Coastal dunes are more than essential habitat for ghost crabs and rare beach mice. They help blunt the force of onshore winds and repel powerful storm surges.
Click here to see how sand dunes are formed.
On the top of a sandy ridge the tracks of a fox join the prints of a mouse, and for a few dozen feet we walk the dunes together—the fox, the mouse, and I. It’s easy to guess what was on the fox’s mind, so I scan the hummocky dunes and shell-littered sand flats for evidence of a scuffle.Despite the fall chill and winds that whip long mare’s tails of sand far down the beach, it’s a sunny day on North Carolina’s Masonboro Island, an eight-mile-long oasis of beach, dune, and tidal flat on the state’s southern coast, near Wilmington. An hour earlier ecologist David Nashand I had hopped off a flat-bottomed skiff and started across the island together. But as beachcombers seem always to do, we’ve parted ways, each drawn to some alluring bit of flotsam on the wrack line or curious object up in the dunes.
Alone, I concentrate on my Leatherstocking skills. Suddenly, the fox tracks quit the trail, diverging into a moist swale lined with saltmeadow hay. I stay true to the smaller track, and follow the mouse to a single stem of young sea oats. When fully grown, sea oats can tower six feet above the sand. But this is a smaller plant, maybe three feet high. All around the base are a confusion of prints, and I can see where the tiny creature climbed the reedy stem until its weight toppled the plant, pulling the seed head to the ground. I lie down on the beach and imagine the scene: the mouse, cheek pouches stuffed with flaxen, pointed seeds, clutching the bent stem, like the little boy swinging from the trees in Robert Frost’s poem Birches.
This close to the ground, the sound of surf recedes, and in the quiet I hear something odd—a sibilance, from down in the sand. I hold my breath and turn an ear toward the sea oats’ slender, lanceolate leaves. That’s it: It is the sound of wind-driven sand rasping against the sea oats. For a coastal sand dune, it is the sound of life and its heartbeat—as it is for entire shorelines along the hurricane-prone coasts of the southeastern Atlantic and Gulf shores.
No one can question that hurricanes are striking Atlantic and Gulf coasts with greater frequency and intensity. According to the National Oceanic & Atmospheric Administration (NOAA), Atlantic Ocean waters saw an average of 7.8 hurricanes and 3.8 major hurricanes per year between 1995 and 2004, up from an annual average of 5 hurricanes and 1.5 major hurricanes between 1970 and 2004. Whether this uptick is due to global climate change or a natural storm cycle is hotly debated, but regardless of the cause, the sober, stolid sand dune is increasingly thrust into a role as the coast’s main line of defense against seas and winds whose extreme natures stretch the limits of imagination. Indeed, along the southeastern Atlantic and Gulf shores, beach dunes are viewed foremost as fortifications to protect manmade structures—homes, condos, boardwalks, and the like—and the infrastructure that accompanies beachfront development. That they are, or can be, healthy ecosystems and the haunt of an intricate web of highly adapted life is a secondary consideration at best.
Truth be told, nothing protects beachfront property like a healthy dune system. Even when storms are absent, sand dunes help beaches deal with the everyday stresses of life on the edge. A two-foot-tall wave breaking on a vertical wall exerts more than three times the force of hurricane-strength winds; healthy beaches handle such forces uncountable times each day. During major storm events, ramparts of sand dunes can blunt the force of onshore winds and turn back storm surges. Even as a dune erodes in the face of high water, it does its job. Sand stored in beach dunes washes into sandbars just offshore. The sandbars, in turn, trim the height of the waves racing toward the beach.
But a perspective of sand dunes as scarcely more than a shoreline’s stiff upper lip leads to a lack of respect for the ecological complexity of dunes, and to poor designs for human-built sand dunes constructed to replace or augment the natural variety. “Too often what passes for a beach dune is just a hump of sand pushed up by a bulldozer,” laments Nash, a coastal management specialist with the North Carolina Cooperative Extension, an educational program associated with North Carolina State University. He has joined me in Masonboro’s dune field, happy to find someone else so taken with sea oats as to study them while lying flat on his stomach. “But a sandy hump is not a dune. Pushing sand around might feel good, but it’s a real bad way of thinking.”
