Protecting Aotea from High-Risk Weeds

NIKLAS ERIKSON (Senior Conservation Advisor, Aotea, Auckland Council)

Aotea / Great Barrier Island is home to many native plant species, some of which are endemic to Aotea. There are also many exotic species present on the motu, the bulk of these exotic plants are pests which often get overlooked as a threat to biodiversity and our natural history.  Many of us have seen firsthand the impacts pest plants can have on New Zealand’s natural environment. Some of my own experiences have shown me the devastating effects of wilding conifers around Lake Tekapo and Twizel, evergreen buckthorn outcompeting coastal vegetation on the inner Gulf Islands, and some of the weediest esplanade reserves South Auckland has to offer, drowning in a sea of climbing asparagus and kahili ginger(1). 

One of the key reasons I took on the challenge of Senior Conservation Advisor for Auckland Council, was the opportunity to work towards eradicating and excluding a suite of pest plants from Aotea, which compared to the mainland, is feasible. 

Historically, Department of Conservation, Auckland Regional Council and Auckland Council from 2010, has had a strong foothold on Low Incidence pest plants on Aotea. An article ‘Weeds Never Sleep’ by George Wilson in Environmental News in 2008(2), summarises efforts that have been made in the past, to collectively stop the ‘weed explosion’ of certain pest plants like kahili ginger, which if left uncontrolled would have a devastating effect on Aotea’s biodiversity, becoming widespread and limiting the feasibility of eradication.

Now, we are lucky to have the support of the Natural Environment Targeted Rate, which has enabled more pest plants to be eradicated, plus we are able to expand our delimiting efforts to provide more certainty. This initiative has also boosted on-island environmental procurement, creating more roles for locals in the pest plant space. Aotea is Auckland’s biodiversity jewel that needs ongoing protection. There aren’t many places in the world where you can successfully say that you have eradicated certain pest plants, especially extremely invasive ones like kahili ginger and climbing asparagus. We hope to make history on Aotea, not fall under the title of “The Weediest City in the World”, which Auckland currently is(1). Waiheke is the second weediest island in the world, dethroned by Hawaii. This is how we will do it… 

Changes to the regional pest management plan

In 2018 Auckland Council adopted a Natural Environment Targeted Rate (NETR) and a new Regional Pest Management Plan (RPMP), which became operational in 2020. Through the RPMP process, the number of pest plant species under Low Incidence Weed Management on Aotea (previously known as Total Control Species) was increased. These species were added based on previous Aotea Pest Plant surveillance data. 

Species-specific management plans have been developed for our pest plants that factor in the potential distance that seed/propagules can be carried or spread from a parent plant thereby necessitating the establishment of search buffers. This has meant that for most of our weed species, a larger area needs to be surveyed to give confidence that all Low Incidence pest plants are encountered and eliminated. Under the RPMP there are three main categories that make up the Low Incidence Pest Plant programme. These are as follows: 

Exclusion Pest Plants: These pests are not known to be present on the Aotea / Great Barrier Island group but have the potential to establish widely and cause adverse effects to the Island’s ecological, economic, and cultural values. Early intervention to manage the ways pests can travel and respond in the event of an incursion is a cost-effective approach to prevent their establishment on Aotea. Examples include, evergreen buckthorn (Rhamnus alaternus), an evergreen shrub, which can grow up to 5 meters high. It is a super coastal invader which forms dense stands, preventing the recruitment of native vegetation. Rhamnus has been an ongoing issue for many of the inner Gulf Islands. If this plant becomes established on Aotea it would pose a significant threat to the native biodiversity on the Island. 

Eradication Pest Plants: These pest plants are present in low numbers within the Aotea | Great Barrier Island group and eradicating them is feasible. Early intervention to prevent further spread by eliminating current small infestations is a cost-effective approach to protect Aotea from these plant pests. A good example of an Aotea eradication species is climbing asparagus. There are currently three infestations on Aotea, and all are being actively managed. On the mainland climbing asparagus is the worst weed in Auckland, and is continuing to spread by birds to some of our most pristine pockets of bush. 

Progressive Containment Pest Plants: These pest plants are present in low numbers within the Aotea | Great Barrier Island group but have the potential to be highly damaging if they become widespread. Eradication may not be feasible in the short term but progressive containment of these pests by reducing their distribution over time is a cost-effective approach to prevent further spread and consequential impact on the Aotea's biodiversity. An example of a progressive containment species on Aotea is royal fern (Osmunda regalis). Currently, there are only four infestations, which have been progressively contained within wetlands at Claris & Whangapoua. This plant is very tricky to manage because of where it is, wetland, and because it spreads by wind dispersed spores.

Auckland Council is responsible for delivering these objectives across all land tenures. For species where these objectives are no longer feasible, pest plant species are generally classified as Sustained Control Species. The intention of the sustained control programme is to reduce the spread of pest plants and their impacts to protect the values of Aotea. Species in this category include pampas and coastal banksia, which are controlled in high ecological areas as part of Site Led Control programmes. 

What’s happening on Aotea now?

The map above illustrates how current weed plants are being managed on Aotea based on the Low Incidence Pest Plant (LIPP) programme.  By dividing weed infestations into these different categories it enables us to balance our priorities, as investing in one species will take away resource from controlling another. As we are in the early stages of the programme, only a few species have been stood down and are now part of our sustained control programme. 

Also, we are now getting more consistent records of pest plant finds since the development of our RURU GIS system. Team members can capture more comprehensive data, which before 2020 was all captured on GPS units and exported into GPX and Shape files. Going forward we hope to see a downward trend of pest plant counts and reduction of mature plants. 

