Tane 36: 85-111(1997)

FLORA AND VEGETATION OF STANLEY (ATIU) ISLAND, MERCURY ISLANDS

G.A. Taylor1 and T . G . Lovegrove2

'50 Kinghorne Street, Strathmore, Wellington, 26 Deuxberry Ave, Northcote, Auckland 1309

SUMMARY

Stanley Island has a vascular flora of 180 species including remnant stands of taraire (Beilschmiedia tarairi) and other coastal hardwoods such as coastal maire (Nestegis apetala), tawapou (Pouteria costata), coastal milk-tree (Streblus banksii) and puriri (Vitex lucens). Most of the main island was modified by past forest clearance. Pohutukawa(Metrosideros excelsa)/mahoe (Melicytus ramiflorus) and pohutukawa/mapou (Myrsine australis) forests have subsequently regenerated over much of the island with occasional patches of manuka (Leptospermum scoparium) shrubland. Two threatened plants are present - Cook's scurvy grass (Lepidium oleraceum) and mawhai (Sicyos australis). Rabbits (Oryctolagus cuniculus) and possibly Pacific/Polynesian rats or kiore (Rattus exulans) have had a profound effect on the vegetation and these impacts are assessed by comparing the main island with the flora of the rabbit-free offshore stacks and other islands in the Mercury Group, and by examining the early signs of vegetation recovery following the successful eradication of both rabbits and kiore in 1991.

Keywords: vascular flora; forest regeneration; browsing mammals; Stanley Island; Mercury Islands; New Zealand.

INTRODUCTION

Stanley Island (lOOha) (also known as Atiu, Kawhitu or Kawhitihu Island), the third largest island of the Mercury Group, lies 11km north-east of Opito Bay, Coromandel Peninsula, New Zealand at Lat 36°38'S, Long 175°53'E. Stanley Island is administered as a Nature Reserve and access is by permit only, controlled by the Department of Conservation's Waikato Conservancy. During the 1980s permission to visit the island was also necessary from the Hauraki Gulf Maritime Park Board. The main landing is at the north-western point (usually Northern Bay), and an unmarked route (this was marked with pegs in 1993) leads from the landing to a bird transfer aviary and the main campsite on the north­west slope (Fig. 1).

The geology of Stanley Island has not been studied in detail, but the New

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Stanley Island

Zealand Geological Survey map (North Island - 1st Edition, 1972, "Geological Map of New Zealand 1: 1,000,000) shows Stanley and adjacent islands in the Mercury Group as having volcanic rocks such as andesite and rhyolite. Nearby Red Mercury Island has a complex sequence of interdigitating basalt flows, breccias, scoria and possible rhyolitic, tuff beds (Hayward & Moore 1972). The exposed cliffs and eroded slopes of Stanley Island suggest a similar complex geology, and give the island a colourful landscape. Various informal place names were given to prominent features such as the Big Red Cave, a large amphitheatre of red rocks with a small cave at the base, located in the eastern bay (Fig. 1).

The landform of Stanley Island is dominated by flat-topped hills at the north­eastern, central and southern ends (maximum altitude 137m asl). A central ridgeline runs north to south with broad plateaux at the northern and southern ends. There are several gentle basins sloping away from the main ridge on the western (Fig. 2), south-western and eastern (Fig. 3) sides of the island. No permanent streams are present, but drainage channels in the Eastern Valley and south-west basin hold water for some time after rain. Steep sea cliffs encircle much of the island; however, there are boulder beaches in Northern Bay (Fig. 4) and Eastern Bay, and a sandy beach in the western bay. There are two small rock stacks off the north-western tip (Fig. 5). The Inner Stack (0.36ha) is connected to Stanley Island at low tide by rock pools and boulders, whereas the Outer Stack (1.25ha) is separated by a 100m wide channel from the Inner Stack (Figs. 1 & 5).

Wildlife surveys of the Mercury Islands were undertaken in the 1960s (Edgar 1962, Skegg 1963, 1972) and Stanley Island was visited briefly during these expeditions. Thoresen (1967) camped on Stanley Island for a month in 1966 while studying the breeding biology of diving petrels (Pelecanoides urinatrix). His paper includes three photos of the northern half of Stanley Island, and provides a useful comparison of the vegetation patterns in the 1960s compared with those of the late 1980s. North Island saddlebacks (Philesturnus carunculatus rufusater) were translocated from Cuvier Island to Stanley Island in 1977 (Lovegrove 1996). Stanley Island was seldom visited again until the late 1980s when a series of trips was organised by T G L , who was undertaking research on North Island saddlebacks (Lovegrove 1992). G A T visited Stanley Island on six trips between 1987 and 1989 to assist T G L with these studies, and to carry out research on the seabird communities (Tennyson & Taylor 1990).

In September 1991, Pacific/Polynesian rats or kiore (Rattus exulans) and rabbits (Oryctolagus cuniculus) were eradicated from Stanley Island by an aerial drop of Talon 20P poison and ground laying of Talon 50WB poison (Towns et al. 1993). Kiore were removed from the Inner Stack during the same operation. We have no evidence that rabbits ever colonised the Inner Stack. The Outer Stack

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Fig. 2. The north-western corner of Stanley Island from the Pa. Note the dense stands of pure mahoe forest and emergent pohutukawa. Cuvier Island is visible in the background. Photo: G.A. Taylor 1988.

Fig. 3. The north-eastern corner of Stanley Island from the Pa. View toward the Eastern Summit and Double Island in the background. Photo: G.A.Taylor 1988.

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Fig. 4. The northern bay of Stanley Island from the Outer Stack. Note the main landing beach in the centre and the Inner Stack. Photo: G.A. Taylor 1989.

Fig. 5. The Inner Stack (at low tide) and the Outer Stack of Stanley Island. Note Cuvier Island in the background. Photo: G.A. Taylor 1988.

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has always been free of introduced mammals (Taylor et al. 1990). In November 1993, G A T visited Stanley Island to establish permanent plots

for monitoring changes in the seabird populations following the eradication of kiore and rabbits. The success of the eradication operation was confirmed on this trip. Stanley Island has eight species of burrowing seabirds (GAT, unpubl.) including grey-faced petrel (Pterodroma macroptera gouldi) and Pycroft's petrel (Pterodroma pycrofti) (Skegg 1963, 1972). Seabird burrows are widely scattered over the forested interior and many thousands of petrels breed on the island (pers. obs.). However, burrow density is still relatively low compared with the Outer Stack of Stanley Island. Here there is a very dense colony of burrowing seabirds, especially diving petrels and fluttering shearwaters (Puffinus gavia) (pers. obs).

