For millions of years, countless sea turtles navigated the world’s oceans, migrating vast distances between foraging sites and natal nesting beaches. But today, those long journeys repeatedly expose them to harmful anthropogenic impacts and disruptive environmental changes. And despite worldwide conservation efforts, all seven sea turtle species are endangered or critically endangered at global or regional levels.

The mass movement of these, and other animals, by land, sea or air, represents one of Earth’s ancient rhythms and one of its great wonders. Those migrations also weave together vital living threads that strengthen ecosystem structure.

Now, for myriad reasons — including human-made physical hazards, climate change, pollution, habitat loss, and much more — the frayed fabric of those global eco-structures is shredding fast. Species are disappearing at unprecedented rates and biodiversity loss is irrevocably altering natural systems, imposing adverse impacts on the world’s migrators.

The burning question: can sea turtles, people and conservation strategies evolve fast enough to protect the world’s epic migrations and the animals that make them?

Crossing planetary boundaries poses multiple threats

Biodiversity is just one of nine planetary boundaries that allow a “safe operating space for humanity,” according to an interdisciplinary team of scientists convened by the Stockholm Resilience Centre. The other eight boundaries that humanity must avoid overshooting are climate change, ocean acidification, land-system change, freshwater use, stratospheric ozone depletion, atmospheric aerosol pollution, biogeochemical flows (imbalances in the nitrogen and phosphorus cycles), and the impact of novel entities (such as chemicals, engineered materials or organisms).

Humanity has already breached the “core borders” of biodiversity loss and climate change, and overstepped the bounds of biogeochemical flows and land-system change. While the overshoot of just one core border could completely destabilize the Earth systems that sustain humanity, the crossing of any single boundary also risks destabilizing others — unleashing a domino effect. In coming decades, human activities will put as many as 1 million more plant and animal species at risk of extinction, according to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) 2019 report.

“Biodiversity and nature’s contributions to people are our common heritage and humanity’s most important life-supporting ‘safety net,’” said Sandra Díaz, an Argentinian ecologist who co-chaired the IPBES in 2019. “But our safety net is stretched almost to breaking point.”

Turtles at added risk due to boundaries overshoot

Sea turtles hold a tenuous and unenviable position in the planetary boundary framework. Their life cycle requires safe passage across sea and land. Leatherback migrations, for example, can traverse the borders of more than 30 countries.

As humanity rapidly approaches the limits of more planetary boundaries, it puts these intrepid mariners increasingly in harm’s way from a variety of sources. If conservation efforts cannot reverse sea turtle losses, their plight could herald extinction for other migratory species, even perhaps for the greatest migrator of all: Homo sapiens.

The best-known environmental threats to sea turtles mostly involve the biodiversity planetary boundary (fisheries bycatch and human take of turtle eggs, for example), and the land-use change boundary (habitat and/or nesting site losses).

But now, redefined within the context of other human planetary boundary transgressions, the turtles face a plethora of poorly understood new hazards, plus looming questions about how conservation can pivot to help.

Assault on nesting beaches

Sea turtles are most visible to us when females come ashore to lay eggs, and that’s their habitat most studied by science. Climate change is one planetary boundary already known to be altering the sandy beaches where turtles nest and spend a brief but critical portion of their lives, posing multiple existential challenges. For example, because the sex of sea turtles is temperature dependent, more females are hatching as global warming pushes temperatures higher on the world’s nesting beaches. Today, females outnumber males three to one at many global sites.

“But how is feminization affecting populations?” asks Mariana Fuentes, a Florida State University marine conservation biologist. “How many males do there need to be to sustain populations? We don’t know.”

Novel entities, another planetary boundary, may be acting synergistically with global warming to turn up beach heat even more. Human-made microplastics mingled with nesting sand could be raising sand temperatures higher, says Fuentes, who is studying sand’s evolving thermal profile.

Clearly, all those females will need nesting beaches with optimal incubating environments — a key factor in the resilience of global turtle populations, Fuentes adds. But another boundary, land-use change, is reducing the availability and suitability of nest sites. As climate change escalates, more severe and frequent storms will erode more beaches, and primary nesting sites may disappear. Simultaneously, sea level rise due to climate change and the “armoring” of coastlines with human development, especially sea walls, will make the nesting situation worse.

Turtles have adapted and shifted to new nest areas in the past, but as humanity increasingly blocks beach access, will there be enough suitable nesting places?

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