Cayman Islands xeric scrub
Satellite view of the Cayman Islands (United Kingdom) Photograph by USGS
The Cayman Islands xeric scrubecoregion is a low elevation very disturbed ecosystem in the Caribbean basin. The Cayman Islands are a small group of low-lying Caribbean islands that serve as an important stop-over and wintering site for migratory land birds. A humid tropical climate provides for twenty-six species of herpetofauna, seventy-five percent of these are endemic to this ecoregion. Little of the original habitat remains. Introduced animals, such as domestic dogs and cats, and rats, pose the greatest threat to the native wildlife. The extinction of several endemic mammals are attributed to these invasive species.
Location and General Description
The Cayman Islands (Grand Cayman, Little Cayman and Cayman Brac) are emergent limestone bluffs situated along a submarine ridge that runs westward from the Sierra Maestra range in southern Cuba. Grand Cayman is the largest of the group at 35 kilometers (km) long and up to 14 km wide, however, there is a large lagoon in the northern section that gives the island an irregular shape, as if a giant bite has been taken out of the northwestern end. Cayman Brac is the tallest island in the group, rising to a height of 43 meters (m) on the eastern end where sheer cliffs drop into the sea. Little Cayman is the smallest of the three islands, at only 14 km long and a maximum height of merely 12 m above sea level. Grand Cayman lies approximately 700 km south of Miami, and 200 km from both Cuba and Jamaica. The lesser islands are located about 130 km northwest of Grand Cayman, and lie 7 km apart. All three islands were uplifted from the ocean floor approximately 10 million years ago and have apparently never been connected to adjacent land masses. The Cayman group is subject to strong trade winds and has a humid tropical climate with a distinct wet season from May-November.
The Cayman Islands support two distinct types of vegetation: evergreen thicket and woodland, and seasonal swamp. Evergreen thicket dominates the eastern sections of Grand Cayman, and is found on the northern slope of Little Cayman and on higher ground on Cayman Brac. The thicket has a discontinuous, two-storeyed canopy with occasional emergents. Dominant species include red birch (Bursera simaruba), Swietenia mahagoni, Picrodendron baccatum, Sideroxylon salicifolium, Calyptranthes pallens, and Chionanthus caymanensis. Palms (Coccothrinax proctorii and Thrinax radiata) are common and climbing cacti (Selenicereus) are well represented.
Biodiversity Features
Levels of endemism in the Cayman Islands are fairly low, probably due to the proximity of other islands and the strong trade winds which assist over-water dispersal. Of the 601 vascular plant species found on the island, only 21 are endemic species. Similarly, most of the bird species are shared with neighboring islands, or are migrants from North and South America. The only endemic bird species found in the islands, the Grand Cayman thrush (Turdus ravidus), has recently gone extinct. The floral community shows strong affinities with both Cuba and Jamaica, as does the breeding landbird fauna and the lepidopteran community. In contrast to these groups, about 75% of the herpetofauna and 30 of the 48 species of non-marine molluscs are endemic to the Caymans. Although the extant land mammals of the Caymans are all bats, most of which are found elsewhere in the Carribbean, there were once at least 5 non-volant land mammals in the Cayman Islands.
The herpetofauna of the Cayman Islands includes 26 taxa: two crocodiles, one turtle, 14 lizards and nine snakes. Only the Cuban crocodile has disappeared in historic times, but populations of many endemic lizards are declining at an alarming rate.
Current Status
The Cayman Islands government has set up a National Trust whose mission is to preserve the natural and cultural heritage of the islands. The Trust owns more than 310 hectares (ha) of natural woodland and bluff habitat on Grand Cayman and has initiated a captive breeding program for the blue Grand Cayman iguana (Cyclura nubila lewisi), which is listed as endangered by the USFWS. The Trust is also conducting research programs assessing the status of the threatened rock iguana (Cyclura nubila caymanensis) and the native parrots. Despite these initiatives, there is a continued need for increased species management, predator control, and habitat preservation.