V E G E T A T I O N A S S E S S M E N T

The vegetation of Stanley Island has not been studied previously. A l l the remaining islands in the Mercury Group however have been visited by botanists and there are published accounts for Great Mercury Island (1860ha) (Wright 1976), Red Mercury Island (225ha) (Lynch et al. 1972), Korapuki Island (17.5ha) (Hicks et al. 1975), Middle Island (13ha) (Atkinson 1964, Cameron 1990), Green Island (2.3ha) (Atkinson 1964) and various small stacks (Taylor et al. 1990). The vascular flora of Double Island (32.5ha) has been listed in unpublished reports by Atkinson (1962) and G A T (unpubl.).

Plant species were noted on Stanley Island and its two offshore stacks by G A T and T G L during the following trips: 3-10 June 1987, 18-25 September 1987, 22-23 August 1988, 7-8 November 1988, 16-22 May 1989, 11-18 July. 1989, 16-25 August 1989 and 4-11 November 1993. Voucher specimens were collected to confirm identification of difficult species. A sample of tree diameters was measured in 1989, and further surveys were undertaken in 1993 to assess the composition and structure of the old forest remnants. Three 20m by 20m square plots were sampled for seabirds in 1993, and all plant species in these plots were also recorded. Additionally, 22 circular plots (each of 50m2 and centred on existing marker pegs) were sampled on a transect line from the northern coast to the interior of the island. This transect helped to determine the distribution and abundance of the more widespread plant species.

V E G E T A T I O N

Stanley Island appears to have been burned or cleared in the past. Stone walls are present and the early Maori occupants probably cultivated kumara on the

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sunny northern and western slopes of the island. A probable habitation site (which we informally called the Pa) exists on the central high point (Fig. 1), and features shallow pits and stone chips.

We identified seven broad vegetation types on Stanley Island and its two outlying stacks. The main island is forested, and comprises two widespread vegetation types. There are also two remnant patches of what is presumably the original coastal forest, and scattered old forest trees in rock-fall areas.

Vegetation types:

1. Pohutukawa/mahoe forest This is the dominant vegetation cover on the main island and is widespread

on the northern, western, and south-west slopes. Here, tall emergent pohutukawa (Metrosideros excelsa) (at least 20m tall) are scattered through almost continuous stands of 10m tall mahoe (Melicytus ramiflorus) with few other species. In some areas mahoe forest is emerging through an understory of low shrubs (l-3m tall) such as mapou (Myrsine australis), N Z flax (Phormium tenax) and Astelia banksii. The ground cover is mainly shining spleenwort (Asplenium oblongifolium) and Doodia media with occasional patches of bracken (Pteridium esculentum). Mahoe shrubs in these sites appear old and stunted and regeneration to tall mahoe forest may have been slowed by exposure to strong coastal winds or poor soils. Where the mahoe forest is taller (generally >5m), the canopy is thick and there are very few species in the shrub layer. Here, virtually the only ground cover is shining spleenwort.

2. Pohutukawa/mapou forest This vegetation type is most widespread in the central eastern and southern

parts of the main island. The tallest stands of mapou forest (c. 8m) occur between the Pa and a rocky basin on the southern escarpment, an attractive, enclosed area which we called the Arena (Fig. 1). Occasional emergent pohutukawa (up to 20m tall) are present in these dense stands of mapou forest. The understory includes mahoe and mapou saplings, with very occasional hangehange (Geniostoma rupestre), mingimingi (Leucopogon fasciculatus) and Astelia banksii along with a ground cover of shining spleenwort, Hound's tongue (Phymatosorus pustulatus) and Doodia media.

3. Taraire forest There are two remnant stands of taraire (Beilschmiedia tarairi) forest. One is

just south of the Pa and the other is on the slopes above the Arena. Both stands are similar in appearance and there are large boulders amongst the canopy trees.

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The composition of these forest remnants was examined in November 1993. At the Pa site, taraire (c. 15m tall) were dominant but tawapou (Pouteria costata) were also common in the canopy along with occasional kohekohe (Dysoxylum spectabile), wharangi (Melicope ternata) and pigeonwood (Hedycarya arborea). There was one karaka (Corynocarpus laevigatus), one puriri (Vitex lucens) and three white maire {Nestegis lanceolata). The understory comprised tall mahoe along with four parapara (Pisonia brunoniana), one Carmichaelia australis, one large supplejack (Ripogonum scandens) vine and occasional coastal karamu (Coprosma macrocarpa). Several ferns and orchids were confined to this forest remnant, e.g. Arthropteris tenella and Drymoanthus adversus.

The Arena site had a similar forest but puriri and karaka were more common in the canopy and there were two coastal maire (Nestegis apetala) in the understory. After spells of prolonged heavy rain a large pond (c.20m across) formed in the basin in the centre of the Arena. A prominent feature of this basin was the very deep leaf litter and peaty soil which occurred amongst the boulders and around the margins of the pond.

4. Mixed coastal hardwood forest Patches of tall remnant coastal forest lacking taraire are present in rocky areas

on the main island. These include the stands of tawapou west of the Pa, and remnant hardwoods growing amongst large boulders north-east of the Aviary; on the northern slopes and on the slopes east of the Arena. These remnant patches include several large puriri, tawapou, titoki (Alectryon excelsus), and coastal milk-tree (Streblus banksii). Epiphytes, e.g. Collospermum hastatum, grow on some of the trees above the Arena. Rhabdothamnus solandri is locally common on the south-east face near the Arena.

5. Manuka shrublands An area of dense manuka (Leptospermum scoparium) shrubland with mixed

hardwoods such as mapou, mingimingi, mahoe, hangehange, Hebe pubescens, Pomaderris phylicifolia along with bracken and Astelia banksii grows on the slopes north-west of the central Eastern Summit. Manuka shrublands also occurs on the ridgeline north of the Pa. Access along this ridge is very difficult owing to the density of this vegetation.

6. Coastal shrublands There are localised areas of dense coastal shrublands on the lower slopes of

the main island, especially above the northern and eastern bays. These shrublands include ngaio (Myoporum laetum), flax, coastal karamu and coastal mahoe (Melicytus novae-zelandiae), with occasional poroporo (Solanum aviculare).

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Taupata (Coprosma repens) is confined to steep rock faces in this vegetation type. The eastern bay also has rangiora {Brachy glottis repanda) and flax along the coastal fringe.

The two offshore stacks are predominately covered in this vegetation type. Ngaio, coastal karamu, coastal mahoe, mahoe, taupata and houpara (Pseudopanax lessonii) are the main canopy species on the Inner Stack; whereas taupata, coastal mahoe, ngaio and karo (Pittosporum crassifolium) are the main canopy species on the Outer Stack.

