Ewen Cameron

  PICTURED: Ewen Cameron  

Botanist Ewen Cameron has a head for heights and sense of adventure

By . Image credit Supplied. 2016 september

For over three decades I’ve been scrambling over rock stacks and islets of northern New Zealand recording their plant life. Most have never been surveyed before so there is an air of excitement when visiting one for the first time. Physical difficulty of access is all part of the challenge which may involve quite an adventure to get ashore, or to reach cliff-vegetation. Unusual native plants or surprising weeds all help to indicate the wider picture: the current distribution of the native and naturalised flora. Islets with seabird populations and no rats make interesting comparisons with those lacking seabird colonies and with rats. Ideal prerequisites for islet study: surfing background; enjoy boating, kayaking and swimming; a head for heights; and a sense of adventure.

There are 351 islands ≥0.1 ha in the Hauraki Gulf and 67% of these are 0.1–1.0 ha in size. Many small islands are not formally owned and exist in legal limbo as “uninvestigated”.

Sicyos mawhai I.Telford & P.Sebastian
Fig 1. Herbarium specimen of the native cucumber, mawhai, collected by Thomas Cheeseman on Otatau [Otata] I. (15 ha) in 1883 (AK 9196). This At Risk species no longer occurs on the Noises Islands, but still occurs on a few outer Gulf islands. It has been shown to be susceptible to Cucumber Mosaic Virus and therefore cultivation of any cucurbits near mawhai put it at risk from CMV.

Historical botanical surveys of northern islets
The Auckland Museum herbarium holds over 30,000 plant specimens, including seaweeds, from the Hauraki Gulf and its islands dating back to a spinifex grass specimen from “Wai-he-ke” collected by Captain James E. Home in 1852 (AK 92728). Thomas Cheeseman collected from several small Gulf islands during the 1880s–1919. (Fig. 1). The label data of all these specimens is available at: http://www.aucklandmuseum.com/collections-research/collections, and many also have images attached.

English naturalist, Mary Gillham, in the late 1950s surveyed many small seabird islands, including the Mokohinau Is. and Horuhoru (Gannet) Rock (1.3 ha). Ecologist Ian Atkinson surveyed many from the 1960s onwards, including Sail Rock (2.1 ha) which he recorded as the least modified of the small Gulf islands so far described. Botanist Alan Esler surveyed many including 28 islands in the 1970s off the west coast of the Coromandel Peninsula. Councillor and ecologist Michael Lee surveyed seven islets by Waiheke I. using a kayak in the late 1990s.

Commonest natives and naturalised species
A comparison of 15 relatively undisturbed, exposed northern New Zealand islands, 0.2–3.6 ha in size, showed the most commonly occurring plants were all native species: taupata, NZ ice-plant, glasswort, coastal spleenwort, Mercury Bay weed, karo, shore groundsel, fishweed (Chenopodium spp.), knobby sedge, coastal mahoe, pohuehue, and Poa anceps. Common features of these plants includes: tolerant of seawater, low stature and often succulent. The commonest naturalised species only occurred on 5–8 of the islands: puha, catsear, inkweed, fleabane, boxthorn, scarlet pimpernel, prairie grass, and black nightshade. Apart from boxthorn most of these species are herbaceous and fairly harmless.

However, aggressive weeds on more modified small islands which suppress the vegetation commonly include: rhamnus, smilax, sweet pea shrub, mile-a-minute (Fig 2.), bone-seed and boxthorn. The indigenous percent of the flora roughly reflects of the health/naturalness of the island, ranging from 100% indigenous on Tatapihi (Groper) I. (Fig 3.) down to 28% on the commercial chert quarry on Karamuramu I. out from Kawakawa Bay (Table).

