The form of our Rata on the summit of Te Ko‘u is the parent of the popular New Zealand ornamental Metrosideros “Spring Fire”. GMcCormack/ 23110320
The inland mountains of Rarotonga maintain one of the most pristine tropical forests in the Central South Pacific. Gerald McCormack of Cook Islands Natural Heritage Trust explains why.
The biodiversity crowning
glory of the inland forest is in the Cloud Zone, which is recognised by the
occurrence of two particular ferns.
Depending upon the
rainfall, they grow down ridges to between 350 and 450 metres elevation. In
1988, I defined the Cloud Zone as above 400m as a compromise to facilitate the
creation of a reserve including Te ‘Atukura (638m), Te Manga (653m) and Te Ko‘u
(588m) within a contiguous block.
While Te Manga and Te ‘Atukura
have pinnacle-like summits, Te Ko‘u is completely different with its basin-like
summit. The oval basin measures about 200m North-South and 300m East-West with
an area of about 5ha (12 acres). This basin is dissected by a small stream
running west to east, where it has cut the summit rim into a deep V to cascade
down a very steep valley into the Avanā.
The basin on the summit of Te Ko‘u viewed westward from Te Manga Cook Islands Biodiversity/ 23110324
During 40 years of periodic
visits to the summit basin, I have always found the small stream emerging near
the basin’s centre and trickling over an impervious clay embankment. This
impervious barrier is probably the main structure holding back a large
groundwater reservoir on the basin’s western side, enabling the stream to run
continuously. The mountain’s high elevation causes frequent cloud cover and a
high level of precipitation of about 4000mm, which is twice the annual rainfall
of coastal Rarotonga.
The 1885 claim by the
Reverend W. Wyatt Gill that Te Ko‘u is the “source of all the streams which
fertilise the island” is false. Its stream is a continuous but relatively small
contributor to the Avanā Stream, the largest stream on the island.
Here, we present
information on a few species that make Te Ko‘u a Cloud Zone highlight.
Rata, Polynesian Metrosideros
In 1899, Thomas
Cheeseman, the Curator of the Auckland Museum, spent three months researching
the plants of Rarotonga, culminating in his Flora of Rarotonga of 1903. He
recorded 267 flowering plants, of which 168 were native or indigenous,
including 17 new species unique or endemic to the island.
He reported collecting
Rata on ‘Ikurangi in his diary on the 27th of May, and on the 18th
of June, on the summit of Te Ko‘u, he collected a different Rata, writing there
are “apparently two species”. However, in his Flora, he accepted only one
species (Metrosideros collina) with “two forms: one, …. has the young
shoots and branches of the inflorescence silky or tomentose; the other … is
nearly glabrous [without hairs], …”
The form he found on Te Ko‘u
had a silky/hairy inflorescence and new leaves, and it is restricted to the
Cloud Zone, where it is common. The other form, without a silky inflorescence,
is common on the drier ridges above mid-elevation, such as on the ridge to the
Needle and on the dry summit of ‘Ikurangi.
Although the silky Rata
of Te Ko‘u has garnered little attention on Rarotonga, it is now known to be
New Zealand’s most popular ornamental Metrosideros, Metrosideros “Sure Fire”.
This connection was established by botanist Peter de Lange in 2011. Although it
could not be determined when our Cloud Zone Rata was taken to New Zealand, the
suspicion points to Cheeseman, who was also known to collect some live plants.
“Sure Fire” must have
been popular before 1945 when it was selected to be planted on the Waitangi
Treaty Grounds beside the Te Whare Runanga (Meeting House), where they are now
about 15m high. After Cheeseman, William Philipson was the next Kiwi botanist
to research Cloud Zone plants in 1969. This is too late, so Cheeseman probably took
it to New Zealand.
Our Rata, Metrosideros collina, is a dramatically
crimson-flowered shrub or gnarled tree to about 5m, also found on islands in
the Australs and Societies. It is an endemic plant of Southeast Polynesia.
Rata Leafminer Moth
The Rata Leafminer Moth (Macarostola
pontificalis) is a tiny, slender moth about 6mm long. Its tiny size is offset
by its bright crimson and yellow markings.
The Rata Leafminer Moth has been found only on the summit of Te Ko‘u. GMcCormack/ 23110319
It has been found only on
Te Ko‘u, where its caterpillar lives within the leaves of the Rata. The young caterpillar
makes a narrow serpentine tunnel, which becomes wide and filled with waste at
the edge of the leaf. The half-grown caterpillar emerges and moves to another
leaf, where it cuts out a half-moon section attached at the end.
On the underside of the
leaf, it uses silk to roll the section around itself into a tepee-like cone
about 12mm high. It lives and feeds inside the cone until it gets its 5th and
last skin, when it bites a hole to emerge. It moves to yet another leaf, where
it folds the edge to make a silken tunnel in which it pupates and transforms
into a moth.
This remarkable animal is
found only on Rarotonga, Rurutu, Rapa ‘iti, Mo‘orea and Ra‘iātea – always at
high elevations on Rata. It is an endemic of Southeast Polynesia.
Montane Emerald Dragonfly
The Cook Islands has
seven dragonflies, and the rarest is the Montane Emerald Dragonfly, found
almost exclusively in the summit basin of Te Ko‘u.
The Montane Dragonfly is the rarest dragonfly in the Cook Islands, and its only substantial population is on the summit of Te Ko‘u. GMcCormack/23110321
The Montane Emerald Dragonfly
(Hemicordulia hilaris) has a distinctive metallic blue-green colour and
a spindle-shaped abdomen. It is a small dragonfly with a 6cm wingspan, which at
rest holds its wings level with its body, rather than forward and down, like
the ubiquitous Small Red Dragonfly.
