|Scientific Name:||Cyclura nubila ssp. caymanensis|
|Species Authority:||Barbour & Noble, 1916|
See Cyclura nubila
|Taxonomic Notes:||Synonyms = Cyclura caymanensis Barbour & Noble 1916; Cyclura macleayi caymanensis Grant 1940; Cyclura nubila caymanensis Schwartz & Thomas 1975; Cyclura nubila caymanensis Schwartz & Carey 1977.|
|Red List Category & Criteria:||Critically Endangered A3bce+4abce ver 3.1|
|Assessor/s:||Goetz, M. & Burton, F.J.|
|Reviewer/s:||Grant, T.D. & Desbiez, A.|
Based on the life history of the Sister Isles Rock Iguana and similarly sized rock iguanas, the minimum estimated generation length for this iguana is 23 years. The majority of the population on Cayman Brac was lost more than three generations ago and is now extremely small and vulnerable. In recent years, the population on Little Cayman is under threat from the same concerns and the rate of decline has increased. The current population size is estimated to be not more than 900 mature individuals for both islands. Based on mortality estimates, high prevalence of invasive alien predators, and the ongoing habitat degradation observed in the last 1.5 generations, the population is expected to have declined by 95-98% by the end of an additional 34 years. Indeed, projections for this iguana indicate functional extinction is possible within one mean generation time if threats cannot be reversed.There is no change in the Red List Category from the previous assessment, however, the Criteria has been updated with recent data. Previous Criteria may previously have been misinterpreted due to lack of survey data on Cayman Brac, the difficulty in estimating population size for iguanas, or underestimating the area of occurrence.
The Sister Isles Rock Iguana is native to two islands: Cayman Brac (38 km²) and Little Cayman (28.5 km²). The islands are 7.5 km apart and are well isolated from other land masses in the area. Both islands have been continuously inhabited by people since the early 1800s. Current human population sizes for Cayman Brac and Little Cayman are approximately 2,000 and 150 respectively. It occurs from sea level up to 45 m.
|Range Map:||Click here to open the map viewer and explore range.|
In 1938, Sister Isles Rock Iguanas were abundant throughout Cayman Brac and Little Cayman (Grant 1940). Since that time, the iguana populations have declined and are clearly in danger of extinction, particularly on Cayman Brac. By 1965, iguanas were abundant on Cayman Brac only along a relatively small section of the southwest coastline (Carey 1966). A comprehensive survey in 2012 counted 87 iguanas on Cayman Brac, most of which were adults distributed around the island’s west, along the south coast, and a smaller subpopulation in the middle of the island (M. Cottam unpublished report 2012). Based on these results, the population consists of not more than 100 mature individuals, with minimal juvenile recruitment, and is under severe threat from automobiles and feral cats and dogs.
Little Cayman still supports a widely, although patchily, distributed iguana population. The main subpopulations are found in the western part of the island and the south coast, while the majority of the inland habitat is sparsely occupied (M. Goetz unpublished report 2010). The population is reproducing and all age classes are seen, although juvenile mortality is probably high due to predation by feral cats. A conservation research project initiated in 2007 currently estimates the number of mature iguanas to be around 800 with an overall population of 1,200-1,500 iguanas. However, due to the iguana’s life history and the inaccessibility of most of its habitat, it is inherently difficult to make good population estimates. The population size is estimated by nest counts, plot sampling, and distribution theory based on the geology of the island. A more thorough estimation might be achieved in the near future by analysing mark/recapture data since about 200 individuals were permanently marked on Little Cayman during the past three years and is ongoing.
Based on qualitative observations in the 1970s (Townson 1980, Stoddart 1980) and 1980s (Blair 1983), the population on Little Cayman appears not to have declined overall in large numbers. Conservatively estimated, this time period comprises one-two generation lengths. Nonetheless, the dense concentrations of iguanas which occupied the mid-northern coast 30 years ago (B. Ryan pers. comm.) and mid-southern coasts 50 years ago (Grant 1940), no longer exist and indicate there has been a relatively recent decline in the number of individuals. A growing population of feral cats, rapidly increasing human development, and especially the recently increased road traffic severely threaten the long-term survival of iguanas on Little Cayman.
|Habitat and Ecology:||
Like other rock iguanas, Sister Isles Rock Iguanas require suitable forage plants, basking areas, retreats, and nesting sites. On Little Cayman, these requirements are met in a variety of coastal and interior habitats and iguanas are widely dispersed. Maximum densities occur in dry evergreen shrublands and thickets growing on exposed and highly weathered limestone or dolomite. These habitats provide a diverse assemblage of forage plants, a mosaic of sun and shade for thermoregulation, and an abundance of solution holes in the rock substrate which iguanas of all sizes use as retreats. However, suitable nesting sites in these habitats are restricted to shallow patches of soil that accumulate in small depressions. Consequently, most of the larger females migrate to coastal areas with relatively deep sandy soils to nest. Five larger communal nesting sites, with up to 60 nests in close proximity, have been recently identified (M. Goetz unpublished data).