7. Grassy clearings Small grassy clearings, predominantly of Rytidosperma racemosum, exist on

the ridgetops above the northern and eastern bays. These sites were heavily browsed by rabbits in the late 1980s and only a stubble of grass sward was present. By 1993, these sites had changed greatly and tall native and adventive grasses were present along with sun-loving herbs, e.g. creeping cudweed (Gnaphalium gymnocephalum). A patch of grassland on the northern slope was found in 1989 which appeared to have been browsed very infrequently by rabbits. It had a mixture of native and introduced grasses and quite a few weeds.

8. Mixed coastal herbfield This vegetation type was severely damaged by rabbits and consequently most

open areas behind the beaches were barren in the 1980s. There was one remnant patch beside the Big Red Cave (Fig. 1), an area which rabbits apparently never reached. This had a variety of grasses and herbs including N Z ice plant (Disphyma australe), glass wort (Sarcocornia quinqueflora), Mercury Bay weed (Dichondra repens) and Einadia trigonos.

Coastal herbfield is fairly localised on the Inner Stack and occurs mostly on steep cliff faces and ledges. The Outer Stack has extensive areas of coastal herbfield and N Z ice plant, glasswort, Mercury Bay weed and Einadia trigonos are abundant in soils heavily burrowed by nesting petrels. Here, shore groundsel (Senecio lautus) is common and Samolus repens and saltwater paspalum (Paspalum vaginatum) are common on the rocky margins of brackish pools.

F L O R A

The annotated species list below documents all vascular plant species seen on Stanley Island and the two offshore stacks. The vascular flora of Stanley Island numbers 180 species, 77% of which are native (Table 1). The Inner Stack has 69 species (80 % native) and the Outer Stack has 45 species (82 % native) (Table 1). There are five species which were only found on the Inner Stack and one

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species which was only found on the Outer Stack. The total vascular flora for Stanley Island and the two offshore stacks is 186 species.

Plants only seen after the removal of rabbits and kiore (e.g. new records from the 1993 visit) are shown with a double asterisk. Abundance is noted using a generalised scale for each taxon: abundant (A), common (C), locally common (LC), occasional (O), scarce (S). The habitat descriptions represent the locations where there are confirmed records in our notebooks. Where a voucher exists to support the record, the herbarium sheet is listed: A K = Auckland Museum, A K U = Auckland University.

F L O R A O F S T A N L E Y I S L A N D

N A T I V E SPECIES:

Ferns and allies Adiantum cunninghamii Occasional, seepage face, north cliffs A. hispidulum Occasional under mapou scrub, coastal cliffs Arthropteris tenella Locally common, on rocks in old forest remnants Asplenium flaccidum Scarce, terrestrial in mapou scrub Asplenium haurakiense Occasional in NW sector, often on base of pohutukawa A. northlandicum Scarce, rock crevices on SE shoreline A. oblongifolium Abundant, the dominant ground cover throughout. AKU

20347 A + B A. oblongifolium x A. polyodon Scarce, clump of 5 plants Blechnum sp. "1" Scarce, seepage face on north cliffs, above western bay and

east of Pa in damp hollow B. filiforme Occasional south of Pa, in tall forest, overhanging banks and

on northern flat Cheilanthes distans Locally common on open banks and cliffs, SE face C. sieberi Occasional on open banks and cliffs, SE face Cyathea dealbata Occasional on western side under forest C. medullaris Scarce, local group above western bay Doodia media Common throughout Hypolepis ambigua Locally common on northern face Pellaea falcata Occasional plants on SE cliffs, locally frequent on rock ledges

on north cliffs P. rotundifolia Scarce, 1 plant by track, top of northern slopes Phymatosorus pustulatus Common Polystichum richardii Occasional on slopes above eastern bay, occasional on bluffs

south of Arena Psilotum nudum Occasional, SE of Pa and on northern flat Pteridium esculentum Occasional Pteris comans Occasional clumps, especially on northern face P. tremula Occasional on northern face, SW and SE faces Pyrrosia eleagnifolia Common

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Dicotyledons Alectryon excelsus

Apium prostration Beilschmiedia tarairi Brachyglottis repanda

Callitriche muelleri Calystegia soldanella Cardamine debilis Carmichaelia australis Cassinia leptophylla Clematis paniculata Coprosma macrocarpa C. repens C. rhamnoides Coriaria arborea Corynocarpus laevigatus

Cotula australis C. coronopifolia Crassula sieberiana Dichondra repens Disphyma australe

Dodonaea viscosa Dysoxylum spectabile Einadia trigonos Entelea arborescens Epilobium nummularifolium Geniostoma rupestre Gnaphalium audax G. gymnocephalum** G. involucratum** Gonocarpus incanus Haloragis erecta Hebe pubescens

Hedycarya arborea

Hydrocotyle microphylla Kunzea ericoides Lagenifera pumila Lepidium oleraceum

Leptospermum scoparium Leucopogon fasciculatus L. fraseri

Scarce, 1 large tree on NW face, 1 tree on NNE slope and 3 trees above Arena Local patches, SE shoreline, Big Red Cave Locally common, Pa and Arena Occasional, eastern bay, northern slopes, slopes and bluffs SW of Arena Scarce, patches in stream-bed behind eastern bay Locally frequent at Big Red Cave, Scarce on eastern bay beach Occasional in clearings Scarce, 2 trees on SE ridge, 1 tree at Pa, one sapling Scarce, northern cliffs Occasional, northern cliffs. AK 229695 Locally common on SE slope, local around fringe of island Occasional on shoreline rocks Scarce, 2 small shrubs Occasional to common, throughout Scarce, trees on NNE face, 1 canopy tree at Pa and 9 large trees at Arena Locally common, SSW boulder beach Scarce, on rocky islet in SW bay Scarce on coastal rocks, eastern bay Locally common, Big Red Cave, occasional elsewhere Locally common, Big Red Cave, occasional elsewhere near shoreline Scarce, 1 shrub north of Pa. AK 229696 Locally common, old forest at Pa and Arena Occasional, Big Red Cave Occasional in forest above western bay Scarce, on clay bank seepage behind western bay. AKU 21024 Occasional shrubs on SE slopes Scarce, northern cliffs Occasional, northern cliffs. AK 229689, AK 229694 Occasional, northern cliffs. AK 229691 Local, slopes around eastern summit Local, few shrubs on SE cliffs and at Big Red Cave Occasional on ridge south of Pa, locally common on bluff south of Arena Common south of Pa, saplings and seedlings present, occasional elsewhere Occasional, stream-banks behind eastern bay Scarce, 1 tree south of Pa Local, patch on slip on SE corner, on cliffs behind eastern bay Scarce, 4 plants on boulder bench below SSW cliffs. AK 229700 Locally common, near eastern summit and ridge north of Pa Occasional Scarce, patch in open area on eastern ridge

