As an isolated, limestone island, Bermuda’s caves are a real treasure, and represent a globally recognised biodiversity hotspot. Caves form from fresh water dissolving the calcium carbonate in limestone over a very long period of time. Many of Bermuda’s caves formed during the Pleistocene Ice Age when much of the Earth’s oceans froze and sea level dropped allowing rainwater to percolate through the limestone and dissolve it. After the last Ice Age the oceans melted, sea level rose and ‘drowned’ some of the caves, flooding them with seawater.
Caves in Bermuda are found across the Island, but most exist in the oldest limestone which is the Walsingham formation. This formation is found between the east end of Harrington Sound and Castle Harbour, at Morgan’s Point in Southampton and at Ireland Island (Dockyard). Caves are also found in the limestone covering the sides of the Bermuda seamount that were exposed to the air when sea level was 125 metres lower than it is today during the Ice Age. Today these deep sea caves are covered by many metres of seawater and are just beginning to be explored.
Bermuda’s Caves are both terrestrial and marine habitats as some are totally dry, some contain pools of seawater, and others are totally flooded.
Dry Cave Habitat
Dry cave habitats have environmental conditions that are fairly stable year round despite seasonal fluctuations in the weather. The further one moves away from the cave entrance, the more stable the environment becomes. Inner caves are completely dark, and the temperature and humidity range far less than outside. Many of Bermuda’s caves remain unexplored and little is known about the fauna of dry caves.
Bermuda’s caves are decorated with cave formations called speleothems. These are formed when fresh water (usually from rain on the surface) moves down through the limestone dissolving calcium carbonate as it goes. The water collects into a drop on the ceiling of the cave until it falls and deposits its load of dissolved calcium carbonate inside the cave as a speleothem.
Speleothems are made of calcite, a crystalline form of calcium carbonate that is shiny (some people mistake it for quartz). Speleothems are actively growing as long as there is dripping water in air-filled caves. Examples of speleothems include stalagmites (growing up from the floor), stalactites (growing down from the ceiling), columns or pillars (occur when the first two grow together), flowstone, curtains or draperies, soda straws and helictites. Bermuda’s caves have some of the best examples of helictites in the world.
Drowned Cave Habitat
Partially drowned caves contain dry areas and pools of water. Fully drowned caves, are found below sea level and are completely filled with water. Many of Bermuda’s drowned caves contain speleothems which do not form underwater, indicating these caves where once dry.Drowned caves are often connected in a continuous system of submerged caves and tunnels which lead to the coast and beyond. Depending on the distance to the coast, the make up of the cave and its depth below sea level, the water in some partially drowned caves will rise and fall with the tide.
It is the inner parts of the cave habitat that hosts truly unique animals, including many endemic species. The inner parts of a flooded cave are completely dark and contain water at a constant temperature and fairly stable environmental conditions. The mouths of submerged caves host similar fauna to shallow coastal bays and marine ponds; which are primarily where the known cave entrances have been found.
Cave Mouths & Limestone Sinks
Limestone sinks (sinkholes) are formed when the roof of a cave collapses, opening it to the sky. This usually happens through a series of several collapses, not just one. As a result sinks often contain piles of broken rock from the former ceiling, creating a complex habitat. Like caves, sinks can be drowned or dry. Devil’s Hole and the Blue Grotto (former dolphin show pool at Blue Hole Park) are examples of drowned sinks; Sears Cave in Smiths is an example of a dry sink.
Collapsed caves that are open to sunlight and the area around cave mouths provide moist, often shaded, rocky habitat that is home to some of Bermuda’s rarest plants. Critically endangered species like the Wild Bermuda Pepper, Bermuda Shield Fern, Bermuda Maidenhair Fern and Bermuda Cave Fern are plants that are found around Bermuda’s Caves. The first three are found nowhere else on Earth.
Why are Caves Important?
Bermuda’s caves, cave mouths and limestone sinks are an important biodiversity hotspot. Bermuda’s system of drowned caves contains more endemic species than any other habitat and it is very likely that there are organisms here that have not yet been discovered. Cave mouths and sinks are also hotspots for endemic and endangered plants and ferns.
Caves also tell us much about the geologic history of Bermuda, including Ice Age and recent sea level change, making them of global scientific value. Bermuda’s caves are also scientifically important because fossils of existing and extinct species such as birds, tortoises and invertebrates have been found in these caves; some date back thousands of years. These did not necessarily live in the cave, but may have been washed into it, covered and preserved.
Several of Bermuda’s caves are also tourist attractions, providing educational and economic opportunities. These ‘show caves’ allow Bermudians and visitors a glimpse into an important habitat and should be treated with care and respect.
