Characterization of the Ashepoo-Combahee-Edisto (ACE) Basin, South Carolina
The ACE Basin study area has an extremely rich bird life. Over half of the species of birds that occur in North America inhabit the 320,000 ha (790,000 ac) of the ACE Basin study area. There are about 8,600 species of birds in the world divided into 28 orders. Of these, approximately 280 species of birds in 17 orders occur in the ACE Basin study area. Many of these birds migrate in tremendous numbers to South Carolina from northern breeding grounds to spend their winters or to rest before continuing their migration to more southern areas. The ACE Basin is well known for its important bird breeding habitats, in particular bald eagles, least terns, and wood storks, three species listed as threatened or endangered by state or federal authorities.
An estimated 30 species of birds utilize the coastal marine subtidal habitat as feeding grounds. Eight of these species are common year-round residents and seven are common in winter. The majority of birds in this habitat are piscivorous and catch fish by aerially diving (e.g., terns, gannets); surface diving (e.g., loons, double-crested cormorants); or surface skimming (skimmers, gull-billed terns). Two species (e.g., Wilson's petrel and northern phalarope) feed on nearshore zooplankton while diving ducks consume benthic organisms (Sandifer et al. 1980). The estimated 44 species of birds that utilize the coastal marine beaches can be divided into three broad categories: (1) marine species such as the royal terns and black skimmers that feed on fish and use the beaches only for resting or breeding; (2) macrobenthic predator species (e.g., sanderlings, American oystercatchers, plovers, and sandpipers) that hunt in the sand for annelids, crustaceans, and mollusks; and (3) beach scavengers such as gulls, crows, boat-tailed grackles, and vultures that feed mostly on dead animal matter that litters the beaches (Sandifer et al. 1980).
Several species of colonial waterbirds (e.g., brown pelicans, laughing gull, sandwich terns, and black skimmers) that feed in the coastal marine habitats nest on bird keys. Bird keys and banks are small isolated islands that usually occur in tidal inlets and broad bays. They are very dynamic habitats because they are susceptible to overwash by storm action and spring tides, and they tend to migrate in response to inlet morphology (Sandifer et al. 1980). Forty-two percent of all colonial waterbird nests counted during 1969 were on Deveaux Bank, the only bird key in the ACE Basin.
The maritime communities (dune, maritime shrub thickets, maritime forests) in the ACE Basin are harsh environments with many stressful physical attributes (e.g., blowing sand, high summer temperature, limited freshwater, and sparse vegetation). In the dune community, the avifauna community is primarily comprised of granivores (grain eaters) and insectivores (insect eaters). Sea oats seeds make up the bulk of the diet for eleven species of granivorous birds such as doves, blackbirds, sparrows, and cardinals. Insectivores (e.g., nighthawk, swallows, chimney swift, and warblers) are the next largest group of bird in the dune habitat. These birds eat a variety of insects including flying ants, mosquitoes, beetles, and gnats. The lack of an understory and the low herbaceous plant density in the maritime shrub thicket community provide little food for granivorous and herbivorous species. Most species in this habitat consume insects (e.g., kingbird, yellow-throat, and sparrow hawk). The maritime forests provide more diverse habitats than the coastal marine and other maritime communities; therefore, they contain a more diverse avian community. Of the 280 birds found in the ACE Basin study area, almost one-third (87) can be found in the maritime forests. Many of the birds in the maritime forests are passerine birds including flycatchers, swallows, crows, nuthatches, wrens, kinglets, thrushes, vireos, warblers, sparrows, blackbirds, grackles, and finches. The painted bunting, a common summer resident, is probably the most visually spectacular bird found in this habitat. Insects make up all or part of the diet of most passerine birds, although grains and fruits also are important. Warblers, swallows, vireos, and flycatchers feed almost exclusively on insects, while sparrows, buntings, and finches feed mainly on vegetarian matter such as fruits, seeds, and grains (Sprunt and Chamberland 1970).
The estuarine subtidal habitat is an open water system used mainly by birds for resting and feeding. All of the birds found in this habitat are waterbirds which feed on fish, benthos, carrion, or insects. Terns, cormorants, and brown pelicans inhabit open water areas to feed on fish. Three species of gulls (laughing, herring, and ring-billed) are important in the consumption of dead animal matter. The osprey is the only bird of prey to utilize this habitat where it feeds on its primary prey, fish. The estuarine intertidal areas provide feeding, nesting, and resting habitats for approximately 87 species of birds. Wading birds such as herons and egrets use intertidal creek habitat for feeding on their primary prey, which includes mummichogs, mullet, menhaden, and penaeid shrimp. Other birds such as rails, swallows, wrens, and blackbirds use the salt marsh habitat as feeding and nesting grounds. The clapper rail is a strict inhabitant of ACE Basin salt marshes. This species feeds, roosts, nests, and raises its young on the Spartina marsh (Sandifer et al. 1980). Eight species of herons and egrets utilize the estuarine intertidal flats as feeding grounds with the great egret, snowy egret, and tricolored (Louisiana) heron being the most abundant. Many of the shorebirds such as the American oystercatcher feed extensively in the estuarine habitat types but breed in others such as beaches and bird keys.
