Tane 37: 91-98 (1999)

F L O R A AND F A U N A O F W O O D E D ISLAND, INNER H A U R A K I G U L F

G.A. Taylor 1 and A.J.D. Tennyson2

'50 Kinghorne Street, Strathmore, Wel l ing ton , 21 L i n c o l n Street, B r o o k l y n , Wel l ing ton

S U M M A R Y

Wooded Island has a vascular flora o f 33 species o f which 70% are native. The island is covered mainly in a low forest o f taupata (Coprosma repens), coastal mahoe (Melicytus novae-zelandiae) and boxthorn (Lycium ferocissimum). There are significant colonies of common diving petrels (Pelecanoides urinatrix) and fluttering shearwaters (Puffinus gavia). Blue penguins (Eudyptula minor) and white-fronted terns (Sterna striata) also breed on the island. Eradication of boxthorn is recommended, as it is having an impact on the survival of the seabirds.

Keywords: Pelecanoides urinatrix; Puffinus gavia; vascular flora; Wooded Island; New Zealand

INTRODUCTION

Wooded Island (0.95 ha) lies 200 m off the northern coast of Tiritiri Matangi Island, inner Hauraki G u l f (Lat 36° 35'S, Long 174° 53 'E) (Fig. 1).

Fig. 1. Wooded Island from Tiritiri Matangi Island, August 1987. Photo: G . A . Taylor.

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The island is sometimes known as Little Tiri Island. Two visits were made to Wooded Island by the authors. On 29 August 1987, G A T , T im Lovegrove and John Dowding landed at 0945 h and spent about two hours ashore. Two adjacent rock stacks were also surveyed on this visit. On 1 February 1989, G A T , A J D T and G i l l Eller landed between 1200-1500 h on the main island and also checked the north-western rock stack. During our landings, we compiled a list of all vascular plant species, seabirds were surveyed, landbirds noted and searches made for reptiles.

V E G E T A T I O N

Fig. 2. The northern coast of Wooded Island, February 1989. Note coastal herbfield and low forest of taupata and boxthorn. Photo: G . A . Taylor.

The main island is covered in a low forest of taupata (Coprosma repens) and coastal mahoe (Melicytus novae-zelandiae) with emergent pohutukawa (Metrosideros excelsa) occasional near the summit. A dense thicket of boxthorn (Lycium ferocissimum) exists on the western and lower northern slopes. The ground cover under the forest is kept relatively barren by the activity o f burrowing petrels. Small-flowered nightshade (Solatium americanum) and black nightshade (Solatium nigrum) were common under the forest in August, along with patches of Einadia trigonos. A fringe o f coastal herbfield covers the northern and eastern side of the main island (Fig. 2) and is present on the two stacks. New Zealand ice plant (Disphyma australe), glasswort (Sarcocornia quinqueflora), Einadia trigonos and shore groundsel (Senecio lautus) are common in the coastal herbfield.

F L O R A

Thirty-three vascular plant species (70% native) were recorded during the two visits. The ten adventive species recorded were mainly herbs and grasses. Most of the plant species found on Wooded Island are typical of small island floras in the

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Hauraki G u l f (pers. obs.). The species list below has the following abundance scale: a = abundant, c = common, o = occasional, s = scarce. * = adventive species.

W o o d e d Island

S W Stack

N W Stack

Ferns

Asplenium haurakiense

Dicotyledons

Coprosma repens Cotula australis Crassula sieberiana Dichondra repens Disphyma australe Einadia trigonos E. triandra Lycium ferocissimum* Melicytus novae-zelandiae Metrosideros excelsa Muehlenbeckia complexa Pelargonium inodorum Phytolacca octandra* Pittosporum crassifolium Polycarpon tetraphyllum* Sarcocornia quinqueflora Senecio lautus Solanum americanum S. nigrum* Sonchus oleraceus* Spergularia media Stellaria media* Tetragonia trigyna Verbena bonariensis* Wahlenbergia vernicosa

Monocotyledons

Arthropodium cirratum Bromus willdenowii* Holcus lanatus* Isolepis nodosa Lachnagrostis littoralis Poa annua* Rytidosperma biannulare

