|
 Who’d
have thought? New Zealand’s largest living gecko species, and one of the
world’s largest geckos, has just been rediscovered at Windy Hill Rosalie
Bay Catchment Sanctuary in January this year - albeit unfortunately dead
in a rat trap. Field worker Rachel Wakefield discovered the decomposing
lizard during a routine check of trap and bait stations. With the aid of
a lizard field guide, she was able to identify it as a Duvaucel’s gecko
and this was verified when the preserved remains were sent to Halema
Jamieson.
• Duvaucelles gecko (Hoplodactylus
duvaucelii) – long life and slow breeding makes
the species highly vulnerable to rats. Photo by Trent Bell
Duvaucel’s gecko (Hoplodactylus duvaucelii)
are very large and heavy-bodied geckos, with the largest recorded
specimens reaching to 32cm long, and up to 118g. Although chevron skinks
are known as one of New Zealand’s longest lizards, they are not as fat
or heavy (40g). This discovery comes on the heels of the rediscovery of
the rare striped skink (Oligosoma striatum) near Windy Hill in 2009.
What is the significance of this
rediscovery? The last – and only – recorded and verified sighting of
Duvaucel’s gecko on the Barrier was exactly 40 years ago in 1971, by
Brian Gill of the Auckland Museum, at Okupu. The closest existing pop-ulations
are those of the Mokohinau Islands in the north, Little Barrier Island
and also the Mercury islands to the south. Anecdotal rumours do persist
of ‘large lizards’ being seen on Great Barrier Island on the odd
occasion but these are usually attributed to chevron skinks.
However, sightings last year of ‘very
large gecko’ in Okupu may have been a Duvaucel’s. Have Duvaucel’s geckos
on Great Barrier Island survived the onslaught by the pirates that ship
rats are? That was considered highly unlikely, but in light of the
rediscovery, is now clear that at least a few have.
Duvaucel’s gecko were once widespread on
the New Zealand mainland, with subfossil or museum records indicating
presence in Northland, Waikato, Hawke’s Bay, Wellington, Canterbury and
even Otago. Distressingly, the species’ current range is now disjunct,
with a 400 km straight line gap between the closest north-eastern island
populations and the southern island populations on the opposite ends of
the North Island. (NB, the Maungatautari gecko found in a rat trap has
been determined by genetic analyses as most likely have been a released
captive-bred animal). The extinction of Duvaucel’s gecko on both the
main islands, and some offshore islands is attributed to predation by
rats, cats and mustelids.
 So
what puts such large geckos at risk of rats and other predatory mammals?
It is thought that Duvaucel’s gecko faces enhanced risks from predation
by rats due to a combination of factors. As the geckos are strongly
nocturnal (active at night), their activity period coincides with rats,
a time where the geckos are most vulnerable while they are foraging for
food or mates without protective and secure refugia nearby. In order to
‘escape’ predators, Duvaucel’s gecko run and ‘freeze’, a strategy
probably effective for birds, but clearly not very smart when
encountering rodents and cats.
Photo taken on Little Barrier Island
Another vulnerability that Duvaucel’s
gecko have are their own large sizes. Being too fat to squeeze into many
of the narrow protective refugia often available to the many other
smaller gecko and skinks leads to limited avenues for escape and
protection from predation. This is especially problematic for heavily
gravid female geckos that often look rather obese, and waddle rather
than scurry when attempting to escape!
Duvaucel’s geckos are also extremely long
lived, probably the longest living gecko species in the world. Most
geckos in the world live about several years, but an individual
Duvaucel’s gecko was recently recaptured 50 years after being first
marked as an adult in 1958 on the North Brother Island, and is possibly
still alive even today. Other individuals have also been recaptured
after 36 and 43 years since their first capture.
Long-lived species must therefore reach
maturity later in life? Yes, that would be correct for this species. In
order to breed for the first time, a female Duvaucel’s must wait six
years before she is sexually mature, and a male also matures at around
seven years. Duvaucel’s gecko also have a ‘low reproductive output’ – in
other words it has been estimated that females have 1.12 young per year
on average, and that every female only breeds every other year.
A study on Ohinau Island prior to and six
months after the removal of kiore from that island found that only 4% of
their population was under 100mm SVL (an adult Duvaucel’s range in size
from 100mm to 161mm snout to vent length, SVL). This contrasts with
kiore-free Korapuki and Green Island populations comprising of 20% and
14% newbies under 100mm SVL. Were the kiore taking the mokopuna of these
geckos? As there are no other invasive mammal species on Ohinau, it is
apparent that Duvaucel’s gecko populations cannot sustain themselves
even from the seemingly innocuous kiore. It is also likely that in the
face of the presence of the more voracious ship rats elsewhere, no
Duvaucel’s gecko are able to survive long enough to even get their leg
over to carry on the family line and inheritance.
