- summit to sea
by John Ogden
Hobson) is the highest point on Great Barrier Island, visible from
almost anywhere on the island, and on a good day from Auckland. The
mountain, buttressed on all sides by rugged volcanic terrain, rises from
sea level to 627m, over less than 4km to the west coast and 5km to the
east. This altitudinal change, over a relatively short distance, on
Hauraki Gulf’s outer-most island, has seen distinct ecosystems
develop—from the summit to the sea.
For many people the
walk to the summit of Hirakimata, and the splendid views of the island,
Little Barrier, the Coromandel Peninsula, north to the Mokohinau and Hen
and Chickens islands, and south to Cuvier are the highlights of a visit
to Great Barrier. Thousands of visitors sign the book on the summit
every summer, while others prolong the experience by spending a night in
the splendid Department of Conservation hut on nearby Mount Heale.
petrel on the nest. Data indicates a slow decline for this species —
the bird’s main breeding colony is on Hirakimata’s summit.
The overnighters, if
they know to look, will see Great Barrier’s most iconic endangered bird,
the black petrel (Taiko. They will surely hear the eerie staccato
calls as the birds come in after dark to their nesting burrows in the
summit forest. They’ll probably see the smaller, paler Cook’s petrels
too, and on fine nights, long-tailed bats.
Forest zonation with
The upper montane
forest1 (Figure 1) that supports these birds is an ancient
mix of conifers and broadleaf trees, found only in a few other places –
mountain summits on infertile volcanic rocks – in Coromandel and
Northland. This zone, from 400m above sea level (a.s.l.) to the summit,
contains yellow silver pine, monaoa, Kirk’s pine, and toatoa, mixed with
the more familiar rimu, miro and kauri. Broadleaf trees include species
more familiar from the South Island’s West Coast, such as tawheowheo,
southern rata, tawairi and the rare creeping rata, Metrosideros
parkinsonii. This forest type was commoner during a cooler, wetter
period over 30,000 years ago — it is a relic ecosystem, with species not
seen elsewhere on Great Barrier.
Figure 1. Distribution of selected
trees, shrubs, tree ferns and other species on transects from sea
level to the summit of Hirakimata (627m). Black bars are species
found only on Hirakimata or a few adjacent summits. Green bars are
species found on all the main mountains of Great Barrier Island and
usually at sea level too. Bars indicate presence only at that
altitude, without reference to abundance.
*Kunzea sinclairii is the
creeping kanuka, found only on Hirakimata.
This tall dense
forest, surviving in the damp clouds which often hide it from view,
passes abruptly into the lower structure of the montane forest zone
below 400m. The sharp boundary was created by fires during European
kauri-logging and would previously have been more gradual – indeed
many species transgress it. More open vegetation is evident from 400
to 200m a.s.l., dominated by shrubby plants or small trees, though
most of it was kauri forest once. A marked change occurs across this
zone, with seven lowland plants being added and six upper montane
Three species found
only on Great Barrier Island inhabit open rocky areas – creeping
tea-tree (Kunzea sinclairii), Allom’s daisy tree (Olearia
allomii) and Rehua’s hebe (Hebe macrocarpa var. rehuarum).
pubescens var. rehuarum) Rehua’s hebe – is found only on
Great Barrier Island.
Creeping tea-tree (Kunzea sinclairii) spreading over
volcanic rocks on Hirakimata.
(shrub daisy) is endemic to Great Barrier Island.
The lower forests,
in the Kaiaraara valley for example are a diverse mix of about 20
tree species, often dominated by puriri, kohekohe and the two types
of tawa (Beilschmeidia tawa, B. tawaroa) but with
emergent conifers; kahikatea in the valleys, rimu on the slopes and
kauri or totara on the ridges.
The montane forest of
Hirakimata contains more native forest bird species than any other
forest area on Great Barrier.
The coastal strip
is dominated by pohutukawa and some coastal shrubs, although
throughout Great Barrier much of the lowland zone has been cleared
and now carries manuka or kanuka (under which are seedlings of
future forest trees).
