Oruawharo Bay Wetland Restoration Project

LOTTE McINTYRE (Project Coordinator, Medlands, Aotea Great Barrier Island)

In late 2018 a small group of Oruawharo/Medlands Bay residents supported by the Ecology Vision formed a community conservation group ‘Oruawharo Medlands Ecovision (OME)’ with the goal of restoring the ecological health and biodiversity of their bay. With financial support from the Local Board and The Department of Conservation they launched an ambitious restoration programme of removing exotic invasive plants, suppressing predators by trapping, and restoring habitat for rare birds such as matuku-hūrepo/Australasian Bittern, mātātā/fern bird, pāteke/brown teal and spotless crake, as well as a range of reptiles and invertebrates. They also plan to improve community access to the wetland with the establishment of paths and boardwalks that will have information boards on the flora and fauna in this important habitat.  

Introduction

Wetlands are often referred to as nature's kidneys due to their natural filtration system and their capacity to mitigate flooding and erosion. Wetlands are a vital link between land and water, shaped by factors such as the underlying geology, soil type and climate, as well as salinity, velocity and permanence or transience of the water. 

In the Auckland region, 97% of wetlands have been destroyed due to drainage and development, so protecting and restoring what we have left is now widely seen as a priority; not only in the Auckland region, but nationwide. Wetlands are home to many native species, both plant and animal, some of which are now endangered due to this huge loss of habitat. These areas act as both nurseries and habitat for native fish and eels, as well as many birds and plants adapted to these special conditions. 

A key focus of our restoration effort in Oruawharo Bay is on the DoC managed areas of ‘The Medlands Wildlife Management Reserve’ at the northern end of the bay behind the Medlands settlement, and the estuary and margins of Waitematuku stream at the southern end. Although this area best fits the wetland categorisation of swamp there is also a strong intertidal element more characteristic of estuaries. These features strongly influence the types of plants that can grow in this reserve. We are also working on the restoration of the estuary and margins of Oruawharo stream at the southern end of Medlands. The Oruawharo estuary area was known to Māori as Waitematuku – meaning the estuary of the bittern, and this bird has indeed been seen there since we started the project. 

This swampland has been heavily modified since the arrival of Māori on Aotea in the 1300s. Remnants of vegetable gardens associated with early Māori settlements have been found in the bay(1). Installation of drains were part of these original farming developments. With the arrival of European farmers on the island, more drainage was installed to improve the land for grazing livestock. In a photo of the wetland area looking toward Sugar Loaf, taken by Henry Winkelmann ca. 1892, it is evident that the natural course of the Oruawharo/Waitematuku stream is rather different from what we see today, where the creek has been redirected south to make way for more usable land for farming and development (2).

Rodney Ngāwaka, Aotea’s local mana whenua storyteller, will tell you that the wetland was once part of a much larger ecosystem, which, before it was drained and used for farming(3), would have spanned from Kaitoke creek in the north to Waitematuku stream at the southern end of Medlands. He also highlights the importance of these wetlands as nurseries for many different species of fish, which were an important source of food for Māori. 

Current state of the wetland

Along with the drainage and development of this bay for farming, came the introduction of many exotic plant species. These included the highly invasive kikuyu (Pennisetum clandestinum) and Pampas grass (Cortaderia selloana) which were introduced to the island in the 60’s to provide additional forage for grazing animals. Both have since spread all over the island, with Pampas grass now dominating the wetland landscape and, unless removed, threatening to completely replace the native vegetation. Along with the spread of these invasive weeds has been the loss of crucial native plant habitat for rare birds such as the matuku-hūrepo/Australasian bittern and pāteke/brown teal which have populated this wetland for centuries. 