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Linum monogynum

Lobelia anceps

Macropiper excelsum

Melicope ternata Melicytus novae-zelandiae M. ramiflorus Metrosideros excelsa Muehlenbeckia complexa Myoporum laetum Myrsine australis Nestegis apetala N. lanceolata Olearia furfuracea Oxalis exilis** O. rubens Parietaria debilis Pelargonium inodorum Peperomia urvilleana

Pimelea cf. urvilleana

Pisonia brunoniana Pittosporum crassifolium

Pomaderris phylicifolia Pouteria costata

Pseudognaphalium luteoalbum agg Pseudopanax lessonii

Rhabdothamnus solandri Samolus repens Sarcocornia quinqueflora Scandia rosifolia Scleranthus biflorus** Selliera radicans Senecio glomeratus S. hispidulus S. lautus S. quadridentatus Sicyos australis

Solanum americanum

Occasional, on shoreline rocks in SE bay, 1 on slip below red cliff on north face,few plants on SW cliffs, locally common on cliffs by Big Red Cave Occasional, cliffs and seepages throughout but especially behind western bay Locally common, shrubs and saplings on slope above SW bay, occasional on SE face Occasional large trees near Pa and Arena Occasional on coastal cliffs, clifftops and margins Abundant, dominant plant cover over much of island Abundant, emergent trees throughout, few saplings Occasional on northern cliffs Occasional on NW face Abundant, dominant tree on eastern slopes and south of Pa Scarce, 2 trees in Arena forest. AK 229697 Scarce, 3 trees at Pa and 2 at Arena. AK 229704 Scarce, 1 small tree south of camp on western slopes Occasional on northern slopes Occasional on northern slopes Local, on SSW boulder beach Scarce, on eastern cliff-tops Locally common on bluffs south of Arena, occasional at northern end of eastern bay Locally common on exposed slopes at south end of island, rare on northern cliffs Scarce, 4 trees at Pa, 7 trees at Arena Locally common, seedlings, saplings and small shrubs on northern cliffs, patch on cliffs above Big Red Cave, a few in eastern bay Scarce, on central eastern summit Occasional large trees on northern face, locally common on slopes west of Pa and near Arena Occasional on slips, also seepages in SE bay Scarce, 1 tree in eastern valley, 4 trees on red slip on northern cliffs, 3 trees SE of Arena Locally common on SE face, occasional near Arena Locally common, eastern bay Occasional on coastal rocks, SE bay Scarce, on SE slopes near Arena Scarce, small patch on rocks at NW landing Scarce, Big Red Cave Scarce, 4 plants on boulder beach at SSW bay Scarce, on rocks on eastern ridge Occasional, Big Red Cave Scarce, SE cliffs Local, patch sprawling over boulders and slopes by NW landing, another patch on northern face near NE corner Locally abundant, SSW boulderbeach, local patches elsewhere (e.g. grass area on northern cliffs)

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S. aviculare

Spergularia media Streblus banksii

Tetragonia trigyna Vitex lucens

Wahlenbergia sp.

Monocotyledons Acianthus oblongus A. sinclairii Arthropodium cirratum

Astelia banksii Bulbophyllum pygmaeum Carex breviculmis C. flagellifera

C. spinirostris Collospermum hastatum Cordyline australis Cortaderia splendens Cyperus ustulatus

Dichelachne crinita

Drymoanthus adversus Earina mucronata Elymus multiflorus var. longisetus Gahnia ?lacera Isolepis cernua I. nodosa

Lachnagrostis billardierei

L. littoralis Lepidosperma australe L. laterale Leptocarpus similis Microlaena stipoides Oplismenus imbecillis Phormium tenax Poa anceps Pterostylis alobula Ripogonum scandens Thelymitra longifolia

Occasional, few by NW landing, small tree near Aviary, few behind west beach Scarce, on point near Big Red Cave Occasional old trees only, amongst rocks near Aviary and above Arena. AK 229699 Occasional, SE slip and shoreline, also Big Red Cave Scarce, 1 large tree at Pa, 6 trees near Arena, 1 tree on southern slopes near eastern summit Scarce, SE cliffs and SW bay

Scarce, by Arena under manuka. AKU 19969 Local, patch on mid-eastern clifftops Locally common on bluffs south of Arena, occasional on eastern and northern cliffs Common throughout Scarce, 1 clump at Pa, large clump on boulders NE of aviary Local, SE clifftop Locally common on northern cliffs and beach, also Big Red Cave and SSW boulder beach Local, south of Pa Local, on boulders and trees above Arena Scarce, 1 by Pa, 1 near southern end, 1 in eastern valley Local, patch on slip near shoreline on SE slopes Locally common behind northern beach, occasional at eastern beach Scarce, 1 plant on each of SE cliffs, SW cliffs and northern cliffs, few plants on SSW boulder beach Scarce, on trees at Pa Scarce, 1 plant on rock at south end Scarce, on boulder ledge on northern cliffs. AKU 22491 Occasional on eastern ridge Local around SE shoreline, common in seepages Locally common, Big Red Cave, occasional elsewhere Locally common at Big Red Cave and SSW boulder beach, also on rocky islet in SW bay Occasional, Big Red Cave, clifftops Local, patches under mapou on SE ridge. AKU 22719 Local patch only, 20-30 plants on NE ridge Locally common on cliffs and seepages behind western bay Occasional, northern and eastern cliffs Occasional, throughout Locally common throughout, dense patches Occasional, blue form present in SE corner Scarce. AKU 19970 Scarce, 1 plant in taraire grove by Pa Scarce, behind western beach coastal cliffs

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A D V E N T I V E SPECIES:

Dicotyledons Anagallis arvensis s. str. Brassica ?oleracea Cakile edentula** Centaurium erythraea Chenopodium pumilio Cirsium vulgare Conyza albida Coronopus didymus Euphorbia peplus Gnaphalium coarctatum Hypochoeris radicata Lotus pedunculatus Lotus suaveolens Lycium ferocissimum Phytolacca octandra Picris echioides Plantago australis P. lanceolata

Polycarpon tetraphyllum Ranunculus sessiliflorus Sagina apetala** S. procumbens Senecio bipinnatisectus S. jacobaea Sigesbeckia orientalis

