Biospeleology in Macaronesia Pedro Oromí

98 AMCS Bulletin 19 / SMES Boletín 7 — 2004 Biospeleology in Macaronesia Pedro Oromí Dept. of Animal Biology, University of La Laguna, Tenerife, Canary Islands Geographical and speleological original basements, were uplifted over caves do not necessarily follow the sur- background the sea level and are now exposed on face topography, and normally open to the surface by the effects of erosion. On outside at cliffs and other steep terrains In the biogeographical sense Maca- other islands like Santa Maria (Azores) due to erosion. Some examples of this ronesia is a subregion of the Western and Porto Santo (Madeira) some lime- kina of caves are Gruta dos Anjos (Santa Palaearctic which includes southwest stone rocks of marine origin have been Maria), Gruta do Inferno (Selvagem continental Portugal, part of the coastal formed and are actually emerged because Grande) or Cueva de la Fajanita (La zone of south Morocco, and the Atlantic of eustatic movements of the sea level. Palma). archipelagos of the Azores, Madeira, These non volcanic rocks are anyway The volcanic pits often derive from Selvagens, Canaries and Cape Verde. very scarce, and have developed such a the emptying of volcanic chimneys when Since the establishment of the term in slight karstification that true caves are the eruption stops and the remaining lava the 19th century by the British botanist not found at all inside them. Therefore, contracts. The spatter cones are hollows P.B. Webb, much has been discussed in the Macaronesian islands the caves with limited dimensions, while other about the validity of Macaronesia as a enough developed as to be considered volcanic pits can exceed 100 m deep, biogeographic unit, about its appropri- of speleological interest occur only in like Algar do Montoso, in São Jorge ate space and boundaries, and about volcanic terrains. (Azores). They are usually bell-shaped, its different meaning for vegetal and Such particular cavities have a gen- though they often show more complex animal organisms. Two continental esis, morphology and a life span very structures with connected cavities and areas and five volcanic archipelagos different than limestone caves. The main multiple vents. The geysers and the vents have generally been identified within types of volcanic caves are lava tubes of gaseous phreatomagmatic eruptions Macaronesia. The islands are of oceanic and volcanic pits, each with their variants can originate remarkable pits, like that of origin with no surface connection with depending on the type of speleogenesis Sima de Tinguatón in Lanzarote. Some other land since they emerged from (see Montoriol, 1973). times the retraction cracks originated the sea bottom. Independently to other The lava tube caves are formed only after cooling trachytic, viscous lavas can biogeographic considerations, in this text in fluid basaltic lavas, never occurring also originate remarkable pits, like the we only pay attention to the strictly vol- in viscous acidic lava flows of trachytic 70 m deep Sima Vicky (Tenerife). canic Macaronesian archipelagos, which nature. They originate after more or less Also lava tubes can be combined with constitute an insular geographic reality permanent lava channels that consolidate volcanic pits in a single but complex very different to that of the continental by cooling of the peripheral layers, and cavity with several levels at different Macaronesian enclaves. From a political are finally roofed when significant speed depths, like it occurs in Sima de Las point of view the Azores, Madeira and differences are established between the Palomas (El Hierro) and Cueva del Selvagens belong to Portugal, the Canary inner and the surface flow. The inner Sobrado (Tenerife). Islands to Spain, and the Cape Verde temperature of the tube allows the lava Speleogenesis and ecological form an independent country, though to keep flowing inside until the emission succession on volcanic terrains with a strong Portuguese character for stops, the liquid empties totally and the obvious historical reasons. system becomes a hollow tube. These Besides their peculiar speleogenesis The Macaronesian archipelagos have caves are therefore usually shallow and when compared to karstic caves, lava common geological features mainly follow parallel to the surface topography tube caves have a geological cycle and derived from their volcanic origin. All at the moment of being formed. Great an ecological succession also very differ- the islands have been built up from the accumulation of further new lavas on that ent (see Howarth, 1996). The formation sea bottom by successive accumula- containing the cave, and changes on the of a lava tube is very quick, sometimes tion of volcanic materials that finally relief by important erosive effects can just a few days, and immediately starts emerged over the marine surface along alter this parallelism between lava tubes its evolution towards definitive destruc- the Tertiary and Quaternary. Actually the and the actual surface upon them. tion as a cave, which will take place volcanism is still active on the Azores, A