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5.3 Effects of Agricultural Land Use on Forest Wildlife

5.3.1 Effects of agricultural/urban interfaces on forest wildlife

For more information about the effects of fragmentation on forest birds, mammals, and herpetofauna, see the previous discussions in Section 5.2.1, Section 5.2.2, Section 5.2.3, Section 5.2.4 and Chapter TERRA-1.


5.3.1.1 Forest wildlife densities and movement along the forest/agricultural edge

Forest wildlife species differ in their responses to forest/agricultural edges. Some wildlife species are limited to forest interior habitats, and avoid edges. Other wildlife species are adapted to edges and forest openings, or may be attracted to special habitats created at forest/agricultural interfaces. Small mammal species exhibited differing responses at forest/field edges (or forest wildlife openings) (Manson and others 1999, Menzel and others 1999, Wegner and Merriam 1979). Increased numbers of mammalian nest predators were found along forest-field edges (Gates and Gysel 1978), higher densities of mammalian predators were found in floodplain forests adjoining residential and agricultural land (Cubbedge and Nilon 1993), and raccoons were found to be more abundant in forest edges adjacent to agricultural fields and streams (Dijak and Thompson 2000). In contrast, Heske (1995) found no differences in the abundance of furbearing and small mammals along forest/farm edges versus forest interiors in southern Illinois.


5.3.1.2 Nest predation of forest nesting birds adjacent to agricultural areas

For information about the effects of small forest fragments and forest edges on the success of forest-nesting birds see Section 5.2.1.2


Some avian species in forests near agricultural areas have reduced nest success rates. Rates of nest predation for songbirds were ubiquitously high in a study site bordering agricultural fields. Mammalian predators (especially raccoons) were abundant throughout the study site and present on all transects surveyed (Heske and others 1999). Similarly, higher predation rates for ground nests were documented in forests fragmented by agricultural land, due to more abundant avian predators (Huhta and others 1996). Increased numbers of nest predators (crows and blue jays) were noted during bird surveys in the Great Smoky Mountain National Park (Wilcove 1988). Apparently, agricultural and other land conversions outside the park boundaries caused an increase of these nest predators, even in this large, relatively contiguous forest area.


5.3.2 Agricultural areas as habitat for forest wildlife

5.3.2.1 Early successional species

Many bird species dependent on open habitats such as grasslands, prairies, savannas, glades and barrens are now in serious decline in the Eastern United States (Hunter and others 2001). Today, many of these early successional and disturbance-dependent species are found associated with active and abandoned farmland, pastures, and other man-made forest clearings. Prior to European settlement, these species were found in naturally occurring and Native-American-maintained forest openings. Many of these disturbance-maintained ecosystems have been lost from the landscape during the last 300 years. Some species dependent on them found suitable nesting habitat in man-made fields following loss of the natural openings. Populations of disturbance-dependent birds and other wildlife vary along with trends in agriculture. Conversions of pastures to more intensively cultivated row crops or intensively mowed, fescue dominated pastures, the maturing of abandoned farm fields in some areas of the South, and the trend to larger fields of cash crops with accompanying loss of fence-row habitat have all affected early successional species. Information from the 1997 National Resource Inventory indicates that the 13 Southern States lost about 2.2 million acres of pasture between 1992 and 1997, a net loss of about 3.4 percent (USDA Natural Resources Conservation, 2000). "These species are in trouble not only because of the intensification of farming and declining numbers of pastures, hay meadows, and abandoned fields, but also suppression of natural disturbances - fires, beaver activity, and floods - that generate natural grasslands and shrublands" (Askins 2001).


The introduction of exotic, cool-season pasture grasses was probably in response to overgrazing of native warm-season species and deteriorating range conditions (Twedt and others in press). Use of "improved" cultivars such as tall fescue, red fescue, Bermuda grass, weeping love grass, and many others began in the mid-1930s. Exotic grasses such as tall fescue can be grazed quite close to the ground and can be hayed during the mid-nesting season of many grassland bird species. Depending on their management, intensively grazed or frequently mowed fescue pastures offer little or no cover for wildlife and can be poor habitat for northern bobwhite (Barnes and others 1995) and other grassland species.


Eastern cottontail populations were found to remain highest in areas with relatively high amounts of pasture, stable woodlands, hayfields, and fields planted in small grains, such as wheat, oats and barley (Mankin and Warner 1999). The presence of pasture seemed to be the most important factor, however. In contrast, increases in row crops such as corn and soybeans were accompanied by declines in cottontails. Pasture environments apparently maintained cottontail abundance because they are closest to their preferred vegetation structure (old fields and early successional shrub lands). Similarly, landscape features such as percentage of woodland on farms, percentage of farmland in nonrow crops, percentage of land in soil-protecting crops, and percentage of land in conservation tillage were used to calculate habitat indices (Ribic and others 1998). These indices are important in determining areas likely to support high populations of northern bobwhites and cottontails. Indices indicating farming disturbance such as percentage of land under grazing and percentage of land on which fertilizers, pesticides, and herbicides were applied were associated with lower populations.


