Just as rising seas pose serious threats to coastal residents and human-built systems and structures, and cause human climate refugees to flee low-lying atolls, these seas are also threatening many diverse species of wildlife.

Mickey Agha with turtles
UC-Davis graduate student Mickey Agha poses with a pair of western pond turtles. (Photo: Courtesy of Mickey Agha)

Coastal species are particularly affected, and some are losing habitat as rising water covers the beaches they rely on for important life functions, like nesting. Another chief concern is the intrusion of salty sea water into freshwater habitats and the impacts that has on some species.

A recent University of California-Davis study predicts up to 90 percent of coastal freshwater turtle species will be at risk from sea-level rise. The study was published in Biological Reviews in March. If sea levels rise three feet by 2100 – a frequently cited estimate well within the range of NASA’s projections – salty seawater is likely to inundate many of the freshwater habitats these turtles need.

Some turtles ‘just continue to lose body fat’

Mickey Agha, a doctoral candidate in ecology at UC-Davis, was the lead author on the study. Using georeferencing, his research team compared maps of projected sea-level rise with range maps showing where coastal freshwater turtles live to see where the two overlap. According to the study, turtles in Oceania and southeastern North America are facing the most serious threats. Many live in freshwater adjacent to brackish ponds where, during periods of high water, the brackish pools can move into the freshwater and raise the salinity. Drought and water diversion can also increase salinity levels. While many turtles can live in brackish areas, many don’t do well with high levels of salinity.

“Their bodies can’t process waters with high-salts compositions,” Agha explains. “So, what they do is they refrain from eating or drinking when in these salty waters, and by doing so they essentially just continue to lose body mass over time.” However, Agha notes some species have adapted to survive in more saline water. He points to the diamondback terrapin, which lives on the East Coast, as being able to live in brackish water by excreting salt from glands near its eyes. Other turtles may exhibit similar adaptations, Agha says, but the topic has not been thoroughly studied.

With suitable habitat available nearby, animals may be able to move short distances to escape high-salinity waters. Northern California’s Suisun Marsh, for instance, is home to a population of western pond turtles, and Agha studies these turtles and how they adapt to the somewhat brackish water in the marsh.

“I believe that they’re behaviorally adjusting to changes in water salinity,” he says. “So when the water salinity is too high, they just move. The issue for them is: if there isn’t any space to move to or freshwater sources to move to, they will just lose the habitat completely. We could start seeing population declines in the areas where they are unable to access freshwater.”

Habitat loss confounds turtles’ easily moving to safety

GPS transmitters on turtles
GPS transmitters on Suisun Marsh turtles track movement. (Photo: Courtesy of Mickey Agha)

Importantly, loss or destruction of habitat can make moving difficult or impossible, and in many coastal areas, flooding, hurricanes, and extreme weather can also drastically change water composition and nearby habitat. Invasive species raise additional concerns, and Agha cites the red-eared slider as a threat to native California turtles, which he says they’re out-competing for resources.

To learn more about how turtles respond to salinity, Agha and fellow researchers have fitted GPS transmitters on some of the Suisun Marsh turtles to track their movements in response to changing salinity, tidal fluctuations, and other factors. They are also conducting lab work to analyze the turtles’ blood chemistry and learn more.

Whether or not the turtles ultimately adapt successfully to saltwater inundation and sea-level rise is a bit of a mystery. Agha notes some species of the long-lived reptiles can take up to 20 years to produce a new generation, so it might be a while before the evidence is clearer. “The question is ‘Can they adapt fast enough to these changing aquatic environments?’” he says.

Look next to Hawaii for another example of a species confronting risks posed by rising sea level.

Northwest of the beach resorts of the main Hawaiian Islands lie a string of small, mostly low-lying islands and atolls home to many species, including endangered Hawaiian monk seals. These islands, and the creatures that inhabit them, are in danger from sea-level rise.

Rare, endangered Hawaiian monk seal faces challenges

The Hawaiian monk seal is endangered and very rare, with only around 1,400 in the world. Most live in the northwestern Hawaiian Islands, where females give birth on sandy beaches, nesting and nursing their pups. And it’s not just seals in need of these low-lying habitats. According to NOAA marine biologist Jason Baker, over 94 percent of Hawaiian green sea turtles use one location – French Frigate Shoals – for nesting.

Earlier models had predicted lower levels of sea-level rise, but many scientists now predict around one meter of sea level rise by 2100. That could leave huge swaths of these low-lying islands under the ocean with even a conservative sea-level rise scenario. “Under almost any of the sea-level rise scenarios, there’s going to be some habitat loss,” Baker says. “We don’t know how much. So I would anticipate that there’s going to be population declines because of that.”

In addition to pupping, the seals also use the islands to molt their fur and skin annually for a few weeks, according to Baker. They may be able to find higher elevation islands to use for all these purposes, but with habitat loss, animals will likely be forced into closer quarters. Baker says the seals have high “site fidelity” and often go back to the islands where they were born, but they also are able to move around the islands. He says it is possible they may swim to other islands in search of new habitat.

However, he says one behavior won’t change. “They’re not going to start having their pups in the water and nursing them in the water,” Baker says. “There’s no pinniped that does that.”

In the eastern U.S., engineers are in a mad rush to ward off the rising seas. Miami Beach has spent $500 million on pumping systems and other methods to help make the city more resilient to rising sea levels, as researchers investigate other innovative engineering solutions.

Low-lying habitat for Florida Key deer in danger

But further south in the Florida Keys, many species of wildlife also are threatened by rising sea levels. The endangered Key deer is a subspecies of small deer – with males weighing no more than 75 to 85 pounds – that inhabits many islands in the lower Keys.

A 2017 article in Conservation Genetics estimates the deer’s population to be roughly 1,000. However, their low-lying habitat is in grave danger.

According to Eric Hoffman, associate professor of biology at the University of Central Florida and co-author of the 2017 study, the primary concern is the “freshwater lens” which allows freshwater to pool on the island. He says that if the sea level rises even minimally, this lens could be breached, causing saltwater inundation, and making the water on the islands unpotable. Without any freshwater, many species are likely to perish.

Male key deer
Male Key deer. Photo credit: Marc Averette.

“The concern is that if the sea level rises just a little bit,” Hoffman says, “basically that [lens] will wash out. And basically, the larger Keys like Big Pine Key, and certainly all the smaller Keys, will no longer be able to hold freshwater pools because they will just be washed in with the saltwater.”

Hoffman says he believes evolution and natural selection may offer the best hope for survival, though he admits the outlook is grim.

“Evolution is a process for which a selection, natural section, will act on the genetic diversity that’s present within the population,” he says. “If there are genetic variants that are present in the population that allow individuals to be able to survive better under certain conditions, then those individuals will be able to leave more offspring and, assuming those traits are genetic in nature, then they’ll pass on these traits to their offspring.”

But Hoffman is quick to point out it’s not just deer in danger – the islands are facing an ecosystem-wide threat. He says other species at risk are the Lower Keys marsh rabbit, snakes, butterflies, and many others. He’s quick to note each of these species is only one part of a larger system.

“It’s not the species so much as the ecosystem that’s being altered and lost and there’s a lot of other species that are directly impacted and the ecosystem as a whole,” Hoffman says. “It’s a symbol of the ecosystem as a whole that’s being changed.”

Kristen Pope is a freelance writer and editor who frequently writes about climate change, ecology, wildlife, conservation, and many other topics for a wide variety of publications. She has a masters degree...