by John Ogden

Great Barrier's Mission Impossible - or is it?


In this issue we feature an article by James Frankham, editor of the New Zealand Geographic, in which he outlines the deliberations of a group of scientists and conservationists meeting to consider the possibilities of eradication of all mammalian pests from the whole of New Zealand. Apparently independently, Rebecca Priestley (Science, The Listener, April 14, 2012) reports on the vision of Sir Paul Callaghan to capitalise on our real point of difference in the world by ridding New Zealand of all its mustelids, rats, rabbits, possums and other mammalian pests. Sir Paul was one of New Zealand’s leading thinkers, and his views, while “crazy and ambitious” were backed up by an outlined plan to achieve the goal. The theme is taken up again, by the editor of The Listener (April 21, 2012), under the heading: “The duty to dream – New Zealand’s unique habitats and species are our greatest attractions”.

John Innes, an ecologist with a lifetime of solid published research in this field, reports that every year 26 million native birds are killed by mammalian predators. This is a staggering figure, but it is probably conservative. In an Email to our local politician and conservationist, councillor Mike Lee, Innes gives the maths, based on a published review of 24 research projects involving 13 native forest bird species – see box.

If we do the same estimates for Great Barrier, assuming the area of scrub and forest is 24,294 ha and that (conservatively) there are only four nesting attempts per ha, we conclude that we are loosing 86,500 native birds to rat and cat predation on Great Barrier every year.

Scientifically, the precise figures mean diddley-squat, but they surely give us, the public, cause for concern – they are based on conservative estimates and are certainly of the correct order of magnitude – they explain why so many species have gone extinct and why bird populations explode so quickly when predators are eliminated.

The forests of Great Barrier Island may still look beautiful to visitors, but: “Basically our forests are dead. Yes, there are still trees in them, but they’re quiet. The birds are gone, they’re totally changed because of introduced mammals” (Charles Daugherty, professor of ecology at Victoria University.) Our recent submission to the Local Board Long Term Plan emphasised the need to address the on-going nature of bird extinctions on Great Barrier with urgency. Since Hutton’s list was made in 1868, Great Barrier has lost 12 bird species, three of these within living memory (whitehead, 1950s; rifleman, 1970s; kokako 1996). A further three bird species are just hanging on here, with populations so small that they could be gone next year (tomtit, kakariki and bittern). The Island has a further 18 birds in the nationally endangered, vulnerable or declining categories, and an additional 12 ‘rare’ birds. We are not turning the tide – we’ve been loosing species continuously since 1868, and there is every indication that we’re likely to loose more very soon unless a serious, and successful, effort is made to reduce predators. Our birds could be one of our great tourist attractions, creating local employment, attracting visitors and stimulating the economy, but a long-term plan with a vision of cooperation between all stakeholders is needed to achieve this.

Silent Statistics

1. Area of native forest in NZ = 5,980,000 ha

2. Average 5 native bird nests per ha per annum = 29,900,000 nesting attempts per annum

3. Mean failure rate (Table 3 in Innes et al. 2010) = 73% = 21,827,999 failures

4. If average 2 eggs or chicks per nest = 42,654,000 dead eggs or chicks

Photo by D Mudge, DOC

5. On average 61% of failures were due to mammalian predators, therefore:


26,628,940 eggs or chicks of native birds lost each year due to mammalian predators.

From: 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.
See also: www.newzealandecology.org.nz/nzje/.