The Caribbean Islands are considered to be a biodiversity hotspot because they harbour a high concentration of endemic species that are threatened by habitat loss and invasive species (Myers et al., Reference Myers, Mittermeier, Mittermeier, da Fonseca and Kent2000). Of the region's herpetofauna, 99% of frogs and 93% of reptiles are endemic (Hedges, Reference Hedges, Woods and Sergile2001). The Critically Endangered endemic dipsadid snake Erythrolamprus cursor (previously known as Liophis cursor, Lacépède, 1789, Plate 1) was common throughout Martinique during the 18th and 19th centuries (Moreau de Jonnès, Reference Moreau de Jonnès1818). It was last observed on the Martinique mainland in 1965, when a single individual was caught near Fort-de-France. There are two potential reasons for its decline: people may have mistaken it for the venomous lancehead Bothrops lanceolatus, which may have led to its eradication, and the small Indian mongoose Herpestes javanicus auropunctatus, an invasive predator, was introduced to the West Indies at the end of the 19th century, resulting in declines and extirpations of reptile species (Henderson, Reference Henderson1992).

Plate 1 Museum specimen (MNHM 1887.0120) of the dipsadid racer Erythrolamprus cursor, endemic to Martinque. Photograph by Stephane Caut.

To the south-west of Martinique, c. 2 km from the coast, lies a volcanic islet (spanning 5.8 ha, with a maximum elevation of 175 m; Fig. 1a; Plate 2) known as Diamond Rock. The last published record of E. cursor is associated with this unique ecosystem (the snake was observed there in 1962; Lazell, Reference Lazell1967), and the last specimen was collected on the islet in 1968 (Breuil, Reference Breuil2009). Diamond Rock was occupied by the English during the Napoleonic Wars (1804), when c. 100 men lived there for 17 months, sleeping in caves. Since this period of occupation few records of Diamond Rock have been kept but it seems to have remained uncolonized by the mongoose. The site is now fully protected and people have been banned from landing on the islet since 2008. Fishermen and tourist boats anchor near Diamond Rock, and there have been reports of snakes basking on shoreline rocks. However, several 1-day surveys for the species failed to locate it (Breuil, Reference Breuil2009; M. Dewynter, pers. comm.). The lack of any confirmed sightings of the snake since 1968 suggests that it is probably extinct (Honegger, Reference Honegger1981). The aim of this research was therefore to carry out an extensive survey to elucidate the species’ status on Diamond Rock.

Fig. 1 (a) Location of Diamond Rock off the coast of Martinique; (b) map of Diamond Rock from the period of English settlement (Commodore Hood's dispatch, 14 August 1804; Stuart & Eggleston, Reference Stuart and Eggleston1978), with the locations of the eight trapping sites (filled black circles).

Plate 2 Diamond Rock. Photograph by Stephane Caut.

Because of the difficulty of exploring the steep slopes of Diamond Rock, and given its protected status, we surveyed the islet during 4–14 June 2014 as a two-person team. We used several methods to search for all potential traces of E. cursor. Firstly, eight snake-specific intercept traps (cylindrical cages 80 × 15 cm with cones at the extremities, modified after Linnell et al., Reference Linnell, Engeman, Pitzler, Watten, Whitehead and Miller1998) were set in various microhabitats (near rocks, tree trunks, crevasses and at the base of stone walls; Fig. 1b) and were checked twice per day (80 trap-nights total; Table 1). Some traps were baited with live mice captured on the islet (8 trap-nights; Table 1). Secondly, we searched for snakes, snake eggs and sloughed skin in all the various habitats on the islet both during the day and at night (12 hours per day per researcher). To examine E. cursor's potential trophic relationships with other species on Diamond Rock (predator–prey and/or competitive interactions) we compiled an inventory of the species we observed during the survey. To confirm the presence of invasive rodents we used INRA rat traps (34 × 13 × 13 cm) and mouse traps (12 × 5 × 5 cm) baited with peanut butter. Trap lines were established in various habitats (six sites; Fig. 1b), 25 and 5 m apart for rats and mice, respectively. For logistical reasons we could only use 14 traps (six rat traps and eight mouse traps), which we moved between sites. Trapping was conducted for 1 or 2 consecutive nights on each line transect (a total of 119 trap-nights; Table 1).

Table 1 The number of trap-nights with rodent traps (for rat and mice traps, see text for details, combined) and with interception traps for snakes (with number of nights on which traps were baited with live mice), with the species caught in each, at the eight survey sites (see numbered locations on Fig. 1) on Diamond Rock (Fig. 1).

