The coral reefs in Mexico’s Islas Marías archipelago are bursting with life.Credit: Octavio Aburto/iLCP

A history of punishments, torture and repression ended in March when the Mexican government closed one of the last island prisons in the Americas. President Andrés Manuel López Obrador has promised to turn the facility into a cultural and environmental education centre. But researchers are pushing for more: they want the government to fully protect the site and surrounding islands, which have remained relatively untouched for more than a century.

The Islas Marías Federal Penal Colony — located on Isla María Madre in the Pacific Ocean — was once a strict no-go zone. That island is a two-hour boat ride from the nearest city, San Blas. And Mexico’s navy regularly patrolled nearby waters to deter prison breaks. But now that the government has shuttered the facility, biologists worry that illegal fishing and wildlife trafficking in the area will increase.

Mexico instituted some protections for the Islas Marías archipelago, a 245-square-kilometre region that includes Isla María Madre, in 2000. But the government allowed some fishing in the area. Fully protecting Isla María Madre and enforcing the rules for the archipelago would guard against destructive practices, such as overfishing, and enable scientists to study relatively intact forests and coral reefs, says Octavio Aburto, a marine biologist at the Scripps Institution of Oceanography in La Jolla, California.

If researchers' push for additional protections succeeds, the former penal colony would join a surprisingly long line of prisons turned nature reserves. Many of these converted detention centres act as time capsules, demonstrating what an area’s ecosystem used to look like before people altered it.

Paper parks

The relatively untrammeled state of Isla María Madre impressed Aburto when he first visited in 2010. “When we went diving ― wow, we saw incredible things,” he says. Colourful corals swarmed with life, schools of fish blocked the sunlight from above and sharks cruised nearby.

Data that Aburto and his colleagues have collected from Mexico’s other marine parks and reserves reinforce their view that no-take zones are the best way of preserving these ecosystems. In 2012, the team reviewed the effects of fishing and tourism on ten marine protected areas in the Gulf of California1. All but one, Cabo Pulmo, where local families banned fishing in 1995, suffered from poor governance and enforcement2. This led to a drop in the populations of predators such as sharks and snappers. The parks and reserves existed only on paper, says Aburto.

Subsequent studies3,4 by Aburto and his colleagues confirmed that Cabo Pulmo and the Islas Marías archipelago contained the only healthy reefs in any of Mexico's marine protected areas in the Gulf of California.

A seabird comes in for a landing on one of the islands in Mexico's Islas Marías archipelago.Credit: Octavio Aburto/iLCP

“The isolation of militarized areas can act as a natural experiment on how ecosystems persist and change when they are not subject to direct human impacts,” says Joe Roman, a conservation biologist at the University of Vermont in Burlington. He proposed turning the US military prison at Guantánamo Bay, Cuba, into a peace park and marine research hub5 when the United States announced plans to close the facility in 2016. (The closure never happened, but researchers continue to study the area.)

These de facto reserves won’t stop the biodiversity crisis, Roman says. “But they can provide hope for the future in areas that are often seen as a stain on human history.”

A different legacy

One such haven is Panama’s Coiba National Park, which once hosted a penal colony that closed in 2004. The island harbours healthy populations of trees, such as the ajo (Caryocar costaricense), that illegal logging and agriculture have largely erased from the mainland. Panama lost 423,000 hectares of forest on the mainland between 1990 and 2015 because of such activities. “There’s barely anything anymore,” says Alicia Ibáñez, an independent botanist who spent six years cataloguing Coiba’s vegetation starting in 1997.

Former penal colonies can also provide a controlled space in which to measure how ecosystems recover from human activities. Before the prison on Colombia’s Isla Gorgona closed in 1984, inmates cut down trees for firewood, broke off pieces of coral to line trails and hunted the island’s sloths, monkeys and coatis, small members of the raccoon family. The prisoners’ activities affected roughly 70% of the island, says Alan Giraldo, an oceanographer at the University of Valle in Cali. He coordinates a project studying the recovery process on Gorgona. Since the prison closed, levels of biodiversity have increased, Giraldo says. “The island is doing great.”

But unless these places are consistently protected, they could go the way of other degraded ecosystems. Last year, when Aburto and his colleagues returned to a less-guarded part of the Islas Marías archipelago, they found hundreds of conch shells lying on the sea floor. Fishermen had drilled into the shells to remove the snails. That is why the scientists are pushing the Mexican government to enforce the archipelago’s protections and prevent fisheries from operating close to the islands.

“It’s an opportunity to do things right,” says island ecologist Federico Méndez Sánchez, director of the Ecology and Island Conservation Group, an environmental advocacy organization in Ensenada, Mexico.

“We can’t afford to lose areas that have been indirectly protected by a prison,” says Aburto. “These places are surviving, and we need to take care of them.”