SCIENCE:

Biodiversity a bitter pill in 'tropical' Mediterranean Sea

Two weeks ago, a group of marine biologists from Israel's National Institute of Oceanography set sail from the country's central coast. Under a full moon, with the lights of hectic Tel Aviv a band on the horizon, they cast their nets into waters that have sustained civilization for millenia in the Levantine Basin, the eastern branch of the Mediterranean Sea.

They had a rich catch that night on the research vessel Shikmona, according to Bella Galil, a senior scientist at the institute. Spilling from the nets were pucker-faced dragonet fish, sprawling octopuses and brown crabs, snapping their claws. On the examination table, it seemed a display of the sea's bounty.

Unfortunately, it was another sea's bounty.

Almost all of the species Galil found that night were natives of the Indian or Pacific oceans. Lured by warming waters and a newly improved route through the Suez Canal, tropical marine species have enacted a slow march into the Mediterranean, displacing native species and disrupting ecosystems.

"We are open to this caravan of alien species," Galil said. "When we trawl the southern Levantine Basin, about 80 percent of the fish we catch are of Red Sea origin."

"This," she added, "is unprecedented."

Israel is not alone in these observations. Over the past century, more than 600 alien species have migrated into the sea, according to conservative estimates and data gathered by the Mediterranean Science Commission (CIESM) in a decadelong survey set to be completed next year. Some species are benign, others invasive. Their arrival has accelerated in the past few decades. And almost all have arrived because of man.

As the sea grows warmer due to climate change -- "tropicalization" is the preferred term -- no native marine life has yet to be driven to extinction. But many species, particularly those near the Suez Canal, have seen their ranges diminished, leaving the sea suspended in a bitter, biodiverse moment as it transitions from what nature intended to what humans have created.

"We are in a peculiar period of time," said Frederic Briand, CIESM's director. "Species of tropical affinity, favored by climate warming, are settling in at a fast rate, while species of colder affinity are not doing very well but are still around. The net outcome is a jump in species richness."

While biodiversity is currently a popular indicator of an ecosystem's health, statistics tell too simple a story in the Mediterranean Sea.

Some scientists have argued that the sea, particularly in its eastern stretches, suffered from an "ecological vacuum" that is now being filled by Suez immigrants. However, this flood of additional sea species does not immediately equate to robust habitats.

Instead, scientists have seen mosaics of well-structured, diverse regions replaced with single species, like meadows of invasive green algae, said Marc Verlaque, a marine ecology professor at the University of the Mediterranean who is coordinating the last segment of CIESM's survey.

"As we say in France, it is like introducing an elephant in a porcelain shop," Verlaque said.

The Mediterranean Sea is a bellwether for the world's oceans, added Ferdinando Boero, a zoology professor at the University of Lecce. Already, one indicator seen in the sea -- a rise in jellyfish numbers -- has been realized as a global phenomenon. Some Italian residents have already begun to adapt, catching the jellyfish for what could become a new Mediterranean diet, he said.

The Mediterranean "is a miniaturized ocean," Boero said. "The velocity of the water processes is higher because the water mass is lower. ... [But] what we are seeing here will happen in the future [everywhere]."

Bitter Lakes no more

For the first century of its existence, the Suez Canal, opened in 1869, was hardly the species superhighway it is today.

In those days, the canal -- which is actually two canals -- was more modest, as were the ships that passed through it. The canals used two lakes, known as the Bitter Lakes for their concentrated salt content, as a bridge. Few species could survive a trip through such saline conditions.

Today, the Bitter Lakes are no longer so bitter. The Suez has been continuously deepened and widened in recent decades. The most recent expansion, meant to accommodate ultra-high-capacity oil tankers, is set to be completed this year. The canal will run more than 70 feet deep, and further dredging is planned.

"We're talking a much larger corridor that now has no barriers," Galil said.

Through these broad waterways, the lakes' salinity has fallen, opening the canal for marine life. Added to this, Galil said, flooding of the Nile Delta has fallen dramatically. These floods, near the Suez's northern outlet, caused the sea to turn brackish, a low-salt counterpart to the Bitter Lakes -- what was once a potent one-two punch. Both barriers are now gone.

The wildlife passing through the Suez have found a region that was, in a way, waiting for them.

About 5 million years ago, the Mediterranean Sea lost its connection to the oceans and dried up. Later, the Atlantic Ocean came rushing back through the Strait of Gibraltar, but many of the cold-water species that came along could not survive in the hotter stretches of the Levantine Basin, Boero said.

This resulted in what marine biologists called an "ecological vacuum" that has now been filled by tropical prawns, rabbitfish and mussels. As one scientist wrote, biodiversity has increased "because we are introducing new species to areas faster than those new species exterminate natives."