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F A U N A

Birds Common diving petrel (Pelecanoides urinatrix) - on 29 August, a diving petrel

incubating an egg (38 mm x 30 mm) was found in a burrow. Another white egg (41 mm x 32 mm) was found nearby on the surface. The incubating adult was in a burrow with an entrance 10 cm wide and 5 cm high. Most o f the petrel burrow entrances on Wooded Island were of similar dimensions. The abundance o f diving petrel burrows was a dominant feature on Wooded Island. The density was estimated as 4-5 burrow entrances per m 2 on the summit plateau, 4 burrow entrances per m 2 on the north-east slope and 1-3 burrow entrances per m 2

elsewhere on the island. If 6000 m" o f habitat is available on Wooded Island (excluding shoreline rocks and stacks) and diving petrel burrows average 2 entrances per m 2 , then 12,000 entrances could be present on the island. If 40% of these burrow entrances hold a breeding pair (Marchant & Higgins 1990), then an estimated 4800 pairs may breed on Wooded Island. We believe that the breeding population is between 1000-10,000 pairs. Less than five diving petrel-sized burrows were noted on the north-west stack. One was in a patch of glasswort, the others were in rock crevices and were revealed only by the presence o f fresh brick-red excreta at the entrance. No diving petrel burrows were seen on the south-west stack.

Fluttering shearwater (Puffinus gavia) - a dead adult fluttering shearwater was found caught up in a boxthorn shrub on 29 August, and a partial skeleton was found during the February visit. We found empty burrows near the summit that were slightly larger and longer than diving petrel burrows. These were possibly being excavated by fluttering shearwaters in August. On 1 February, two fluttering shearwater chicks were found in burrows under ice plant on the lower northern slope. The first chick (banded X-3008) was covered in down, but the primaries and tail were well emerged. The chick was still heavy and was probably 1-3 weeks off fledging. The second chick (banded X-3009) was in fresh plumage with only a few wisps of down present. It was very close to fledging. Quite a few empty shearwater-sized burrows with down feathers present were found, suggesting that most chicks had already departed by early February. Most of the fluttering shearwater burrows found in February were under ice plant near the shoreline. From the density of burrows observed in February, we estimated that up to 200 pairs may breed on Wooded Island.

Blue penguin [Eudyptula minor) - penguin burrows were scattered over the island from the shoreline to the summit. Three birds were seen in burrows in August, and moulted feathers were noted outside c. 12 burrows on the island's summit in February.

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Little shag (Phalacrocorax melanoleucos) - one seen in August. Arctic skua (Stercorarius parasiticus) - one pale-phased bird with pointed tail

feathers flew over the island during the February visit. Southern black-backed gull (Larus dominicanus) - two were seen in August, and

four birds and an empty nest were noted in February. Red-billed gull (Larus scopulinus) - two adults and two large chicks were seen on

the north-west stack in February. White-fronted tern (Sterna striata) - a nesting colony of c. 30 nearly-fledged chicks

and several adults was present on the north-west stack on 1 February. Empty nest sites were found scattered all over the stack.

Red-crowned parakeet (Cyanoramphus novaezelandiae) - one was seen in August and three were seen in February.

Welcome swallow (Hirundo tahitica) - two were seen in August. House sparrow (Passer domesticus) - two were seen in August. Hedge sparrow (Prunella modularis) - one was seen in August. Bellbird (Anthornis melanura) - at least three were seen in August. Tui (Prosthemadera novaeseelandiae) - six birds were seen and a tui corpse was

found under the forest in February. Grey warbler (Gerygone igata) - three were seen in February. Silvereye (Zosterops lateralis) - two were seen in August and two were noted in

February

Starling (Sturnus vulgaris) - one was seen in August and six were seen in February.

Mammals

We saw no sign of any introduced mammals. Rodents are almost certainly absent. Reptiles

N o skinks or geckos were seen despite searching under rocks, fallen branches and in other likely habitats. The apparent absence o f reptiles was unexpected on this island, as reptiles are usually common on petrel islands free o f introduced mammals (Whitaker 1978). Further searches, especially at night, may be more successful.