Delimitation is key for successful eradication

Controlling pest plants on the spot can be challenging but ensuring that there are not any pest plants further afield brings another key element to the process. For bird and wind dispersed plants we have to delimit 500 m from the outermost known plants. Examples of bird dispersed species include kahili ginger and climbing asparagus with their very palatable viable berries. Examples of wind dispersed species include climbing gloxinia and moth plant with its ‘hand grenade’ like seed pods. Human mediated dispersal requires forward and back tracing, informed by a literature review of the highest risk goods. On Aotea, we have seen a lot of movement of the invasive water pennywort (Hydrocotyle umbellate), which is classified under the LIPP as an Aotea eradication pest plant, through earthworks and mowing. For freshwater pest plants we trace upstream from the origin of the waterbody and downstream to the saltwater/intertidal zone where the plant can still be viable. We also take into account flooding zones and side catchments.  

We continue to create a buffer of 500m from the outside of any further finds, until it is decided whether the infestation is too far spread or there are budget constraints. These delimiting ‘rules’ are based on:

• A literature review of potential spread distances based on different mechanisms of dispersal 

• Discussions with a TAG

• Literature review for each plant species. 

• Importantly, these are not hard and fast rules and that discretion is required depending on site-specific factors and budget constraints.

Aotea approach for controlling, surveying, and recording data 

Contractors grid-search predefined weed control areas that are linked to infestation points of the target species. These are seen via an App called ‘Fieldmaps’ on their work phones. The team use best practice methods for control.  The majority of the control is done manually using spades/trowels to ensure all viable parts of the plant are removed and disposed of appropriately. Weed areas and infestation points are mapped to the Auckland Council ArcGIS system “Ruru” and can be viewed by the contractor once they are assigned by the project manager. While working the weed control areas all field team members must hold a GPS unit or other portable device that is able to record tracks while grid searching. Track logs are reviewed to ensure that the consistent delimitation is achieved, and we are meeting the delimit rules for each species.

Communication strategy for Aotea High Risk Weeds

We have worked to increase community awareness of Low Incidence Pest Plants and have been lucky to have the creative and pest plant passionate mind of Sarah Matthews from Black Dog Conservation Ltd behind it. The initiative, Aotea High Risk Weeds, raises awareness of these pest plants and aids passive surveillance to be part of this new initiative.  Residents and visitors to the Island are encouraged to join the iNaturalist project ‘Aotea High Risk Weeds’(3), which is a project that lists and links all of the plants we need to look out for.

The project is monitored, and new detections are built into the programme. Since the programme was established in 2023 there have been 37 observations reported which have resulted in control and further surveying carried out.  We hope to have more members join and utilise this awesome resource, available to all, so please sign up. 

Feasibility of success of Aotea 

One of the key reasons to the success of the Aotea Low Incidence Pest Plant programme is the geographic isolation of Aotea, which reduces the likelihood of mainland species colonising or re-colonising the Island(4). With limited pathways for incursion, controlling prescribed plant pest species becomes more manageable compared to on the mainland and to islands closer in proximity to the mainland. However, we still need to ensure people aren’t bringing in pest plants to Aotea. Recently, we have seen seedlings of tree privet, which is in the Eradication category, coming up in newly landscaped gardens from contaminated soil from the mainland. We cannot eradicate pest plant infestations we don’t know about, so community buy-in and communication is extremely important.

Surveillance 

In the early 2000’s Auckland Regional Council carried out a Pest Plant Survey from which they established a list of priority weeds they would manage before they became too widespread to control. Surveillance is key to Biosecurity Management. In addition to managing active sites, we continue to survey the Island on a rotational basis, and as the Aotea RPMP has evolved so has the wider scope of species under survey. Importantly, we rely on the support and good will of landowners to allow us access to their properties to carry out weed surveys. As part of those visits, we are always keen to provide recommendations on other species to keep on top of and provide the know-how, tools and resources the landowners can use themselves. 

Local knowledge and support 

Being able to retain local knowledge of sites, and keeping the option of procurement open for Aotea residents who are passionate about controlling pest plants, helps keep the momentum rolling for the programme. The long term support of landowners allows teams to commit to ongoing work on their land. This is essential for the success of the programme, and we are very fortunate for the collective and ongoing support we receive. 

To conclude, my take home message to all GBI Environmental News readers is to join the Aotea High Risk Weeds project on iNaturalist, and help grow that data base. Review the species lists and log any potential weedy finds or report them directly to Auckland Council. Who knows, you might be the first to find a new pest plant on Aotea that we don’t yet know about (here’s hoping not!). Ongoing surveillance is vital to the programme, so the more pest plants eyes and ears we have on the motu the better. Let’s all work together to eradicate all High-Risk Weeds from Aotea! 

References

1. Philip Hulme: “New Zealand is one of the weediest places on the planet. RNZ, 6th May 2018: https://www.rnz.co.nz/national/programmes/sunday/audio/2018643664/philip-hulme-nz-is-one-of-the-weediest-places-on-earth#:~:text=Introduced%2520plant%2520species%2520are%2520already,says%2520bioprotection%2520specialist%2520Philip%2520Hulme

2. George Wilson (2008). Weeds never sleep. Aotea Great Barrier Environmental News. Issue 15: https://www.gbiet.org/en15-weeds-never-sleep 

3. iNaturalist. Aotea high risk weeds: https://www.inaturalist.org/projects/low-incidence-pest-plants-of-aotea-great-barrier 

4. RNZ Mark Leishman interviews Niklas Erikson. New project launched to battle weeds. Afternoons with Jessie. 7th June 2024: https://www.rnz.co.nz/national/programmes/afternoons/audio/2018941713/new-project-launched-to-battle-weeds