San Salvador iguana
Cyclura rileyi rileyi

By William Hayes

Description

Schwartz and Carey (1977) concluded that the San Salvador iguana is the largest subspecies of C. rileyi, with a maximum size of 307mm SVL. More recent studies (Hayes et al., unpublished) indicate that many individuals of the San Salvador iguana on Low Cay, a satellite of San Salvador Island, exceed 350mm SVL, and attain a size of up to 395mm SVL (890mm total length). Although iguanas on Guana Cay within San Salvador's Great Lake also average over 300mm SVL, those on other cays rarely do.

The species is characterized by the absence of azygous scales in the prefrontal suture, rostral scale always in contact with nasals, first prefrontal never in contact with precanthal, dorsal crest scales on the neck always higher than on the body, body crest scales almost never higher than postsacral crest scales, and a variety of other scale-count features (Schwartz and Carey 1977). Characters that distinguish San Salvador iguanas from the other two subspecies include several features of facial scalation and poorly defined postsacral crest scales.

Dorsal coloration of San Salvador iguanas is striking but variable. Dorsum colors of red, orange, yellow, green, or brown are usually punctuated by darker markings and fine vermiculations. Males generally exhibit more color (red, orange, or yellow) and contrast than females, especially at warmer body temperatures. Juveniles are solid brown or gray, often with a slightly paler middorsal band having faint longitudinal stripes or indistinct darker areas near the middorsal crest. Juveniles lack the brighter coloration and vermiculations of adults, as well as the dorsal chevrons or pale diagonal markings present on juveniles of other taxa.

Distribution

Fossil remains found by Olson et al. (1990) indicate that San Salvador iguanas once occurred throughout the island of San Salvador (area 150km2). Today, however, sightings on the mainland are exceedingly rare, occurring most often on the eastern side between Great Lake and Storrs Lake. Although the Lucayan Indians may have hunted iguanas in earlier centuries, the extensive agricultural practices and other human activities of the last 100 years likely represent the greatest contribution to the iguana's demise. At present, San Salvador iguanas appear to be restricted largely to five tiny offshore cays (Gaulin, Goulding, Green, Low, Manhead) and two cays within Great Lake (Guana and Pigeon; Hayes et al. 1995). They were presumably extirpated on at least six additional cays, with two extinctions occurring in recent decades (discounting an unconfirmed sighting on High Cay in 1991). The seven inhabited cays range in size from 1-12ha and total approximately 26ha of marginal to excellent habitat.

Status of populations in the wild

Recent censuses by Hayes et al. (1995) suggested that approximately 500-600 individuals remain. However, they suspected that juveniles were underestimated in their surveys, and have since learned that a moderate population thrives on Pigeon Cay, the one known population they had not yet visited. Nevertheless, this subspecies likely numbers fewer than 1,000. Populations on the isolated cays vary from perhaps as few as 10 (Gaulin Cay) to as many as 250 (Green Cay). Several populations are threatened by human-related causes and appear to be declining.

Ecology and natural history

The habitat on cays presently occupied by iguanas varies greatly. Vegetation on offshore cays is similar in varying degrees to coastal rock, sand strand and sea oat, and coastal coppice plant communities described on the mainland (Smith 1993). However, for cays within inland lakes, the vegetation resembles the blacklands coppice (Guana Cay) and mangrove (Pigeon Cay) communities on the mainland. Habitats on the mainland of San Salvador are highly diversified and suitable enough to harbor large iguana populations, but feral animals are numerous in many areas.

The number of plant species on each cay varies from ten on Green and Gaulin Cays to more than 40 on Guana and Low Cays (Moyroud and Ehrig 1994; Hayes et al., unpublished data). Mean and maximum body sizes of iguanas vary significantly from cay to cay, and are positively correlated with plant diversity, suggesting nutritional constraints on body size (W. Hayes et al., unpublished data). Iguanas are largest on Low Cay and smallest on Manhead Cay. Previously reported measurements of body size (Schwartz and Carey 1977) were limited to samples from cays having low plant diversity. Thus, prior recognition of C. rileyi as the smallest of the rock iguanas may be an artifact of historical extinctions resulting in extant populations being confined today largely to the most inaccessible cays having minimal plant diversity.

