Fall 2005

Edge Effects

by Claire Payne

Fragmentation is the principal cause of ecological change in forested urbanizing landscapes, according to The Southern Forest Resource Assessment Summary Report. Editors, David Wear and John Greis, say designing development so some forest connectivity is retained could provide important habitat and other benefits, especially for neotropical migratory birds.

Sustainable forests include associated parts: trees, soil, water, plants, animals, timber, and minerals. These interdependent components are ecological capital. The infrastructure they create includes species biodiversity and human economic health. The ecological effects of fragmentation are most easily discerned at its edge.

Birds, Spiders, and Mammals

Susan Loeb, ecologist and project leader at the Southern Research Station (SRS) Endangered, Threatened, and Sensitive Wildlife and Plants in Southern Forests unit in Clemson, SC, found that early successional small mammals that live in vegetation regenerating after an area has been cleared, such as old-field mice and cotton rats, depend on patch size to maintain abundance and diversity. Populations on smaller patches are more susceptible to local extinction than on patches with larger population densities. Because large clearcuts may fragment latesuccessional forests to unacceptable levels, a balance must be maintained between the number and size of early and late-successional patches to maintain a full complement of species within an area.

Fragmentation effects can’t be generalized across all species—a small rodent might not be able to cross an agricultural landscape, whereas a bird could fly without restriction.

Fragmentation thus results in another selective filter for the distribution of plants and animals, according to SRS scientists John Pye and Vic Rudis, and colleagues. Pye is an ecologist with the SRS Economics of Forest Protection and Management unit in Research Triangle Park, NC. Rudis is a forester with the SRS Forest Inventory and Analysis unit in Knoxville, TN.

Carolina wrens, indigo buntings, hooded warblers, and eastern towhees were more numerous in the small openings and interior edge habitats created when Hurricane Opal in 1995 caused blowdown conditions in the Blue Ridge Mountains. Cathryn Greenberg, ecologist with the Bent Creek Experimental Forest, says, “We suggest that canopy gaps increase bird diversity at a landscape scale by providing habitat patches for some species that require young, secondgrowth forest, and serve as magnets for recruitment and foraging.” These positive effects occurred within a complex forested landscape.

At the Savannah River Site near Aiken, SC, more green tree frogs were found in canopy gaps than in closed canopy forest. The tree frogs benefited from an increase in insects and flies, as well as the early successional vegetation of the edge habitat, according to SRS researchers Scott Horn, James Hanula, Michael Ulyshen, and John Kilgo. Hanula leads the SRS Insects and Diseases unit in Athens, GA, where Horn and Ulyshen conduct research. Kilgo, a wildlife biologist based at the Savannah River Site, works with the SRS Center for Forested Wetlands.

However, Kilgo found that gaps at the Savannah River Site did not result in an increased abundance of arthropods, a primary food source for many birds, including hooded warblers. Kilgo says, “more than half (52 percent) of all spiders collected were greater than 328 feet from a gap edge, suggesting that spiders, like arthropods in general, were negatively impacted by gaps. My results indicate that foliage-dwelling arthropods are negatively affected by group-selection harvest gaps in bottomland hardwood forests during early summer.” Hooded warblers apparently encountered fewer prey and presumably foraged less efficiently near gaps. Hooded warblers foraging for fledglings maintained a constant attack rate, regardless of greater distances between areas of prey abundance. Does foraging faster and farther to maintain the necessary food supply for fledglings exert an energetic burden on warblers? Kilgo says that although the birds have adapted to exploit small canopy gaps within mature forests, research needs to consider the possibility of energy depletion during postbreeding and migration periods.

Cerulean warblers are declining across their breeding range, largely because of the extensive loss and fragmentation of their breeding and wintering habitat, according to Paul Hamel, wildlife biologist at the SRS Center for Bottomland Hardwoods Research in Stoneville, MS. These sky-blue birds winter along the northern Andes Mountains in northern South America and breed throughout much of Eastern North America, but are limited to large patches of mature, deciduous forest for nesting. Although cerulean warblers were formerly among the most abundant breeding warblers in the Ohio and Mississippi River Valleys, their numbers plummeted in the 1900s. The birds are extremely sensitive to deforestation because they nest so high in the forest canopy, and because habitat fragmentation aids brood parasitism by brown-headed cowbirds, as well as nest predation. Also, small fragments exhibit substantially lower prey abundance than larger fragments.

Soil, Roads, and Water

Soil disturbance related to forest fragmentation frequently leads to erosion and sedimentation. Erosion control blankets use stitching and netlike mesh fabrics of various materials (plastic, nylon, twine) to hold materials (straw, coconut husk, jute, wood, etc.). These products provide an organic matrix to retain soil moisture, promote seed germination, and disperse erosioncausing energy from raindrops. Christopher Barton, formerly with the SRS Center for Forested Wetlands in Charleston, SC, used rolled erosion control blankets to help restore 15 degraded Carolina bay wetlands on the Savannah River Site. The blankets were effective for their stated purpose, but were hazardous to snakes. The products’ mesh sizes of 10 and 20 mm2 provided easy entry for the black racer, rat snake, water snake, corn snake, and eastern hognose. Once inside the mesh, the snakes couldn’t escape. Fourteen of the nineteen trapped snakes died while tangled in the mesh, either due to lacerations from twisting and thrashing, overheating, or being unable to escape predators, including fire ants. A smaller mesh size would be safer for snakes and possibly other wildlife.