Types and Severity of Threats
The two major threats to native species in the Caymans are habitat loss and introduced species. Very little native woodland remains in the Cayman Islands, as most of the large trees have been felled for timber and for fuel. The few remaining stands of trees need to be protected from future logging. In addition to the loss of native woodland, native evergreen thickets on Grand Cayman are now being replaced by logwood (Haematoxylum campechianum). This deliberately introduced species spreads by stolons, and will probably continue to invade native thickets and displace native plants.
Recent declines in the populations of iguanas and other reptiles throughout the Cayman Islands have been linked to the increased number of domestic cats and dogs on the islands, and introduced rats (Rattus rattus and Rattus norvegicus) have been implicated in the extinction of at least 2 of the 5 native non-volant land mammals, particularly the two endemic Nesophontes species. Introduced mosquitoes do not pose any obvious threat to native species in the Caymans, nor do they pose a significant health threat, but it is worth noting their success in the islands. Mosquitoes were accidentally introduced sometime soon after European settlement of the islands, and their populations subsequently exploded. In the 1970’s, mosquito density in the Cayman Islands was more than twice the maximum recorded anywhere in the United States; in a standard measure of mosquito density, one researcher recorded a total of 600 bites per minute on one arm. Mosquito populations have been greatly reduced in recent years through an intensive control program and regular aerial spraying.
Justification of Ecoregion Delineation
The Cayman Islands were divided into dry forest and xeric scrub ecoregion, which were then distinguished from the mangroves. The lines to delineate the xeric scrub were derived from both the Department of Tourism map which distinguished between woodlands and open areas of low growth, and the Stoddard map which distinguishes mangrove and swamp habitat from terrestrial habitat. In this case the lines were drawn to encompass potential xeric scrub as open areas of low growth.
References
- Bradley, P.E. 1994. The avifauna of the Cayman islands: an overview. Pages 377-406. in M.A. Brunt and J.E. Davies, editors, The Cayman Islands: natural history and biogeography..The Netherlands: Kluwer Academic Publishers.
- Brunt, M.A. 1994. Vegetation of the Cayman Islands. Pages 245-282. in M.A. Brunt and J.E. Davies, editors, The Cayman Islands: natural history and biogeography. The Netherlands: Kluwer Academic Publishers.
- Davies, J.E. 1994. Rare and endemic plants, animals and habitats in the Cayman islands, and related legislation. Pages 527-542. in M. A. Brunt and J. E. Davies, editors, The Cayman Islands: natural history and biogeography. The Netherlands: Kluwer Academic Publishers.
- Davies, J.E. 1994. Mosquitoes of the Cayman Islands. Pages 357-376. in M.A. Brunt and J.E. Davies, editors, The Cayman Islands: natural history and biogeography. The Netherlands: Kluwer Academic Publishers.
- Davies, J.E. and M.A. Brunt. 1994. Scientific studies in the Cayman Islands. Pages 1-12. in M. Brunt and J.E. Davies, editors, The Cayman Islands: natural history and Biogeography. The Netherlands: Kluwer Academic Publishers.
- Department of Tourism, Cayman Islands. 1989. Visitors Map: Cayman Islands. Map 1:50,000. Cayman Islands Government & Ordnance Survey, Southampton, England.
- Grant, C. 1940. The herpetology of the Cayman Islands. Jamaica: The Institute of Jamaica.
- Seidel, M.E., and R. Franz. 1994. Amphibians and reptiles (exclusive of marine turtles) of the Cayman Islands. Pages 407-434. in M. A. Brunt and J. E. Davies, editors, The Cayman Islands: natural history and biogeography. The Netherlands: Kluwer Academic Publishers.
- Stoddart, D.R., M.A. Brunt, and J.A. Davies, editors. 1988. The biogeography and ecology of the Cayman Islands. W. Junk Publishers, Dordecht, The Netherlands. Maps 1:25,000 included.
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