Sonchus asper S. oleraceus Solanum nigrum

Monocotyledons Bromus arenarius** Bromus hordeaceus Cortaderia jubata C. selloana Dactylis glomerata Holcus lanatus Paspalum dilatatum P. vaginatum Poa annua Rytidosperma pilosum** R. racemosum Sporobolus africanus Vulpia bromoides**

Occasional around shoreline and in clearings, red flowers only Occasional, Big Red Slip. AK 229703 Scarce, NW beach Occasional, banks above eastern beach Scarce, grass slope on northern cliffs. AK 229701 Occasional throughout, cliffs and clearings Scarce, grass slope on northern cliffs Local, Big Red Cave Locally common, slopes behind northern beach Scarce, northern cliffs Local, on rock outcrop on eastern ridge Locally common, Big Red Cave and southern point Locally common, Big Red Cave and southern point Scarce, 1 plant on eastern beach, 2 shrubs on northern cliffs Locally common, dense patches at northern end Scarce, SSW boulder beach Local, SSW boulder beach and cliffs behind northern beach Locally common, eastern beach, Big Red Cave, SSW boulder beach Occasional, cliffs throughout, Big Red Cave Scarce Scarce, northern coastline Local, patches behind eastern beach Scarce, northern cliffs Scarce Locally common on NE slopes, also grass slope on northern cliffs. AK 229698 Scarce, 3 plants behind western beach. AK 229702 Occasional, slip on SE corner Scarce, local patch on grass area, northern cliffs

Scarce, small patch on northern face. AK 229693 Scarce, grassy clearings. AKU 21357 Scarce, 1 plant on NE summit, 1 behind west beach Scarce, 1 in valley below northern red slip Scarce, SE coast Scarce, slip at NE end Local, grass slope on northern cliffs Locally common, Big Red Cave Scarce Occasional, coastal cliffs. AK 229688 Common, coastal cliffs, clifftops and clearings. AK 229690 Local, SW cliffs and on grass slope on northern cliffs Occasional. AK 229692

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FLORA OF T H E INNER AND OUTER STACKS, STANLEY ISLAND

* = adventive species INNER STACK OUTER STACK

Ferns Asplenium haurakiense O-C, lower slopes and summit () A. northlandicum - S, 1 plant on cliff above sea A. oblongifolium O-C, lower slopes and summit 0, patches on summit ridge Pellaea falcata 0, summit only -Phymatosorus pustulatus S, summit only -

Pyrrosia eleagnifolia S, tree trunks on summit -

Dicotyledons Apium prostratum - S, on boulder beach Brassica l oleracea* LC, summit -Calystegia sp. S, 2 seedlings on summit -Cardamine debilis S, lower slopes, 1 patch on summit -Carmichaelia australis - 0, shrubs and saplings Cirsium vulgare* S, lower slopes, 0, summit S, 1 on summit Clematis paniculata S, summit only -

Conyza albida* 0, lower slopes and summit S, summit Coprosma macrocarpa C, trees and shrubs on summit S, 1 tree C. repens C, fringe of small trees on lower A, large trees, stunted shrubs

slopes and seedlings, abundant around fringe of island

Coronopus didymus* S, 1 plant -Cotula australis S, lower slopes, C, summit -Crassula sieberiana 0, lower slopes, C, open summit -

Dichondra repens 0, lower slopes, C, summit LC Disphyma australe A, lower slopes, C, clifftops A Einadia trigonos C, lower slopes, 0, summit A, throughout Geniostoma rupestre S, 1 shrub on summit -Geranium retrorsum L, summit clearing -Haloragis erecta S, 2 shrubs on summit -Hebe pubescens S, 2 shrubs on summit -Hypochoeris radicata* S, summit only -Lepidium oleraceum 0, 11 plants at northern end S, 7 plants present in

of summit. AK 229707 1988. AK 229708 Linum monogynum 0, 12 plants northern end S

of summit AK 229706 Lycium ferocissimum* 0, cliffs and summit, large shrubs S, several large shrubs but many

seedlings Macropiper excelsum S, several shrubs on summit -Melicytus novae-zelandiae C, lower slopes, A, summit A, dominant cover over summit M. ramiflorus S, lower slopes, C, summit S, 1 shrub Metrosideros excelsa 0, lower slopes and summit 0, large shrubs, several large

trees Muehlenbeckia complexa 0, lower slopes and summit -Myoporum laetum A, lower slopes and summit C, summit Myrsine australis S, 1 plant on summit -Orobanche minor* - S Oxalis rubens S, lower slopes, 0, summit S, 1 patch (no flowers) Parietaria debilis 0 S Peperomia urvilleana 0, on cliffs -

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Phytolacca octandra* Pimelea cf. urvilleana Pittosporum crassifolium Polycarpon tetraphyilum* Pouteria costata Pseudognaphalium luteoalbum agg. Pseudopanax lessonii Samolus repens Sarcocornia quinqueflora Senecio sylvaticus*

S. hispidulus S. lautus Silybum marianum* Solanum americanum S. aviculare

S. nigrum Sonchus oleraceus* Spergularia media Streblus banksii

Tetragonia trigyna

O, lower slopes, C, summit

S, lower slopes, 5 trees on summit C, lower slopes and summit S, 1 tree on summit S

LC, at least 17 trees on summit

O S, c.12 plants on summit clearing. AK 229705 O, lower slopes and summit O S, lower slopes only O, lower slopes and summit

S, summit only C, lower slopes, S, summit

S, lower slopes, A, summit

S, 2 plants O, open areas at north end C, throughout S, summit only

O, large trees LC, especially northern end C, near shoreline

S C

C O, 1 on boulder beach, few on lower slopes and summit ridge

O LC, north end S, 1 tree (10cm basal trunk, 3m tall) S, summit

Monocotyledons Arthropodium cirratum Astelia banksii Bromus hordeaceus* Carex flagellifera Cortaderia splendens Dichelachne crinita Elymus multiflorus Isolepis cernua Isolepis nodosa Lachnagrostis billardierei L. littoralis Microlaena stipoides Oplismenus imbecillis Paspalum vaginatum* Phalaris aquatica* Phormium tenax Poa anceps

Rytidosperma sp. Trisetum antarcticum agg. Vulpia bromoides*

LC, cliffs, O, summit S, lower slopes and summit S, local 0, under scrub

O, summit C, summit

S S, lower slopes and summit O, southern slopes S O-C, summit only

O, southern slopes O, lower slopes, LC, summit S, several patches on summit

O, summit S, summit AKU 22489 O

0 O, 18+ plants on island

O AKU 22491 O O

O

LC

O, under forest, summit ridge, north end

DISCUSSION

Comparison with other islands in the Mercury Group

Stanley Island has the second largest published list of vascular plants for

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islands in the Mercury Group (Table 1). Although 180 species were found, this represents only 43% of the species recorded on nearby Great Mercury Island, including only half the number of dicotyledons and a third of the monocotyledons. This discrepancy is not surprising as Great Mercury Island is 18.6 times larger (1860 ha) and has a much greater diversity of habitats than Stanley Island. Wright (1976) identified 12 main plant communities including open exotic pasture, wetlands and sand-dunes, all of which are absent on Stanley Island. Also, old stands of kanuka (Kunzea ericoides) forest were common on Great Mercury in 1975 but only one kanuka tree was found on Stanley Island.

