When Hurricanes Hit Habitat

Katrina may have been the harbinger of a windy new world in which global-warming-powered storms threaten wildlife as well as people

08-01-2006 // Roger Di Silvestro

PERHAPS THE MOST frightening aspect of major hurricanes such as Katrina is that storms of that magnitude may be the wave of the future. Warm tropical seas power hurricanes, and studies indicate that in the wake of global warming sea surface temperatures worldwide have increased by up to 1 degree F since the 1970s.

Peter Webster, a climatologist at the Georgia Institute of Technology, and his colleagues published a report earlier this year in the journal Science in which they showed that during the past 35 years the average annual number of category 4 and 5 hurricanes--the two most powerful categories--has nearly doubled worldwide, from 10 in the 1970s to 18 since 1990. This change "most definitely stems from the sea surface temperature increasing from global warming," Webster says.

The news that global warming may be spawning more muscular hurricanes is bad not just for humans but also for wildlife. Although hurricanes can have positive ecological benefits, such as stirring up nutrients in wetlands and bays, they also batter natural habitats.

Consider the Chandeleur Islands, a 60-mile-long crescent-shaped archipelago, most of it within the Breton National Wildlife Refuge, that lies in the Gulf of Mexico about a dozen miles off the coast of Louisiana. Prior to Katrina, the Chandeleurs provided nesting areas for large numbers of birds, says Tommy Michot, a biologist with the U.S. Geological Survey who conducts aerial canvases of Louisiana birds. Flying over the Chandeleurs a year ago, he would have expected to find a wide variety of nesting birds, including the largest known concentration of nesting sandwich terns in the world, an estimated 50,000 to 100,000 birds, and perhaps 3,000 nesting brown pelicans. Seagrass beds sheltered behind the islands, on the mainland or western side, provided feeding and nursery areas for fish, shrimp, crabs, mollusks and a variety of waterfowl, including 10,000 to 20,000 redheads.

Then the eye of Hurricane Katrina passed nearby, rendering the Chandeleurs one of the most ravaged sites in the hurricane's path, drowned under a tidal surge 28 feet deep--well above the island's 20-foot dunes. "All kinds of nesting sites were lost," Michot says. The islands themselves were reduced from 5.64 square miles to 2.5. Then came Hurricane Rita, and they were cut to 2. "A lot of sand was lost to the open bays behind the islands," says Kerry St. Pé, director of Barataria-Terrebonne National Estuary Program in Louisiana. This sand buried much of the seagrass beds, Michot says.

The problem was not limited to the Chandeleurs. Herman Fritz, a civil engineer at the Georgia Institute of Technology who has been studying storm surge, says that barrier islands up to 12 miles off Alabama and Mississippi were inundated. Some were cut in half, and trees suffered from saltwater intrusion.

Scientists do not yet have the data needed to predict how the damage will affect local wildlife, says James Harris, a biologist at the Breton refuge. "Any birds left are going to be more vulnerable to spring surges and tidal events," he says, adding that the birds will probably suffer "some pretty significant declines, which is as exact as I can be right now."

Barrier islands erode and rebuild naturally, and many species that use them, particularly birds, adapt to their destruction by moving to undamaged habitat. But development has altered seashore zones to such an extent that wildlife inhabiting these areas have increasingly fewer alternatives, according to biologists who study coastal habitats. This is particularly true of Louisiana coastal marshes, which were already collapsing even before Katrina. These marshes shrink naturally from erosion and subsidence, but, in the past, silt from rivers restored them. Levees and canals have altered river flow, however, allowing marshes to erode at a rate of 25 square miles per year in recent decades.

Katrina compounded this threat. The storm surge, Harris says, brought salt water into freshwater marshes, which, followed by two months of drought, helped kill trees and reduce food resources for waterfowl by 70 to 80 percent. Most of the food plants are annuals, however, and should recover quickly. Longer lasting is the loss of cypress trees in some areas, such as Bayou Sauvage National Wildlife Refuge. The loss of forest has been extensive, Harris says, and most native trees were affected.

At Jean Lafitte National Historic Park and Preserve, Katrina may have caused permanent damage, says Thomas Doyle, a research ecologist with the National Wetlands Research Center in Lafayette, Louisiana. Partial and complete blow down of hardwood trees has opened wooded areas, allowing sun-loving invasives such as Chinese tallow to gain a stronger foothold. Marsh erosion in the area averages about 20 feet per year, Doyle says, but Katrina took out more than 100 feet in some places--five years of erosion in a matter of days. He and other researchers are still trying to determine if an influx of salt water will kill off cypress forests, transforming them into marshes, which in turn erode into open water.

Forests in other states were also hard hit by the storm. National forests in Mississippi, for example, suffered damage to 50 to 80 percent of their trees, according to a U.S. Forest Service report.

