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Biodiversity

On the Tracks of Cave Dwellers

Last year, the Santo 2006 expedition was able to compile an inventory of underground fauna on Espiritu Santo Island, in Vanuatu. Neglected for many years, the ecosystems in this island's caves abound in species new to science and often spectacular in nature.

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© L. Deharveng

The most extensive underground fauna in tropical regions was found in the Maros karst (Sulawesi Island, Indonesia) where a huge number of caves have developed, like Gua Bakti.


 

It was in September 2006 that the researchers first reached the caves. But before venturing inside, much time was spent sieving the tropical forest's litter and soil around the entrance. Their first stop inside the cave was to study the bat colonies and take a few samples of the millipedes, cockroaches, and other arthropods feasting on the thick layer of guano on the ground.

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© L. Deharveng

Isolated in southwestern Vietnam, the caves and soils of the limestone hills of Hon Chong contain numerous endemic species. These have been catalogued to determine if the limestone mining industry in the area–which will result in the complete destruction of some of these hills–may lead to endemic species extinctions.


Then, guided by their headlamps, they followed the galleries as far underground as they could go. As part of the Santo 2006 expedition,1 and with the objective of compiling a vast inventory of the biodiversity of Espiritu Santo Island in Vanuatu, a team of about twenty researchers thus explored the underground environment of this Southern Pacific island. They focused on the topography of its cavities, the collection of fossilized pollen, paleontology, and an in-depth study of the fauna in the soil and caves.

“It will take months to sort through all the samples. But for springtails alone–a group of invertebrates close to insects–we should be able to distinguish approximately a hundred species, fifty of which will be new to scientists.” And Louis Deharveng, the director of the Origin, Structure, and Evolution of Biodiversity Unit2 and coordinator of one of the four research themes of Santo 2006–namely “Karst,”3–is no neophyte. An experienced field collector, he organizes expeditions every year to explore and sample fauna in tropical caves, with a predilection for South-East Asia.

 

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© L. Deharveng

Centipede (diplopod) often found in the caves of northern Vietnam. Like many strictly cave-dwelling species, it is depigmented and its appendices are extremely long, but it has retained an ocular system. Very large (more than 4-cm long), it exhales a fetid odor of no known function.


“Long neglected, underground fauna–both aquatic or terrestrial–constitutes a much larger source of new species than any other environment on the Earth's surface,” he explains. “Like an island, each karst massif is a potential source of endemism–in other words, we can hope to find species that exist nowhere else in the world.” Until the 1980s, it was thought that no organisms specific to underground environments existed in humid tropical regions. The climatic conditions inside these caves were deemed too similar to those on the surface to isolate populations enough and thus allow genetic divergence and species development. “Today, we know that these types of organisms, called troglobites, also exist in places like Espiritu Santo Island, even though they are less numerous than in temperate environments.”

To determine whether the organisms discovered belong exclusively to an underground environment, the scientists systematically sample the soil of the forest at the cave mouths. “In Espiritu Santo, we discovered that numerous animals found in forest litter are dragged toward galleries, mainly by percolating water from rainfall and by river sinking into the rocks. In the long run, if a population finds itself isolated in the caves of a karst massif for long enough periods of time–for example following a rise in underground water levels–it may give rise to a troglobite species.” Aquatic troglobites often originate in a marine environment. “In the karsts of Sulawesi, we have discovered several strains of organisms derived from marine species, in particular two closely related crabs, one of which had adapted to fresh underground water and the other to moderatly salty water. The sea probably entered these cavities where they live not very long ago, before withdrawing. Since then, evolution went to work.” The extreme conditions of the underground environment, especially the absence of light, have often led to spectacular evolution. Loss of pigmentation, regression of the eyes (and, if appropriate, of the wings) are usually observed. “In parallel, there may be a 'sensory compensation' phenomenon, where the tactile organs–bristles, feet and antennae–become longer. In addition, cave species have a longer lifespan than their surface-inhabiting 'cousins.' They reproduce later, and their eggs are less numerous but larger. This last reproductive behavior reflects an ecological strategy usually observed in highly stable environments.”

 

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© L. Deharveng

A young tailless whip scorpion (Amblypygi) , a common predator of Rhaphidophoridae orthoptera in the caves of tropical Asia (Indonesia, Sulawesi Selatan).


 

Another characteristic of underground ecosystems is that they count very few species. “Twenty troglobite species in one cave is already a lot.” This is a tremendous advantage for scientists who can envisage to understand all their interactions, something that would be impossible on the surface. “In a way, caves are like life-sized laboratories, and we are only just starting to exploit their potential.”

 

Marie Lescroart

Notes :

1. SANTO 2006 is a scientific expedition to document the fauna and the flora, both marine and non-marine, of the Espiritu Santo (or Santo) island. It is organized by the Institut de recherche pour le développement, the Muséum national d'histoire naturelle and the NGO, Pro-Natura International. Biological expeditions in Indonesia are organized in collaboration with researchers from the Museum Zoologicum Bogoriense in Java, Vietnam's Institute of Tropical Zoology and Le Cong Kiet University.
2. “Unité Origine, structure et évolution de la biodiversité” (CNRS / MNHN).
3. A karst is a limestone massif hollowed out by erosion.

Contacts :

Louis Deharveng
Laboratoire “Origine, structure et évolution de la biodiversité,” Paris.
deharven@mnhn.fr


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