The Trust organised two public lectures on Great Barrier in January. Alan Saunders is an internationally recognised authority on rodent eradication from islands, currently working with the Invasive Species Group of the IUCN (International Union for the Conservation of Nature) based at Auckland University. His main area of work at the moment is in the Pacific islands. Alan covered the negative ecological, economic and social impacts of invasive species, with examples mostly from the Pacific region. He pointed out that, unlike many environmental ‘disasters’ – such as oil spills, invasive species do not ‘clear up’ over time – on the contrary they get progressively worse. While the advanced countries have generally got some control over rat-carried diseases (remember the black death?), the economic impact of rats throughout the world is still considerable – 17% of Indonesia’s rice crop is eaten not by humans, but by rats! (17% of my peaches were too – and the rest). Alan also stressed how island ecosystems are particularly vulnerable to invasive species, because they have evolved ‘in isolation’. This of course applies to New Zealand more than any other place on earth – we are the last major land area to be invaded, and our native ecosystems are at huge risk.

Alan defined "eradication" as the complete removal of all individuals of an invasive species in a set time from a defined area. The time frame is important – if it is indefinite then we are talking about "control" not eradication. The difference is fundamental, because control implies on-going expenditure and on-going economic risk from the pest. Eradication may also involve on-going expenditure, but on different aspects (such as border bio-security) which will allow the restoration of the native ecosystems and new economic opportunities. On a small scale, the eradication of rats from Tiritiri Matangi Island has allowed ecological restoration of that island and generated a small but successful nature tourist venture. If rats returned to Tiri, stitchbirds, saddlebacks, kokako etc would be eliminated, visitors would not come, and the business would collapse.

Most of the rat eradications from the off-shore islands of New Zealand have been on uninhabited islands. The largest of these (Campbell Island) is more than one third the area of Great Barrier, and equally rugged. However some of the islands (eg Kapiti) did have small human populations. Alan stressed that over 100 islands around New Zealand have had invasive mammals (mostly rats) eliminated from them, and that no attempted eradication initiated by the Department of Conservation has ever failed. The technology for rat eradication is highly advanced and successful. The outcomes, in terms of biodiversity increases, are also proven, on Tiri, on Kapiti, on Mana Island, on Little Barrier, and dozens of others. The economic advantages for human inhabited islands remain to be proved, but the evidence is surely clear, and some isolated inhabited islands such as Norfolk and Lord Howe, are starting to act on it.

The second public lecture was by John Innes, who is a senior Scientist working for Landcare Research in Hamilton. John has a long history of research on endangered birds in New Zealand, and especially on the effects of rats on them. John began by describing the three different rat species present in New Zealand, pointing out that they have different ecologies and different effects. The ship rat is the most nocturnal and the most acrobatic climber. Females are highly territorial; males may range more widely. John also stressed the need to consider all the inter-actions – cats – rabbits – rats – mice etc involved in an eradication attempt. However, the take home message for me was just how secretly destructive ship rats have been to our native birds and lizards. (In my article "Rats eat Forest" in the last Environmental News I presented research showing their scarcely acknowledged detrimental effects on the vegetation too). Most of the time we don’t know that ship rats are there, but every hectare contains at least 1 of them (usually 4 - 7) and they are such good climbers that there is no place a bird can build a nest that they cannot reach. And they are so active that they will find all the nests in their territory. The wonder is not that many bird species have gone quietly extinct since 1840, but that some have survived! John’s motion videos of predation events – rats eating eggs and chicks – were the clear evidence of what goes on every night, and has done so for over 100 years – no wonder the bush is silent.