Iguanas are locally most common in the vicinity of limestone rock outcrops and/or patches of sea grape (Coccoloba uvifera). On some cays they are numerous in patches of buttonwood (Conocarpus erectus) where they ascend into the foliage to browse. On Pigeon Cay they are frequently encountered basking on the limbs of mangrove trees, often several meters or more above the ground. The iguanas share their habitat with nesting seabirds on several cays, most notably on Gaulin Cay where brown noddies (Anous stolidus) and sooty terns (Sterna fuscata) are extremely dense (100+ nests/ha).

Aspects of feeding and reproductive ecology remain unstudied but are likely similar to other rock iguana species. Adult males appear to be territorial throughout the year. As in other Bahamian taxa, courtship and mating probably occur in May, followed by nesting and egg-laying in June or July. Copulations have been observed by investigators visiting the cays during the last week of May (W. Hayes, unpublished data). Stejneger (1903) reported a clutch that numbered five eggs. Hatchlings probably emerge from nest burrows in September or October. Like other rock iguanas, San Salvador iguanas presumably require sandy areas for nest construction. Such habitat appears to be limited on Guana Cay, but the presence of several juveniles in 1994 is indicative of successful nesting there.

Habitat

On most cays, iguanas range widely throughout all available habitats. However, on Low Cay iguanas are largely restricted to areas of sea grape that comprise a relatively small portion of the island. Although a manmade structure is under construction on High Cay where iguanas are thought to be extirpated, none of the other cays are inhabited and, at present, are seldom visited. On several cays where iguanas have disappeared the habitat appears suitable for reintroduction; however, these cays may harbor feral rats that could be incompatible with reintroduction efforts unless they are first extirpated.

Considerable habitat has been lost on the main island of San Salvador. Nevertheless, extensive areas of excellent but very remote habitat remain which could support large populations of iguanas if development on the island could be halted (which appears unlikely). Thus, while habitat availability does not presently limit the mainland population, it may well hinder the prospect of increasing the total population size via future reintroductions.

Threats

Although remote and relatively difficult to access, populations on the cays are still threatened by human-related causes (Hayes et al. 1995). All size classes of iguanas are readily seen on Goulding, Green, Manhead and Pigeon Cays, which suggest the presence of healthy, stable populations. However, juveniles are conspicuously scarce on Guana and Low Cays, and possibly absent on Gaulin Cay. The scarcity of juveniles on Low Cay is probably attributable to the presence of feral rats only recently detected there (Hayes et al. 1995). The iguana population consists almost entirely of large, aged adults. More recently, rats have also been seen on Guana, High, and Pigeon Cays. Considering the apparent impact of rats on insular populations of the tuatara (Sphenodon punctatus), an iguana-sized burrow-nesting reptile in New Zealand (Cree et al. 1995), rats probably pose a serious threat to survival of iguanas on several cays and need to be exterminated soon. Rats may also impact iguana populations indirectly by affecting vegetation, especially on cays with low plant diversity.

The once dense population on Guana Cay (Ostrander 1982) has become greatly reduced in recent years. A mysterious die-off occurred in spring 1994, as evidenced by the discovery of eight adult carcasses and an estimated surviving population of only 24 individuals (Hayes et al. 1995). Because the carcasses all appeared to be in similar states of decay, they may have died within a narrow time frame from similar, but unknown, causes. Although natural disease is a possible cause, so too might be mosquito control efforts, recently implemented for the benefit of the growing tourism industry. The ticks which infest these iguanas have not been found on any other cay, and may have rendered the lizards more vulnerable to the agent(s) causing their deaths. Juvenile iguanas may also be scarce as a consequence of the die-off; their carcasses may have decayed quickly or escaped detection. However, nesting habitat appears to be restricted, which could limit the numbers of juveniles and affect future recruitment. Nesting habitat is also limited on Pigeon Cay and nesting failure is inevitable in wet years when lake surface level inundates the nests. It is unclear why the population on Gaulin Cay is so low (possibly fewer than 10) and juveniles appear to be absent. This small, potentially inbreeding-depressed population may no longer be viable.

The larvae of a moth (Cactoblastis cactorum) introduced decades ago to the West Indies are now rapidly devastating prickly-pear cacti (Opuntia stricta), an important food source for iguanas, on several cays. The dense population of lizards on Green Cay is especially vulnerable, particularly since destruction of the cacti will be nearly complete within a matter of years, there are no known means of controlling the moth, and the remaining vegetation (nine plant species) represents a meager diet compared to other rock iguana species (Auffenberg 1982). The impact of this ecological disturbance needs to be closely monitored.