The flora of Red Mercury Island is clearly under-recorded as the published list has only 75% of the total species found on Stanley Island (Table 1). Red Mercury is twice as large as Stanley, has more diverse habitats and has always lacked rabbits (Lynch et al. 1972). The flora of Red Mercury Island has a strong similarity to that on Stanley Island. There are extensive stands of mapou and pohutukawa/mahoe forest as well as regenerating manuka shrublands. The majority of the species (75%) found by Lynch et al. (1972) on Red Mercury Island also occur on Stanley Island. Notable absences from Stanley Island which are present on both Great Mercury and Red Mercury Islands are tawa (Beilschmiedia tawa), Pittosporum umbellatum, fivefinger (Pseudopanax arboreus), N Z jasmine (Parsonsia heterophylla) and the sedge Morelotia affinis. Nikau (Rhopalostylus sapida) has been reported from Red Mercury Island but has not been found on Stanley or Great Mercury Islands (Wright 1976).

The flora of both Double and Korapuki Islands also appears to have been under-recorded, as both islands have fewer listed plants than nearby smaller Middle Island (Table 1). Despite this, three species on Atkinson's (1962) Double Island list were not found on Stanley Island; N Z jasmine, fivefinger and Morelotia affinis. Only one species recorded on Korapuki Island by Hicks et al. (1975) (Veronica plebeia) was not located by us on Stanley Island.

The vegetation of the smaller islands in the Mercury Group has already been compared with other small unmodified (rodent-free) northern offshore islands by Taylor et al. (1990). The Inner Stack has all ten plant species found on most of these small unmodified islands and the Outer Stack has nine of these species; only wire vine (Muehlenbeckia complexa) is absent. The flora of the Outer Stack is of a similar size to that of Green Island (cf. 49 species on Green Island, 45 species on the Outer Stack) (Table 1). However the composition is different with only 60% of the plant species shared by both islands. The larger and more sheltered Green Island had several additional species of coastal trees and shrubs such as tawapou and kawakawa (Macropiper excelsum), whereas the vegetation on the more exposed Outer Stack was lower in stature (up to 4m) and had correspondingly more herbs and grasses.

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Table 1. Comparison of the vascular flora of islands in the Mercury Group.

Island Size (ha)* Ferns Conifers Dicots Monocots TOTAL % Native Source

Great Mercury 1860.00 63 6 226 119 414 61 Wright 1976

Red Mercury 225.00 23 1 86 26 136 86 Lynch et al. 1972

Stanley 100.00 25 0 111 44 180 77 this paper Double 32.50 10 0 60 16 86 90 Atkinson 1962

Korapuki 17.50 12 0 60 13 85 85 Hicks et al. 1975 Middle 13.10 9 0 65 22 96 74 Cameron 1990 Green 2.30 3 0 39 7 49 85 Atkinson 1964 Outer Stack, Stanley I 1.25 3 0 34 8 45 82 this paper Inner Stack, Stanley I 0.36 5 0 47 17 69 80 this paper Stack West of Green I 0.15 1 0 13 4 18 94 Taylor et al. 1990 "Haunted House" Middle I stack <0.1() 1 0 17 6 24 87 Cameron 1990

* Island areas (ha) from Taylor (1989)

Past vegetation cover

Stanley Island was probably burned by the early Maori occupants, and stone walls on the western slopes suggest that kumara crops were probably cultivated on the island. The current mosaic of forest types may reflect a series of fires in the past century, or different regeneration patterns in response to wind exposure, soil fertility or soil moisture gradients. The brief descriptions of the island in the 1960s (Edgar 1961, Thoresen 1967) reveal that Stanley Island had not been burnt for some time prior to their visits. These accounts of the 1960s mention that the vegetation was dominated by extensive areas of dense scrub. While this is true of some areas today, clearly much of the shrublands have grown into taller forest in the past 30 years.

The two taraire groves and scattered large trees of tawapou, titoki, coastal milk tree, karaka, wharangi and kohekohe suggest that the former cover of Stanley Island was a diverse coastal forest. The occurrence of taraire on Stanley Island was a surprise as this species is absent from nearby Red and Great Mercury Islands. However, Hayward (1976) found fossilised taraire leaves on Great Mercury Island, suggesting that the Stanley Island groves are remnants of the original forest cover in the Mercury Group. The nearest extant taraire forests are on the northern Coromandel Peninsula (Wright 1984). One taraire tree was found on Cuvier Island by T G L (unpubl.). Ian Atkinson (pers. comm.) also found a taraire seedling there in 1993 and mentioned that a number of taraire seedlings have established recently on Double Island in the Mercury Group. The southern geographical limit of taraire is at East Cape (Wright 1984). Most of the other coastal hardwood species on Stanley Island are also reported elsewhere in the Mercury Group. Therefore these species are presumably remnants of the original forest cover on Stanley Island. Currently, no conifers are present on Stanley Island although a small number of kauri (Agathis australis) have been found on nearby Great Mercury Island (Wright 1976). These are the only native conifers reported in the Mercury Group.

We measured samples from 15 tree species in the forest remnants in 1989 to determine their diameter at breast height (dbh) where breast height is 1.5m (Table 2). The tawapou and taraire trees sampled reveal a pattern of middle- or old-aged trees with very little recent recruitment (Fig. 6). These species and puriri were present only as canopy trees in these remnants and prior to the rabbits being removed in 1991, we found no seedlings or saplings of either species. However, seedlings of all these species were present in 1993. One large puriri had a basal girth of approximately 77cm and ten large limbs sprawling over rocks.

Kohekohe had smaller dbh measurements than the previous species (Fig. 6)

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T A W A P O U (n=37)

Number of

trees

CM 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 dbh (cm)

T A R A I R E (n=43)

Number of

trees

0-4 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 dbh (cm)

K O H E K O H E (n=29)

Number of

trees

0-4 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45^9 50-54 55-59 dbh (cm)

Fig. 6. Diameter at breast height (dbh measured at 1.5m) of tawapou, taraire and kohekohe trees on Stanley Island, Mercury Group. Tawapou and taraire were mainly large, with little evidence of recent regeneration whereas kohekohe were mainly small to medium-sized trees with very few large (old) trees present in 1989.

suggesting that kohekohe may have been able to establish seedlings and saplings despite rabbit browsing. For example, we found numerous kohekohe saplings in the vicinity of the Arena. Perhaps the rabbit population on Stanley Island

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Table 2. Mean diameter in cm ( ± standard deviation and range) at breast height (1.5m) of a sample of trees on Stanley Island. For multi-limbed trees, the largest limb was measured 1.5m from the base.

