Ascension Island Feral Cat Eradication and Seabird Recolonization

Ascension Island (97 km²), a UK Overseas Territory within Saint Helena, Ascension and Tristan da Cunha, had cats introduced in 1815 by early settlers. The cats decimated seabird colonies over two centuries, confining roughly 6,250 breeding pairs of the endemic, IUCN Vulnerable Ascension frigatebird to the tiny (5.3 ha) offshore stack of Boatswain Bird Island by 2002.

The RSPB and the Ascension Island Government Conservation Department ran a three-phase eradication — knock-down (February–October 2002), mop-up (October 2002–January 2004), and confirmation (January 2004–January 2006) — combining cage traps (42,008 trap-nights, 70 cats caught), leg-hold traps and poison bait; the last feral cat was killed on 31 January 2004. The programme, funded by the UK Foreign and Commonwealth Office at a cost of about GBP650,000, ran alongside a parallel domestic-cat neutering programme led by the Ascension Island SPCA.

Peer-reviewed in Oryx (Ratcliffe et al., 2010), the study documents mainland seabird colonies being reoccupied for the first time in roughly 180 years: masked booby pairs rose from 2 (2002) to 152 (2007), brown booby from 6 to 29, and brown noddy from 0 to 79. Ascension frigatebirds nested on the mainland for the first time in 2012 (2 pairs), reaching 44 nesting attempts by 2014 and 1,493 recorded nesting attempts in the 2020–21 season per Ascension Island Government monitoring. Sooty tern adult deaths in colonies fell from over 4,500/year pre-eradication to negligible levels.

About 38% of the island's registered domestic cats (69 animals) were killed accidentally by poison bait, causing local backlash. The Oryx paper itself documents post-eradication rat predation on some seabird eggs and chicks, but explicitly rules out classic mesopredator release, finding rat numbers tracked rainfall and food availability rather than cat removal. Breeding success of recolonizing species remains lower than at long-established colonies elsewhere.

Read the full analysis: https://doi.org/10.1017/S003060530999069X