The Invertebrates and Introduced Animals of Saint Brandon

ÎLE COCO — THE DOCUMENTED CASE

The Cole surveys identified a significant number of non-native fauna species historically introduced to Coco. Several are flagged as priority for active removal.

Mus musculus (House Mouse)

Present on Coco. Flagged in the Cole survey as a priority removal target. The note in the data reads: Remove, but first archipelago needs biosecurity, followed by archipelago-wide eradication, for mouse control at peak seabird breeding season. Mice predate on seabird eggs and chicks, particularly the smaller tern and noddy species, and at peak breeding density a single mouse population can do enormous damage. The Cole recommendation is the use of A24 traps developed by Goodnature, a New Zealand company specialising in self-resetting predator traps for offshore islands.

Rattus rattus (Black Rat)

Not detected at Coco at the most recent survey — a meaningful finding, and one of the reasons the island remains so important to the wider conservation programme. Rats are catastrophic for ground-nesting seabirds. Their absence from Coco must be defended through biosecurity at every arrival.

Oryctolagus cuniculus (European Rabbit)

Not detected at Coco. But — as the diary records — rabbits remain a problem on L’île du Sud, where they were introduced decades ago and never eradicated. The Cole survey flags them as a priority removal target if and when confirmed at additional islands.

Gallus gallus domesticus (Chicken)

Present on Coco and on the other inhabited islands. Surprisingly, the Cole survey does not recommend their removal. The note reads: May need control, but do not remove, as they control ticks. The chickens, brought to the islands by fishermen as a low-effort food source, have settled into a useful niche — they keep tick populations down, which matters to the human inhabitants and possibly to the bird colonies as well. The chicken is one of the rare introduced species at Saint Brandon that is allowed to remain.

Hemidactylus frenatus (Common House Gecko)

Present on Coco. The pale, almost translucent gecko that climbs the walls of every building in the tropics. Non-native but not flagged for removal. Considered benign at present densities, though continued monitoring is recommended.

Solenopsis geminata (Tropical Fire Ant)

Present on Coco. Flagged for removal. Fire ants are aggressive, painful when disturbed, and can affect seabird chicks at ground level. One of the more serious invertebrate invasives at Saint Brandon.

Pheidole megacephala (Big-headed Ant)

Present on Coco. Not flagged for removal but on the watch list. Big-headed ants are one of the world’s most successful invasive ants and have caused ecosystem-wide damage on other tropical islands.

Periplaneta americana (American Cockroach) and Blattella germanica (German Cockroach)

Both present on Coco. Inevitable companions of human occupation, found in and around the structures rather than in the native vegetation. Not priority targets for active removal; their numbers are limited by the small habitable footprint.

Amblyomma / Carios spp (Ticks)

Present on Coco. The reason chickens are kept on the inhabited islands. Tick populations are controllable as long as the chicken population is healthy. I can confirm the tick presence directly — I was bitten during my time on the islands, and the resulting red welt took several days to subside. The bite itself was unremarkable. The principle it illustrated was not: that even on a coral atoll four hundred kilometres from anywhere, an introduced parasite has established itself in the soil and the vegetation, and the only sustained control mechanism is another introduced species. Conservation at Saint Brandon, like conservation everywhere, is a negotiation between imperfect options.

THE NATIVE INVERTEBRATES RECORDED AT COCO

The Cole surveys also confirmed the presence of a small native invertebrate fauna. The full list is short, which is itself a finding — the islands lie too far from anywhere for the wider terrestrial invertebrate fauna of the western Indian Ocean to have colonised them in any depth.

Heliotrope moth — Utetheisa pulchelloides

A native moth with delicate orange-and-white spotted wings, fluttering through the vegetation in the cooler hours. The most commonly observed native invertebrate on the island.
The fuller native invertebrate community — spiders, beetles, native bees, dragonflies, terrestrial isopods — has not yet been thoroughly catalogued. Several genera have been listed as not detected in the Cole surveys, which most likely reflects survey scope rather than absence. A proper invertebrate baseline for Saint Brandon would almost certainly add dozens of species to the record.

THE OTHER ISLANDS — WHAT WE KNOW

Cole’s methodology has not yet been formally extended to the rest of the archipelago. What follows is drawn from the 1998 World Bank Management Plan, informal observations by the fishermen and conservation visitors over the years, and my own diary.

Île Raphael

Rats confirmed present. The largest human-inhabited island and therefore the highest biosecurity risk in the archipelago. House mice almost certainly present. Common house geckos present. As of 2025, no formal species-level survey has been completed.

L’île du Sud (South Island)

Rabbits confirmed present and reproducing — the population observed and noted during our visit. Mice likely present though not formally surveyed. The herb-mat damage from rabbit grazing is visible across the inhabited interior. The 1998 World Bank Management Plan recommended rabbit eradication; the recommendation remains outstanding. South Island is the most urgent rabbit problem in the archipelago.

Avocaré

Largely uninhabited at present, though historically used as a fishing base. Less likely to support a large rodent population given the limited current human use. Not formally surveyed.

The Northern Guano Islands — Albatross, Puits à Eau, Poulailler, Frégate, Pearl, Sirène

Rats are confirmed on Poulailler and Puits à Eau, and likely on Pearl and Sirène. Cats may have been historically present on some of the inhabited islands but are not believed to be present today. The Mauritian Wildlife Foundation has, since 2016, identified rodent eradication on these islands as the single most impactful conservation action available at the archipelago — though the logistics of eradicating rats from islands without permanent human presence, with limited landing access, are considerable.

Île Coco itself

Beyond the documented species, the Cole 2019 survey recommends continued biosecurity protocols: thorough cleaning of all arriving vessels, careful inspection of all incoming supplies, and quarantine of any organic material. The atoll, in conservation terms, is only as protected as its most recently arriving boat.

WHAT THIS LIST DOES NOT YET INCLUDE

The Cole surveys catalogue what we currently know. They do not yet catalogue what we do not. The full invertebrate fauna of Saint Brandon — the spiders, the beetles, the moths, the marine and terrestrial isopods, the springtails, the parasitic insects of seabirds and turtles — remains largely undescribed. The Wikipedia and Saint Brandon Conservation Trust accounts both note that the atoll is biologically under-surveyed and almost certainly harbours species not yet known to science. The 2013 discovery of Novaculops alvheimi — a new wrasse described from Saint Brandon’s lagoon — is the marine equivalent of what is almost certainly waiting in the terrestrial invertebrate community. Restoration cannot wait for that work to be completed. But restoration without that work is also incomplete.

Editorial & Commissioned Work

If you would like to be in touch — for editorial commissions, place writing, long-form journalism, or design and brand work — write to me at river@welovethesea.co

I am available to be sent to places. I take a small number of commissions a year, for editors, publications, and brands who want a particular kind of attention paid to a particular place. Travel writing in the conventional sense is not what I do. My work is written from inside a place rather than across the surface of it, paired with original research and field notes from the ground.