Mean dbh S Range Tree Species n (cm) (cm) (cm)

Alectryon excelsus 3 25.8 15.0 11.8 - 41.7

Beilschmiedia tarairi 43 29.8 10.0 7.5 - 59.2

Coprosma macrocarpa 1 - - 17.2

Corynocarpus laevigatas 9 17.9 10.5 4.5 - 32.8

Dysoxylum spectabile 29 18.8 12.0 5.1 - 54.4

Hedycarya arborea 5 21.2 12.9 5.4 - 39.5

Melicope ternata 11 18.1 6.3 9.5 - 30.2

Myoporum laetum 3 40.8 X.7 35.7 - 50.9

Nestegis apetala 2 - - 24.2 - 44.6

Nestegis lanceolata 5 40.4 7.9 26.7 - 46.2

Pisonia brunoniana 11 12.4 4.5 4.1 - 19.1

Pouteria costata 37 34.5 11.6 16.6 - 54.1

Pseudopanax lessonii 3 22.6 6.3 15.6 - 28.0

Streblus banksii 9 24.9 14.8 6.3 - 55.7

Vitex lucens 6 39.2 8.9 26.4 - 53.4

declined in recent years as the forests regenerated and the areas of open grasslands reduced. If this occurred, it may have enabled kohekohe seedlings to establish in the past decade or two.

Other trees measured showed they were old. The largest coastal milk tree was 9m tall and had a dbh of 55.7cm. Another milk tree was 11m tall and 34.7cm dbh. The two largest white maire were found at the Arena and were 16-18m tall and 46.2 and 45.5cm dbh respectively. Nearby were two coastal maire; one was 13m tall and had a 24.2cm dbh, the other was 12m tall and had a 44.6cm dbh (Table 2). These large trees presumably established prior to rabbits being introduced to Stanley Island. No seedlings were found of either maire species in 1993.

Eleven parapara were present in the two taraire groves. A l l were large trees (maximum dbh 19.1cm) or saplings; no seedlings were present (Table 2). Parapara is largely confined to the northern offshore islands (Sykes 1987). Elsewhere in the Mercury Group, the species occurs on Great Mercury Island where Wright (1976) reported large old trees between 10-20cm dbh.

Adult parapara are present locally on Double Island and seedlings were common in some plots measured on a recent trip (I. Atkinson, pers. comm.

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1997).

Effects of introduced mammals on the vegetation

Kiore and rabbits are likely to have had a profound effect on forest regeneration on Stanley Island. Visitors in the late 1980s could be forgiven for thinking that fewer than 20 plant species occurred on the island for that was about all the species that could be seen on a walk from the northern landing to the campsite. The forest understory, where it occurred, was bare of seedlings and regeneration, and consisted mainly of dense single species stands of sapling mahoe or mapou. A high proportion of plants were scarce or very locally distributed. In the 1980s the best way to interpret the effects of the introduced mammals on Stanley Island's vegetation was by comparing the flora of the main island with that on the offshore stacks or other islands in the Mercury Group. The eradication operation in 1991 provided an opportunity for a more direct comparison by enabling observations of early changes in the flora during the 1993 visit.

A comparison of the vegetation on the stacks provided an initial guide to the impacts of rabbits and/or kiore. Several species of plants were common on both offshore stacks (where rabbits have always been absent) but were rare on the main island. These included ngaio, coastal mahoe, taupata, Asplenium haurakiense, N Z ice plant, puha (Sonchus oleraceus), Mercury Bay weed, Einadia trigonos and glasswort. The main factor limiting these plants on Stanley Island was probably rabbits although kiore may have had some effect through predation of fruit and seeds (Campbell 1978). A possible test for this hypothesis would be to examine the flora of nearby islands in the Mercury Group. For example, rabbits were absent from Red Mercury Island (but kiore were present in the 1980s), whereas Korapuki Island previously had both rabbits and kiore. Lynch et al. (1972) reported that ngaio, taupata, puha, Mercury Bay weed, N Z ice plant and glasswort were common on Red Mercury, but coastal mahoe and Asplenium haurakiense were only found occasionally and Einadia trigonos was rare. Observations from Korapuki (Hicks et al. 1975) support the hypothesis that these species are browsed mainly by rabbits, as all the above species were rare on Korapuki, or mainly confined to cliffs.

Four plant species (karo, shore groundsel, Carmichaelia australis and Spergularia media) were common on the Outer Stack but were scarce or occasional on the Inner Stack and main Stanley Island. Possibly kiore limited the spread of these plants directly by barking trees, chewing foliage or seed predation (Atkinson 1986); or indirectly by predation on small petrel eggs and chicks (Imber 1978, Booth et al. 1996). The dense concentrations of seabird burrows

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on the Outer Stack provides soils rich in nutrients (Atkinson 1964) and some of these plant species may prefer nutrient rich soils. Again to test these ideas, it is instructive to compare the abundance of these plants on nearby islands. Surprisingly, all of the species are common on Red Mercury except Spergularia media which has not been recorded there (Lynch et al. 1972). On Korapuki Island, all species were reported by Hicks et al. (1975) as frequent except Carmichaelia australis, which was not found by Hicks et al., but is present on the island although not abundant (I. Atkinson, pers. comm., 1997).These confounding results do not support the view that kiore affect the distribution of these species.

Atkinson (1986) considered that the regeneration of karo and coastal milk tree was limited by kiore. Possibly the substrate of the island may influence the effects of kiore on plant regeneration. For example, Ian Atkinson (pers. comm., 1997) has suggested that some recruitment may be possible if a proportion of seeds fall into crevices on a talus slope and are out of the reach of kiore. The differences tend to be expressed in reduced rates of recruitment rather than total presence or absence of a species. Atkinson (1964) reported that karo and shore groundsel were abundant on Middle and Green Islands (both rat-free), and this finding does suggest some influence of kiore on these plant species on Stanley Island.

De Lange et al. (1995) considered kiore to have an impact on forest regeneration on Fanal Island, Mokohinau Group. Kiore are the only introduced mammals on this island. Although mature trees were sometimes common, de Lange et al. (1995) found no karo or coastal milk-tree seedlings, only one parapara seedling, and puriri, tawapou, taraire and tawa seedlings were scarce. Clearly, the regeneration processes and habitat preferences of plant species on islands with and without kiore needs further study.

