What Does It Mean to Downlist a Species?

In October 2025, the International Union for Conservation of Nature (IUCN) downlisted the green sea turtle from Endangered to Least Concern. This has been celebrated as a conservation victory, and it is, but there are some caveats worth talking about.

The IUCN created the Red List of Threatened Species in 1964 to serve as a central location for global biodiversity and species health data. Over 172,600 species are currently on the list, and countless organizations and governments use the list to inform conservation goals and policy-making. The IUCN Red List is the go-to source for how a species is doing and what its threat level for extinction is, with species being listed in one of nine categories:

• Not Evaluated

• Data Deficient

• Least Concern

• Near Threatened

• Vulnerable

• Endangered

• Critically Endangered

• Extinct in the Wild

• Extinct

The Red List is updated several times a year, but species typically undergo assessments every 5-10 years. Experts in each species work together with IUCN committees to determine which Red List designation the species should be given based on several factors, including current population, threats, and geographic range.

Can we get back to the turtles already?

OK, so now that you've had your primer on the IUCN red list, let's talk about green sea turtles. Green sea turtles are globally distributed, and they are the second-largest species of sea turtle. They are arguably the most recognized species of sea turtle, thanks to pop culture portrayals like Crush from "Finding Nemo." Green turtles' populations had declined severely by the time the Endangered Species Act was passed in 1973. Despite being hunted sustainably for millennia by indigenous peoples, Europeans and colonists over-hunted them in many parts of the world, especially the Americas. In addition, increased fishing and boating have put all species of sea turtle in growing danger from things like entanglement in fishing gear and boat strikes. Every previous year that the green sea turtle was assessed (starting in 1982), it was designated as endangered.

Green sea turtles, like this one, have had an overall population rebound, but they're not out of the woods completely.

In response to this designation, global efforts have been undertaken to help protect all seven species of sea turtle. In the United States, the Endangered Species Act of 1973 put in place laws that protect sea turtles and their habitats from undue harm, and sea turtles have seen dramatic population increases as a direct result. Perhaps the most encouraging trend is that population growth doesn't seem to be leveling out any time soon: green sea turtle nests in Florida have had an almost parabolic increase over the past 40 years.

Data from the National Oceanographic and Atmospheric Administration show that green sea turtle nests in Florida have had the most dramatic increase over the past 10 years, indicating a cumulative benefit from state and federal protections. Image from https://www.fisheries.noaa.gov/feature-story/green-turtle-conservation-successes-and-continuing-challenges

Encouraging numbers like these are what ultimately led to the IUCN downlisting green sea turtles from Endangered to Least Concern, but there is still a lot of nuance to this situation that shouldn't be ignored. While the IUCN makes global status determinations, many species, including green sea turtles, have regional populations that make up the global ones (greens have 11 such populations). The East Pacific, Southwest Pacific, and Indian populations are listed as Vulnerable, the Central West Pacific and Mediterranean populations are Near Threatened, and the Central South Pacific population is still listed as Endangered. These areas are more seriously affected by dangers like habitat fragmentation, geographic isolation, and bycatch than populations living in larger areas with more contiguous nesting beaches.

For many scientists, especially those of us whose governments have recently made it clear that conservation is not a priority, this change in IUCN status is a bit of a double-edged sword. It is beyond exciting to see conservation measures pay off in our lifetime. Ultimately, it would be a dream come true to have all endangered species rebound out of endangered status. However, the ICUN Red List is a major factor when policymakers are deciding what protections wildlife should get. While it is unlikely that green sea turtles will stop being protected by laws like the Endangered Species Act, being listed as Least Concern could have other detrimental effects. For example, research projects seeking funding are often prioritized based on how endangered the research subject is. There can also be a shift in public perception, leading to the false belief that we as a society no longer need to care about green sea turtles and their conservation. The unfortunate reality is that sea turtle populations are still significantly depleted compared to pre-colonial estimates. Meeting the criteria for Least Concern status doesn't mean the population is completely sustainable and worry-free.

A weird byproduct of science working properly is its lack of visibility. A great example of this is the effectiveness of vaccines. The smallpox vaccine was the first vaccine ever developed, and it is responsible for the eradication of one of the most deadly human diseases in history. Since smallpox is no longer a concern, it is easy to forget just how critical the vaccine was to human history. So, while we should celebrate the victory of bringing a species back from the risk of extinction, we also must remain vigilant and committed to green sea turtle conservation. Such is the Möbius strip of a scientist's emotions.