The tropical species have been efficient in overtaking the Levantine Basin. They settle in with a success rate of 50 percent, far higher than the 10 percent rate typically expected with alien species, Galil said. And their progress has not been benign.

For some, it has been an economic boon. Commercial fishers setting out from Egypt now find that alien prawns constitute almost their entire catch -- and they're happy for it, as the prawns are far more financially viable than the native prawn. The newcomers now reach as far as Tunisia.

As for the native prawn? No one is quite sure what will become of it, said Verlaque of the University of the Mediterranean.

"Has there been addition or substitution of species? The question is open," Verlaque said. "In the few cases studied, the Red Sea species have taken the place of their Mediterranean counterpart."

To most scientists, it is now apparent that for many native species, the progress of the "Erythrean invasion," as it is sometimes called, will not stop at filling vacant niches. A symphony of man-caused drivers -- climate change, pollution, nutrient saturation and overfishing, among others -- appears to have, at the least, permanently marginalized native species.

For one example, take two species of rabbitfish, the dusky and marbled spinefoot. The schooling, algae-chomping fish have outcompeted native counterparts off the coasts of Lebanon, Libya and Israel and possibly in the Aegean Sea. Prior to their arrival, there were few grazing fish. Now, rabbitfish can be found all the way to Sicily. Their arrival, scientists believe, caused grazing pressure on rocky algae that benefited another Red Sea dweller, the pharaonic mussel.

In the early 1970s, the mussel was extremely rare compared to its native counterparts on the Israeli coastline. By the late 1980s, its numbers grew, and researchers predicted the mussels could coexist. But recent surveys have found that the larger, thick-shelled invader has completely displaced its Mediterranean cousins -- and is possibly causing an increase in snail populations. The implications ripple out from there.

While ecosystems like the Israeli coast have been drastically altered, it will be difficult for this web of man-caused change to eliminate native species, scientists said. Marine life is resilient.

"I have the feeling it is very difficult for a species to totally eliminate another," Verlaque said. "Perhaps only man and pathogens are able to do that."

Extinction, though, is a relative term, Boero added.

"There are species that live in the northern part of the Mediterranean -- especially in the northern Adriatic -- that have not been found for 150 years," he said, adding: "If it's a tiny beast, people don't care."

Green tides

While it is estimated that nearly two-thirds of the alien species in the Mediterranean Sea stem from the Suez Canal, other, more common sources of introduction -- such as ship ballast and aquaculture -- have also introduced some of the most aggressive species.

Most spectacular has been the arrival of several breeds of green and red algae. In particular, a tropical green algae accidentally introduced by the Oceanographic Museum of Monaco two decades ago has spread to Spain, Italy, Croatia and Tunisia.

The algae have flourished in warm waters at the expense of other algae and fish. Their dense and creeping meadows -- which feature up to 14,000 algal blades per square meter -- crowd out other life. And, if that were not enough, the algae release toxins that kill nearby mollusks, fish and sea urchins. Once they have spread, species like this are nearly impossible to eliminate, Verlaque said.

And, of course, there are those jellyfish. Just this year, Boero said, sea walnut jellyfish have been found in Italy, flowing down from the Black Sea, where they were introduced by ship ballast in the 1980s. The sea walnuts have devastated the tuna population of the Black Sea, and, while it remains to be seen how they will survive in the more complex Mediterranean Sea, that experiment has now begun in earnest.

Most disheartening to the scientists chronicling the Mediterranean's tropical future, there are few steps that can be taken locally to halt the process. Galil advocates setting up a simple saline lock system for the Suez Canal -- the Panama Canal has conducted little in the way of invasive species partially because of its locks -- but canal authorities and governments have shown little concern, she said.

Perhaps it will not be until a beloved marine animal becomes endangered that some sort of action is taken, according to Verlaque.

"In France we say: 'Loin des yeux, loin du coeur' -- 'Far of eyes, far of heart,'" he said. "The impact of invasive [algae] is not visible from the shore, so politicians and populations feel few concerned by the problem."

In the end, "if temperatures are increasing, there is nothing you can do at the local stage," Boero said.

Surrounded by historic ruins in Israel -- like much of the Mediterranean -- Galil fancies the classics. There is one mosaic she finds particularly poignant. It is from Pompeii. Staring out from its browning tiles are prawns, squid and varieties of open-mouthed fish. It is sad to think, Galil said, that half the fish now caught off her coast did not swim in Mediterranean waters during ancient times.

"Yes, mankind probably has much more pressing demands on its time," Galil said. "But we pride ourselves on taking care of our environment. It's time that we take care of the Mediterranean."

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