Last Updated: December 1, 2006
Introduction
Paget Marsh, Bermuda, UK (Photograph by Catherine Mastny)
The Bermuda islands were once covered in dense forest of endemic tree species, with mangrove forests lining the coasts and inland saltwater ponds. The islands are distinguished as having the northernmost mangrove forests in the Atlantic, which is made possible by the warm Gulf Stream current. Bermuda’s isolation led to the evolution of many endemic species, including the endangered Bermuda petrel (Pterodroma cahow), the Bermuda skink (Eumeces longirostris), and many endemic invertebrates. Restricted to this small archipelago, all endemic species are especially vulnerable to introduced predators and alien pests, and unfortunately the islands have seen the extinction of many species since the time of human settlement. Due to intense human activity, only very small areas of natural habitat remain on Bermuda today. Though the islands have a well-managed and well-funded system of protected areas, this is one of the world’s most densely populated regions. Additionally, Bermuda is subject to intense pressure from a heavy tourist industry.
Location and General Description
The Bermuda ecoregion is a crescent-shaped chain of some 150 coral limestone islands and islets, formed along the submerged rim of a long-extinct volcano. It is situated in the western Atlantic Ocean a little over 900 kilometers from the North American coast. Most of the land area is composed of 7 main islands that encompass 53 km2. This land area is surrounded by about 750 km2 of shallow reef platform, the most northerly in the Atlantic. The climate is subtropical, with mean air temperatures ranging from 19 to 30 °C. The average humidity is 77 percent, and the annual precipitation (146 cm) is distributed relatively evenly throughout the year.
The ecoregion is situated on a more than 75-meter-thick coralline limestone cap, covering a volcanic pedestal that rises to just below sea level. Roughly 110 million years ago (MYA), volcanic action along the mid-Atlantic ridge created a 2000-meter-high seamount, which was topped off by a second eruption some 33 MYA. Over time, changes in sea level led to the deposition on this seamount of alternating layers of soil (paleosols) and wind-blown calcareous sand. These dunes solidified during the Pleistocene into a cover of soil and highly porous limestone.
Paget Marsh, Bermuda, UK (Photograph by Catherine Mastny)
Bermuda's topography is moderately hilly and low, with the highest elevation reaching only 79 meters (m). The shoreline is composed of bays and inlets, with coral sand beaches lining the shores, primarily in the south. The ecoregion has no rivers or surface streams, and the limestone supports a shallow freshwater lens in only 20 percent of the land area, resulting in fewer than 20 brackish and freshwater ponds. This includes a total of 17 peat marshes.
The ecoregion originally contained two predominant vegetative features: a once densely forested plateau and mangrove forests. The woodlands were characterized by three endemic trees, each of which is listed as threatened by the IUCN: Bermuda cedar (Juniperus bermudiana), Bermuda palmetto (Sabal bermudana), and Bermuda olivewood (Elaeodendron laneanum). Also present are the threatened yellow-wood (Zanthoxylem flavum) and southern hackberry (Celtis laevigata). Wax myrtle (Myrica cerifera) dominates the understory of peat marshes, along with St. Andrew’s cross (Ascyrum hypericoides), Bermuda sedge (Carex bermudiana), saw grass (Cladium jamaicensis), and ferns. Bermuda maidenhair fern (Adiantum bellum) is found in cave damps.
Warmed by the Gulf Stream, the ecoregion’s mangrove forests are the most northerly in the Atlantic. These forests are isolated, small in size, and relatively low in tree diversity, with only three species present: red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), and buttonwood (Conocarpus erectus). An unusual feature of these mangroves is that roughly one-third are situated around inland saltwater ponds, which are mostly tidal and connected to the sea by submarine fissures. These inland forests are typically monospecific, with either red or black mangrove present. The remaining two-thirds of mangroves are located in coastal bays, where they display typical mangrove zonation. The most frequent coastal bay mangrove associations are with red algae, green algae, and three flowering plants: Asparagus sprengeri, Borrichia frutescens, and Sesuvium portulacastrum. The different pond mangroves tend to display unique assemblages of species.
Biodiversity Features
As many as 320 bird species have been observed in the ecoregion. But Bermuda’s position as an isolated oceanic island has contributed in large part to an overall low diversity of species in most taxa, low endemism, and absence of native mammals, amphibians, or large predators. Low floral diversity in particular is attributed to isolation and lack of anthropogenic influence prior to the 1600s. The ecoregion’s biological composition is also a product of prehistoric sea-level fluctuations of up to 150 meters, which alternately favored marine and terrestrial species and led to the demise of land snails, many endemic birds, and a tortoise. Other factors limiting biodiversity include the ecoregion’s low elevation, small size, geologic composition and age, as well as regional ocean currents and past and present climate and weather patterns.
