The news release may also be found online at http://biology.usgs.gov/pr/newsrelease/1998/12-8.html
NOTE TO NEWS EDITORS: Reproducible photos of Puerto Rican parrots, the Chandeleur Islands, and coral reefs may be found at:
Natural systems in the Caribbean and the southeastern United States have been weathering hurricanes for millennia, with forests and species adapted to take advantage of the changing quilt of landscapes left behind by hurricanes. But the impact of these periodic disturbances on plants, animals and ecosystems may be greater now than ever before, according to U.S. Geological Survey scientists who study the biological and environmental legacy of storms like Hurricanes Georges.
Now when large tropical storms course across the Caribbean and the Gulf of Mexico, their damaging effects are often magnified by ongoing, human-caused disturbances to coastal and island ecosystems. Particularly vulnerable are already stressed ecosystems, such as coral reefs, and rare species such as the red-cockaded woodpecker, sea turtles and the highly endangered Puerto Rican parrot, for which the combination of increasing human pressures and the natural cycle of hurricane disturbance can be devastating.
Puerto Rican Parrot
It is still too early to tell what the lasting effects of Hurricane Georges will be on the parrot, which has been the subject of an intensive recovery effort involving scientists from the USDA Forest Service, the U.S. Fish and Wildlife Service, the U.S. Geological Survey, the Puerto Rican Department of Natural Resources and other organizations. When Georges slammed into Puerto Rico’s Caribbean National Forest on September 21, less than 43 individuals of this colorful species remained in the wild. Approximately the same number of birds existed in 1989, when Hurricane Hugo reduced the wild parrot population by half.
Dr. Jaime Collazo, leader of a group from USGS’s North Carolina Cooperative Fish and Wildlife Research Unit, which is providing technical support to the USFWS-led recovery effort, says it now appears that most of the wild parrots survived the storm. "To date, 36 of 40 birds have been counted during preliminary surveys," Collazo says. Two aviaries in Puerto Rico where captive parrot populations are maintained were damaged by the hurricane, but no birds were lost.
Georges was a slow-moving storm that passed lengthwise across the island of Puerto Rico, causing extensive damage to both forests and populated areas. This time, however, the parrots may have been lucky. "Unlike Hugo," Collazo says, "several valleys were spared — relatively speaking. One of these is where most of the breeding activity takes place and where most of the nonbreeding flocks are seen." He adds that many of the tree cavities the parrots use for nesting appear to be intact.
Biologists caution, however, that the detrimental effects of hurricanes are often indirect, and not felt immediately. Dr. J. Michael Meyers, a wildlife research biologist at the USGS Patuxent Wildlife Research Center, who has been involved in the parrot recovery effort, says the experience of Hurricane Hugo demonstrated a variety of ways in which the birds can be affected.
"Food may not be available, and the parrots will have to travel farther," says Meyers. "Cover is less, and so predation on the parrots will be higher." Meyers notes that although the birds bred successfully in the years immediately following Hugo, damage to their preferred species of nesting tree may have resulted in the parrots abandoning a section of their former range a few years later.
If parrots move out of their preferred habitat in the Luquillo Mountains, says Meyers, they will encounter feral species of parrots and parakeets, whose populations are doing well. In fact, says Meyers, some of these introduced bird species use habitat right to the edge of the forest where Puerto Rican parrots occur. "As it is, disease from recently released pet parrots could also be a big problem for the few wild Puerto Rican parrots that remain," Meyers adds.
Once numbering in the tens or even hundreds of thousands, the wild population of Puerto Rican parrots reached an all-time low of 13 in 1975; additional individuals were in captivity. Scientists believe that extensive deforestation in Puerto Rico is the main cause of the decline, along with hunting and the collection of parrots for pets. Heavy logging in newly opened forest areas following an 1899 hurricane reduced the parrot to a remnant population on the eastern end of the island.
Collazo says a planned attempt to increase the wild population of Puerto Rican parrots through the release of birds raised in captivity will probably now be delayed by about a year. In the meantime, biologists will make the most of this opportunity to study how parrots in the wild respond to hurricane damage.
As a "dress rehearsal" for the Puerto Rican parrot reintroduction effort, Collazo has been leading a team of biologists conducting experimental releases of the closely related Hispaniolan parrot in the Dominican Republic. Before the arrival of Georges, the team was tracking 34 Hispaniolan parrots marked with radio transmitters. Three days after the hurricane passed through the Dominican Republic they were able to locate 26 of the birds, and the search continues for others.
"The bottom line," says Collazo, "is that this ’rehearsal’ of ours has provided an opportunity to monitor post-hurricane survival and movement patterns, issues of paramount importance for Puerto Rican parrots, for which we have no direct information." Over the next year other researchers in Puerto Rico will monitor the recovery of the forest and try to assess how habitat changes resulting from the hurricane may affect the ability of wild parrots to elude predators, and to forage and nest successfully.
