Warmed, overfished and polluted, the small Mediterranean Sea is giving scientists a look at what the future may hold for the rest of Earth's oceans — and it's not pretty.

Beneath its surface, a transformation is taking place. Food webs are shrinking, with rich ecosystems that supported valuable commercial fisheries giving way to barrens dominated by jellyfish and tiny invertebrates. Mass die-offs and disease are now common.

"The predicted effects of climate change are being met in the Mediterranean. The results are more obvious and dramatic, but the drivers are the same all over the world," said Pierre Chevaldonné, a University of the Mediterranean biologist.

Chevaldonné is a co-author of a review of more than 100 studies on the Mediterranean's changing ecological dynamics. Published last Monday in Trends in Ecology and Evolution, it describes the convergence of climate change and human impacts in waters that had been stable since the time of Aristotle.

During the latter half of the 20th century, the Mediterranean's deep northern regions, a traditional source of cold waters that flowed south into warmer basin currents, warmed by one-fifth of a degree Fahrenheit. Shallow northwest waters — an intermediate zone more productive than any other region of the Mediterranean — warmed by 1.8 degrees Fahrenheit. Some of the warming was expected, but it appears to have accelerated in the last 20 years, as the unusually hot 1990s coincided with natural cycles.

With that overheated decade came anomalies in surface temperature and rainfall. These appear to have disrupted deep-water hydrology, changing its composition and currents. That disruption has now rippled to the western shallows. Compounding the problem, runaway population growth has packed 132 million people around the sea's rim, with habitat destruction, pollution and fishing pressure increasing apace.

The effects of these interacting stresses make the Mediterranean a model system for the rest of Earth's oceans, which are also overfished and, in many regions, warming at comparable or greater rates. Scientists say warming will continue for decades even if greenhouse gas emissions soon fall to a fraction of current levels. And though it will take longer for disruption to become visible in those larger waters, the lessons are the same.

"It's difficult to know exactly what's going to happen elsewhere, but the principles can be extrapolated," said Marta Call, a Dalhousie University marine biologist who has modeled the interactions of Mediterranean species. In a paper published last year in Ecosystems, she and her colleagues described Mediterranean food webs as "in an advanced state of degradation."

Degradation in the Mediterranean has taken place on multiple levels. Many large fish species, including top-level predators like sharks and tuna, have been fished to functional extinction. A few still swim, but they no longer have the same ecological role. Coll's models and other research on predator interactions suggest that they helped stabilize food webs, and their absence now leaves other species prone to wild fluctuations.

Mass die-offs of dozens of invertebrate species are now common in the northeast. They're stressed by rising temperatures and vulnerable to disease, and the most common invasive species are not new predators, but microbes. Most strikingly, soft corals that once carpeted the northwest seafloor, forming a literal underwater forest, have in many areas been wiped out altogether. Replacing them is what Chevaldonné calls "lawns" of algae and short-lived invertebrates.

The prevailing dynamic is what scientists call "brittleness," or a decline in "robustness." Historically complex food webs cannot find balance. In their place have emerged simpler food webs dominated by species that Coll and her colleagues characterize as "unpalatables" and "detritus" — algae, invertebrates and jellyfish. There are still some fish, but they're relatively few in number, and small. Much of the Mediterranean catch is now processed and sold as animal feed.

"In terms of biomass and production, the Mediterranean is basically impoverished," said Coll.

These conditions probably represent a transitional period for the Mediterranean, though it's likely a one-way transition. Neither Chevaldonné nor Coll claims to know exactly what the sea's next stable ecological configuration will look like, but this may be a preview, just as the Mediterranean may be a preview of the profound shifts likely elsewhere.

"In the future, we may get only jellyfish. Then we'll find a way of consuming jellyfish," Coll said. "The problem is, do we want that?"

Images: 1) In some regions of the Mediterranean, traditional food webs have collapsed, and the new ones are dominated by bacteria, small invertebrates and jellyfish./jetzt_ist_immer/Flickr.

2) A map of Mediterranean species movement./Trends in Ecology and Evolution.

See Also:

Citations: "Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea." Christophe Lejeusne, Pierre Chevaldonne, Christine Pergent-Martini, Charles F. Boudouresque and Thierry Perez. Trends in Ecology and Evolution, published online, Dec. 1, 2009.

"Structural Degradation in Mediterranean Sea Food Webs: Testing Ecological Hypotheses Using Stochastic and Mass-Balance Modelling." By Marta Coll, Heike K. Lotze, and Tamara N. Romanuk. Ecosystems*, Vol. 11 No. 6, Sept. 2008.*

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.