From the coast to
the summit, a continuum can be seen, of ever-changing vegetation
composition dependent on the altitudinal preferences of species, and
their responses to past disturbances, especially fire.
distribution with altitude
pattern is reflected in other components of the ecosystem, such as
the birds. The montane forest of Hirakimata contains more native
forest bird species than any other forest area on Great Barrier
(Figure 2). There are tomtits and now a few robins, two species
which are not readily seen or heard elsewhere on the island. The
robins have colonised since being re-introduced at Windy Hill and
Glenfern sanctuaries, and may survive in this high altitude forest
due to reduced predation by rats. Of course birds move between
zones, so that while kakariki and long-tailed cuckoo are
characteristic of the upper forest, they also occur at lower
altitudes —indeed kakariki nest in the splendid remnant of lowland
forest at Okiwi.
Hirakimata other native ecosystems of high conservation value are
linked to the mountain through Maori tradition, and by their
geology, hydrology and ecology. Visible from the summit ridge of
Hirakimata are a series of ‘satellite reserves’. To the north is
remote Te Paparahi, the final refuge of kokako before the last pair
were removed to Hauturu in 1996. Ngati Rehua Ngati Wai ki Aotea are
planning to restore the area and return this iconic bird. This is a
project which will require large-scale pest control.
To the north-east
of Hirakimata are the Whangapoua Estuary and Okiwi spit. The estuary
contains a well-documented transition from mangroves, through
saltmarsh to fresh-water ecosystems3. It is an essential
feeding ground for New Zealand dotterel, pateke, variable
oystercatcher and various migrant waders4. The area has
great cultural significance and is still an important source of
kaimoana. The enclosing dunes of Okiwi spit have the best native
dune vegetation on the island, and contain evidence of past
sea-level fluctuations, tsunami and Maori middens.
Figure 2. Overall number of native
bird species recorded (light grey); or combined (all species)
density per hectare estimates (dark grey) for different locations2.
The Department of
Conservation manages the Okiwi Basin to conserve pateke. Clearly
visible from the beach are the cliffs of Rakitu, also a DoC reserve,
mooted as the next big pest eradication project on the island.
Looking to the
south east, the Kaitoke Swamp (Te Puea) is the largest area of
semi-natural wetland in the Auckland region and is a ‘Special Place’
in the island’s Conservation Management Strategy. It is probably the
last refuge for the nationally endangered bittern5 on
Great Barrier and the spotless crake, as well as pateke and banded
rail. The largest population of fern birds6 can also be
found here. Plant communities and the vegetation history of the
swamp have been described in detail7 and include at least
one orchid regarded as nationally critical (Prasophyllum hectorii).
value of the Kaitoke Swamp cannot be over-stated including as a
significant freshwater reservoir7. Unfortunately, this
ecosystem is being invaded by alien plants such as Mexican devil.
Rats and feral pigs are also present.
country lies to the south and west of Hirakimata’s summit, with some
remnant forest and much regenerating kauri in the Wairahi Reserve.
Some of these kauri stands are infected with Phytophthora
taxon Agathis (PTA), a soil-borne pathogen capable of killing mature
and younger kauri trees. Steep streams in this hill country, such as
the Kaiaraara, descend to flooded valleys with small areas of
saltmarsh and mangroves.
Much of the
coastline is still wild, and there are likely to be small colonies
of fluttering shearwater and other birds on the rocky headlands.
Forming a ‘twin’ to Rakitu, Motu Kaikoura stands across the entrance
to Port FitzRoy with Glenfern Sanctuary just to the north. Both
these areas have well documented flora and fauna, and both are
managed for conservation and outdoor education.