Other Caribbean Wildlife
Dr. James Wiley, leader of the USGS Grambling Cooperative Wildlife Project in Mississippi, has studied hurricane effects on wildlife in Puerto Rico and on other islands in the Caribbean. In addition to the parrot, Wiley says, people should be concerned about the recent storm’s impact on other threatened wildlife. "In many ways Georges was more damaging to the island’s wildlife and habitat than was Hugo," says Wiley. "This one ripped down the Cordillera Central and over southwestern coast of Puerto Rico, passing through the habitats of plain pigeon and yellow-shouldered blackbird."
Both of these species are critically endangered, with populations numbering in the hundreds. Like the Puerto Rican parrot, the endemic pigeon and blackbird have been reduced from large to remnant populations through loss of habitat and, in the case of the plain pigeon, hunting. Wiley says he has received one report of plain pigeons coming into urban areas to feed on royal palm trees after the hurricane. "That is abnormal behavior, and suggests that food resources in normal foraging areas are a problem," he says.
Both Wiley and Collazo say that a small population size and limited distributional range can make any species extremely vulnerable to chance events such as hurricanes. In general, says Collazo, "hurricanes are important and beneficial environmental events for our forests." Many Caribbean species are adapted to take advantage of the changing patchwork of habitats created by these disturbances. But this evolutionary dynamic is upset when there is no acceptable habitat for individuals driven out of one area to move into, or when there is no larger population to recolonize disturbed regions.
In fact, studies conducted in Puerto Rico by the University of Puerto Rico and the USDA after Hurricane Hugo showed that arboreal lizard species in the upper forest canopy faced increased predation and fewer available food resources because of the reduction of cover and suitable habitat caused by downed trees. The lizards compensated by moving to the lower canopy, but there they faced increased competition from other lizards better adapted to exist in that habitat.
Coral Reefs When Georges swept across the U.S. Virgin Islands, it battered coral reefs in Virgin Islands National Park and Buck Island Reef National Monument. Dr. Caroline Rogers, a USGS research biologist at the Virgin Islands Field Station, who helped survey the reefs following the hurricane, says the damage was less widespread than that caused by Hurricane Hugo. Nevertheless, some reefs were hit hard. Rogers and others found fractured corals, as well as sponges and sea fans ripped from their bases.
The effects of Georges on Florida coral reefs were similar, said Dr. Gene Shinn of the USGS Center for Coastal Geology in St. Petersburg, Fl. Georges broke and reduced to rubble many of the large branching corals, such as elkhorn and staghorn coral, that had died from various diseases over the past 10 years. In addition, those few branching corals still living were also broken and transported over the reef top. "Still," said Shinn, "although the damage was devastating, it was not nearly as severe as the damage inflicted by Donna in 1960 and Betsy in 1965."
Under normal circumstances, say Rogers and Shinn, this kind of disturbance can be advantageous for some species. For example, broken pieces of elkhorn coral can reattach to the bottom and start new colonies. "The main way they reproduce is through fragmentation," says USGS marine biologist Ginger Garrison. But now, the three scientists say, multiple stresses may be preventing normal patterns of recovery from taking place. In recent years, poor land-use practices have increased the flow of sediments into shallow ocean waters, raising nutrient levels and stimulating algal growth throughout the Caribbean. At the same time, overfishing has reduced the numbers of algae-eating fish that would normally keep this growth in check. Because coral regrowth can only occur if sections of the reef surface remain clear of algae, scientists will be watching closely to see if coral larvae will be able to settle on rock surfaces newly scoured by the hurricane. In addition to these stresses, elkhorn coral has suffered catastrophic losses throughout the Caribbean from white band disease, a disease that was first noticed in the late 1970s, and which makes the effects of hurricanes on elkhorn even more devastating. A study done at Buck Island Reef National Monument in St. Croix, for example, showed that a combination of storm damage and white band disease caused the numbers of elkhorn coral in some areas to drop from 85 percent down to 5 percent. Rogers notes that some reef zones have suffered nearly total loss of elkhorn coral.
"Reefs have persisted for thousands of years and can usually regenerate after hurricanes," says Rogers. "However, reefs which are subject to a combination of natural stresses and harmful human activities may not be able to bounce back."
On the Gulf Coast of the U.S., Hurricane Georges caused extensive damage to the Chandeleur chain of barrier islands off eastern Louisiana and western Mississippi. The islands buffer this section of coastline from winds and storm surges, and create a calm-water environment where seagrasses flourish. They are the first line of storm defense for eastern Louisiana, especially New Orleans, and western Mississippi; they buffer the mainland from both the wind and storm surges associated with hurricanes and winter storms.
Dr. Thomas Michot, a research wildlife biologist at the USGS National Wetlands Research Center in Lafayette, La., says the seagrass beds of Chandeleur Sound are a critical resource for many species. "They form the basis of an elaborate food web that supports a highly productive system of organisms," Michot says. The web includes many species of marine snails and bivalves, crustaceans such as shrimp and crabs, numerous species of fish, as well as sea turtles, marine mammals and birds.
Flying over the Chandeleurs shortly after the hurricane, Michot observed dunes and beaches washed away, and large areas completely submerged. Material swept off the islands was carried into Chandeleur Sound, burying seagrass beds. Michot says the extent of the burial will have to be determined from a close analysis of aerial photographs, but he guesses it may be comparable to that caused by Hurricane Camille in 1969. "I think it’s safe to estimate about a 25 percent loss of seagrass beds from Georges," Michot says. "Hopefully, if we can go several years without a major storm, those grass beds will come back."