The diet of all age classes consists almost entirely of leaves, flowers, and fruits. However, iguanas occasionally scavenge on animal carcasses (land crabs, for example) or prey on slow-moving insects (e.g., butterfly and moth larvae).
Courtship and mating occur April through June, coinciding with the end of the dry season when temperatures and photoperiod are increasing. Males compete intensely for territories that they occupy in all seasons. Male territories are large, on the order of one hectare, and can overlap as many as 10 female territories. The adult sex ratio appears skewed toward females.Females lay a single clutch of 7-25 eggs (mean 15) annually. The nesting season lasts about six weeks, sometime between May and August, coinciding with the beginning of the wet season in a given year. Hatchling emergence occurs from early August to October after an incubation averaging 72 days (range 63-80). First reproduction usually occurs at the age of 3.5 years, but is generally possible with good nutrition from the age of 2.5. The maximum age for this subspecies is unknown, but other Cyclura species have been known to live up to 70 years and this is assumed to be true for C. nubila caymanensis as well.
Threats to the Sister Isles Rock Iguana include habitat destruction from road construction, commercial and residential real estate development, disturbance and development in sensitive nesting areas, and increasingly from road kills. Since the construction of a municipal power generating station on Little Cayman in the early 1990s, habitat destruction associated with road construction and real estate development have increased dramatically. The human population, although still small, has increased several-fold. There is no controlled development plan for Little Cayman and the proposed major development sites overlap areas of prime iguana habitat on the western half of the island where the majority of iguanas occur. The continued destruction and disturbance of coastal nesting areas on Little Cayman is of particular concern as nesting opportunities for iguanas in the interior appear to be limited due to the paucity of suitable soil patches. Increasing vehicle traffic and paving of gravel roads, which dramatically increases vehicle speeds, is causing significant losses, mainly of adult iguanas, and particularly during the nesting season when gravid females cross the roads on their way to coastal nesting sites. In 2010, the estimated annual loss of iguanas due to road-kill was 6-10% of the overall population.Iguanas are also threatened from predation by invasive alien cats and dogs (both feral and free-roaming domestic) and possibly Norway Rats (Rattus norvegicus). In general, rats are of concern mainly as predators of eggs and nests, but are known to cause harm to iguana hatchlings sleeping at night. The increasing number of feral cats on Little Cayman is an immediate threat to population recruitment (M. Goetz, unpublished data) as they predate upon hatchlings and juveniles up to the age of about one year. The exact impact and extent of mortality by cat predation is extremely difficult to establish, since juvenile iguanas, especially hatchlings, are extremely hard to observe and monitoring changes in this age class has not been possible. High level of predation is inferred based on the large number of cats observed. Dogs have been observed to attack and kill iguanas of all size and age classes.
There is no current or recently historic human use or trade of this species.
The Sister Isles Rock Iguana is protected within the Cayman Islands by the Animals Law of 1976, but protection of native habitats is lacking. The Development and Planning Law of 1971 provides a legal mechanism to prevent the destruction of terrestrial habitats in the Cayman Islands, but this has never been implemented (Davies 1994). Currently, the only protected areas on the two islands are the Cayman Brac Parrot Preserve (a 65 hectare tract of potentially important iguana habitat) and the Little Cayman Ramsar Site (an 82 hectare preserve encompassing Booby Pond and surrounding mangroves). The iguana is included in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
The National Trust for the Cayman Islands has had an active iguana conservation program since 1990. However, due to limited resources and a higher extinction risk at the time, efforts have largely concentrated on the Grand Cayman Blue Iguana (Cyclura lewisi). A field study conducted on Little Cayman by G. Gerber in 1993 produced the first baseline natural history data of Sister Isles Rock Iguanas (Alberts 2002). In 2007, the Durrell Wildlife Conservation Trust initiated a multi-year conservation research project on Little Cayman and Cayman Brac focussing on estimating population size, genetic variation analysis, threat assessment, mapping nesting sites, and home range determination in a variety of habitats (Goetz 2008). This data has sparked a renewed interest in the iguana by the local National Trust and a conservation-planning meeting was organized on Little Cayman in 2011, involving all stakeholders, and resulted in the development of a three-year Species Action Plan.
Immediate conservation actions are prioritized for habitat protection (particularly for coastal nesting sites), invasive alien predator control, and enforced speed limits on roads. All three measures need support of the local communities and will require education and raising awareness to achieve success. Protecting habitat of sufficient size and quality is challenging. The most desirable inland habitat (for example, toward the west end of Little Cayman) as well as coastal nesting areas are equally sought after as prime real estate for future development. A long-term management plan for controlled development on the islands is needed that considers the ecological roles and conservation needs of local species.Future research will need to address the inherent difficulty in obtaining a reliable population estimate on each island and monitoring population trends.
|Citation:||Goetz, M. & Burton, F.J. 2012. Cyclura nubila ssp. caymanensis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 07 December 2013.|
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