5.3.2.2 Importance of vegetated fencerows, hedgerows, and wooded corridors

The presence of woody fencerows in agricultural areas provides important habitat for many wildlife species (Bolen and Robinson 1995). In areas where agriculture constitutes a majority of the land use, fencerows with a continuous row of trees and shrubs can provide habitat for up to 36 species of birds per 6.2-mile segment, whereas fencerows without woody vegetation support 9 or fewer species over the same distance. Forest edges bordered by multiflora rose hedgerows had higher bird species diversity than open forest edges, but habitat generalists and forest-edge species provided most of the increased bird diversity (Morgan and Gates 1982). Forest edges with hedgerows had more cover in the first 6 feet above ground level than open forest edges, and retained more of this cover during the winter. In addition, cottontails were also more frequent in forest edges where hedgerows were present compared to open forest edges (Morgan and Gates 1983). Similarly, farmstead shelterbelts were documented to be valuable habitat for small mammals in agricultural areas (Yahner 1983).


Vegetated fencerows may be important for the movement of some wildlife species, allowing them to reach isolated forest patches across a matrix of open agricultural fields. Chipmunks and white-footed mice tend to move between wooded habitats down vegetated fencerows rather than crossing open fields (Wegner and Merriam 1979). Similarly, many forest-nesting bird species move from one wooded habitat to the next along vegetated fencerows rather than flying directly across open fields. Even when woodland birds such as eastern pewee, red-eyed vireo, and wood warblers foraged in open fields, they first moved from the woods down fencerows, then from fencerows into the open fields. MacClintock and others (1977) documented that a narrow, disturbed "corridor" of grazed woods and early second-growth forest could reduce the isolation of a forest patch, allowing it to maintain a high diversity of forest-nesting birds.


Fencerows in agricultural areas may have negative effects on some species, however. Nest survival for loggerhead shrikes in fencerows was documented to be lower than for those nesting in the adjoining pastures due to higher nest predation (Yosef 1994). Most of the potential nest predators observed during the study either flew or walked along fencelines, and appeared to avoid crossing open pastures. Similarly, area-sensitive grassland bird species avoided nesting in grassy pastures within the first 165 feet of wooded fencerows (O'Leary and Nyberg 2000). Sensitive grassland nesters included two conservation priority species -- Henslow's sparrow and bobolink.


5.3.2.3 Foraging habitat for forest wildlife

Agricultural areas including grain fields, pastures, fruit orchards, gardens, and vineyards are important forage areas for many wildlife species (Martin and others 1951). Not all forage use of agricultural land by wildlife results in damage to crops. Foraging by insectivorous birds and mammals and consumption of weed seeds by wildlife is beneficial to agriculture. Wildlife often consume waste grain left behind by mechanical harvesting machines or consume fruit that has fallen on the ground. In other cases, however, loss and damage to crops by wildlife have been clearly documented. Martin and others (1951) documented the value of several agricultural commodities for wildlife. Corn is consumed by over 100 species of wildlife, including 17 species of upland gamebirds, 59 species of songbirds, 10 species of fir and game mammals, 6 species of small mammals, and 3 species of hoofed browsers. Wheat is consumed by more than 94 species of wildlife, and oats are consumed by at least 91 different species. Rice and apples are other important agricultural commodities eaten by foraging wildlife in the South.


Fallow fields were the most common habitat selected by bobwhite, even though crop fields, wildlife management plots planted annually in small grain and woods managed by prescribed burning were available nearby (Yates and others 1995). Apparently, insects were the most important food resource for feeding bobwhite hatchlings. Insect sampling revealed that fallow fields had more insects than other available habitats.


Black bears in the Southeast feed more in agricultural areas than in other parts of the United States, but their use of these areas may increase their vulnerability to hunting, lowering the overall rates of survival especially for males (Hellgren and Vaughn 1994). In coastal North Carolina, corn crop damage by black bears amounted to about 0.6 percent of the total area surveyed (Maddrey and Pelton 1995). Most of the damage was within 165 feet of the forest edge. In questionnaires completed by coastal North Carolina farmers, deer were the major cause of crop depredation (Maddrey and Pelton 1995). Crop damage by black bears, birds, and raccoons was reported less frequently.


Raccoons frequently use agricultural areas for foraging. One study found that raccoons in an agricultural area foraged mainly on corn which accounted for up to 76.2 percent of their diet (Sonenshine and Winslow 1972). Coyotes were found to be well adapted to agricultural areas in Vermont (Person and Hirth 1991). They preferred hardwood forests in the winter and spring, and farmland during the summer and fall.


Great horned owls are habitat generalists that prefer open cropland and pastures for foraging (Morrell and Yahner 1994). Barn owls also prefer to forage in pastures and grass-dominated agricultural areas (Bolen and Robinson 1995).