Invertebrates Only incidental records of invertebrates encountered are listed here.

Tenebrionid beetles (Mimopeus sp.) were common under rocks. Shore earwigs (Anisolabis littorea) were present. A honeybee (Apis mellifera) was seen feeding on a boxthorn shrub and a bumblebee (Bombus sp.) was noted. German or common wasps (Vespula sp.) were also noted. Silverfish and cicadas were recorded on the February visit. A small centipede (c. 16 mm long) and 3-4 mm isopods were seen on the north-west stack.

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DISCUSSION

Wooded Island is an important breeding ground for seabirds. It has the only known large diving petrel colony in the inner Hauraki Gulf. The nearest large breeding colony of diving petrels is on Sail Rock (Atkinson 1972), although a few birds nest on Tiritiri Matangi Island (presumably an overflow from Wooded Island), and also on Maria Island in The Noises group (Cunningham & Moors 1985). Fluttering shearwater colonies are also rare in the inner Hauraki Gulf. A few birds nest on Tiritiri Matangi Island (pers. obs.). There is also a small colony on Horuhoru Rock off Waiheke Island (pers. obs.). The nearest large colonies are on the North-west Chicken Islands (where a few thousand pairs were estimated by A J D T on a visit on 9 December 1992) and the Bream Islands near Whangarei (P. Miller pers. comm.).

The densely-burrowed colony of diving petrels and fluttering shearwaters on Wooded Island provides a stark contrast with the sparse populations present on neighbouring Tiritiri Matangi Island, which at the time of our visits, still had a population o f Pacific rats or kiore {Rattus exulans). These rodents were eradicated in 1993 (C.R. Veitch pers. comm.). O n 20 November 1989, we located c. 15 fluttering shearwater burrows on the north-western headland opposite Wooded Island. There was a broken egg in one o f the burrows. A few probable diving petrel burrows were also found on this headland. On the next headland to the east, a few probable fluttering shearwater burrows were found amongst a scattering of diving petrel burrows. A large downy diving petrel chick was present in one of these burrows, so evidently a few pairs were successful in spite o f the rats. However, in our experience, dense colonies of diving petrels occur only on islands where mammalian predators are absent.

The apparent absence of reptiles was unexpected on Wooded Island. If reptiles are present, they must be in very low densities. Similar small northern offshore islands with dense seabird colonies typically have at least 3-5 reptile species (see Whitaker 1978 and Taylor et al. 1990). A nocturnal visit is still needed to check for reptiles, including the nocturnal forest-dwelling Cyclodina species.

Most of the plant species found on Wooded Island also occur on adjacent Tiritiri Matangi Island (cf Esler 1978). Three native species {Pelargonium inodorum, Spergularia media and Rytidosperma biannulare) and one adventive species {Verbena bonariensis) were present on Wooded Island, but not recorded by either Esler (1978) or Cameron & West (1986) on Tiritiri Matangi Island. Wahlenbergia vernicosa may or may not be the same taxon as W. gracilis recorded by Esler (1978) for Tiritiri Matangi Island.

Boxthorn is the major problem weed on Wooded Island. Large trees are present and form impenetrable thickets which may threaten some wildlife or make the habitat less favourable. Esler (1978) stated that many boxthorn plants grew on the island off the north-east bay of Tiritiri Matangi Island in the early 1970s

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(presumably referring to Wooded Island). Although petrel burrows were seen under these thickets in the late 1980s, boxthorn may eventually prevent birds from using their burrows, i f the shrubs form a dense low ground cover. Diving petrels prefer to land by their burrow entrances (Richdale 1965). This behaviour puts the birds at risk of being snared on boxthorn spines when the plants overtop the burrows. Esler (1978) mentioned that seabirds had become impaled on the spines of boxthorn plants on Tiritiri. We saw one fluttering shearwater corpse caught in a boxthorn shrub in August 1987 and three dead diving petrels were seen in February 1989. We have also seen seabirds entangled in boxthorn on other islands. For example, we saw a dead white-faced storm petrel (Pelagodroma marina) snared in a boxthorn shrub on Maria Island, The Noises group, on 30 October 1988. A s boxthorn spreads on Wooded Island, we predict that increasing numbers o f birds wi l l become entangled on the thorns.