Unfortunately, many members of the public confuse our native toetoe (Austroderia toetoe) with Pampas grass, thereby through inaction have allowed the latter to take a foothold in many locations. A number of easy-to-spot features distinguish one from the other(4). Native toetoe flowers in spring/early summer, produces pure white flower heads, and has a distinctive secondary vein between the midrib and margin of the leaf. Pampas begins flowering late January and some have purple flower heads. The leaves are rough to touch and have a single midrib. Removal of Pampas grass from the swamp is a high priority for OME, but as it is a difficult plant to eradicate this will take considerable time and effort. A recent survey (transect) through the southern end of ‘The Medlands Wildlife Management Reserve’ identified 30 different plant species, only six of which were native!(5) The two main native vegetation types that were present in this swamp were raupo (Typha orientalis) and jointed twig-rush (Baumea articulata) with each associated with differences in sediment depth, suggesting the vegetation type is controlled by hydrological history (water depth).

Another weed that has taken a major hold along the Oruawharo Stream is grapevine, which is thought to have originated from the original Medlands homestead garden site. It has since spread and is now smothering large areas of native vegetation. 

Besides the weeds, the other big environmental issue for the wetland, like much of the island, is the high density of mammalian predators, including ship (Rattus rattus) and Polynesian (R. exulans) rats as well as feral cats. To reduce rat numbers to protect both birds and invertebrates in the wetland, OME began trapping in January 2021. Monitoring of rat densities 11 months later revealed that 60% of tracking tunnels had rat prints, indicating that we still have a long way to go to meet a target of 5%, which is the standard set by Windy Hill and Glenfern Sanctuaries, considered a good measure of predator suppression. By comparison, tracking tunnels in the nearby dunes, where OME have been trapping since November 2019, had more desirable scores of around 7.5%. However, the August ‘Covid lockdown’, which halted trapping for a period of around six weeks, will have impacted on these numbers. 

While it is easy to be pessimistic about the current state of the wetland, there are pockets of natural regeneration such as the beautiful mature tī kōuka/cabbage tree groves. These will flourish once the pressures of weeds and predators are reduced. The driving force for OME is to intervene in a way that accelerates those natural processes.  

The Restoration Project

In 2020, OME successfully applied for funding from the Department of Conservation for a three-year project to begin restoration of the DoC-managed wetland. The first steps to this restoration work include reducing introduced pest species, plant as well as animal, and encouraging the natural regeneration of plants in this area, through planting of native plants that will create vegetation cover to outcompete the kikuyu and shade the waterways, as well as stabilise the creek banks to reduce erosion.

Establishing a rat trap network was one of OME’s first objectives, which was started in January 2021. We have since kept up with weekly checks until the Covid alert level 4 over the winter period gave the rats a short break. We have added a few more traps recently to include a picturesque area along the main creek. There is still potential to put more traps along the trap lines to provide more effective control. With not all areas currently accessible, there will be some gaps in the network.

An extensive weed eradication process has begun focussing on treating the difficult-to-remove Pampas grass with a glyphosate based herbicide, which is relatively specific for grasses. Given the high density of Pampas grass, completely eradicating them will take some time as seedlings continue to emerge within the wetland from the bank of seeds in the ground and from surrounding areas, but the benefits for the wetland will be considerable. The plants we have managed to treat so far are showing great results.

Feral cats also need to be controlled given they prey on many of the native birds as well as invertebrates. Reducing their numbers within the wetland is a high priority for OME who are working closely with Council and DoC in support of their feral cat management plan. Given their nocturnal nature they are not often seen, but cat scat is frequently observed within the wetland, and their distinctive prints have been detected in the tracking tunnels.

The wetland is also a favourite place for wild pigs, who like the muddy areas around the creeks. They do immense damage to the vegetation covering the stream banks. While fencing the area to exclude the pigs would be the ideal solution, it is also very expensive, so OME has been in discussions with the Local Board and the community to find solutions that minimise damage to the vegetation, especially the new plantings: this process has proved to be not straightforward.   

