Rangitoto Island and Motutapu Weed Control Contract

Rangitoto and Motutapu Islands are two of the most ecologically significant islands in the inner Hauraki Gulf.  The islands were declared pest-free in 2011 and pest plant control is a priority for the Department of Conservation. 

Te Ngahere has been contracted since 2005 to provide environmental weed control services to:

  • reduce the number of Class One weed species on Rangitoto Island (2330 ha), Motutapu Buffer Zone (20 ha) and some additional work across the rest of Motutapu Island;
  • identify new weed sites; and
  • control Class Two weed species to a sustainable level.

To undertake this contract, our field staff are based on Motutapu Island, usually for one week periods.  The work is undertaken in remote conditions in teams of up to eight staff. 

Within the available hours, work is prioritised based on output of previous work.  For example, frequency of checks to control specific weed infestation points is determined by the results of previous searches (whether weed incursions are still active) and the priority of the species. 

To efficiently target pest plants, two main methods are used:

  • Locating and searching within a point specific radius of recorded infestations.Point information is recorded and entered into our database, including descriptions of the extent of the site and coverage for each size class (seedling, juvenile and mature); and
  • For evergreen buckthorn (Rhamnus alaternus), the contract area has been broken into blocks.Areas of high density or outliers are identified as priorities for control (through past control and field surveys of stem density to produce hot spot GIS mapping).The work is undertaken in teams, grid searching with GPS tracklogs to demonstrate the area covered.

The contract manager conducts regular checks of the contract work area, including isolated sites, to ensure the work is carried out the appropriate standard outlined in the contract register.

Several innovations have been implemented over the period of work on Rangitoto to improve efficiency and quality of work notably:

  • sharing of point data and providing specialist GIS support to produce hot spot mapping of evergreen buckthorn stem density to target weed control more efficiently;
  • managing weed infestations by using a best practice spatial database and GIS analysis to support decision making and optimise efficiency in the field;
  • use of ourPoint Database as a method to manage what sites should be prioritised for weed control. The database allows for tracking of different age class and coverage of weed species and whether a species is extant depending on time since last positive find. This contract was central to the development for this best practice database; and
  • trialling the use of pre-emergent herbicides at sites with panic veldt grass.This species produces seed throughout the year and by reducing germination the frequency of site checks and therefore cost can be reduced.

Hot spot mapping of evergreen buckthorn stem density on Rangitoto Island

Hot spot mapping of evergreen buckthorn stem density on Rangitoto Island