Mediterranean Sea Biodiversity - part 1
The Mediterranean Sea is a biodiversity hotspot containing between 15,000 and 20,000 marine species, nearly a quarter of which are endemic. The causes of the high Mediterranean biodiversity lie primarily in the turbulent geological history of the basin during the Tertiary and in the dramatic climatic fluctuations of the Quaternary. Both induced a rate of environmental change, and hence species occurrence, which acted as a biodiversity pump. As a result, species with different biogeographic origins and affinities are found in the basin. Although the Mediterranean Sea as a whole constitutes a distinctive province of the Atlantic-Mediterranean biogeographic region, a great variety of climatic and hydrologic situations is found in its fairly isolated sub-basins. Thus, 12 different biogeographic sectors can be recognized: Alboran Sea; Algeria and north Tunisia coasts; Tyrrhenian Sea; Balearic Sea to Sardinia Sea; Gulf of Lions and Ligurian Sea; northern Adriatic Sea; central Adriatic Sea; southern Adriatic Sea; Ionian Sea; Levant Sea; southern Aegean Sea; northern Aegean Sea.
The high biodiversity of the Mediterranean Sea may be explained by historical, paleogeographic, and ecological reasons. Paleogeographic reasons are probably most important, as the whole basin has experienced a turbulent geological history which has led to an enhanced rate of environmental change, and hence species occurrence, with few equals in the world.
The Mediterranean Sea is frequently, although rather imprecisely, considered as the right heir of the ancient Tethys Sea.
The Tethys was a once-extensive, wedge-shaped, eastward-opening equatorial ocean that indented Pangea in the Mesozoic. Towards the end of the Mesozoic, in the Cretaceous, the Tethys came to separate the two supercontinents of Laurasia to the north and Gondwana to the south. The expansion of the Tethys became the Mesogea, a sea stretching between the two supercontinents. After the opening of the Atlantic Ocean, the Mesogea was connecting, through an uninterrupted equatorial belt (the so-called Tethyan seaway), the newly born ocean to the older Indo-Pacific Ocean (the ancestral Panthalassa).
According to geological reconstruction, the ultimate fate of the Tethys has been sealed by plate tectonics. Todayâs configuration of the Tethys is the formation of remnant and successor seas, including the Gulf of Mexico, the Caribbean, the Mediterranean, the Red Sea and the intertropical zone of the Indian Ocean. A significant portion of the former Tethys, however, uplifted during the Cenozoic geodynamic evolution marked by the Alpine orogeny, which practically ended in the Oligocene. The formation of the Alps, Carpathians, Dinarides, Taurus and Elburz mountains separated the main body of the Tethys, to the south, from the Paratethys, to the north.
The Paratethys was a large shallow sea that spread over a large area from Central Europe to western Asia. The Black Sea, the Caspian Sea and the Aral Sea are all that is left of that once vast inland sea.
The overall convergence of Africa and Eurasia, which involved several successive local rifting processes and collisions during the Tertiary, led to the shrinkage of the Tethys in the Oligocene. The consequent diminution of its warming influence in the world oceans produced cold water conditions elsewhere. This may have resulted in higher extinction rates outside the Tethys and the Indo-west Pacific than within these regions. At that time, the Tethys was harboring a highly diverse warm-water biota. Throughout most of its geological history, Tethys had been the global center of marine biodiversity - a role assumed today by the area between the Philippines and Indonesia. Can the high biodiversity of the present-day Mediterranean Sea be directly linked to that of its Mesozoic ancestors? The answer is probably no.
Up to the Burdigalian, in the Miocene, the connection between the shrunken Mesogean Tethys, which in a sense could be considered as the first sketch of the Mediterranean, and the Indo-pacific to the east and the Atlantic to the west was maintained. However, the between the Indo-West Pacific and the Caribbean-East Pacific biogeographic regions. The connection that previously existed between the proto-Mediterranean and the Indian Ocean in the area of the Mesopotamian trough ceased when the Isthmus of Suez was raised during the Miocene orogeny.
Marine interconnections only remained in the west, leaving the Atlantic as the only biological reservoir for the adjacent proto-Mediterranean. The marine biota of the Tethys was typically tropical. Coral reefs were present up to the Tortonian. Towards the end of the Miocene, the connection with the Atlantic was also interrupted on several occasions, and the Mediterranean become a virtually isolated sea, with a degree of enclosure of over 99 %. The segregation of the Mediterranean basin from the Atlantic took place because of the closure of the Betic and Rif straits, roughly in the same region as todayâs Gibraltar. The negative water balance should have nearly desiccated the Mediterranean, which was probably transformed into a series of large evaporitic lakes during the so-called âsalinity crisisâ at the end of the Messinian age.
The reconstruction of the series of events that accompanied the Messinian salinity crisis has been the object of many controversies and differing interpretations. Most authors, however, agree that within about six hundred thousand years the Mediterranean, which was secluded from free hydrological exchanges with the world ocean, shifted from anoxic pre-evaporitic to evaporitic hyperhaline, then to hypohaline conditions. After the main phase of massive evaporite precipitation, the ending of the Messinian salinity crisis was characterized by the so-called âLago Mare (= Lake Sea) eventâ, i.e. the progressive and generalized establishment of brackish to freshwater aquatic environments throughout the Mediterranean, possibly linked to the flow of the brackish waters of Paratethys into the dry basins.
The peculiar hydrological conditions of the isolated Mediterranean during the Messinian salinity crisis should have produced dramatic effects on the biota of ancient Tethyan origin. Again, there is not general consensus about what happened. Some authors believe that the biota was completely annihilated, others consider that at least part of it may have survived through the Neogene. Survival of Tethyan biota should have been possible in fully marine refuges that may have persisted from permanent links with the Atlantic via the Betic portal or in satellite basins within the western Mediterranean, possibly in southern Spain. The existence of seaways between the eastern Mediterranean and some oceans at that time has also been postulated.
Possibly, it is from the remnants of the Tethyan biota that what are today called the Mediterranean paleoendemics should have evolved. Alleged Mediterranean paleoendemics are known in invertebrate groups and have especially been described among macrophytes.
The idea that during the Messinian high salinity crisis all the marine life of the Mediterranean was exterminated is not correct. Marine life, with many documented examples especially in echinoderms and fishes, survived into the Pliocene in near-shore environments. The classical Tethyan relic, a Messinian survivor, is the endemic Mediterranean seagrass Posidonia oceanica, which however did not leave any fossil evidence in the Mediterranean basin prior to the lower Pleistocene.