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Bermuda's Caves - A Biodiversity Hotspot


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cave shrimp

Biodiversity hotspots are defined as relatively small areas with exceptional concentrations of endemic species. Bermuda's marine caves readily qualify as a biodiversity hotspot of global significance due to their remarkably rich and diverse community of cave-limited animals. More than 60 species have been identified from Bermuda caves including two new orders of crustaceans, one new family and 15 new genera. Even on a small island like Bermuda, caves are not evenly distributed; most of the 150 known caves are located in the narrow strip of land separating Harrington Sound from Castle Harbour where the island's oldest limestone is exposed. Many of Bermuda's cave-dwelling animals are even more restricted, having been found only in a single cave or cave system. Due to their limited distribution, the fragile nature of the marine cave habitat, and severe water pollution and/or development threats, 25 of these species have been listed as critically endangered.

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stalagmite stump

Observations and explorations of Bermuda's caves date from the earliest periods of colonization (Forney, 1973; Iliffe, 1993). William Shakespeare's play "The Tempest," inspired by the shipwreck of Sir George Somers on Bermuda in 1609, takes place in and around a cave. The first published reference to Bermuda's caves was by Captain John Smith in 1623, when he notes finding "in some places varye strange, darke, and combersome Caues." The first scientific cave study done in Bermuda was in 1864 when David Milne Home calculated the apparent age of a 3 m high stalagmite from Admiral's Cave as 600,000 years. Home (1866) did this by measuring the volume of the newly deposited knobs on this stump 44 years after his father had removed the stalagmite from the cave. Accounts of visits to Bermuda's caves date back over 150 years and include descriptions by such eminent scientists as the Scottish naturalist Sir C. Wyville Thomson (1878) and the American naturalist and explorer A. Hyatt Verrill (1908).

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Church Cave lake

Bermuda's caves began forming about one million years ago during glacial low stands of sea level (Palmer et al., 1977). At this time, sea level was as much as 125 m below present levels such that the entire top of the Bermuda Platform was dry and the islands' total landmass was about 20 times as large as it is today. Consequently, a sizable fresh groundwater body, necessary for cave formation, was present. As post-glacial sea levels rose, large portions of the caves were drowned as the encroaching seawater displaced the freshwater. Continuing collapse of overlying rock into the large solutionally-formed voids created the irregular chambers and fissure entrances that are commonly seen in Bermuda's caves.

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drowned stalagmites

Bermuda caves consist of integrated networks of submerged passageways developed primarily at depths between 17 and 20 m below present sea level that connect otherwise isolated saltwater pools. Above sea level, the caves are characterized by extensive collapse that produced fissure entrances and comparatively short cave sections. Underwater, large labyrinth-like passages only accessible to divers are well decorated at all depths with a variety of impressive stalactites and stalagmites. Since such mineral deposits can only be formed by dripping water in air-filled caves, their presence in now underwater caves indicates that that these caves must have been dry for considerable periods of time. The deepest depths that have been encountered are 24 m below sea level. Since the island's volcanic basement rock is on average at only -30 m, it appears that most of Bermuda's caves are developed at or just above the limestone-basalt interface.

Bermuda's cave pools have a thin brackish layer at the surface, overlying fully marine waters at depth. Tidal exchange of subterranean waters with the sea takes place in those caves bordering Harrington Sound. In other areas, caves contain slowly moving or near stagnant waters. The input of food in most caves is primarily derived from the sea itself in the form of plankton carried in by tidal currents.

A number of Bermuda caves, including Crystal, Island, Cathedral, Admiral's, Castle Grotto, Leamington, Wonderland, Walsingham, and Tuckers' Island Caves, have been operated as commercial tourist attractions. The Devil's Hole, a collapse cave that was first opened for public exhibition in 1843, is utilized as a natural fishpond containing sharks, groupers, and sea turtles. Blue Grotto, another water-filled collapse cave, was recently the site of trained dolphin shows. Prospero's (previously known as Island) Cave contains an underground bar and discotheque.

