smugglers Cove with occasional seedlings quite
but eaten by wallabies. Most seedlings do not establish. Viable seeds
found in seed rain studies very remote from present remnants. (Will
not be a serious problem.)
Example. Well dispersed
infestations under higher light canopy and at margins and power line
clearings. Major (and one of the original) infestation at Vivian Bay.
(Birds feed on fruit.)
Example. Patches of dense
ground cover in old settled areas, such as Newtons to Cargaud Point.
infestations in many valleys and seedlings common. (not presently able
to thrive in drier locations.
between North Cove and Vivian Bay, and at Sunny Bay.
Example. North Cove,
Starboard Arm Valley. Various
small infestations. (Birds feed on fruit.)
Caster Oil Plant
Example. Sharp Point
inland. Moores Bay.
Example. Large areas in
vicinity of Grey’s Lookout and in Swansea/Mt Taylor Valley. (locally
Cape honey flower
Example. Patches from
close to beach margin to elevated dry positions including Pah Farm and
Cove-Vivian Bay Isthmus. (Birds feed on fruit.)
Nicotiana - Tobacco
Example. Common and
widely dispersed in dense patches with full sun to partially shaded.
Example. Common and
east, South Cove, Pah Farm,
Hokimai Bay, North Cove.
Example. Widely dispersed
in semi-open areas such as open canopy kanuka and tracksides.
Example. Obvious spread
east from Mansion House Bay Valley. A few seedlings now appear North
of Bon Accord (with reduced wallaby numbers) and are pulled when seen.
Tuber ladder fern
Example. Fairly common at
older dwelling sites.
Example. Widely dispersed
distribution from foreshore to exposed clifftop situations.
Pea Flower Shrub
Common in open dry areas,
Often found in similar location to Boneseed.
Elephant Ear Lilly
Example. Patches in cool
shady places associated with dwellings/occupation.
Example. Planted, but
forms dense and spreading cover eg. Cable Bay.
Example. Mansion House
Bay Valley. (Birds feed on fruit.)
Example. Patches and
individual plants in cleared or open areas. (Birds feed on fruit.)