We did not detect any signs of the snake. The intercept traps functioned well because we captured all the reptile species that we observed during our visual surveys (Table 1). Although some remote areas of the islet remain unexplored for logistical reasons, our survey covered more of Diamond Rock's surface than previous surveys. We ascended steep slopes, whereas previous researchers focused on accessible rocks at low elevations (Lazell, Reference Lazell1967), surveyed for only a few hours, and did not explore the islet extensively. In reports written during the English occupation of Diamond Rock (Stuart & Eggleston, Reference Stuart and Eggleston1978) snakes were described as abundant near the edge of the water tank (site 2, Fig. 1). The last time E. cursor was observed on Diamond Rock was in July 1962 (Lazell, Reference Lazell1967). Weather and temperature conditions during our survey in June (including some precipitation) should have been favourable to observing the snake. Some seabirds (e.g. bridled tern Onychoprion anaethetus, red-billed tropicbird Phaethon aethereus, brown noddy Anous stolidus) breed on Diamond Rock during this season, which could have enhanced the likelihood of spotting the snake if it had adapted to consume eggs or juvenile birds.

Proving the absence of a species is notoriously difficult, and previous work on Erythrolamprus ornatus on Maria Major Island, Saint Lucia, suggests that snakes are difficult to survey, with as few as one located per 30 days of searching (J. Daltry, pers. comm.). Although, it seems likely that E. cursor is extinct, what allowed the species to be abundant on Diamond Rock in the 18th century and persist until the 1950s? Given the aridity on the islet and its distinct prey communities E. cursor would have had to modify its ecology to persist on the islet. Although the genus Erythrolamprus contains 48 species little is known about their ecology and diet, and living E. cursor have never been studied. Studies of the species’ trophic ecology (Henderson & Bourgeois, Reference Henderson and Bourgeois1993; Henderson, Reference Henderson2004) were based on the stomach contents of preserved specimens, indicating consumption of frogs (Eleutherodactylus spp.) and lizards (Anolis spp.) and a small proportion of insects. The closest living related species, Erythrolamprus juliae (Jowers et al., Reference Jowers, Caut, Garcia-Mudarra, Alasaad and Ineich2013), has a similar diet, comprising mainly anurans (Schwartz & Henderson, Reference Schwartz and Henderson1991; Breuil, Reference Breuil2002). There have not been any reports of frogs on Diamond Rock and we found no evidence of them, and therefore E. cursor probably underwent a dietary shift to prey on the species available (e.g. four reptile species; Table 1). We found that the Martinique anole Anolis roquet was common, especially at lower elevations. Other potential prey present at high densities included seabird chicks, juvenile Antillean fruit-eating bats Brachyphylla cavernarum, and invasive house mice Mus musculus.

Another cause of the snake's disappearance could be the introduction of non-native predators. Diamond Rock was granted full protection only in 2008. During our survey we found no evidence of the presence of superpredators on the islet (e.g. mongoose or feral domestic cat). We did not capture any rats Rattus spp., and based on our experience trapping rodents on other oceanic islands we are confident there are no rats on Diamond Rock and probably never have been. However, mice appear to be present at high densities. They can have devastating, irreversible and ecosystem-changing effects on islands and are known to consume invertebrates, seabird eggs and juvenile seabirds (Angel et al., Reference Angel, Wanless and Cooper2009). On Diamond Rock mice could have affected the snake population directly by consuming juvenile snakes or snake eggs, or indirectly by decreasing prey availability. Egg clutches of E. cursor may have contained no more than five eggs (Arlington & Henderson, Reference Arlington and Henderson2004). However, as the mouse population dates from the time of the English occupation in 1804, why was the snake still present on the islet <50 years ago?

Lack of data on the species’ abundance in the 1950s is a significant limitation to evaluating the cause of the species' extinction. There may never have been a large population of E. cursor on Diamond Rock, or the population may have decreased during the English occupation. If E. cursor on Diamond Rock lived at the same density as E. ornatus on Maria Major (1.7–8.3 individuals per ha; J. Daltry, pers. comm.), this would imply <50 individuals in total. Such a low-density population would probably suffer from inbreeding depression (e.g. reduced fertility, congenital defects, reduced resistance to disease) and be vulnerable to stochastic forces such as a skewed sex ratio, natural disasters and chance fluctuations in reproduction and mortality (Traill et al., Reference Traill, Bradshaw and Brook2007). Alternatively, the disappearance of E. cursor may have resulted from a particular event (e.g. climatic change, species invasion) that took place between the 1950s and 2008, when the islet became fully protected.

Although the complexity of the Diamond Rock ecosystem makes it difficult to explore, we conclude that E. cursor may now be extinct, given that 10 days of extensive surveying and trapping yielded no signs of its presence. It is more difficult to declare a species extinct than to describe or rediscover species. Erythrolamprus cursor is just one example of several reptile species known to survive only on one small island, at a low density (e.g. E. ornatus, Alsophis antiguae, Cnemidophorus vanzoi, Ameiva polops). There is an urgent need to assess these species and to ensure that their habitats are free from invasive species and other potential threats. The invasion of oceanic islands by non-native predators can lead to dramatic effects on island ecosystems and cause numerous extinctions. From a conservation perspective, the Caribbean, a biodiversity hotspot with a large number of species at risk of extinction, is one of the most important areas in the Western Hemisphere.