There were also other plant species that were apparently browsed by rabbits on Stanley Island which were common on nearby Red Mercury Island (suggesting rabbits were the main factor limiting their regeneration). Kawakawa (Macropiper excelsum) was only locally common on Stanley Island and seedlings were absent except on rock outcrops. Tawapou were only present as mature trees and seedlings and saplings were scarce. However, tawapou of all ages were noted on Red Mercury Island in 1991 (GAT pers. obs.). Hangehange, rangiora, Cassinia leptophylla and Calystegia soldanella were all relatively uncommon on Stanley Island but were abundant or common on Red Mercury Island (Lynch et al. 1972). These species and the others noted above are all expected to show rapid regeneration on Stanley Island now that rabbits have been removed.

By 1993, there were already obvious changes in the vegetation as a result of the eradication of rabbits and kiore. The open grasslands on the clifftops had tall

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flowering grasses and herbs where there had only been short stubble in the late 1980s. The zone behind the northern landing had developed into a mixed coastal herb field. N Z ice plant was common and other browse-sensitive species had appeared, e.g. shore groundsel. The most remarkable change was on the large open area at the southern tip of the main island. Instead of being virtually bare soil and rock, this area had a lush growth of Pimelea cf. urvilleana, grasses and herbs including N Z ice plant and shore groundsel but also a profusion of weeds, e.g. Lotus pedunculatus and puha.

Rabbits probably browsed out most of the palatable species in the pohutukawa/mahoe forest as the diversity and structure of this forest type was starting to change by 1993. Coastal karamu (up to 0.5m tall) was becoming common in more open areas and pigeonwood, taraire and ngaio seedlings were encountered in sites well away from any parent trees. Seedlings of puriri, tawapou, pigeonwood, karaka and taraire were found amongst the litter in the remnant forest groves. New Zealand pigeons {Hemiphaga novaeseelandiae) were present in the old forest remnants on most visits and the presence of this important disperser of large-seeded species, e.g. tawapou, taraire, is a good sign for future forest regeneration on Stanley Island.

Threatened plant records

Two threatened plant species were found on Stanley Island. Cook's scurvy grass (Lepidium oleraceum), listed as Endangered by Cameron et al. (1995), is now largely confined to offshore islands. Four plants were found on a boulder bench below cliffs in the south-west bay in a site inaccessible to rabbits. A group of 11 plants was discovered on the summit of the Inner Stack in 1989 and seven were present on the Outer Stack in 1988. The small number of Cook's scurvy grass plants found on these islands was surprising in view of the abundant petrels and high soil fertility, which usually favours the growth of this species (Norton et al. in press).

Mawhai (Sicyos australis), listed as Vulnerable by Cameron etal. (1995), was present in two places on Stanley Island, sprawling over vegetation behind boulder beaches. This species is declining in New Zealand and is now also mainly confined to northern islands (Cameron 1992). The Stanley Island plants still looked healthy in 1993, but they showed no early sign of spreading following the removal of rabbits.

Problem weeds

The intensive browsing of rabbits apparently had the effect of eliminating

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many weeds before they established on Stanley Island. Very few of the 43 adventive plant species recorded on Stanley Island and its two offshore stacks pose a long-term threat to plant regeneration or the indigenous biota. The majority of the adventive species we recorded were herbs and grasses that only colonise disturbed soils or open ground. However the long-term impact of rabbits on the island's ecology may soon become apparent, as the open vegetation structure induced by rabbits will make habitats on the island much more susceptible to weed invasion in the future.

Privately-owned Great Mercury Island is the largest and most modified island in the group and appears to be a weed seed source to the other less disturbed islands. Wright (1976) recorded 161 naturalised or planted species on Great Mercury (see also Table 1), which is the only island in the Mercury Group that has not been left to totally regenerate. Thirty-four of the 45 adventive plant species found on Stanley Island and its two larger stacks were also recorded on Great Mercury in 1975 (Wright 1976).

Three weed species warrant close attention and should be controlled in the near future. Two species of pampas grass (Cortaderia selloana and C. jubata) were found on the main island. Only three plants were located in 1993; one on the north-east summit, one in the valley below the northern red slip and one behind the western sandy beach. Seedlings from these mature plants are quite likely to establish in the near future in open sites and all these plants should be removed urgently.

The remaining problem weed is boxthorn (Lycium ferocissimum). The removal of rabbits from Motunau Island in North Canterbury bought about a rapid increase in this weed once browsing pressure stopped (Cox et al. 1967). Only three boxthorn plants were found on Stanley Island but large shrubs and seedlings were present on the cliffs and summit of the two offshore stacks. The thorns of this species pose a serious threat to small burrowing petrels (Bell & Blackburn 1960). Boxthorn has been present in the Mercury Group since at least 1962 when Atkinson (1964) found a single plant on Green Island. Boxthorn probably reaches the islands in bird excreta, as the fleshy fruits are eaten by birds such as starlings (Sturnus vulgaris) (Cameron 1990). Grace (1976) reported that large flocks of starlings (up to 300 birds) were present on Great Mercury Island, and at dusk birds flew south-east to roost on other islands in the Mercury Group. Cameron (1990) reported starlings roosting on Middle Island in their hundreds during his visits in 1983. There wil l need to be on-going control of boxthorn, especially on the two offshore stacks and other islands in the Mercury Group.

On-going environmental weed monitoring should be carried out (+ every 2 years) on all the important Mercury Islands as other species of environmental weeds will continue to spread from modified mainland sites. For example, on the

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Poor Knights Islands (20km offshore), Mexican devil (Ageratina adenophora), mist flower (Ageratina riparia), pampas grass and moth plant (Araujia sericifera) have all managed to reach these islands in recent years (P.J. de Lange & E . K . Cameron pers. comm., 1996).

ACKNOWLEDGEMENTS

Permission to visit Stanley Island was granted by the Hauraki Gulf Maritime Park Board and Department of Conservation, Northern Region (1987-89) and Waikato Conservancy (1993). Neil Hopkins provided reliable transport on MV Maire. Ewen Cameron (Curator of Botany, Auckland Museum) helped with plant identifications and provided many helpful comments on earlier drafts of this paper. Thanks also to the volunteers who helped on visits to Stanley Island, especially Terry and Lindsay Hatch, Alan Tennyson and David Riddell. Ian Atkinson provided unpublished field observations from his long-term studies in the Mercury Islands and made constructive comments on the manuscript.

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I l l

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