The reason, Nash explains, is because coastal dunes exist in a state of equilibrium with the rest of the beach. They are the most visible manifestation of a symbiosis that exists between all parts of the beach—the dunes, dry beach, wet sand below the high-tide line, and underwater sand deposits for hundreds of feet offshore. “You can’t separate one from the other; they are all a part of the same system,” Nash says. “Moving sand from one place to the other is borrowing from Peter to pay Paul. And most of the time, that borrowed sand is going to go right back where it came from. ”
Authentic, natural coastal dunes, on the other hand, migrate and morph and grow with the wind. They are veined with the intricate root webs of plants adapted to this very niche of habitat—sea oats, American beachgrass, bitter panicum, the federally threatened seabeach amaranth. Their slopes are an admixture of beach sand and the organic detritus of annual and perennial vegetation, pocked and aerated with ghost crab tunnels and sand wasp burrows, trellised with the tracks of beach mice and waterbirds. Natural dunes are alive in the way that tidal pools are alive.
A far better alternative to the bulldozer is, in Nash’s words, “to artificially build a natural dune.” Sitting cross-legged, he picks up a handful of sand and quietly lets it stream from his palm. “Now we’re learning how to make something as good as a natural dune,” he says. He pauses for a moment, then adds: “Actually, it is a natural dune. We’re just giving nature a jump start.”
The basic processes that birth and build beach dunes are fairly straightforward. “Essentially, three things are required to make a dune,” Nash explains. “You need a source of sand, wind to move it, and an object to slow down the wind so the sand can start piling up.”
Every natural beach dune begins with a breeze and just a few grains of sand. As wind sweeps the beach, it picks up individual sand grains and whips them into the air-stream a few inches above the beach. Those sand grains don’t stay aloft for long, however. Gravity tugs them back to the beach, and each grain of sand strikes the surface like a minuscule missile. They don’t create much of an impact, but it’s enough to knock another sand grain or two off the beach surface and into the airflow above, where the breeze drives them farther downwind. Eject, fly, crash, eject. The process is called “saltation,” from the Latin root word meaning, “to jump,” “to leap,” or “to dance.” Over and over again, untold millions of times, sand grains are ejected into the wind stream, ride the breeze, then crash-land onto the beach, sending more grains into the wind.
Sand grains larger than one millimeter in diameter are too heavy to move very far in a typical beach breeze. Instead, they’re nudged along by the saltating sand grains working in concert with the wind. When a sand grain falls out of the wind and back to the surface, it may strike another sand grain too large to be knocked into the air. The impact, however, might provide just enough of a shove to get the larger sand grain rolling. Once it’s dislodged, the wind keeps it moving. This bumper-cars process is called “surface creep.”
But sand and wind are just two-thirds of the formula. The third factor is an object to slow the wind and trap the sand. As wind travels over an obstacle, its energy is reduced on the lee side of the object. As wind speed slows, sand grains fall out of the wind stream and into a “sand shadow” on the obstacle’s downwind side. These sand grains pile up to create an even higher windbreak, which siphons more sand from the wind. Sand grain by sand grain, a dune is born.
Among the most effective sand-trapping mechanisms are, not surprisingly, dune plants. The best in the business is that mealtime staple of mice, marsh rabbits, and migrating songbirds: sea oats, Uniola paniculata, the beach plant that is synonymous with dunes throughout the South, from Virginia to Florida and west along the Gulf Coast to Texas. (North of Cape Hatteras, North Carolina, the American beachgrass is the primary dune-forming plant.) Tolerant of wind, abrading sands, and salt spray, sea oats send out large, rhizomic root mats that can spread out for more than 20 feet to create a dune-anchoring web.
What’s more, the growth of a dune that forms around sea oats isn’t limited to the height of a static object such as a sand fence. As the dune grows taller, so, too, do the sea oats. “That’s the really beautiful part of the relationship between sand dunes and sea oats,” Nash explains. “They grow in harmony, dune and dune plant, up and out together.”
