- The Great Barrier Slaughterhouse
by John Ogden
In the last issue I stressed that the
forests of New Zealand – including Great Barrier Island – are cathedrals
without choirs – the birds have largely gone and the whole ecology has
been changed by introduced predators. Great Barrier currently has 18
bird species declining in numbers or vulnerable to extinction
nationally. A few simple calculations based on published data (Innes et
al. 2010) indicate the cause: on Great Barrier at least 85,500 native
birds are killed by rats (mainly) and feral cats every year. Imagine
what it would be like if even 10% of that number survived and bred every
year! I have estimated that the island has at least a quarter of a
million rats, and at least a thousand feral cats (Great Barrier Island
SOE, 2010, Chapter 14). This army of predators has to be fed every night
of the year. Sure a few birds are increasing (kaka and tui for example);
these have benefited from protection and increased food availability as
gardens have become more established and forest cover extended. But,
while rats remain, the overall picture for native biodiversity remains
This fact was brought home to me the other
day. For years I have been monitoring a population of grey-faced petrels
above the cliffs at Awana. These are large seabirds (c. 1m wingspan).
Although they are not rare by national standards, and many of the
islands off the coast of northern New Zealand support large colonies,
there are very few grey-faced petrels nesting on Great Barrier. The
birds nest in burrows, laying a single egg in mid-winter. This is
incubated, mostly by the male, for nearly two months. After hatching,
the chick slowly grows to the size of an adult and eventually leaves for
the sea in December. All the activity at the burrows takes place during
the night, so we’re generally unaware of it.
The number of grey-faced petrel burrows at
Awana has been increasing since 1998, and, although I have found a
couple of young birds killed by cats, I was optimistic about the future
of the colony. However, the other day I found three eggs rolled out of
burrows and predated by rats (see photo), and in the cold wind, another
explanation hit me. Three eggs represent about 10% of those laid per
annum, and this is almost certainly not all the predation occurring.
What is probably happening is this: Great Barrier is surrounded by
pest-free islands with large grey-faced petrel colonies such as Cuvier
(visible from the Awana colony), the Mokohinaus and Little Barrier.
These island colonies are doing really well and exporting surplus birds.
Perhaps the birds are trying to establish on Great Barrier, but a
significant proportion of their nesting attempts are destroyed by rats
and cats. The same may apply to the smaller Cook’s petrels, diving
petrels, fluttering shearwaters and fairy prions. At night our
pohutukawa clad sea-cliffs should be noisy with nesting sea-birds,
coming to greet their partners and their young after days out at sea
catching fish to feed them.
* Grey-faced petrel burrow with
egg predated by rat (yolk eaten out from hole at right end of
egg). Awana, Sept 2012. The egg is the size of a big
Instead, the only sound is the wind in the
branches and the waves below. When I saw those predated eggs at Awana I
couldn’t help but feel that the colony would be a sad place that night.
This is not just speculation. For example,
rats were eradicated from Little Barrier Island in 2004, and the
following year there was a major influx of bellbirds onto Great Barrier.
A few survived and at least one pair bred, but although annual inputs
continue from Little Barrier (in the north), and Cuvier (in the south),
this species cannot establish a viable population here in the presence
of rats. A similar influx of bellbirds from Little Barrier to Tawharanui
peninsula (from which rats have been eliminated) has resulted in a large
viable breeding population there.
To protect Great Barrier’s forest and
birds, we currently have biosecurity tracking tunnels at wharves and
airports so that we can prevent noxious animals such as possums and
mustelids from getting here. How long before this biosecurity is seen
not as preventing animals getting in, but more as preventing rats from
getting out? Great Barrier Island is already a black-hole into which the
surplus from the surrounding rat-free islands disappears, and we may
well become a biosecurity risk to those islands.
To end on a lighter note: did you hear
about the German sausage maker who made sausages from seabirds then got
sick? No? He took a tern for the worst.
* The author recording grey-faced petrel
burrows on a fine day at Awana in 2006.
Innes, J., Kelly, D., Overton, J. M &
Gillies, C. (2010). Predation and other factors currently limiting New
Zealand forest birds. New Zealand Journal of Ecology 34(1): 86-114.
Great Barrier Island Charitable Trust. State of
Environment Report, 2010.
Our founding Chairperson John Ogden has
been awarded the “Ecology in Action” award of the New Zealand Ecological
Society . Partly in recognition of his involvement with community and
ecology on Great Barrier, and partly for his history of activity in
other environmental and ecological issues over many decades. For example
the campaign to stop the logging of native forests at Whirinaki in the
1980s. John’s award also demonstrates an increasing level of national
publicity for the Great Barrier Island Trust.
John was nominated by the GBICT and other
bodies and will be presented with the award at the annual general
meeting and dinner of the Society at Lincoln in November.
The award comes with $500 donation for the
environmental cause John chooses, and support to attend a joint
conference of the New Zealand and Australian Ecological Societies in
2013, where he will present a talk about the work of the Trust in
furthering the vision of a rat-free Great Barrier.
Congratulations John — Great Barrier’s
residents (human and otherwise) are privileged to have a conservation
advocate of your stature in residence.