OME has also initiated a ‘Wai Care’ water quality monitoring program with four sampling sites, two at either end of Oruawharo Bay. This is an Auckland Council freshwater initiative that enables communities to undertake simple tests to measure pH, dissolved oxygen, nitrate levels and a macro invertebrate check, to assess the ecological ‘health’ of rivers, lakes and streams(6). These tests are done quarterly on-site at the sea-end of the creeks which have brackish water, and in the upper freshwater-reaches. These tests indicate that within the areas tested the streams entering Oruawharo Bay are in a very healthy state, with scores falling well within the guidelines provided by the ‘Wai Care’ programme. However, this may not invariably be the case for the seaward ends of these streams.

In October last year, OME finally managed to have their first planting day after postponing twice, firstly due to flooding and the second time due to a Covid 19 lockdown. Our OME community volunteers planted 50 tī kōuka/cabbage tree (Cordyline australis), 50 mānuka (Leptospermum scoparium)/kānuka (Kunzea ericoides), 10 kōwhai (Sophora microphylla) and approximately 60 harakeke/flax (Phormium tenax), split over two sites. There are plans to have at least one planting day per year during the three years of the project, depending on availability of sustainably sourced plants.

Oruawharo Medlands Ecovision vision

There are many challenges working in a wetland with unpredictable conditions.

Earlier this year, the wetland was flooded with sea water, subsequent to a storm that blocked the creek exit at high tide and left all the brackish stormwater trapped for a couple of weeks. The dead mature mānuka one can see from a plane, highlight the impact of this flooding on the vegetation. This was just prior to OME’s first planned planting day, and we were very grateful for the timing. Imagine if we had planted first! This particular couple of weeks also saw most of our rat traps floating in the area, which was a bit of a novelty, though not very conducive to catching rats. The ever-changing conditions require an adaptable approach as we find the best ways to restore this complex ecosystem.

Over time, OME envisages the area becoming more accessible and inviting to the community, with paths and information signs highlighting the plant and animal species of the area, to encourage everyone to see it as a beautiful, rich habitat to be valued. The abundant rainfall in the Winter and Spring of 2021 has shown us just how incredible the wetland looks with a lot more water, compared with the previous drier seasons of 2019 and 2020. The many ponds that have formed across the wetland created safe niches for the nesting birds. We have sighted white-faced heron (Egretta novaehollandiae), paradise shelducks (Tadoma variegata), pied shags (Phalacrocorax varius), little black shags (Phalacrocorax sulcirostris), and of course the ubiquitous pūkeko (Porphyrio melanotus) frequenting the ponds within the wetland, and welcome swallows (Hirundo neoxena) zooming above catching insects. Up to 18 pāteke/brown teal (Anas chlorotis) ducks have been observed in the wetland this spring. Although the endangered matuku-hūrepo/Australasian bittern (Botaurus poiciloptilus) have been spotted in Oruawharo Bay on several occasions, we are yet to sight them in the wetland itself. We hope to see that situation change. OME would like to see DoC work towards enabling a large pond to become a more permanent feature, though the very nature of this area is that the water levels will always change with the seasons. 

For years this area has been a bit of a wasteland with few efforts made to protect and restore what was once a splendid wetland. OME’s goal is to see this wetland restored to its former glory as the jewel in the crown of Oruawharo Bay, brimming with native plant and bird life, and breeding fish, and delighting visitors who come to enjoy the biodiversity of this special Aotea natural feature. 

References:

1. Michelle Benson, personal communication.

2. Winkelman H (1892). Auckland Museum Photographic Records.

3. Rodney Ngāwaka, personal communication.

4. https://www.tawapou.co.nz/about-native-plants/native-toe-toe-or-pampas

5. Ogden J. Preliminary survey of the biota and stratigraphy of the Department of Conservation wetland at Medlands, Great Barrier Island. Unpublished report.

6. Wai Care: https://waicare.org.nz/Files/3%20-%20Field%20Manual.pdf