Editorial - The Great Barrier Slaughterhouse

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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 grim.

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 hen's (68x47mm).

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.

References:

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.
 

ECO BRIEF

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.