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 magnificent. 

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 most.

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 then.

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 wallabies.

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.

 VERTIBRATE PESTS

(Operations Manual Section 10, May 1996)

The vertibrate pests identified are:

Possum     Wallaby (4 species)     Rat (ship)     Stoat     Feral cat     Mouse*

Mynah     Magpie                                       

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.

 GENERAL ANIMAL PEST STRATEGY

 Possum.

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.

 Wallaby.

Successful establishment 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:

(a) 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.

 Rat.

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.

 Stoat.

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 cat.

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 toxins.

The aim is to target feral cats, eradicate them, and explore community acceptable restrictions to prevent reinfestation.

 Mouse.

Established mouse 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. 

 Magpie and Mynah.

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 native birds.  

The aim is to monitor numbers and to encourage control and eradication by individuals.

WEEDS

(Operations Manual Section 11, May 1996)

THE GENERAL WEED SITUATION

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.  

WEED SPECIES

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 biodiversity.

Some of the other main weeds (local site infestations) and their common names are:

Acacia species                                                

Tasmanian Blackwood    Example. In sector between Lookout, Cemetery, Schoolhouse Bay Road.                                     

Golden Wattle   Example. Schoolhouse Bay Valley. Little Vivian Bay to Vivian Bay.

Thistle 

Californian  Example. Common on any cleared or open land. Power line routes.     

Wild iris “Stinking iris”

Both yellow and rust coloured flowers Example. Clumps on sides of tracks. Open areas under pohutukawa.

Privet species 

Example. Large numbers centred on flanks of Mansion House Valley. Isolated  tree privets elsewhere, where birds have spread seeds. (Birds feed on fruit.)                                                                                

Pampus

Example. Widespread with some dense infestations. Infestations on East Coast cliff areas.

A serious and invasive weed problem.

Acmena

Example. Inland from Pembles Bay. (Birds feed on fruit.) 

Climbing asparagus

Example. Not common but dispersed in occasional clumps. (Birds feed on fruit.)

Palm species

Example. Occasional seedlings from sources at Smelting House Bay and Sunny Bay.

Tecoma

Example. Various remnant hedge plantings at Swansea Bay, Kidds Cove and other dwelling sites.

Gorse  

Example. Remnants (declining) inland from Smugglers Cove with occasional seedlings quite widespread 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.)

Smilax

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.)

London pride  

Example. Patches of dense ground cover in old settled areas, such as Newtons to Cargaud Point.                                      

Arum lilly

Example. Dense infestations in many valleys and seedlings common. (not presently able to thrive in drier locations.

Wild ginger                 

Example. Infestations between North Cove and Vivian Bay, and at Sunny Bay.

Wooly Nightshade                                                      

Example. North Cove, Starboard Arm Valley. Various small infestations. (Birds feed on fruit.)                                                          

Caster Oil Plant

Example. Sharp Point inland.  Moores Bay.

Agapanthus

Example. Large areas in vicinity of Grey’s Lookout and in Swansea/Mt Taylor Valley. (locally serious)

Cape honey flower     

Example. Patches from close to beach margin to elevated dry positions including Pah Farm and Swansea Bay.       

Ivy                                                       

Example. North Cove-Vivian Bay Isthmus. (Birds feed on fruit.)                 

Nicotiana - Tobacco

Example. Common and widely dispersed in dense patches with full sun to partially shaded.

Cassia                         

Example. Common and widely dispersed. Coppermine valley east, South Cove, Pah Farm,

Hokimai Bay, North Cove.

Ragwort                                              

Example. Widely dispersed in semi-open areas such as open canopy kanuka and tracksides.

Pinus species

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.  

Boneseed                                                       

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.

Hydrangea

Example. Planted, but forms dense and spreading cover  eg. Cable Bay.

Lantana

Example. Mansion House Bay Valley. (Birds feed on fruit.)

Ink Weed