Bermuda is home to 165 native vascular plants, and 15 of these are endemic. Endemic trees include: Bermuda cedar (Juniperus bermudiana), Bermuda palmetto (Sabal bermudana), and Bermuda olivewood (Elaeodendron laneanum). Other vascular plant endemics are: Bermuda spike rush (Eloecharis bermudiana), Bermuda sedge (Carex bermudiana), wild pepper (Peperomia septentrionalis), wild Bermuda bean (Phaseolus lignosus), St. Andrew’s cross (Ascyrum hypericoides), Bermuda snowberry (Chiococca bermudiana), Darrell’s fleabane (Erigeron darrellianus), and Bermudiana (Sisyrinchium bermudiana). There are also 4 endemic ferns: Governor Laffan’s fern (Diplazium laffanianum), Bermuda maidenhair fern (Adiantum bellum), Bermuda shield fern (Goniopteris bermudiana), and Bermuda cave fern (Ctenitis sloanei). The two endemic mosses are Campylopus bermudiana and Trichostomum bermudanum. Also present are 40 endemic fungi and ten endemic lichens.
Bermuda contains no native mammal or amphibian residents, although four species of visiting bats have been recorded. Two of the ecoregion’s bird taxa are endemic, representing a low endemism rate of less than one percent. These are the cahow, or Bermuda petrel (Pterodroma cahow), and the locally-termed "chick-of-the-village" (Vireo griseus bermudianus), a subspecies of the white-eyed vireo that has shorter wings, a larger head, and stout legs. The only native terrestrial reptile is the endemic Bermuda skink (Eumeces longirostris), a rock lizard that is also unique in being the sole non-avian, native land-vertebrate. There are at least 41 endemic insects, including 11 species of Lepidoptera, 17 Diptera, seven dragonflies, and three damselflies. There is also one potentially endemic spider, 11 endemic terrestrial mollusks, and one (now-extinct) endemic nemertean, Pantinonemertes agricola.
The ecoregion has witnessed the recent extinction of many endemic species, including the Bermuda spike rush (Eloecharis bermudiana) and the only endemic nemertean, Pantinonemertes agricola. An estimated 16 insect species have disappeared in the last century, including the flightless grasshopper, Paroxya bermudensis, and seven Diptera. The near-eradication of Bermuda cedar in the 1940s led to extinction of two associated endemic insects, the cicada Tibicen bermudiana, and the geometrical moth Semiothisa ochrifascia. Other losses include the apparent extinction of at least two endemic land snails, Poecilozonites spp., probably due to the introduction of carnivorous snails. Among birds, at least ten presumed endemics are known from Pleistocene fossils including the crane Baeopteryx latipes, the duck Anas paschyscelus, four species of rail, a woodpecker, a hawk, a heavy-billed passerine, and a small owl. Most of these birds are thought to have gone extinct due to sea-level changes before human settlement, although the passerine and owl are were present at the time of the first settlers. Finally, scientists are also preparing a description of a recently excavated fossil land tortoise.
Highly endangered endemics include the Bermuda petrel (Pterodroma cahow), whose nesting sites are currently restricted to a few outlying islets, and the Bermuda skink (Eumeces longirostris). Rare endemic plants include Bermuda olivewood (Elaeodendron laneanum), wild Bermuda bean (Phaseolus lignosus), and Bermuda sedge (Carex bermudiana), which is confined to Paget Marsh and 5 upland sites. The endemic fern Diplazium laffanianum is extinct in the wild, but a few individuals still survive in the Bermuda Botanical Gardens.
Current Status
Currently, some ten percent of the ecoregion’s land area is covered by forest or woodland. Within this, only very small and fragmented areas of natural habitat survive. An estimated 95 percent of the surviving population of native Bermuda cedar (Juniperus bermudiana) was destroyed between 1946 and 1951, following the accidental introduction of two coccoid scale insects. Only an estimated one percent of the original cedar forest survived the blight. Subsequent reforestation using a scale-resistant strain has returned the cedar to roughly ten percent of its former abundance, though these efforts have been hampered by the introduction of fast-growing casuarinas and other exotics into much of the cedar habitat. Of the 116 hectares of inland peat marshes present in the ecoregion in 1900, only some 48 hectares remain, including the 19.6 hectare Devonshire Marsh, Pembroke Marsh, and Paget Marsh. Only small, scattered areas of mangroves remain, totaling 16.7 hectares in 1980. The largest mangrove forests are found at Hungry Bay Mangrove Swamp and Mangrove Lake.
