The once great floating gardens of Mexico City, which filled the bellies of the Aztecs, are dying of serious neglect.

On this point, everyone sadly agrees.

The ancient plots and their life-giving canals are weedy and abandoned, overrun by cattle, invaded by exotic fish, sucked dry by urban sprawl — and a dozen agencies of government have failed to save one of the wonders of the world.

A few farmers continue to till their little corners of Eden. They grow marigolds for El Dia de los Muertos, the Day of the Dead.

“Which is appropriate,” said Luis Zambrano, a biologist at the National Autonomous University of Mexico, who is trying to save the indigenous life of Xochimilco, “since the place is dying.”

The gardens have been sick for a long time, ever since the Spanish conquistador Hernan Cortes arrived in 1519 and began draining the lakes.

The problem is that they are now dying quickly, and there are worrying signs that the ecosystem is crashing.

Asked whether the place can survive, a respected historian of Xochimilco, Gloria Valek, answered, “I would like to think so, but it might be impossible.”

The canals that once fed 50 square miles of gardens are overwhelmed by foreign fish, African tilapia or Asian carp, thriving in the dirty waters. The fish are loaded with heavy metals, fed by wastewater treatment facilities — the lake’s only water source, now that the 2,500 artesian springs have dried up, trying to slake the thirst of the megacity. Maybe half the original wetlands used by the Aztec vassals here remain, much of them degraded. But the land could bounce back.

Fisherman Roberto Altamirano has been working to save Xochimilco (pronounced so-chi-MIL-co), which is a U.N. World Heritage Site. For two years, he has been netting exotic species from the canals. “We have removed 650 tons of fish,” he said, “which is a lot of fish.”

But people don’t want to buy the tilapia and carp the fishermen net when they learn they come from Xochimilco, which has a bad reputation because of the pollution.

In one of the experimental reserves, the farmers had to stop using the tilapia as fertilizer — because they were too toxic.

The Valley of Mexico is a bowl surrounded by mountains and volcanoes. The Aztecs built their empire in the middle of a series of interconnected lakes. Their capital of palaces and pyramids, Tenochtitlan, reached by causeways, was an amazing sight to the conquistadors, who said it was more impressive than any city in Europe.

For a thousand years, farmers staked out small rectangular plots in the shallow lakes around the Aztec capital. They built their artificial islands of wattle and willow and filled the gardens, called chinampas, with fertile muck.

The farms were irrigated by an immense grid of miles of shallow canals, just wide enough for a canoe. The gardens produced a bounty: five or six crops a year, an abundance of chiles, greens, cactus and herbs. The canals were exploited for crayfish, frogs, fowl, fish.

It was an ingenious system, and what is amazing is that a visitor can see a glimpse of the old ways even today.

Bird watchers still come to look at the white pelicans, in one of the last big green spaces in a metropolis of 22 million. There are bicycle paths and an underwhelming ecological center. The area continues to shrink.

The gardens and canals were filled with rubble from the 1985 Mexico City earthquake; the wetlands were cut in half by a perimeter highway; after the springs dried up, wastewater was rerouted from the sewage treatment plants.

On the weekends, the canals are packed with hundreds of gondoliers poling their brightly painted launches through Mexico City’s version of Venice. The boats are packed with revelers, drinking beer and eating tacos, as barges filled with mariachi bands pull beside them and sing for $8 a song.

But beneath the surface of the water, scientists fear that the rare and endangered axolotl (pronounced ACK-suh-LAH-tuhl), a foot-long salamander that can regenerate its lost limbs, is nearly gone from the wild.

“We are very sad,” the biologist Zambrano said. “In 1998, we surveyed 6,000 axolotl per square kilometer. In 2008, there were 100 per square kilometer.”

They haven’t been able to find one in months.

The remarkable salamanders were once so abundant that locals remember them well. “There were axolotls everywhere 20 years ago,” farmer Anastacio Santana said. “We used to eat them, wrapped in corn husks, cooked with onion and epazote herbs.”

“Some studies suggest they will be extinct in the wild in 10 years,” Zambrano said.

The university scientists have erected several refuges for the salamanders — small stretches of canal that they empty of exotic fish and cover with netting to keep the birds away.

“This is it,” said Zambrano, pointing to one small ditch where they have placed 20 axolotl raised in the laboratories. “It is not much. But it is a start.”

Martha Teresa Delgado, the environmental secretary for the Mexico City government, said that the solutions for saving Xochimilco are well known, but that funding has been elusive and that there are too many agencies with too little responsibility, all making promises and passing the blame. She said there is a proposal to create a kind of czar to coordinate recovery efforts.

“Things have been bad for a long time. But now we fear that the destruction is accelerating, that within our lifetimes this very special place will no longer exist,” Zambrano said. “It will just be a few dirty canals for the tourists and will mean nothing.”

Researcher Gabriela Martinez contributed to this report.