Although it is too early to assess in detail Katrina's damage to natural habitats, the effects of past storms offer some indications. Hurricane Andrew, when passing through Louisiana in 1992, stripped leaves from trees and shrubs and dumped the plant material into wetlands, where it rotted and reduced oxygen in the water. Result: 184 million fish died in Louisiana's Atchafalaya Basin alone. Andrew also wiped out about 25 percent of the state's public oyster seed grounds along with other bottom-dwelling creatures that provide food for birds and other animals.

In South Carolina, Hurricane Hugo in 1989 damaged 4.5 million acres of state forest. Although birds and insects returned to damaged areas soon after the storm, amphibians and reptiles remained significantly scarcer for a longer period. At Francis Marion National Forest, 477 colonies of endangered red-cockaded woodpeckers were reduced to only 100.

Given the recent increase in severe storms and its apparent link to human-generated greenhouse gases, how do we prepare for more powerful hurricanes? In Louisiana, says St. Pé, we might consider restructuring the system of levees and water distribution that stops the delivery of silt to marshes. "We need sediments to restore our wetlands," he says. Otherwise, these habitats, which buffer winds and surge, will disappear.

On a larger scale, Webster suggests, reducing the hurricane threat requires the political will to reduce global warming and to restructure coastal development. It would be more economical to spend $100 billion now on preventative construction along the coasts, he says, than to spend some $200 billion after every massive hurricane. "The category five storms of 2005 were not storms of the century," he says. "There were five of them. That's what you build policy on. Otherwise, you're in denial."

Roger Di Silvestro is a senior editor for this magazine.

Away Goes The Wind

Hurricanes draw power from warm seas. Once they hit land, their winds may lose more than half their speed in about 7 hours, quick enough to reduce 150 m.p.h. winds to below hurricane strength, which is defined as sustained winds of at least 74 m.p.h.

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Hurricanes Batter Some Species

One might reasonably expect that species native to hurricane-prone areas would be adapted to the storms and capable of surviving them. For many species, however, adapting means moving elsewhere, a challenge in today's world, in which many of the "elsewheres" have been developed. The result: stricken animals have few alternatives for new habitat. And as global warming enters the mix, potentially increasing hurricane severity, wildlife's ability to weather the storms becomes less certain.

A key group of species with an uncertain fate, thanks at least in part to shrinking habitat, is wading birds, such as egrets, ibises and herons, that both migrate through and breed in Louisiana. Even before Katrina and Rita, 10 species of wading bird that nest along Louisiana's eroding and fragmenting coast declined, says Tommy Michot, a biologist with the U.S. Geological Survey who conducts aerial canvases of Louisiana birds.

For example, in 1976, Louisiana biologists counted 40,000 breeding pairs of little blue herons, 70,000 pairs of tricolored herons and 9,000 pairs of black-crowned night herons. By 1990, the first two had sunk to 25,000 breeding pairs each and the black-crowned to about 7,000. By 2001, the birds had sunk further, to 8,000 breeding pairs of little blue herons and 1,000 each of tricolored and black-crowned. From 1976 to 2001 (the last year for which numbers are available), great egrets, cattle egrets and snowy egrets dropped by about 30 to 50 percent. Biologists cannot explain the declines. "They could have moved elsewhere, or it may be a true decline," Michot says.

These restricted populations, with their debilitated habitats, make the species more vulnerable to high-powered hurricanes. The birds simply have fewer numbers in reserve, and their eroded habitat gives them fewer areas in which to seek shelter.

Reptiles may rank high among species most vulnerable to hurricanes. In South Carolina, Hurricane Hugo in 1989 damaged 4.5 million acres of state forest. Although birds and insects returned to damaged areas soon after the storm, reptiles, as well as amphibians, remained significantly lower. Gary Hopkins, a biologist at Gulf Shores National Seashore, composed of barrier islands off Louisiana, says, "After Katrina and early this year I saw no reptile activity on the islands." However, he says, as spring progressed, reptiles began to emerge. Alligators, powerful enough to withstand the battering of waves, "are still there," he says.

In Florida, sea turtles are a major reptilian concern. They're vulnerable on their nesting grounds, where storm surge can drown eggs. Because about 80 percent of loggerhead sea turtles nesting in the southeastern United States do so in Florida, loss of eggs there can hit the species hard. In 2003, three weeks of tropical storms and one hurricane wiped out sea turtle production in parts of Florida for the entire year. However, sea turtles are long lived and presumably can adapt to such losses. "Certainly they have a life history that should tolerate hurricanes," says Jeannette Wyneken, a Florida marine biologist. The problem begins, she says, if such losses become more frequent. "Probably losing one isolated year of reproduction is not going to be a big deal, but is one year out of every five going to be a big deal?"--Roger Di Silvestro

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