Bermuda holds the distinction of having passed the first conservation laws in the New World, protecting the cahow (Pterodroma cahow) and other birds as early as 1616 and limiting the uses of native cedar as early as 1622. A comprehensive and well-managed protected areas system currently exists, comprising 12 nature reserves that cover some 48 hectares, as well as 63 parks. The 25-acre Paget Marsh Nature Reserve is the best surviving example of native cedar, palmetto, and mangrove forests. The largest wildlife sanctuary is Spittal Pond, covering some 60 acres and home to at least 25 species of waterfowl. Other important protected areas include Devonshire and Pembroke Marshes, Warwick Pond, and the upland hills of Castle Harbour and Walsingham.
One important conservation success story has been the recovery of the endemic Bermuda petrel. Early visitors to Bermuda had been terrified by the cahow’s screeching cries, but they soon found the bird easy to catch and good to eat. Birds that were overlooked by humans were quickly consumed by introduced pigs, and as a result the cahow was thought to be extinct as early as the mid-1600s. But in 1951, 18 breeding pairs were rediscovered off the island’s East End, and subsequent recovery efforts have raised that number to 55 by 1998. The birds are currently protected in off-limits offshore sanctuaries.
Bermuda (the UK) is party to a number of relevant international environmental treaties, including: the World Heritage Convention, the Ramsar Convention on Wetlands, CITES, and the Bonn Convention on Migratory Species.
Types and Severity of Threats
In 1514, Spanish chronicler Oviedo y Valdes commented on the abundance of fish and bird life in Bermuda, and in 1593 shipwrecked English sailors joyfully discovered "an unbroken forest of cedar" from which to build their new vessel. But four centuries of settlement have all but eliminated Bermuda’s native flora and fauna. Predominant threats to the ecoregion include conversion of habitat for agriculture and development, introductions of exotic plant species, and sea-level rise.
By the end of the 17th century, settlers on Bermuda were already clearing the native land cover to make room for fields, gardens, hedgerows, and houses. They brought new pets and pests and planted ornamental plants, herbs and spices, and crops like tobacco and cotton. The conversion of land to agriculture continued through the 19th century, when Bermuda became a market garden for rapidly growing New York City. Over the centuries, residents cleared mangrove forests for development and used the bark in tanning and as fuel. The native cedar was felled for housing, shipbuilding, furniture, and export, while the thick-trunked palmetto was used for roof thatch, to weave fashionable hats, ropes, and baskets, and even to concoct a potent alcoholic beverage.
Over the past century, Bermuda’s agricultural importance has declined, but the pressures of development continue to grow. Twenty of Bermuda’s 150 islands are currently inhabited, and the country remains one of the most densely populated in the world. As the population has expanded over the last 20 years, and demand for housing has grown, undisturbed areas of natural vegetation have diminished rapidly. An estimated 10 percent of the land area is now covered by houses, roads, and other surfaces, and many peat marshes have been converted to landfills. The pressure on biodiversity is further intensified by the arrival of an estimated half million tourists each year.
Introductions of exotic species continue to crowd out native wildlife. An estimated 1500 plant species have been brought to the island over the centuries. Exotic trees like guava and mulberry threaten to replace native plant communities with monocultures, though conservationists employ culling and other techniques to keep these invasions at bay. Meanwhile, the activities of introduced birds like the great kiskadee (Pitangus sulphuratus) (brought in 1957 to control Anolis lizards) continue to aid in invasive plant dispersal.
In the future, sea-level rise will pose another serious threat to native vegetation as global temperatures increase, likely contributing to accelerated retreat of the region’s remaining mangrove forests.
Justification of Ecoregion Delineation
Bermuda’s position as an isolated oceanic island has contributed to a flora and fauna quite distinctive from any continental area or from Neotropical islands to the south. The ecoregion’s unique biological composition is also a product of prehistoric sea-level fluctuations as well as a lack of anthropogenic influence prior to the 1600s.
adapted by : http://www.eoearth.org/article/Bermuda_subtropical_conifer_forests
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