But Hirakimata is
the central and most significant reserve on Aotea. The mountain’s
ecosystem is like no other, is surrounded by a network of accessible
reserves linking the summit to the sea, including Okiwi Park and
the spit, Whangapoua Estuary, Kaitoke wetland and Glenfern
Sanctuary, mostly located close to the centre of Department of
Conservation’s activities – the new headquarters at Okiwi (Figure
Figure 3: Map of part of Great Barrier
Island, centred on Hirakimata. The black outline encloses most of
the main ridge of Hirakimata and the main black petrel nesting
colony. Blue circles indicate the ‘satellite reserves’; starting at
the top, moving clockwise: Te Paparahi, Whangapoua Estuary and Okiwi
spit, Rakitu, Kaitoke wetland, Wairahi Forest Sanctuary, Motu
Kaikoura, Glenfern Sanctuary, Kotuku Peninsula and Okiwi Park. The
circles are not proportional to size but give an indication of
location. The red square is the location of the new Department of
the central and most significant reserve on Aotea.
Awareness of the
linkage and interactions between the terrestrial forest ecosystems
and the health of the marine environment could be realistically
promoted through what is already known from studies on Whangapoua
Estuary and elsewhere8.
The reserve assets
within and around Hirakimata should be promoted as places where
visitors can experience the full range of the island’s biota, and
the linkages between them should be made apparent.
But, these special
places are managed in what seems like a piecemeal way – an inevitable
consequence of the Department of Conservation’s much reduced on-island
A plan to link the
Hirakimata track system to the reserves around it, perhaps by additional
tracks in broad corridors of vegetation would be a worthy priority for
the new Aotea Conservation Park Advisory Committee.
From the summit to sea — view
west from Hirakimata
— Port FitzRoy, Glenfern Sanctuary/Kotuku Peninsula and
Motu Kaikoura, Little Barrier Island (Hauturu) in the distance.
Photo: E Waterhouse
Issue 36 Winter 2016
relates to mountains.
Cook. (2013) An investigation of the population size and distribution of
tomtit and red-crowned parakeet on Mount Hobson/Hirakimata and the
results of five minute bird counts at four different sites on Great
Barrier Island. Report to GBIET, 2013.
et al. 2006. Application of palynology to describe vegetation succession
in estuarine wetlands on Great Barrier Island, northern New Zealand.
Journal of Vegetation Science 17: 765-782. Also: Deng, et al. 2006.
Journal of Biogeography. 33: 592- 608. Also: Deng et al 2004.
New Zealand Journal of Botany 42: 568-588. Also: Ogden et al. 2006.
Sequential impacts of Polynesian and European settlement on the
vegetation and environmental processes recorded in sediments at
Whangapoua Estuary, Great Barrier Island, New Zealand. Regional
Environmental Change 6: 25-40.
(2016): In Great Barrier Island Environmental News 2016 # 35. Also:
Ogden, J., & Dowding J. E. 2013. Population estimates and conservation
of the New Zealand dotterel (Charadrius obscurus) on Great
Barrier Island, New Zealand. Notornis 60: 210-223.
A., Corin, S. & Ogden, J. 2012. Monitoring Report: Australasian Bittern;
Great Barrier Island. Department of Conservation Report: GRBAO 22380.
S. H. & Ogden, J. (2003). The bird community of the Kaitoke wetland,
Great Barrier Island. Notornis. 50(4): 201-209.
M., Ogden, J., Nichol, S.L., Alloway, B.V., Sutton, D.G. (2000)
Palynology, sedimentology and environmental significance of Holocene
swamps at northern Kaitoke, Great Barrier Island, New Zealand.
Journal of the Royal Society of New Zealand. 30: 27 - 47. Also:
Pegman, A. P. McK. & Ogden, J. (2005). Productivity-decomposition
dynamics of Typha orientalis at Kaitoke Swamp, Great Barrier
Island, New Zealand. New Zealand Journal of Botany 43: 779-289;
Also: New Zealand Journal of Botany 44: 261-271. Also: Rutherford, G. N.
(1998). The current vegetation of Kaitoke Swamp, Great Barrier Island (Aotea).
Msc. Thesis, University of Auckland.
J. et al. (2006). Sequential impacts of Polynesian and European
settlement on vegetation and environmental processes recorded in
sediments at Whangapoua Estuary, Great Barrier Island, New Zealand.
Regional Environmental Change 6 : 25-40.