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primitive cave crustacean

Bermuda's cave pools and extensive submerged passageways are inhabited by an unexpectedly high diversity of unusual and previously unknown marine invertebrates. Included among this fauna are extremely ancient relict organisms that can be legitimately referred to as "living fossils" (Sket & Iliffe, 1980). Some of Bermuda's cave-dwelling species have close affinities with Old World cave and groundwater fauna and probably colonized the subterranean habitats on Bermuda early in the island's history, tens of millions of years ago, when the Atlantic was much narrower (Iliffe et al., 1983). Other animals inhabiting Bermuda caves have close relatives occurring in caves on other isolated oceanic islands from both the Atlantic and Pacific (Holthuis, 1973). Finally, some are closely related to deep-sea species. Members of the cave fauna are providing important links in establishing the evolution and dispersal of present oceanic species. For a list of Bermuda's cave species see: CaveBiology.com

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fossil bird bones

In addition to the living fauna, Bermuda's caves also hold important clues about Ice Age history, and global warming. Dead End Cave, a small system of crevices at Government Quarry, contains evidence for a short-lived sea level stand, more than 20 meters above today's, about 420,000 years ago (Hearty et al., 1999). Recently, a complete sequence of bird fossils dating back several hundred thousand years has been discovered from a soil bank in Admiral's Cave. Storrs Olson (Smithsonian Institution) and Paul Hearty (Townsville, Australia) have begun to excavate the talus cone in the cave and think this is one of the best and most complete Ice Age deposits in the world. Yet the cave is threatened by continuing blasting in a nearby quarry.

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lake in Gov. Quarry Cave

Belying the considerable age of both caves and the obligate cave-dwelling organisms that inhabit them, this environment is one of the rarest and most delicate on Earth (Iliffe, 1979). The potential impact of Man on caves is profound - over time, even mere visitation of caves can result in irreparable damage. The four primary threats to Bermuda caves are: (i) construction and quarrying activities, (ii) water pollution, (iii) dumping and littering, and (iv) vandalism. Quarrying has destroyed numerous significant caves, particularly at Government Quarry. Construction of luxury townhouses directly on top of Church and Bitumen Caves may destroy their endangered fauna. The Karst Waters Institute twice named these two caves in their list of the Top Ten Most Endangered Karst Ecosystems on Earth (IUCN, 1996). The pool of Bassett's Cave, referred to in 1837 as the longest and geologically most instructive cave in Bermuda, was used by the U.S. Navy as a cesspit for disposal of raw sewage and waste fuel oil. Many Bermuda caves have been used as dumping sites. The bulldozing of large piles of partially burned rubbish into the pool of Government Quarry Cave resulted in depletion of dissolved oxygen and anaerobic production of poisonous hydrogen sulfide (Iliffe et al., 1984). Groundwater circulation transmitted this pollution to at least five other caves as much as half a kilometer or more away. In such polluted caves, all cave-adapted species are exterminated. Since many of Bermuda's cave species are endemic and are often restricted to only one cave or cave system, pollution or destruction of these habitats can result in the extinction of entire species. Finally, few of Bermuda's larger caves have escaped the effects of vandals breaking and removing fragile stalactites and stalagmites or defacing cave walls with their names.

Construction of homes and other developments in the Walsingham area, where most of the island's caves are found, threatens them with destruction or pollution. Organic matter and nutrients are polluting not only the caves, but Bermuda's fresh groundwater as well. Since many caves are important tidal conduits to the island's inshore waters, nutrients leaking from cesspits into caves can readily be transported to open waters. Thus, cave pollution can have momentous impact on groundwater quality and pollution of inshore waters, in addition to resulting in the extinction of Bermuda's unique cave fauna.

