By Zoë Lennon
Islands are some of the world’s most biologically diverse areas. Due to their isolation from the mainland, islands can be incredible habitats for unusual endemic species; you need look no further than the many strange inhabitants of islands like Madagascar to see this. Although they have less than 5% of the world’s land area, islands contain 20% of global biodiversity. Sadly, they have also experienced over half of recent extinctions. Island ecosystems are at risk of destruction, and their greatest threat is invasive species. Invasive mammals in particular are responsible for many extinctions, and one of the most widespread invaders is the rat. Of particular note are the black rat and the brown rat. These two species have spread worldwide, directly causing or contributing to the extinction of many species including birds, mammals, invertebrates, reptiles, and plants, and having huge impacts on island ecosystems.
To see the destructive effects that invasive rats can have on island ecosystems, we can look at the example of New Zealand. An island nation with no native land mammals (besides three species of bat), New Zealand is home to, among other endemics, many remarkable and rare birds. Due to the historical lack of mammals, many New Zealand endemics are ‘naïve’ to mammalian predators such as rats, and have evolved no defences against them. Endemic species are therefore hugely threatened by mammal invaders, and native invertebrates, reptiles, and birds have suffered extinctions. Conservationists are fighting back against invaders using offshore islands, which have been the testing grounds for world-pioneering conservation. New Zealand has been the location of one third of global mammal eradications; often using new methods and in areas previously thought impossible or impractical to clear. Islands are an ideal place to begin eradications, as the water surrounding them provides a natural barrier. Once cleared of dangerous invasive species, islands can be used as safe havens for species that are threatened or extinct on the mainland. Crucially, measures must also be taken to stop reinvasion of islands once invaders have been eradicated.
There has been a long history of research into invasions and eradications, but until recently little into in the problem of reinvasion following eradication. If reinvaders are not detected and removed before they establish new invasive populations, all the effort put into initially eradicating pests will be for nothing. Figure 1 from a 2008 study1 shows that reinvasions rise following eradications; these reinvasions could have disastrous effects on conservation efforts. In order to prevent reinvasion, it is vital to understand how pests behave when they first invade an island, as this behaviour can be quite different from that of pests in high density. Several studies in New Zealand have investigated the reinvasive behaviour of rats.
- Figure 1: Reinvasions of rats on New Zealand islands rise following eradications. From Clout & Russell 2008 http://www.publish.csiro.au/?paper=WR07091
In 2008 a group of scientists took an experimental approach to the question of invasive rat behaviour2. Male brown rats were radio-collared and experimentally released onto New Zealand islands where rats had been eradicated previously. This experiment allowed scientists and conservationists to learn about how quickly rats could be detected and removed under safer conditions than if they waited for an actual invasion to occur. By using only desexed males the risk of new populations of rats becoming established was minimised. The results suggested that the best approach to combating rat reinvasions was an integrated approach using a variety of different surveillance methods and traps. This should be able to kill most invading rats within 14 days of arrival and prevent the establishment of new populations. Rat-tracking dogs were employed where all other efforts failed, allowing rats that had otherwise evaded detection to be located. Using these methods, the researchers found that even islands with relatively high reinvasion rates can be kept rat-free.
The movement and behaviour of brown rats experimentally released onto New Zealand islands was investigated in a related study3 using similar methods; the rats were tracked via radio collars. The scientists found that, once released, the invading rats moved randomly and widely over the islands. These results are not what was predicted from computer models or from watching rat behaviour in the lab. The rats also behaved differently to rats in large invasive populations, ranging more widely. These findings highlight how important it is to understand the behaviour of invasive animals in a real-life context. Previously, received wisdom about how to prevent reinvasion was based partly on assumptions that reinvading animals would behave similarly to animals that are part of an established invasive population. Now that conservationists have a clearer understanding of how reinvaders actually behave, they can use this knowledge to keep eradicated islands pest-free.
The management of a real reinvasion, and the effects this can have on endemic birds, was examined in a study earlier this year4. Stewart Island robins are a subspecies of the protected endemic New Zealand robin. After the robins suffered population decline in their native range following the first invasions of rats, conservationists established populations on nearby predator-free islands. Ulva island is one such island, having been ‘rat-free’ for the past 14 years. However, in 2011 a new invasion of brown rats made its way to Ulva island, and the population began to increase exponentially. The rats were eventually killed following an aerial poison drop; however, this strategy also resulted in a sharp decline in robin numbers as the poison killed birds as well as rats. This example shows the importance of detecting and stopping rat reinvasions before numbers reach a point where large-scale poisoning becomes necessary. Following this reinvasion, New Zealand’s Department of Conservation has increased biosecurity on the island, using methods similar to those found to be effective in the 2008 experiment above.
Islands are areas of breathtaking biodiversity and are of significant use to conservationists. However, their ecosystems are threatened by invasive animals such as rats. A relatively recent, though important, conservation focus has been on preventing reinvasions following eradications. Understanding the dynamics of reinvasions is vital if we want to maintain islands as the havens for extraordinary endemic species they have been for millennia.