This emerald dragonfly is
a strong flier with a wide distribution from New Caledonia and Vanuatu through
Fiji, Tonga, and the Samoas to Rarotonga. The genus is still under taxonomic
revision, and if the Tahitian species, Hemicordulia oceanica is the
same, this would extend the range of our species to the Society Islands.
Dragonflies are usually
seen around ponds of all sizes into which they lay their eggs, and their larvae
(nymphs) live on the bottom, where they ambush their prey with their long,
extendable mouthparts. When the nymphs reach their final stage, they crawl out
of the water on some object. There, their skins split open, and the soft-bodied
adults emerge to pump themselves into adult shape before their skins become
rigid and coloured.
There are no ponds on Te
Ko‘u, and where our young emerald dragonflies live is unknown.
Te Ko‘u Landsnail
Native landsnails have
not done well on Rarotonga, with more known extinctions than all other animal
groups combined. Most native coastal landsnails are extinct, and many mountain
species are gone.
This is the only known photo of a live Te Ko‘u Landsnail. It was taken in 1984, and no more live animals have been found. Shell Length (SL) 9mm. GMcCormack/23110323
Te Ko‘u has proven to be
a natural refuge for several species, and around 2007, snail expert Fred Brook
found four new landsnail species surviving in the summit basin. They are not yet
Rarotonga's most famous
native landsnail was found on the summit Te Ko‘u in 1964 and 1965. Kiwi Laurie Price
found it for the American landsnail expert Alan Solem, who named it after the
mountain and its finder Tekoulina pricei.
Price found numerous specimens in areas of Cloud Blechnum within the summit
basin but not elsewhere on the island.
All landsnails, including
the Te Ko‘u Landsnail, are bisexual (hermaphroditic) with mature male and
female reproductive parts. They produce young by mutual cross-fertilisation,
although self-fertilisation can sometimes occur.
They typically produce
large nutrient-filled eggs in which the embryos develop after being laid, although
in many cases, the adult keeps the eggs inside where the young hatches to be
born free-living. In such cases, each embryo grows on the nutrients within its
egg and is not connected to the mother. This widespread type of live birth is
known as ovatrophic viviparity (“egg-fed, live birth”) or ovoviviparity (“egg,
In contrast, our Te Ko‘u
Landsnail feeds its embryos while they grow in her uterus. The eggs are small
with minimal nutrients, and the embryos are fed through a stalk or umbilicus
attached to the uterus wall. This is called matrotrophic viviparity (“mother-fed,
Mother-fed live births
are incredibly rare in the world of snails. It occurs in only eighteen species
worldwide, and our Te Ko‘u Landsnail is in this elite group.
The last live Te Ko‘u
snail was seen and photographed by the author in 1984. Brook undertook thorough
searches in 2005, 2006 and 2007 and found no live snails, although a few fresh,
empty shells indicated that this endangered species might still exist.
Tuna Kavi, the Polynesian Longfin Eel
Rarotonga has three
freshwater eels, the most common being the uniformly dark swamp eel Tuna Māori,
the Pacific Shortfin Eel (Anguilla obscura). The common eel in the
streams, especially at lower elevations, is the marbled eel Tuna Pupu, the Giant
Marbled Eel (Anguilla marmorata).
Tuna Kavi, the Polynesian Longfin Eel, lives in the upper reaches of streams and has been recorded in the summit stream of Te Ko‘u. GMcCormack/ 23110322
The upper reaches of
streams are the home of the slender Tuna Kavi, the Polynesian Longfin Eel (Anguilla
megastoma). Over the years, a few hikers have reported seeing eels in the
summit stream, and I found a 70cm eel in 1991. It was uniformly grey-brown
above and had a yellowish belly.
There are two amazing
things about Tuna Kavi being on the summit of Te Ko‘u. The first is the
unbelievably steep valley it climbed to reach the top: a valley of several vertical
waterfalls. I once attempted to hike up this stream and found it impossible to
work my way around the edges of the waterfalls. How do eels sometimes make that
The second thing is that
I have not seen any prawns or fish for it to eat. I have heard that Tuna Kavi
is the most vicious eel, and it has been found with the remains of rats inside.
On the top of Te Ko‘u, the odd rat might be its main food source, although I
imagine a few landsnails and insects also fall into the water or venture too
A final point about any
Tuna Kavi on Te Ko‘u is that when it is about 20 years old, it will change from
its usual yellow belly to silver, stop eating, and slither down the mountain to
take the Avanā Stream to sea and use its body fat to undertake an ocean journey
to a still unknown area where it will breed and die. Research in Samoa and
elsewhere has shown that migrating eels swim about 800m deep during the day to
avoid predators, mainly sharks, but come up to about 200m at night to warm up.
Finding the deep-water
breeding sites of freshwater eels has been very difficult. The sites for only
three species are known: the European, American, and Japanese Eels. For
example, the European Eel leaves European streams as a silver eel to swim on
its stored fats for about five months, covering 6000km to its breeding site
about 500m deep in the Sargasso Sea, east of Bermuda. There, they spawn and
die. Their leaf-like larvae take about three years to float on the Gulf Stream
back to the Continental Shelf of Europe, where they become swimming “glass eels”
which enter streams to become small pigmented eels (“elvers”) and then yellow
eels. They change into migratory silver eels when they are about 20 years old.
The deep-water breeding
sites of the tropical South Pacific eels are yet to be discovered. Tahiti has
the same three eel species as Rarotonga, and their young transparent “glass eels”
arrive during the early Wet Season (December-February). Their small size of about
50mm suggests they are only about four months old, indicating relatively nearby
breeding sites. This might apply to Rarotonga eels also. There is a lot to