Some like it hot – or how Mediterranean marine species face global warming with diverse fortunes

The scientific Report 'Climate warming and related changes in the Mediterranean biota' produced by the Mediterranean Science Commission (CIESM) highlights that global warming is transforming the Mediterranean into a much different sea than it was only 20 years ago. This well-documented, 152-page Monograph is the outcome of a CIESM Exploratory Workshop where the multidisciplinary expertise of international scientists was drawn together to produce a synthesis on the impacts of climate change on Mediterranean marine species.

Since the 1980s the Mediterranean marine biota has known rapid, dramatic changes, illustrated by alteration of food webs, mass mortalities, or population explosions such as jellyfish outbreaks. According to experts, these changes cannot be ascribed solely to the intense anthropogenic activities. As the Mediterranean Sea hosts species with affinity to cold waters (of boreal origin), as well as species with affinity to warm waters (of sub-tropical and tropical origin), these two sets predictably respond to climate warming in a different way.

The parrotfish Sparisoma cretense, a native warm-water species typical of southern Mediterranean now fast expanding northwards.
The gorgonian Paramuricea clavata, a slow-growing soft-coral that cannot stand high sea temperatures; a) a healthy colony; b) a colony bleached following a sudden and steep warming of the water column in the Ligurian Sea in 1999.

As described in the Monograph, native thermophilic species, usually restricted to the southern, warmer sectors of the Mediterranean Sea are now moving northwards. This phenomenon (meridionalization) is particularly evident in fish, where over 30 native species have already spread in the northern areas of the Basin. Similarly, climate warming facilitates the establishment and spread of tropical, exotic species that are introduced via the Suez Canal or maritime transport. This process (tropicalization) is fast advancing and more than 500 exotic species have been recorded of late in the Mediterranean Sea [1].

Experts are worried that if the Mediterranean Sea continues to warm up at the same rate, all the sub-regional biological peculiarities may rapidly disappear, to be replaced by a more homogeneous, tropical ecosystem. Already entire replicas of tropical communities from the Red Sea have been recorded in a few Mediterranean locations, as illustrated in the CIESM Monograph.

Shallow-water species of cold-water affinity that are endemic (i.e. found nowhere else) to the northern, and coldest, areas of the basin such as the North Adriatic and the Gulf of Lyons are facing a serious risk of decline and even extinction, as they are prevented to move further north or deeper in the water column to escape rising sea temperatures. As pointed out by Prof. Ferdinando Boero (Univ. of Salento, Italy) who led the workshop discussions, such risks are real for the already rare species like the hydroids Tricyclusa singularis and Paracoryne huvei whose fate is open to question.

To quote Prof. Frederic Briand, Director General of CIESM, “continuing warming trends, combined with the deepening of the Suez Canal and intense maritime traffic, lead us to think that the question is no longer whether the Mediterranean Sea may turn into a giant tropical aquarium, but how fast?
And will there be a place then for ‘relict’ Mediterranean species representative of today’s ecosystems?”

This richly illustrated Monograph is available from the CIESM Bookstore.


[1] For more information, see the CIESM Atlas of Exotic Species in the Mediterranean
See also CIESM Tropical Signals Programme