Roads and vehicular traffic in eastern Texas have depressed populations of the Louisiana pine snake and the timber rattlesnake. Roads with moderate use affect snake density by as much as 50 percent up to a distance of about 500 yards, according to Craig Rudolph, Richard Conner, and Richard Schaefer of the SRS Integrated Management of Wildlife Habitat and Timber Resources unit in Nacogdoches, TX. Seventy-nine percent of the landscape of the Angelina National Forest is within 500 yards from a highway or Forest Service system road corridor. A substantial proportion of the expected snake fauna have been eliminated across the landscape due to road-related mortality.

Fragmented River, Freshwater Mussels

Pearly mussels are among the most endangered animals in fresh waters. These bivalve creatures live in the sediment of rivers, streams, and lakes. Most pearly mussels live from 5 to 6 years to decades; some can live a century. Mussels play important roles in freshwater ecosystems and are economically valuable for their shells and pearls. The Cumberland River basin in Kentucky and Tennessee supported one of the world’s most diverse mussel faunas, including species exclusive to that river system. When a natural or anthropogenic catastrophe occurred, mussels could travel to another tributary to recolonize. When the Cumberland River was dammed in the 1950s, tributaries were isolated and connectivity lost.

Three or four Cumberland River tributaries persisted as mussel refugia in 1985. The Little South Fork in Kentucky and Tennessee was most significant because it supported an intact example of the 26 species unique to the river basin. The Little South Fork also had the largest populations of endangered and imperiled species, according to Wendell Haag, fisheries research biologist, and Mel Warren, fisheries research scientist and team leader for the SRS Forest Hydrology Laboratory in Oxford, MS.

Rapid mussel population declines during the early 1980s were associated with surface mining on the lower section of the river during the late 1960s and 1970s. By the late 1980s and early 1990s, oil extraction activity was implicated in mussel declines on the upper section of the river. Loss of the Little South Fork as a conservation refugium resulted in the loss of populations of at least nine mussels of global conservation significance, including the littlewing pearly mussel and the Cumberland bean.

Nature Hates a Void

Invasive species thrive where fragmentation interrupts forests. Melaleuca, also known as paperbark trees, grow at a voracious pace in sparsely forested south Florida. In the Southcentral States, forest fragment size associated with bottomland hardwoods was directly linked with livestock grazing in small areas and logging in intermediate-sized sections, according to Rudis in 1995. The 1997 Forest Inventory and Analysis survey of Georgia indicated the presence of kudzu, honeysuckle, and privet two to seven times more frequently when edged by nonforest land. “This suggests that conditions at the edge provide the access and light needed to grow and propagate selected nonnative species,” says Rudis.

Fragmentation provides an opportunity for invasion by pests and diseases. Decline disease syndrome is a progressive interaction of abiotic events, such as site, soil type, and climate change, and biological factors or agents that eventually can lead to individual tree death and widespread forest mortality. Decline diseases involving climate may be of particular concern for future southern forests if predictions of extremes in atmospheric temperature and precipitation resulting from increased greenhouse gases hold true. Dan Wilson, Ted Leininger, Bill Otrosina, David Dwinell, and Nathan Schiff report that if there are major systematic changes occurring in the climate, they will likely give rise to more numerous decline-related insect and disease problems. Wilson and Schiff conduct research with project leader Ted Leininger at the SRS Center for Bottomland Hardwoods in Stoneville, MS. Bill Otrosina works with the SRS Biological Foundations of Southern Forest Productivity and Sustainability unit and is stationed in Athens, GA. David Dwinell recently retired from the SRS Insects and Diseases of Southern Forests unit in Athens, GA.

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For more information:
Richard Conner at 936-569-7981 or rconner@fs.fed.us
Cathryn Greenberg at 828-667-5261 or kgreenberg@fs.fed.us
Wendell Haag at 662-234-2744 or whaag@fs.fed.us
Paul Hamel at 662-686-3167 or phamel@fs.fed.us
Jim Hanula at 706-559-4253 or jhanula@fs.fed.us
Scott Horn at 706-559-4249 or shorn01@fs.fed.us
John Kilgo at 803-725-0561 or jkilgo@fs.fed.us
Ted Leininger at 662-686-3178 or tleininger@fs.fed.us
Susan Loeb at 864-656-4865 or sloeb@fs.fed.us
Bill Otrosina at 706-559-4295 or wotrosina@fs.fed.us
John Pye at 919-549-4013 or jpye@fs.fed.us
Vic Rudis at 865-862-2009 vrudis@fs.fed.us
Craig Rudolph at 936-569-7981 or crudolph01@fs.fed.us
Richard Schaefer at 936-569-7981 or rschaefer01@fs.fed.us
Nathan Schiff at 662-686-3175 or nschiff@fs.fed.us
Carl Trettin, for Christopher Barton, at 843-769-7002 or ctrettin@fs.fed.us
Mel Warren at 662-234-2744 or mwarren01@fs.fed.us
Dan Wilson at 662-686-3180 or dwilson02@fs.fed.us