Threats to Bermuda’s Cave Habitats
The formations in Bermuda’s caves, like stalactites and stalagmites, are formed by dripping water over thousands of years. These beautiful features are extremely fragile and are easily broken by human activities. Intentional vandalism has sadly been seen in a number of caves, as well as accidental damage during development.
Bermuda’s caves are under threat from construction, which breaks them open, and often leads to them being filled in. Quarrying of limestone is also a serious threat, as is the associated blasting indirectly. Development of hotels, over-exploitation for tourism, even lights which cause algae to grow in the darkness, are all threats to local caves.
Litter is also a serious problem as many caves are connected in underground systems that stretch for kilometres. Floating garbage from the sea makes its way into the cave system with the tide. Some caves were historically used as dumping areas for garbage and waste, the most notorious being Bassett’s Cave at Morgan’s Point and Bitumen Cave at Tuckers Point. Most of this dumped waste remains.
Groundwater pollution is also a serious threat to caves. Pollutants like oil, chemicals and sewage drip into caves with the ground water contaminating the habitat.
In the future Bermuda’s caves will likely be affected in unforeseen ways by climate change, ocean acidification and rising sea level.
Some protection of cave habitats is offered by the Cave Protection Area zoning (Chapter 22 Caves (CAV) page 137) under the Bermuda Plan 2008. A number of nature reserves featuring caves include Walsingham Nature Reserve, the Idwal Hughes Nature Reserve, Blue Hole Park, and Sears Cave Nature Reserve.
Potential Species in Cave Habitats
As previously stated, cave habitats contain more endemic species than any other Bermudian habitat. Some species that may be encountered in cave habitats include:
Endemic cave invertebrates
Bermuda’s drowned caves contain at least 84 species of identified invertebrates such as crustaceans (shrimps, pill bugs, etc.), mites, worms and snails. A list of species is available here: http://www.tamug.edu/cavebiology/fauna/Bermudafaunalist.html
Many of these species and genera are known from only one cave, making these caves critically important habitats and globally significant. These animals are obligate cave-adapted species – meaning they evolved in these caves over millions of years and can not survive elsewhere. Therefore, if their cave habitat is destroyed they can not just move elsewhere. Destruction of a single cave or cave system could result in extinction of one or more species.
The Bermuda Protected Species Act 2003 (Protected Species Order 2007) protects the following 23 endemic species of cave invertebrates:
7 species of Copepods
1 species of Ostracod
3 species of Isopods (pill bugs)
6 species of Amphipods
4 species of shrimp
1 species of Mysid (opossum shrimp)
1 species of Mictacean shrimp
2 species of segmented worms
Cave mouths and limestone sinks provide critical habitat for endangered fern species. Today most of these species of cave ferns are only found in the undeveloped Walsingham karst area between Castle Harbour and Harrington Sound. The Bermuda Protected Species Act 2003 (Protected Species Order 2007) protects the following endangered species of native and endemic ferns:
Governor Laffan’s Fern – endemic and extinct in the wild
Bermuda Shield Fern – endemic and critically endangered
Bermuda Cave Fern – native and critically endangered
Long Spleenwort – native and endangered
Toothed Spleenwort – native and endangered
Other important species of fern that may be found at cave entrances include;
Bermuda Maidenhair Fern – endemic and common
The rugged and rocky nature of karst terrain (particularly the Walsingham area in Hamilton Parish) meant that most of this habitat was never cleared for agriculture or developed, providing a key refuge for many native and endemic plants (see Woodland section for species list). Cave mouths and limestone sinks provide critical habitat for a number of threatened native and endemic flowering plants. These include:
Wild Bermuda Pepper – endemic and critically endangered
Wild Bermuda Bean – endemic and critically endangered
Turnera – native and locally rare
Marine caves that are open to the sea or sinkholes that are connected directly to the sea by underground fissures and cave systems can often support species of marine fish, turtles, corals and algae that enter the system as juveniles. Marine species are only found where continuous supplies of food are available; therefore most caves don’t have them.
Anchialine Caves and Cave Fauna of the World: Bermuda Introduction. Texas A&M University at Galveston. Available at: http://www.tamug.edu/cavebiology/Bermuda/BermudaIntro.html
A list of Bermuda Cave fauna can be found here: http://www.tamug.edu/cavebiology/fauna/bermudafaunalist.html
Read about Sears Cave Nature Reserve on the Bermuda Audubon Society website
See the Texas A&M University Cave Biology website, featuring some of their research in Bermuda.