Rapid development on the island of San Salvador will undoubtedly threaten the populations further. Feral dogs and cats are already numerous in local areas, but will increase as more resorts and housing tracts are constructed. This would seriously jeopardize any possible reintroductions of iguanas to the mainland, unless protected areas of considerable size could be set aside. Pollution of Great Lake due to environmentally unsound landfill practices may threaten the fragile mangrove community that harbors a moderate iguana population on Pigeon Cay. Eventually, tourists may discover the attributes of the iguanas themselves, which could increase potentially detrimental human-iguana contacts.

Current conservation programs

At present, W. Hayes and R. Carter are collecting baseline data on all populations of C. rileyi to aid conservation management decisions. Initial efforts involve population surveys, assessment of threats to survival, and genetic sampling. Genetic analyses are essential to resolve the taxonomic identities of the nominate taxa, to assess the degree of divergence among individual populations, and to evaluate heterozygosity (which may reveal inbreeding depression). Divergence may be sufficient that the genetic identity of most or all populations will need to be maintained.

Further steps include concentrated searches for isolated colonies on the mainland and on the southernmost lakes, as well as reintroductions of iguanas to previously inhabited cays. For San Salvador iguanas, candidate cays for reintroduction (and source animals) include Barn Cay (from Guana Cay), Cato Cay (from Green Cay), Cut Cay (from Manhead Cay), and High Cay (from Low Cay). However, further research is necessary to determine the suitability of each cay, and to assess what corrective actions would be necessary to render each suitable for reintroduction (e.g., removal of feral rats or supplementation of nesting habitat). Reintroduction of iguanas on the mainland should be undertaken only if protection of a large area can be assured. Excellent mainland areas presently uninhabited by humans include the land bridge east of Storrs Lake (where a major resort and marina are planned), the area between Storrs Lake and Great Lake, and the peninsula east of Pigeon Creek. Additional comparative research planned for the isolated populations of this taxon includes vegetation analyses and studies of reproductive strategies, seasonal dietary shifts, and behavioral ecology.

At present, no legal captive breeding programs exist outside the Bahamas. The Bahamian government has wisely refused to issue export permits for any rock iguana taxa. However, Ardastra Gardens in Nassau (New Providence Island, Bahamas) currently holds two juveniles and plans to implement an in situ program. Captive programs could be highly valuable for repatriation efforts, particularly if the genetic integrity of individual populations needs to be preserved.

A public relations campaign is planned to heighten awareness and appreciation among island residents for their endemic iguana. Brochures have the potential to provide basic information and to promote the need for complete protection of the cays on which the iguanas live. Residents should be alerted to the protected status of the iguanas and urged to report to authorities anyone seen visiting the cays. Bringing feral animals to any cay should be legally forbidden. If possible, especially if the tourism industry continues to escalate, the Bahamas National Trust should declare the cays a land and sea park and hire a warden to patrol the region.

Critical conservation initiatives

• Limitation of access to cays, particularly to discourage feeding of iguanas by tourists.
• Further protection of the cays by incorporation into a national park by the Bahamas National Trust. As some of the cays are privately owned, this will require working with landowners.
• Institutional strengthening of responsible agencies to develop enforcement capabilities.
• Initiation of a national education program for tourists and residents.

Priority projects

1. Continue to sample and survey individual populations on an annual or biannual basis.
2. Eradicate rats on infested cays.
3. Monitor the impact of the Cactoblastis moths and rats on vegetation.

Contact persons

William Hayes
Department of Natural Sciences
Loma Linda University
Loma Linda, CA 92350 USA
Tel: (909) 824-4300 ext. 48911
Fax: (909) 824-4859
E-mail: whayes@ns.llu.edu

Ronald Carter
Department of Natural Sciences
Loma Linda University
Loma Linda, CA 92350 USA
Tel: (909) 824-4300 ext. 48905
Fax: (909) 824-4859
E-mail: rcarter@ns.llu.edu

David Blair
Cyclura Research Center
PMB #510, 970 West Valley Parkway
Escondido, CA 92025 USA
Tel: (760) 746-5422
Fax: (760) 746-1732
E-mail: critter@herpnut.com