Wintering flocks of grackels, red-winged blackbirds, starlings, and brown-headed cowbirds use fields and feedlots for foraging. One such wintering flock removed 1,300 to 7,000 tons of corn each winter from a total foraging range of about 541,000 acres (White and others 1985). In a control measure, over 1 million birds were killed with the surfactant PA-14 one winter. Recruitment of birds from surrounding areas caused the roost to return to prekill levels within about 2 weeks. Roost fidelity for such wintering flocks averages only 3.5 to 4.4 nights per individual. Thus, the daily population turnover rate for the roost is about 23 percent.


5.3.2.4 Hazards of agriculture to wildlife

Although agricultural areas are habitat for many wildlife species, they can also subject them to hazards not encountered in natural areas. Mowing equipment and nighttime mowing has increased the mortality of eastern cottontails, bobwhite, and other wildlife attracted to pastures and hayfields (Bolen and Robinson 1995).


Many wildlife species forage in agricultural fields, but crop losses have resulted in lethal and nonlethal depredation control measures (Bolen and Robinson 1995). Under some conditions, certain crops may be harmful to wildlife. Geese that consume dry soybeans may harm or obstruct their esophagi as the swelling soybeans cause hemorrhaging and necrosis, or prevent the passage of food to the stomach. Aspergillosis is a fungal infection of the respiratory tract, contracted by birds exposed to molding crops. Once contracted, the infection can be spread to other birds, causing sizable die-offs.


Wildlife living and foraging in agricultural areas are exposed to insecticides, herbicides and fertilizers (Bolen and Robinson 1995). Many insecticides are not species-specific, and can be lethal to wildlife through direct exposure or through ingestion of contaminated prey species. Some of the more toxic pesticides, including the chlorinated hydrocarbons DDT, Aldrin, and others, are now banned in the United States, but because of long residual times and heavy pesticide buildups it has taken some time for their deleterious effects to fade. Most herbicides approved for use today are not directly toxic to forest wildlife if applied correctly. Indiscriminant use can indirectly harm wildlife, however, by reducing important vegetation for cover and forage. Fertilizers in granular form can resemble seeds or grit and offer a potential hazard to birds that might ingest a large number of granules.


5.3.3 Old field successional areas

Some areas of the South are likely to experience a reduction in agricultural land uses with a subsequent return to forest habitat (see Chapter SOCIO-1 for more information). Many of these increases in forest acres will undoubtedly be in the form of pine plantations rather than natural forest types, however (see Chapter TIMBR-1). See Chapter TERRA-4 for a discussion of the influence of pine plantations on forest wildlife and habitats.


Abandoned agricultural land undergoes a series of vegetation changes that provide important habitat for a number of wildlife species. The return to old-field habitat benefits many disturbance-dependent bird species. Successful management for many of these rare and declining birds will require adequate space for area-sensitive species, connecting corridors between early successional habitat areas, and availability of areas in specific vegetation stages to offset natural plant succession (Hunter and others 2001). Breeding bird density and species composition shift as abandoned farm fields undergo natural vegetative succession to mature forests (Johnston and Odum 1956). A few species, such as the cardinal, persist through many plant successive stages, but most birds appear to have a definite range of vegetative stages. Browsing mammals such as deer also benefit as abandoned agricultural areas undergo the vegetative transition into scrub-shrub habitats (Adams 1994).


Old-field habitats can vary in vegetative structure. The presence of exotic vegetation in agricultural environments is an influence that persists long after fields are abandoned. Previous type of agricultural use can influence the vegetative structure, and hence the wildlife habitat, in a particular abandoned field. Abandoned pastures differed markedly in their vegetation compared to previously cultivated old fields (Stover and Marks 1998). Exotic herbaceous plants in an old-field environment reached their peak abundance within 65 feet of the forest edge (Meiners and Pickett 1999).


Restored bottomland hardwood forests failed to regain their wildlife habitat value relative to mature forests even 50 years after agricultural usage (Shear and others 1996). Although the regenerating forests had similar structural attributes to the uncut forests, the lack of heavy seeded, mast-producing tree species (oaks and hickories) made them generally less useful for mast-dependent forest wildlife. Conversely, bottomland hardwood reforestation efforts that rely solely on oak planting are slow to produce a substantial 3-dimensional forest that provides useful habitat for nongame species, including many neotropical migrants (Twedt and Portwood 1997). More naturally invading species became established in bottomland hardwood restoration areas sown with acorns than in areas planted with oak seedlings (Twedt and Wilson in press).


5.3.4 Other general effects of agriculture on forest wildlife

Agricultural land uses have resulted in fire suppression and interruption of presettlement forest fire patterns. Lack of fire in most forest habitats has greatly affected the quality of wildlife habitat. For more information on the effects of fire suppression and prescribed burning, see Chapter TERRA-4.


Agricultural disturbance permitted introduction of a great many exotic plant and animal species. See Section 5.1 of this Chapter for information about the impacts of exotic plant and animal species on forest wildlife.


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