“In reserves off Spain and Italy, we found the largest fish biomass in the Mediterranean. Unfortunately, around Turkey and Greece, the waters were bare” – Enric Sala, National Geographic Society.

The Threatened Seagrass Posidonia oceanica. Photograph by M. San Félix, illustration to PLoS ONE study. The healthiest shallow rocky reef ecosystems in the Mediterranean have both large fish and algal biomass, says parallel PLoS ONE study – and seagrass is important.

Geography is good. It creates the diversity requirements for a living planet, like weather, landscape variation, water cycles – a planet’s surface with not just water or just land, but instead oceans, and continents.

To create a planet rich in life and high in biodiversity Earth’s scheme has been to carefully balance connectivity and isolation. Depending on plate tectonics and sea level rises and drops, land bridges went and came between continents, and sea straits formed and closed between oceans.

On the million years’ timescale of evolution this geological heartbeat became life’s rhythm – a planet covered with a wealth of interconnected yet largely autonomous ecosystems.

The climatological timescale

Some land bridges form on the timescale between ice ages and interglacials, as these lead to sea level fluctuations of approximately 100 meters. The famous example is the Pleistocene land bridge across the Bering Strait, connecting Siberia and Alaska. It is thanks to the Bering Land Bridge [not to be confused with the Bering Dam] that North America and Eurasia share many animal species, like the gray wolf, and it is thanks to the Bering Strait that these subsequently evolved into distinct subspecies, like the Northern Rocky Mountain gray wolf, and the Eurasian gray wolf.

The same goes for rhino subspecies on different South East Asian isles – connected at the height of Pleistocene ice ages, separated during interglacials that flooded the land bridges and recreated the archipelago.

The tectonic timescale

Other biodiversity crossroads oscillate at a much slower pace. That is the case with connections created by tectonic forces, like the Panama Isthmus between North and South America.

Marine species have depended on birds carrying them across from the Gulf of Mexico and the Caribbean Sea to the East Pacific. As a consequence of this very rare and exclusive migration route subspecies evolved and overall biodiversity increased on either side of the land bridge. One could say the recipe for high biodiversity is some connectivity – combined with a lot of isolation.

In case of the Panama Isthmus such migrations were limited to small species like snails – and even for these species occurred just once in every 72,000-750,000. Until of course in 1914 they completed the Panama Canal.

Inventory of Mediterranean marine life

Let’s get to the Mediterranean. A new study in PLoS ONE has compared the ecological health of 14 different marine protected areas and 18 open-access sites, setting up transects to count fish and take samples of plants and animals living on the seafloor.

It was conducted by a team of 27 scientists from universities and other research institutes in Spain, Italy, Greece, Turkey and the US.

In their publication they distinguish between four states of Mediterranean marine ecosystems – as shown in the image above: large fish biomass and non-canopy algae (‘predator-dominated’) ecosystems (A); ecosystems with lower fish biomass, but abundant algal canopies and suspension feeders (B); ecosystems with lower fish biomass and extensive barrens (C); areas covered by turf algae (D).

The study’s main conclusion is that overfishing is very damaging to Mediterranean marine ecosystems and that in general well-enforced marine reserves of Spain and Italy in the western part of the Mediterranean Sea have higher biodiversity and biomass densities than the zones further to the east.

Overfishing is bad, conservation is good

From this one can simply conclude overfishing is bad – and establishing and protecting conservation zones is important. But there is another reason why the ecosystems to the west are relatively healthy compared to those further to the east.

The Mediterranean – like many other of the world’s seas – suffers from the damaging influence of the influx of invasive species, fish and other creatures that have never had contact with the waters between Europe and Africa and are therefore not part of the slowly evolved local food chain equilibrium.

As you will have guessed after this lengthy introduction, in the Mediterranean these non-native species invade from the Red Sea, indeed swimming through the lockless Suez Canal, which in 1869 was dug through the Suez Isthmus.

And that is why in the PLoS ONE study we can read of Singanus luridus, or dusky spinefoot, a rabbitfish species. This alien fish comes from the Red Sea, and does not adequately fit in the East-Mediterranean predatory scheme.

The spinefoot is herbivorous [which according to the researchers is a very rare feat in the Mediterranean, only two native fish would be herbivores] and depletes algal biomass in the Eastern Mediterranean – on the southwest coast of Turkey up to the point of creating complete barrens.

“The decrease of these algal communities can also affect the recruitment rate of numerous rocky fishes that select algae as settlement habitats, having a potential cascading effect on the whole community,” the researchers write.

The dusky spinefoot is just one example of 600 invasive species the researchers have found – none of which of course were considered when the Suez Canal was dug now almost one and a half century ago, when the word globalisation hadn’t even been coined.

Who is to call these old pipe-smoking canal diggers naïve and irresponsible could be overlooking the modern world is interconnecting exponentially faster – and we are of course nothing the wiser to ignore the consequences for future ecosystems.

The researchers warn the Mediterranean is also increasingly affected by climate change. Next to the spread of invasive species, “seawater temperatures are steadily increasing, extreme climatic events and related disease outbreaks are becoming more frequent and faunas are shifting.”

Lush seagrass meadows teaming with life

To end with a positive note – from the study we also learn what Mediterranean conservation efforts should aim for:

“In the Mediterranean, we would expect total fish biomass to be […] the single most important indicator of the health of fish populations, with biomass increasing with decreased fishing pressure.”

The researchers specifically name the importance of having a large abundance of predatory fish [come to say, what happened to those schools of Mediterranean tuna – did we really order them on our pizzas??] as these maintain low and predictable levels of sea urchins.

Sea urchins in turn eat away at fucales, long brown or green algae, which – depending on size – create underwater meadows (‘seagrass’) to forests (like kelp).

Ecosystem recovery only in protected areas

The fucales [shown on image at top of story] are important: “Mediterranean shallow benthic communities harbor hundreds of species of algae and invertebrates, but they tend to be dominated in cover and biomass by algae – the least impacted communities are often dominated by canopies of Fucales, mostly Cystoseira spp.”

“Examples of successful recovery at the ecosystem level are rare for the Mediterranean and are systematically related to the presence of marine protected areas.”

© Rolf Schuttenhelm | www.bitsofscience.org