Dipogon lignosus
Mile-a-minute Dipogon lignosus
Fig 2. Mile-a-minute (Dipogon lignosus) – a South African climber with white or pink flowers covers much of Ruapuke (Maria) I. of the Noises group. Present on the island for over four decades, it is proving the most difficult weed to eradicate from the group. It has also established on other Gulf islands, e.g. Motukaha, Moturekareka.
Tatapihi, Groper I 1994
Fig 3. Tatapihi (Groper I.) Mokohinau Is., Sep 1994 – Auckland’s most remote island, at 3.0 ha it is the largest Gulf island with no exotic plants, and supports 20 vascular plant species, 2000 pairs of diving petrels and seven lizard species.

Threatened species
At least four Nationally threatened plant species occur on the small Gulf islands, e.g. Senecio scaberulus, Cook’s scurvy grasses (Lepidium flexicaule, L. oleraceum) and Geranium retrorsum (Fig 4). Another nine species are Nationally At Risk. The rat-free islets are often also home to threatened fauna including: seabirds, lizards and invertebrates (Fig 5).

Geranium retrorsum
Fig 4. The Nationally threatened Geranium retrorsum, hangs on the edge of the exotic grass sward with plantains and onion weed on the flat summit of Watchman I. (Te Kākāwhakaara), near the Harbour Bridge, 26 Sep 2006. It also occurs on several other small Gulf islands.
mokohinau-stag-beetle-jan-1984-ec
Fig 5. Male Mokohinau stag beetle (Geodorcus ithaginis), Critically Endangered – only known from one small stack in the Mokohinau group, presumably eliminated by rats from the rest of the group. The entire Mokohinau group is now rat free.

Ornithocoprophilous species
Gillham noted that many plants thrived on these guano-rich islands, e.g. ice-plant, taupata, shore groundsel, Cook’s scurvy grasses (Fig 6) , fishweed (Chenopodium species), glasswort and sea celery (Apium prostratum) and she used Robert Ornduff’s term ‘ornithocoprophilous’ for these guano-associated plants. Many of the special “island” plants are restricted to, or are at their most luxuriant, on islands with high amounts of seabird guano. The loss of guano and vegetation disturbance, coupled with the increase of weedy grasses, appears related to the decline of several ornithocoprophilous species, including the coastal threatened Cook’s scurvy grasses. The tiny terrestrial green alga (Prasiola stipitata) usually < 5 mm tall occurs as “green fuzz” (Fig 7) on rocks by guano ooze associated with seabird colonies.

Cook’s scurvy grass Geranium retrorsum
Fig 6. White-flowered Cook’s scurvy grass, a Nationally Endangered species, growing with the yellow-flowered native shore groundsel in the guano ooze from the gannet colony above. Mahuke I., Broken Is., Great Barrier I., Nov 2009.
Gannets on Horuhoru rock
Fig 7. The tiny guano alga, Prasiola stipitata, grows to 5 mm tall and appears as green fuzz on coastal rocks by seabird colonies. Horuhoru (Gannet) Rock, Sep 2007.

Conclusions – Small islands:

  • special habitats, home (refugia) to many threatened biota
  • ignored by most people
  • weeds slowly increasing (islands may act as ‘stepping stones’)
  • most pests (plant or animal) can easily be removed
  • baseline recording of biota will lead to future changes being able to be quantified
  • ideal study sites to monitor change
  • their biological importance is out of proportion to their small size.

For a fuller account with references see:
http://bts.nzpcn.org.nz/bts_pdf/ABJ61%282%292006-99-108-Offshor.pdf

 

Table. Comparing the native and naturalised components of six small Gulf island floras.

 

Island Size (ha) Total vascular flora (% native) Feature of the island
Tatapihi (Groper) I., Mokohinau Is. 3.0 20 (100) Remote seabird island
Sail Rock, Bream Bay 2.1 45 (87) Remote seabird island
Horuhoru (Gannet) Rock, near Waiheke I. 1.3

15 (73) A gannet colony
Ruapuke (Maria) I., Noises Is. 1.1

71 (44) A seabird island
Watchman I. (Te Kākāwhakaara), Waitemata Harbour 0.1

27 (30) Highly modified inner Gulf island
Karamuramu I., out from Kawakawa Bay 7.3

99 (28) A chert quarry

 

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