We here propose to initiate a multidisciplinary investigation and survey of Bermuda's cave resources involving:

  • GIS (geographic information system) survey to map cave and cave faunal distributions as part of the Bermuda Biodiversity Program. This project would involve obtaining cave entrance locations with GPS, followed by surveying of both the dry and submerged portions of the caves to produce a three dimensional map of cave distribution. One or more graduate students would conduct this project in cooperation with GIS-cave mapping experts.
  • Cave water quality monitoring program would assess horizontal and vertical variations in physical and chemical parameters including temperature, salinity, dissolved oxygen, pH, redox potential, nutrients and fecal coliform bacteria from both potentially threatened and non-threatened (control) sites.
  • Assessment of threats to the cave ecosystem including solid and liquid waste disposal, quarrying activities, housing and other developments, etc.
  • Biotic cave survey to include:
    • Faunal inventory of the marine caves is still yielding a number of new species as exemplified by the recent discovery of several new marine worms and other invertebrates.
    • Biological collections of terrestrial cave invertebrates promises to yield possible new species and significant ecological and biogeographic information.
    • Paleontological studies of cave fossil deposits including birds, snails, and land crabs.
  • Geological cave investigations will include isotopic age dating of submerged stalagmites to generate ancient sea level curves for the Pleistocene ice age period.

All data including cave maps, water quality information, potential threats and living and fossil species distributions will be plotted and analyzed with the aid of GIS. Tom Iliffe (Texas A&M University, Galveston) and graduate students Darcy Gibbons and Rebecca Belcher will carry out marine cave biology and GIS mapping work in cooperation with Wolfgang Sterrer (Bermuda Aquarium Museum and Zoo) and the Bermuda Biodiversity Project staff, and with Bernie Szukalski (ESRI) and Bob Richards (Texas). Storrs Olson (Smithsonian Institution) and Paul Hearty (James Cook University, Townsville, Australia) will coordinate paleontological excavations of the talus cone in Admiral's Cave as a long-term program, possibly with help from Earthwatch. Katrine Worsaae and Martin Sørensen (University of Copenhagen, Denmark) will look for new species of marine worms inhabiting Bermuda caves. Robert Dill (California) and Yemane Asmerom (University of New Mexico) will carry out dating and analysis of stalagmites. Other scientists will be added as appropriate. Local Bermuda cave divers will assist with cave mapping and other scientific studies. A cave diver certification course will allow interested Bermudians to obtain the specialized training needed for cave diving and thus be able to assist with this work.


BIBLIOGRAPHY
  • Forney, G.G., 1973. Bermuda's caves and their history. J. Spelean Hist., 6:89-103.
  • Hearty, P.J., P. Kindler, H. Cheng and R.L. Edwards, 1999. A +20m middle Pleistocene sea-level highstand (Bermuda and the Bahamas) due to partial collapse of Antarctic ice. Geology, 27:375-378.
  • Holthuis, L.B., 1973. Caridean shrimp found in land locked saltwater pools at four Indo-West Pacific localities (Sinai Peninsula, Funafuti Atoll, Maui and Hawaii Islands), with the description of one new genus and four new species. Zool. Verh., 128:1-48.
  • Home, D., 1866. Notice of a large calcareous stalagmite brought from the island of Bermuda in the year 1819, and now in the College of Edinburgh. Roy. Soc. Edinburgh Proc., 5:423-428.
  • Iliffe, T.M., 1979. Bermuda's caves: A non-renewable resource. Environ. Conserv., 6:181-186.
  • Iliffe, T.M., 1993. Speleological history of Bermuda. Acta Carsologica, 22(4):114-135.
  • Iliffe, T.M., C.W. Hart, Jr. and R.B. Manning, 1983. Biogeography and the caves of Bermuda. Nature, 302:141-142.
  • Iliffe, T.M., T.D. Jickells and M.S. Brewer, 1984. Organic pollution of an inland marine cave from Bermuda. Mar. Environ. Res., 12:173-189.
  • IUCN, 1996. 1996 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland, 379 pg.
  • Palmer, A.N., M.V. Palmer and J.M. Queen, 1977. Geology and origin of the caves of Bermuda. In: Proceedings of the Seventh International Congress of Speleology, Sheffield, England, pp. 336-339.
  • Sket, B. and T.M. Iliffe, 1980. Cave fauna of Bermuda. Int. Rev. gesamten Hydrobiol., 65:871-882.
  • Thomson, C.W., 1878. The voyage of the "Challenger": The Atlantic, v. 1. New York: Harper, xxix + 424 p.
  • Verrill, A.E., 1908. The caverns of Bermuda. Tropical and Sub Tropical America, 1:107-111.

Issued by the Bermuda Aquarium, Museum & Zoo, 12 Dec. 2001


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