Chagos Archipelago Rat Eradication & Seabird-Driven Reef Recovery (British Indian Ocean Territory)

Rats introduced via historic shipping and coconut plantations are present on 26 of the Chagos Archipelago's islands. In 2014, the Chagos Conservation Trust (CCT), the BIOT Administration, RSPB, the Royal Botanic Gardens Kew and Biodiversity Restoration Specialists Ltd, with logistical support from British Forces on Diego Garcia and funding from the UK's Darwin Plus Initiative, eradicated rats from Ile Vache Marine (Peros Banhos Atoll) and the islets of Île Sel and Île Jacobin (Salomon Atoll) — confirmed successful in 2017, the first completed eradication in the territory.

A peer-reviewed Nature study (Graham et al., 2018) analysed a natural experiment across 12 islands (6 rat-free, 6 rat-infested) to measure the ecological effects of rat eradication. A follow-on study in Current Biology (Benkwitt et al., 2021) examined recovery trajectories at other regional eradication sites, including Tromelin Island (rats removed 2005) and Île du Lys (rats removed 2003).

Graham et al. (2018) found seabird densities 760x higher, nitrogen deposition rates 251x higher, and reef fish community biomass roughly 50% larger on or near rat-free islands, with herbivory/bioerosion rates over 3x higher near large seabird colonies. Benkwitt et al. (2021) found seabirds increased roughly 8-fold at Tromelin Island (with 6 species resuming breeding) and roughly 10-fold at Île du Lys, with the seabird-derived nitrogen signal reaching reef fish within about 16 years, though fish growth rates had not measurably improved over that period.

Territory-wide eradication — potentially covering up to about 1,400 hectares of rat-affected islands — remains an ongoing priority rather than a completed task; funding for scale-up is not secured, and reinvasion risk requires continued biosecurity. The wider political context — BIOT's creation through the forced depopulation of Chagossians in the 1960s-70s to establish the Diego Garcia military base, and ongoing UK-Mauritius sovereignty/resettlement negotiations — is a separate but relevant governance backdrop to the conservation programme.