Research has indicated that on
kiore-inhabited islands, Duvaucel’s gecko populations were restricted to
shore and cliff habitats, and nearly absent in forest. Interestingly,
upon eradication of kiore on Ohinau, nearly 70% of Duvaucel’s gecko
suddenly switched their habitat within six months, by leaving their
cliff-bound coastal refuges to explore the forests that once comprised
part of their kingdom. The geckos also spent less time in their
predator-secure refugia and also came down to the ground more often. Did
a similar phenomenon occur with the Duvaucel’s gecko caught in a rat
trap in forest at Windy Hill, about 3 kilometres away from the Rosalie
Bay’s rugged coastal bluffs, where a remnant population may potentially
still exist? This may indeed be the case.
So what do Duvaucel’s gecko do in their
spare time? The species is a habitat generalist occupying coastal
boulders, bluffs, scrub and also forest, and is both arboreal and
terrestrial. These geckos like to take up refugia in hollows, crevices
and even the odd empty petrel burrow. Dietary analysis indicate that
these geckos feed on a variety of invertebrates, including moths, weta,
crickets, cockroaches, beetles and earwigs. However, this is when things
start to get really interesting. Duvaucel’s gecko have been observed
congregating on pohutukawa, ngaio and flax in bloom, engorging
themselves on nectar, and thus spreading pollen to other trees,
considering the pollen’s tendency to attach to the gecko’s chins,
undersurfaces and limbs for at least 12 hours, and the geckos ability to
disperse to distances greater than 50 m in short time frames. The
ecological services of this gecko do not end there. They also feed on
fruits of several tree and shrub species, and excrete viable seeds in
what is often a suitable location for germination. The sweet tooth of
Duvaucel’s gecko seem to know no limits, with geckos also recorded
feeding on honeydew on ngaio and kanuka. As such is their appetites,
Duvaucel’s have also been recording eating other geckos and even
shearwater eggs. Since Duvaucel’s gecko have been recorded in densities
up to 750 geckos per hectare on some predatory mammal-free islands, they
must have played a very important role in many ecosystems in which they
formerly occupied.
Duvaucel’s gecko are thought to have large
home ranges, although as no precise estimates are available yet, this is
probably around 200-500m2. These geckos can cover great distances, with
one animal on the Poor Knights islands recaptured 77m away from the
original capture location three days earlier. In another account, a
gecko released on Mana Island was found 1 kilometre away from the
release location! Yet, despite the gecko’s apparent ability to range far
and wide, they also show tenacious site fidelity, with a single animal
relocated in the exact same refugia on the North Brother Island after 36
years! Not only do they like their homes, they also like to share.
Duvaucel’s gecko like to aggregate, and
often appear to have a ‘family group’ of geckos sharing the same
crevice, potentially suggesting paired adults along with
differently-aged offspring or a tolerance for younger geckos.
The news is not all bad for Duvaucel’s
gecko. The gecko is still widespread and abundant along the (mainly
rat-free) north-eastern islands and on several small islands in the Cook
Strait. Duvaucel’s gecko have also been reintroduced to Mana Island,
Tiritiri Matangi and Motuora Islands, where they appear to have
successfully established.
Unfortunately, it is highly unlikely that
the Barrier’s newly rediscovered gecko species are currently able to
sustain their own mokopuna - or even themselves. Although Duvaucel’s
gecko populations seem to have somewhat persisted on the Barrier to
date, these populations are almost certainly doomed. The rats will get
to them eventually, if we don’t get to the rats first.
The rediscovery of Duvaucel’s gecko as yet another piece of the rare
taonga of Aotea also brings home the realization that in order to
preserve all of the faunal elements of the Barrier into perpetuity, the
rats must go. The question is not if, but when?
Since we have already lost the tuatara,
kokako, hihi, rifleman, and saddleback, among other species from the
Barrier, we cannot afford any more of Aotea’s treasures to be further
plundered. As long as the impasse over the rats remains, we run the risk
of losing even more of our valuables.
 For
those who want to find out more about Duvaucel’s gecko, a visit to
Landcare Research’s NZ Lizards Database is well worth the time (http:/nzlizards.
landcareresearch. co.nz). Click on the Species Synopses tab to reach a
list of NZ lizard species, and look for Hoplodactylus duvaucelii. For
references to the facts within this article, contact Trent.
TRENT BELL is a consultant
herpetologist with EcoGecko Consultants. He is a frequent visitor to the
Barrier, with a long-term lizard monitoring programme at Windy Hill.
Trent is also following up a Duvaucel’s gecko translocation to Mana
Island, Wellington. Trent may be contacted at trent@ecogecko.co.nz, or
visit his website at www.ecogecko.co.nz |
