The conservationist's dilemma - 7

Written by: Martin Harper
Published: 7 Sep 2020
Topic: Science, policy and practice of the impact of predators on wild birds

The Gough Island Restoration Programme is a good example of the tough choices that conservationists face when managing wildlife populations. This project also leads into the topic of the annual update of vertebrate control that the RSPB undertakes on its nature reserves.

Trigger warning: graphic image and text relating to mice predating live birds.

Gough is a remote, uninhabited island in the Tristan da Cunha archipelago in the South Atlantic. It is one of the UK’s Overseas Territories and is a World Heritage Site. It's relied upon by millions of seabirds, some of which breed nowhere else in the world.

Mice were accidentally introduced to the island, most probably by sailors during the 19th Century. Since arriving on Gough they have learnt to exploit all the available food sources on the island, including seabirds. Horrifically, video cameras have shown mice eating the flesh of live seabird chicks. A Tristan Albatross chick can weigh up to 10kg, but mice biting them and inflicting wounds every night often causes them to die. Mice have also been recorded attacking adult seabirds, harming the long-lived breeding adults. Tristan Albatrosses, and other highly threatened species are being pushed to extinction as over two million seabird chicks are lost from the island every year.

This is why the RSPB has intervened.

One of the world's most challenging island restoration projects was developed to remove the mice and restore the fortunes of the island's birds. This work was planned by the RSPB with partners the Tristan da Cunha and UK governments, the South African Department of Environment, Forestry and Fisheries, BirdLife South Africa, Island Conservation, BirdLife International and the Royal Zoological Society of Scotland. The RSPB's hope is that the removal of mice will prevent the deaths of defenceless chicks year after year, allowing populations to recover from unsustainable levels of decline.

Removing invasive non-native species from islands provides a perfect example of how the mistakes of the past can be addressed, giving nature a chance to recover.

It also demonstrates why lethally controlling vertebrates is sometimes necessary for conservation.

The RSPB’s approach to any type of lethal control means that evidence of a problem is sought. If there is a viable non-lethal solution, then that is implemented. In many cases (as with Lapwing conservation) this does the job.

But on Gough, that wasn't an option. Given the severity of the problem, and the urgency to act, the solution was to drop poison by helicopter to eradicate the mice. This is a similar approach to other projects such as the South Georgia Heritage Trust Habitat Restoration Project, which successfully removed all the rats from the island.

Seabirds, and other ground-nesting birds, can be particularly vulnerable to predation. This was highlighted in the RSPB's 2018 review of the impact of predation on wild birds. The review was based on 81 relevant scientific papers and reports covering 908 cases where the effect of a predator on changes in the numbers of a prey species had been measured. It was a UK analysis and showed: 

• Predator numbers have increased;
• That the UK has very high densities of Red Fox and crows compared to other European countries;
• That seabirds, waders and gamebirds are limited by predation
• That pigeons, birds of prey, owls, woodpeckers and songbirds are not limited by predation;
• That there is a real need for research to understand how landscape-scale management could be used to provide longer-term sustainable solutions to reduce the numbers of generalist predators and their impacts on species of conservation concern.

The science determines the RSPB's actions, which means it is sometimes necessary to carry out lethal vertebrate control on RSPB nature reserves to conserve vulnerable species, but only when four criteria are met:

• That the seriousness of the problem has been established; 
• That the seriousness of the problem has been established; 
• That killing is an effective way of addressing the problem; 
• That killing will not have an adverse impact on the conservation status of the target or other non-target species.

Only when the above quastions have been answered, as in the case with Gough, does the RSPB make a decision.

The number of vertebrates killed by the RSPB and its contractors during 2018–19 on RSPB reserves is shown in the table below. This includes those killed as part of major projects that the RSPB is leading, such as the Orkney Native Wildlife Project.