particular type of lava tubes are within a period of 100,000 to 500,000 the Canaries and the Cape Verde islands. those originated by the emptying of years depending on the local climate Almost all the rocks forming these archi- a dyke. They usually have a different and erosion (Howarth, 1973). Volca- pelagos are volcanic. However, in some morphology and since their origin are nic pits, however, can last longer time. of the Canary Islands (i.e. La Palma, La located much deeper below surface than The cycle of lava tubes is very short Gomera and Fuerteventura) there are the so called rheogenetic lava tube caves in geological terms, compared to that plutonic rocks that belonged to their (Socorro & Martín, 1992). These dyke of limestone caves (millions of years) AMCS Bulletin 19 / SMES Boletín 7 — 2004 99 which needed at least 100,000 years to some troglobites have been found in the richness of sap-sucking plant-hoppers initiate its formation. On the other hand, deepest passages, while lavicolous spe- (Cixiidae and Meenoplidae) on three of lava tube caves usually have much less cies are the only inhabitants in the rest of the archipelagos, while in Europe and permanent water than limestone caves the cave (Ashmole et al., 1992; Martín, North Africa these groups are unknown (which need it for their formation and 1992). In limestone caves instead, the in the caves. It is also peculiar of these is only absent in fossil, inactive caves), oldest habitats are closer to the surface island cave-dwelling communities the and are in general much shallower so that and ecological succession progresses presence of troglobitic species belong- the roots of surface plants often reach downwards. ing to taxonomical groups absent in and invade the cave. The older is a lava tube, the higher caves of the nearby mainland, and even A recently formed lava tube starts probability to be covered by further lava very rare all over the world. This is with a juvenile phase characterized by its flows, which keep the cave away from the case for landhoppers (Amphipoda: dependence on the outside climate due to surface. In such conditions the roots Talitridae), earwigs (Dermaptera) and the network of cracks of the lava, easily do not reach the cave, and provision of thread-legged bugs (Hemiptera: Redu- connecting the cave to the exterior. As organic matter by percolating water is viidae) which have troglomorphic spe- ecological succession goes on over the more difficult. Consequently, lava tubes cies only in the Canary Islands and in lava, the soil seals the surface, shallower occurring under many lava flows hold a Hawaii. The diversity and abundance passages retain moisture and the cave poor fauna or are even abiotic, as it also of troglobitic cockroaches (Blattaria) in enters in a mature phase with the sub- happens in dyke caves. the Canaries contrasts with the absence sequent climatic isolation (temperature Animal communities of these insects in caves of the whole and humidity). Thus, there is a simulta- in volcanic caves Palaearctic. neous ecological succession inside and Troglobitic species are unable to sur- outside the cave, which is very impor- When a lava tube has attained maturity, vive outside their hypogean environ- tant to determine the living community its environmental conditions are similar ment, and therefore they cannot colo- inhabiting this environment (Ashmole to that of limestone caves: absence of nize other islands. This implies that all et al., 1992). There is a particular way light, temperature stability, humidity troglobites in Macaronesia are always to accelerate this process when a thick close to saturation. Scarcity of organic endemic to a single island. The pres- layer of small-sized pyroclasts (cinder matter is also severe, with lesser provi- ence of a troglobite in two islands could and lapilli) are deposited upon the re- sion by water than in limestone caves but only be explained when these islands cent lavas, which isolate the cave from frequently compensated by the presence had been connected in relatively recent outside temperatures, keep the moisture of roots (if there are). In the Macaron- past times due to regressions of the sea and allow many plants to grow up and esian islands bat colonies are very few level (for example Pico and Faial in the provide roots to the cave. Many caves inside the caves, therefore the guano is Azores; Fuerteventura and Lanzarote in in recent or very dry areas of the Canary negligible. the Canaries). and the Cape Verde islands have good Volcanic pits are usually richer in food Types of caves and conditions for troglobites thanks to be because they operate as pitfall traps for biological richness covered by ash fields. As time goes by many organisms; on the contrary, in lava the erosion leads the cave to a senile tubes the input of energy through the It is very common to find a troglobitic stage, in which silting of the network of entrance only affects a few metres inside, species in different, distant caves formed cracks and voids in the surrounding lava and hardly progresses into the cave.

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