Supreme Court Ruling Tests Whether the Endangered Species Act Can Outpace Sea Level Rise

When the U.S. Supreme Court ruled in 2018 that land designated as critical habitat under the Endangered Species Act must actually be habitat, conservationists warned that one of the law’s most forward-looking tools had been placed in jeopardy. A new study in the journal Nature Conservation now offers one of the first detailed examinations of what that ruling, and the regulatory whiplash that followed, has meant in practice. Its subject is the U.S. Fish and Wildlife Service’s handling of three South Florida reptiles whose entire world is being slowly swallowed by the sea, and its findings reveal both the surprising resilience and the quiet erosion of a legal provision that many had hoped would shield biodiversity from climate change.

The provision at stake is the designation of unoccupied critical habitat. The Endangered Species Act, ratified in 1973 and widely regarded as the strongest wildlife conservation law in the world, obliges federal agencies to avoid destroying or adversely modifying critical habitat of listed species. Crucially, the statute allows the government to protect areas a species does not yet occupy if those areas are deemed essential for its conservation. In an era of rising seas and shifting climates, this power matters enormously: it lets managers identify higher ground, inland refugia or other suitable sites before a species needs them, paving the way for reintroductions and assisted migrations as conditions deteriorate. Studies have shown that listed species with designated critical habitat are more than twice as likely to show population improvements than those without it.

But in Weyerhaeuser Co. v. the U.S. Fish and Wildlife Service, the Supreme Court held that critical habitat, whether occupied or not, must first qualify as habitat, leaving the meaning of that term to the agencies. The first Trump administration then codified a rigid definition of habitat in December 2020, describing it as the setting that currently or periodically contains the resources and conditions necessary to support a species’ life processes. Critics argued this effectively required habitat to be move-in-ready, an impossible standard in a landscape of mass habitat destruction, and imposed a more stringent two-step process requiring reasonable certainty that unoccupied areas contain essential physical or biological features and will contribute to conservation. The Biden administration repealed the habitat definition and reverted to the less stringent 2016 rules, a federal court briefly vacated the 2019 regulations and then reinstated them, and in November 2025 the second Trump administration proposed reverting to the 2019 framework once again.

Against this backdrop, the study’s author examined proposed critical habitat designations for three cryptic reptiles: the Florida Keys mole skink, the rim rock crowned snake and the Key ring-necked snake. All three are imperilled primarily by climate change, above all sea level rise, saltwater intrusion and intensifying storms. Recent modelling suggests that more than 80 percent, and possibly up to 90 percent, of Florida’s islands could be completely inundated by the end of the century, making the Keys a dire case study in the conservation decisions a warming world forces upon us.

For the Florida Keys mole skink, a small, pink-tailed lizard that spends much of its life underground, the 2022 species status assessment projected that even the lowest sea level rise scenario would destroy or degrade more than half of its potential habitat by 2040, with the Lower Keys, where most of the population lives, suffering the greatest losses. The Service proposed listing the skink as threatened and designated 19 critical habitat units totalling about 7,068 acres, five of them unoccupied, on higher-elevation islands such as Plantation Key and Loggerhead Key. These areas were selected precisely because they should remain above the waterline, providing potential reintroduction sites should the skink be extirpated from its current haunts by rising seas or hurricanes.

Here the study’s textual analysis becomes revealing. Although the mole skink proposal was finalised under the less stringent 2016 regulations, the Service repeatedly asserted that each unoccupied unit constitutes habitat because it contains the physical or biological feature necessary for the species’ life history, language that closely mirrors the stricter 2019 criteria. Under the 2016 rules, showing that an area is essential for conservation should have been sufficient. A separate memo applying the 2019 regulations used nearly identical reasoning, suggesting that the more demanding framework had seeped into the agency’s analytical habits even when it was not officially in force. Whether this reflected new internal norms or a strategic effort to cover their bases amid ongoing litigation remains unclear.

The rim rock crowned snake, quite possibly the rarest snake in North America, tells a similar story with a twist of timing. Over 90 percent of its pine rockland and tropical hardwood hammock habitat has already been destroyed, and the Service identified climate change as the predominant threat to its survival. Because so much of the snake’s former range is urbanised, the agency proposed two unoccupied critical habitat units, Camp Owaissa Bauer and Navy Wells, inland in south-eastern Florida and largely managed by Miami-Dade County’s Environmentally Endangered Lands programme, where sea level rise projections are less severe. Due to a court stay, the 2019 regulations were governing law when the proposal was submitted, and the Service adopted its memo applying those rules as its primary justification, while retaining that the outcome would have been the same under the older regulations.

The Key ring-necked snake, however, is where the limits of the provision became stark. This small yellow-bellied subspecies occurs only in the lower Florida Keys, and the Service’s own assessment concluded that its populations may be extirpated as early as 2040 and will very likely be extirpated by 2080. Yet when the agency listed the snake as endangered, it proposed only four occupied critical habitat units, all of which it acknowledged are vulnerable to sea level rise, saltwater intrusion and storms, and declared that it had not identified any unoccupied areas essential for the species’ conservation. Conservation groups argued this determination was arbitrary, pointing to higher-elevation upper Keys sites that could provide the redundancy and geographic representation the snake will desperately need, though any assisted migration there would have to weigh the risk of hybridisation with the southern ring-necked snake, whose range extends through the upper Keys. The study cautiously advocates designating unoccupied habitat for the snake, noting that scholars increasingly view hybridisation risks as overstated and manageable.

The broader lessons are mixed. On the one hand, the provision still functioned: the Service proposed unoccupied critical habitat for two climate-imperilled reptiles even under an administration with an especially stringent reading of the court’s ruling, and claimed its analysis would have been identical under either regulatory regime. On the other hand, the textual evidence suggests the stricter 2019 criteria now shape agency reasoning regardless of which rules formally govern, a shift that could prove decisive in future cases where unoccupied areas need restoration before they are habitable, as happened with the dusky gopher frog wetlands at the heart of Weyerhaeuser itself. In July 2026 the Service finalised endangered listings for all three reptiles without designating critical habitat at the time of listing, and the second Trump administration’s proposal to reinstate the 2019 regulations looms over future decisions.

The study calls for the Service to reconsider its refusal to protect unoccupied habitat for the Key ring-necked snake and for research on how recovery planning, climate adaptation and assisted migration can be woven into the Act’s implementation. It also poses a larger question: can environmental law respond not only to the symptoms of the biodiversity and climate crises, but to their upstream drivers, from fossil fuel extraction to industrialised agriculture? For the reptiles of the Florida Keys, and for countless coastal and island species worldwide, the answer may determine whether refuges on higher ground exist before the water arrives.

Subject of Research: Unoccupied critical habitat designation under the U.S. Endangered Species Act as a climate adaptation tool for sea-level-rise-threatened Florida reptiles

Article Title: Can the U.S. Endangered Species Act protect biodiversity from climate change effects? Post-Weyerhaeuser unoccupied critical habitat designations

Article References: Gunderson, R. (2026). Can the U.S. Endangered Species Act protect biodiversity from climate change effects? Post-Weyerhaeuser unoccupied critical habitat designations. Nature Conservation, 64, 297-327. https://doi.org/10.3897/natureconservation.64.210893

Image Credits: AI Generated

DOI: 10.3897/natureconservation.64.210893

Keywords: Endangered Species Act, unoccupied critical habitat, Weyerhaeuser v. FWS, sea level rise, Florida Keys, Florida Keys mole skink, rim rock crowned snake, Key ring-necked snake, climate change adaptation, assisted migration, conservation law, U.S. Fish and Wildlife Service