In February 1989, we tried unsuccessfully to cut down one boxthorn tree. However we were able to remove a number of seedlings. Eradication of boxthorn on this island is urgently needed. Removal of boxthorn wi l l initially cause some damage to the underlying petrel burrows. However i f the plants are removed between mid-February and mid-March, there should be few, i f any, petrels ashore. Both diving petrels and fluttering shearwaters are away at sea moulting during this period (Falla 1934, Marchant & Higgins 1990). Regular visits wi l l be needed to prevent boxthorn re-establishing, as seeds ingested by starlings and other landbirds wi l l continue to be carried to the island.

Currently there is little formal protection o f Wooded Island and its wildlife. The island is not directly managed by the Department o f Conservation although many people probably consider it forms part of the Tiritiri Matangi Island reserve. Visits by people are probably limited by the difficulty in landing, except in calm weather. The dense, tight native and adventive vegetation would also deter people from exploring the interior of the island. However, we did find a few wooden boxes (possibly used to grow Cannabis) on the summit in August 1987. The seabird burrows are in very friable soil and are very prone to damage by trampling. The risk of rodents escaping from boats and reaching the island is probably low, because the rocky shoreline makes it difficult for any but the smallest boats to anchor near the island. However, i f ship rats (Rattus rattus) or Norway rats (Rattus norvegicus) colonised Wooded Island, they would devastate the colony o f diving petrels. Some monitoring for rodent presence is needed and permanent poison bait stations should be established there. Sea-based threats to the petrel colony also exist in the form of set-nets. Tennyson (1991) recommended a 1 km set-net exclusion zone around Wooded Island to protect this important seabird colony.

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A C K N O W L E D G E M E N T S

W e thank T i m Lovegrove, John D o w d i n g and G i l l El ler who contributed observations on the plants and animals and took care o f the boat whi le we were ashore on the islands. The Zoo logy Department, A u c k l a n d Universi ty, loaned their boat for both visits to W o o d e d Island. E w e n Cameron (Auck land Museum) k ind ly assisted with plant identification. T i m Lovegrove and Ewen Cameron also provided comments on an earlier draft o f this paper.

R E F E R E N C E S

Atk inson , I . A . E . 1972: Vegetation and flora o f Sail Rock , H e n and Chickens Islands. New Zealand Journal of Botany 10:545-58.

Cameron, E . K . & West, C . J . 1986: Notes on the flora o f Tirit iri Matangi Island, Hauraki Gul f . Tane 31: 121-123.

Cunningham, D . M . & Moors , P.J. 1985: The birds o f The Noises Islands, Hauraki Gul f . Notornis 32: 221-243.

Esler, A . E . 1978: Botanical features o f Tir i t i r i Island, Hauraki Gul f , N e w Zealand. New Zealand Journal of Botany 16:207-26.

Falla, R . A . 1934: The distribution and breeding habits o f petrels in northern N e w Zealand. Records of Auckland Museum and Institute I: 245-259.

Marchant, S. & Higgins , P.J. 1990: Handbook o f Australian, N e w Zealand & Antarctic Birds . V o l . 1. Ratites to Ducks . Oxford Universi ty Press, Melbourne.

Richdale, L . E . 1965: B io logy o f the birds o f Whero Island, N e w Zealand, with special reference to the d iv ing petrel and the white-faced storm petrel. Transactions of the Zoological Society of London 31: 1-85.

Taylor, G . A . , Lovegrove , T . G . , M i s k e l l y , C M . , McFadden , I. & Whitaker, A . H . 1990: A n ecological survey o f small islands in the Mercury Group. Tane 32: 151-167.

Tennyson, A . 1991: Forest and Birds set net policy - in part. New Zealand Professional Fisherman (July): 50-51.

Whitaker, A . H . 1978: The effects o f rodents on reptiles and amphibians. Pp. 75-88 in: D i n g w a l l , P.R., Atk inson , I . A . E . & Hay , C . (eds.), The Eco logy and Control o f Rodents in N e w Zealand Nature Reserves. New Zealand Department of Lands and Survey Information Series, No.4.

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