In a curious turn of logic, the right kind of dune-inhabiting animal—say, an endangered mouse—can save the very developments that threaten it. Given that these particular beaches—the keys and Gulf Shore barrier islands of Florida and Alabama—are highly sought by human beachgoers, it’s no surprise that these mice are in trouble. Six of the eight known subspecies of beach mice are on the federal list of endangered and threatened species, and one is already extinct.
An isolated form of the oldfield mouse, beach mice have adapted to life in the sand. Like their dune-dependent cohort the ghost crab, their sand-tinted coloration matches the dune environment, lending camouflage from preying foxes and owls. Beach mice forage heavily on sea oats and locate their burrows near clumps of the tall reeds. Tunneling three feet into the dunes, these small mice with furry tails avoid the brutal heat of summer.
What they can’t seem to handle, however, is a high degree of human development, and that is precisely what they’re faced with in their native habitats. The sandy shores of the Florida Keys and the Gulf Coast barrier islands attract heavy beachfront development. That not only destroys the favored frontal dune nesting sites, it introduces free-ranging cats that prey on the hapless mice, which evolved in cat-free environments and haven’t developed avoidance behaviors.
Not surprisingly, beach mice are a source of constant friction between coastal conservationists and developers. But when Hurricane Ivan roared across Gulf Shores, Alabama, in September of 2004, beachfront property owners had to tip their caps to Alabama and Perdido Key beach mice. In areas where development setbacks were in place to protect their critical habitat, intact dunes blunted the storm’s damaging winds and surge. “Thank God for the beach mouse,” University of South Alabama engineering professor Scott Douglass told The Birmingham News. “The developers hate that thing, but it saved their developments.”
On September 14, 2005, a few months after my visit to Masonboro Island, Hurricane Ophelia brushed the North Carolina coast near Cape Lookout. I called Nash a few hours before the storm passed his home on Oak Island. He spoke in breathless spurts, pulling furniture off his front porch in preparation for the storm. On the heels of Hurricane Katrina’s pummeling of the Gulf Coast, Ophelia’s glancing blow of 87 miles-per-hour winds seemed relatively benign. But this “is quite an unusual storm,” Nash told me. “This thing has stalled offshore and has churned out there for 18 hours. It’s just blowing and blowing and blowing.”
At 6 a.m. the next day Nash was in his car, picking his way down the storm-battered coast. Washed-out roads and downed trees forced him far inland. Vast stretches of coastal plain bottomlands were under water.
A year earlier Nash had cobbled together 90 volunteers to plant sea oats in a yawning gap in the dunes fronting the North Carolina Aquarium at Fort Fisher, about 15 miles south of Wilmington. On a single spring day, vacationers, beach property owners, and aquarium personnel hand-planted 15,000 seedlings in bare, flat sand. In a single year, the plants had trapped between two and two and a half feet of sand. “I was tickled pink with that,” he told me. “But after Ophelia’s incredible winds, I wondered if those dunes were still standing. For all I knew, the beach had been scoured clean. Driving down, I was pretty nervous about what I might see.”
I heard from Nash within minutes of his arrival back home. “As I was walking up the shoreline,” he reported, his voice crackling with excitement, “I saw this little fringe of green far up the beach. That’s how I knew the dunes were still standing. When I got there, I could hardly believe my eyes. In a single storm event the dunes had trapped enough sand to grow by another foot to a foot and a half. It was spectacular! In some places the dunes nearly doubled in height.”
Yet Nash knows, as well as any, that the resilience of dunes stands to be tested time and again, by forces as seemingly gentle as the wash of each successive wave or as powerful as single storms that chew through entire hemispheres. This cycle of sand dune growth and destruction, even among the plots he has carefully planted and tended, doesn’t cause him anguish when storm warnings are posted along his neck of the beach.
“It’s a dynamic system,” he says. “It’s supposed to work this way. All day, all night, 24/7, sand is moving along the beach. Whether the sand is in the dune, on the beach, or in an underwater sand bar just offshore isn’t nearly as important as whether we manage our beaches in a way that allows nature to take its course.”
T. Edward Nickens is a freelance writer based in North Carolina and a frequent contributor to Audubon.