The conservationist's dilemma: an update on the science, policy and practice of the impact of predators on wild birds (7)

Readers of this blog will be aware of the Gough Island Restoration Programme, but for those unfamiliar with the story I want to give a quick recap.  As Patrick Barkham wrote in the Observer earlier this year, it is a good example of the tough choices that conservationists have to face when managing wildlife populations and it is the hook to provide my annual update of vertebrate control that the RSPB undertakes on our own nature reserves.

Gough is a remote, uninhabited island in the Tristan da Cunha archipelago in the South Atlantic and it is one of the UK’s Overseas Territories.  It is a World Heritage Site and is 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 available food sources on the island, including seabirds.  Video cameras have revealed how the mice eat the flesh of seabird chicks.  Tristan albatross chicks weigh up to 10kg, but open wounds inflicted over successive nights frequently lead to their deaths.  Mice also attack adult seabirds, exacerbating the threat to the seabirds by removing the long-lived breeding adults.  Over two million seabird chicks are lost from the island every year, which is pushing some highly threatened species – including Tristan albatross - towards extinction. 

This is why the RSPB has chosen to intervene.

Great shearwater being predated by an invasive, non-native mouse in its nest, taken by a nest cam on Gough Island.  Image courtesy of Ben Dilley (rspb-images.com)

We have, with partners the Tristan da Cunha and UK governments, South African Department of Environment, Forestry and Fisheries, BirdLife South Africa, Island Conservation, BirdLife International and the Royal Zoological Society of Scotland, developed one of the world's most challenging island restoration projects to remove the mice and restore the fortunes of the island's birds.  We were due to carry this out this year, but as I reported in March the pandemic meant that we had to delay the project.  We intend to return, and when we do (which we hope and are planning to be next year), 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 we can address the mistakes of the past and give nature a chance to recover.

It’s why lethally controlling vertebrates is sometimes necessary for conservation.

The RSPB’s approach to any type of lethal control means that we first seek evidence of a problem, check whether there is a non-lethal solution and if so implement that.  In many cases (as we have shown with lapwing conservation) this does the job.

But on Gough, that isn’t an option.  Given the severity of the problem and the urgency to act we will need to use poison dropped by helicopter to eradicate the mice, similar to other projects such as the South Georgia Heritage Trust Habitat Restoration Project which successfully removed all the rats from the island.

Seabirds are one of the groups, alongside other ground-nesting birds, that can be particularly vulnerable to predation.  This was highlighted in our 2018 review of the impact of predation on wild birds 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 it showed that predator numbers have increased in the UK over the last decades; 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 whereas pigeons, raptors & owls, woodpeckers and songbirds are not limited by predation; and 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.

So, the science determines our own actions which means it is sometimes necessary to carry out lethal vertebrate control on RSPB reserves (>210 sites covering >160,000 hectares) to conserve vulnerable species, but only when four criteria are met:

• That the seriousness of the problem has been established;
• That non-lethal measures have been assessed and found not to be practicable;
• 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.

If we can satisfy ourselves of all these things then, as we believe is the case with Gough, we can be sure to make the right decision. 

Today, as in previous years and in the interests of transparency, I report the numbers of vertebrates killed by us and our contractors last year on RSPB reserves or as part of major projects we are leading such as the Orkney Native Wildlife Project. 

As ever, if you have any comments on the approach we adopt to address this dilemma, I would be keen to hear from you.