The palustrine forested wetlands has the highest avian diversity of any environment in the ACE Basin study area; these wetlands provide an estimated 132 species of birds a wide variety of habitat types. The occurrence of wet and dry tree species and both grassland as well as closed canopy sites contribute to a high diversity of birds (Sandifer et al. 1980). Of the 132 bird species, 34 are year-round residents and 23 are winter residents. Ten species of birds, seven of which are warblers, are closely associated with forested wetlands. The seven species (blue-winged warbler, golden-winged warbler, Tennessee warbler, Swainson's warbler, black-throated warbler, gray-cheeked thrush, Louisiana waterthrush, and worm-eating warbler) feed on the large number of insects that occur in this habitat; however, these birds are rare or uncommon in the ACE Basin study area. Palustrine forests are also important nesting grounds for wading birds such as herons, ibises, and egrets. Nesting populations of wood storks have steadily increased since 1981 and in 1997, 653 wood stork nests were identified in the ACE Basin study area.
Ninety-two of the 280 avian species in the ACE Basin study area are estimated to occur within non-forested wetlands. These wetlands provide nesting grounds for a variety of birds including gallinules, wrens, swallows, red-winged blackbirds, and king rails. Wading birds such as herons, egrets, and ibises use freshwater wetlands as feeding grounds. Waterfowl are abundant in this habitat because the freshwater vegetation is often preferred over salt marsh vegetation for food. They eat the seeds of water lilies, pondweed, water milfoil, and widgeon grass along with mollusks, crustaceans, and insects. The osprey is the only bird of prey to utilize this habitat where it feeds primarily on fish.
The riverine system as discussed here is limited to the open water areas of rivers and does not include adjacent wetland areas. The avifauna of the riverine systems of the ACE Basin study area is made up of species that occur in other habitats and use the rivers for feeding or resting (Sandifer et al. 1980). Species found here forage in the rivers for aquatic plants or animals. About eleven species of ducks use the river to forage for aquatic vegetation such as pondweeds, wigeon grass, wild rice, eelgrass, and marsh grass. Grebes and wading birds hunt for fish either by diving (grebes) or by fishing from shore (wading birds). Shorebirds such as sandpipers and plovers also fish from shore in the rivers for crustaceans, mollusks, fish, and aquatic insects while gulls and terns forage on the rivers for similar prey. The osprey is the only bird of prey to utilize this habitat extensively. The osprey not only hunts in the riverine waters for fish but it also commonly nests on dead snags, channel markers, and power line poles in rivers.
Impoundments are estuarine or freshwater wetlands which have been diked to create managed bodies of water. Most impoundments in the ACE Basin study area are managed for waterfowl and are characterized by brackish or freshwater vegetation. The waterfowl in impoundments consists of geese (Canada geese); puddle ducks (e.g., mallards, teals, gadwalls, wigeons, and shovelers); and diving ducks (e.g., ring-necked ducks, buffleheads, mergansers, and ruddy ducks). Puddle ducks are the most abundant group of waterfowl. These species preferentially feed on wild rice, spikerush, pondweeds, smartweeds, bulrushes, and wigeon grasses and, therefore, many impoundments are managed for these plant species. Wading birds and shorebirds, as well as birds of prey (e.g., bald eagle and osprey) utilize this habitat type.
Pine-hardwood forests in the ACE Basin study area have more bird species than the other upland communities. These mixed upland forests have extensive subcanopy and understory growth that greatly augments the habitat types available and, therefore, more birds can be found here. The abundance of nuts, seeds, and insects provides ample food for a variety of granivores (grain eaters), insectivores (insect eaters), and herbivores (plant eaters). The avian fauna in pine forests of the ACE Basin study area is less diverse because of the lack of dense understory vegetation. Fifty-two species of birds are estimated to occur in pine forests and almost half are considered common year-round residents. Insect-eaters, generalists, and seed-eaters are represented by warblers, bobwhites, and the brown-headed nuthatch, respectively. Woodpeckers are abundant in this habitat, with the red-bellied being most abundant. Seven birds of prey can be found in pine forests. The screech-owl, which often builds its nest in woodpecker holes, is a dominant owl species in the pine forest habitat (Sandifer et al. 1980). Seventy-four birds are estimated to occur in old-field habitats of the ACE Basin study area. Diversity and densities of birds tend to be low in newly abandoned farmlands because these areas lack a shrub layer. The edge community supports a high diversity and density of avifauna. Many of the 74 birds that are estimated to occur in old-field habitats fulfill part or all of their dietary needs from the seeds, grains, and fruits. Others (e.g., Carolina wren, common yellowthroat, brown thrasher, and eastern meadowlark) consume the insects that are feeding in this habitat.
Birds are rarely restricted to one environment and are often found in a variety of habitats (Potter et al. 1980). Typically, they rely on the resources of several habitats for their survival. For example, a bald eagle pair builds its nest in a wooded area, but to fulfill the daily requirement of their young, the nest is located within one mile of large bodies of water such as impounded marshes. Adult wild turkeys occur in a variety of habitat types, preferring mixed pine/hardwood stands interspersed with open areas. Yet the daily water requirement of the young necessitates that the nest site be located within 0.4 km (0.25 mi) of a water source. Thus, the most serious conservation issue for birds of the ACE Basin study area is loss of habitat diversity.
The loss of bird diversity would impact humans because of the wide range of ecological roles filled by birds. The insectivorous birds consume large quantities of insects each day. For example, the pine warbler forages for insects common to upland forests; the chimney swift feeds over the river, catching beetles, flies, and ants; while the ubiquitous Carolina wren feed on insects found in a variety of plant communities (Bent 1964; Sprunt and Chamberlain 1970). The populations of nuisance animals (e.g., rodents and rabbits) are regulated by the raptor species such as hawks, eagles, and owls. Scavengers such as vultures and gulls play an important role in removal of dead animals from all habitats and the recycling of nutrients.