Meanwhile, some of the 20,000 redhead ducks that frequent the area each winter to feed on shoalgrass (one of five seagrass species that form the beds) may have to find other wintering grounds. Michot says many other bird species will also be affected by the loss, including herons and egrets, shorebirds, gulls and terns, and pelicans.
Damage to the islands themselves may impact species such as the sandwich tern, which uses the isolated beaches for nesting. Biologists will be keeping a close eye on how the birds respond to changes in their nest areas when they return next spring. Michot says the breeding population of sandwich terns in the Chandeleurs "is the largest known concentration in the world, with numbers ranging from 50,000 to 100,000 birds." The colony has been forced to move from island to island in response to past hurricanes.
Nesting sites for brown pelicans on North and South Grand Gossier Islands were also heavily damaged or destroyed by the storm. More than 8,000 nesting pairs of pelicans used the islands in 1998, according to Larry McNease of the Louisiana Department of Wildlife and Fish. Next year, he says, "the majority are probably going to have to find another place to nest," particularly if winter storms further damage the islands.
Dr. Glen Guntenspergen, a research ecologist at the USGS Patuxent Wildlife Research Center, has studied how hurricanes affect Gulf Coast ecosystems. Barrier islands like the Chandeleurs, he says, have experienced continual storm-driven erosion over time. Today, however, the problem is worsening due to a rise in sea-level and altered patterns of sediment deposition. As a consequence of this sea-level rise and the lack of sediment to replenish the islands after storms, scientists predict that the Chandeleurs will disappear within 200 years.
As the islands that protect Louisiana’s coast shrink in area, large hurricanes are having a heavier impact on Louisiana’s threatened coastal wetlands and the organisms that depend on them. For example, says Guntenspergen, "There was a lot of physical damage following Hurricane Andrew in 1992. Salt water from the storm surge was pushed inland, killing some of the above-ground vegetation." In bottomland swamps, millions of fish died when the decay of foliage stripped from trees lowered oxygen levels in the water.
Guntenspergen notes that a mixture of natural and human-induced processes are contributing to the loss of wetlands in the region. "Hurricanes tend to exacerbate these processes," he says. Human-caused changes in patterns of water flow and sediment movement can short-circuit the marshes’ natural mechanisms of recovery from hurricane disturbance.
When Hurricane Andrew swept through Louisiana’s Atchafalaya Basin it ravaged some of the last remaining bottomland hardwood forest in the lower Mississippi floodplain. Guntenspergen says species such as cottonwood and willow, which tend to dominate in areas where flood-control measures have resulted in high sedimentation rates, proved particularly susceptible to storm damage.
Ecologist Dr. Bob Keeland of the USGS National Wetlands Research Center also studied the long-term effects of Andrew on lowland and wetland forests. He found that many trees that are extensively damaged by hurricanes remain alive; some recover, and some eventually die. Surprisingly, tree mortality was highest from one to two years after the hurricane.
Overall, trees near the edges of forest habitats fared the worst. Keeland notes that as wooded areas are encroached upon and subdivided for other uses, more vulnerable "edge habitat" is created. "The patchwork nature of agricultural fields interspersed with mature forest results in more damage to the vegetation," he says.
In forests, as in marshes, the dynamics of recovery from hurricane damage can be altered by other kinds of changes to the landscape. One potentially important factor in determining what a hurricane-hit area will look like years later is the presence of non-native species of vegetation. Many exotic plant species have the ability to rapidly colonize disturbed areas, and out-compete slower-growing native trees and plants. Guntenspergen and Keeland say it appears that non-native trees such as Chinese tallow may be spreading in areas heavily impacted by Andrew.
Dr. Thomas Doyle, an ecologist at the USGS National Wetlands Research Center, uses historical climatological and biological data to build computer models showing the likely effects of hurricanes under different sets of conditions. Among his model habitats are montane rain forests in the Caribbean. His simulations show that while hurricanes may be destructive, these systems would be impoverished without them.
"Biodiversity is highest under some moderate recurrence of hurricanes," he says, "but it declines in the absence of hurricanes, or when they become too frequent and severe." While good data on hurricane characteristics before around 1900 do not exist, Doyle has found evidence in the size distribution of south Florida mangroves suggesting that hurricane frequency and intensity may have been less in the 19th century than in recent decades.
Doyle says that while scientists have no sure way of predicting hurricane patterns in the future, they do know that hurricanes will have a greater harmful impact on more species and ecosystems as long as current land-use trends continue to fragment habitats and disrupt natural processes. "Parceling conservation areas into smaller non-contiguous land units makes them more susceptible to greater hurricane damage," says Doyle. Rare and narrowly distributed species like the Puerto Rican parrot are already experiencing the implications of this basic ecological lesson.
The USGS serves the nation by providing reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.
**** www.usgs.gov ****
Links and contacts within this release are valid at the time of publication.