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12.1 Introduction

Even as the amount of early successional habitats in the central hardwood forests of the USA (McNab, Chap. 2; Fig. 2.1) diminishes, some even-aged forest management practices (e.g. clearcutting) used to promote disturbance-dependent species are declining (see Loftis et al., Chap. 5; Shifley and Thompson, Chap. 6). Implementation of less intensive silvicultural systems in their stead, combined with “clean” agricultural practices that discourage early successional growth in weedy, ruderal areas, is ­resulting in loss of early successional habitats (Confer 1992; LeGrand and Schneider 1992; Whitman and Hunter 1992; Trani et al. 2001). Despite conservation efforts to include early successional habitats and promote habitat diversity, these management trends and habitat losses are unlikely to be reversed in the foreseeable future. In this chapter, we explore novel means for creating early successional habitats and managing the disturbance-dependent species that require them. Specifically, we focus on utility rights-of-way (hereafter referred to as ROW).

Utility rights-of-way are ubiquitous anthropogenic landscape features that occupy millions of hectares in the USA. They lie across a diversity of landscapes within the Central Hardwood Region. Because utility companies must maintain transmission line ROWs to deliver electricity, natural gas, or other products and services, they manage these linear landscape features to keep them clear of obstacles and hazards that could interfere with construction, operation, and maintenance of facilities. Efforts to keep trees from growing into ROWs result in large expanses of land that are managed in a state of “perpetual” early succession, from grass-dominated areas to shrub-scrub habitats.

Although the primary function of ROW is to distribute service, these areas increasingly are being enhanced as early successional wildlife habitats. Extensive early work compiled by Lancia and McConnell (1976), Arner (1977), Bramble, and a host of co-workers (1972, 1979a, b, 1985, 1986, 1990, 1991) forms a basis for our understanding potential benefits of ROW for wildlife. These and several other investigators have shown that early successional habitats within ROW are used by vertebrate species such as songbirds (Hanowski et al. 1993; Confer and Pascoe 2003), game birds (Arner et al. 1993), raptors (Denoncour and Olson 1984; Bridges and Lopez 1993), white-tailed deer (Odocoileus virginiana) (Harlow 1991; Harlow et al. 1993), small mammals (Cavanaugh et al. 1976; Lauzon et al. 2002) and even sensitive, threatened or endangered species (Lowell and Lounsberry 2002; McLoughlin 2002; Thomas 2002). Birds are perhaps the most thoroughly studied wildlife that use ROW habitats. Highly charismatic organisms with important ecological functions, they are more easily observed than other vertebrate wildlife and the potential benefits of managing for them carries significant “green” capitol for industrial landowners.

Unlike birds, insects are rarely the focus of landscape-scale management activities that will promote their proliferation. The potential exceptions are butterflies (Lepidoptera). Arguably the most charismatic and publicly accepted insects, butterflies are critical pollinators and prey in temperate terrestrial ecosystems. Many species require features of early successional habitats (e.g. openness, abundant sunlight, flowering plants, bare ground) that occur in many ROW. Like birds, butterflies are often viewed as indicators of ecological health (Ries et al. 2001). Managing for these insects therefore also carries benefits for industries and natural resource managers striving to conserve biodiversity at all levels.

Many bird species, especially passerines, are inextricably linked to insects, especially butterflies, because Lepidoteran larvae (caterpillars) can make up much of their diet during breeding and migratory periods. As many songbirds and butterflies share requirements for early successional habitats, we will link the two groups in this chapter as we present the case for considering ROW as an option for managing some early successional species. In the case of birds, we present a brief review of the state of knowledge regarding avian use of ROW, including species diversity, ecological costs and benefits, and some potential management implications. For butterflies, there is scant literature for review, so we present a case study that provides compelling evidence for considering ROW as opportunities for managing this important, but frequently overlooked, group.

12.2 Songbirds and Rights-of-Way Management

Over the past several decades, many early successional (scrub/shrub, shrubland) songbird populations have declined (Robbins et al. 1989; Petit et al. 1995; Hagan et al. 1992; Hussell et al. 1992; Hunter et al. 2001; Askins 1994, 1998, 2000). The National Audubon Society lists several early successional, shrub-scrub birds among its highest priority species for conservation attention (http://www.audubon.org/bird/stateofthebirds/shrublands.htm, April 1, 2010). As habitat loss due to chronic conversion and aforestation can be a primary factor in these declines (Askins 1998, 2000), areas managed intensively to maintain vegetation in a state of arrested succession could contribute significantly to conservation of shrubland birds.

12.2.1 Songbird Species Diversity in Rights-of-Way

Most studies investigating shrub-scrub bird use of powerline ROW have been conducted in the northeastern USA (e.g. Bramble et al. 1992, 1994; Confer et al. 1998; Confer 2002; Marshall et al. 2002). The list of species using these habitats is impressively diverse (Table 12.1). It includes species ranked high on conservation prioritization lists such as the 2007 Audubon Society Watch List (http://web1.audubon.org/science/species/watchlist/browseWatchlist.php) and species with declining state or region-wide population trends (see Sauer et al. 1999). Several species of conservation concern, such as the Golden-winged Warbler (Vermivora chrysoptera), Blue-winged Warbler (V. pinus), Prairie Warbler (Dendroica discolor), Chestnut-sided Warbler (D. pensylvanica), Black-and-white Warbler (Mniotilta varia), Field Sparrow (Spizella pusilla), and Eastern Towhee (Pipilo erythropthalmus), are listed frequently in the ROW and bird literature as breeders in utility line corridors (Bramble et al. 1992, 1994; Confer 2002; Marshall et al. 2002).

Table 12.1 Rights-of-way bird diversity in Pennsylvania and Tennessee (1982–2004)
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As researchers and natural resources managers have come to understand the importance of managing early successional habitats for wildlife in general and songbirds in particular, recent studies highlight the potential for shrub-scrub (and other) bird conservation in ROW. Confer and Pascoe (2003) documented high bird species diversity in northeastern (New Hampshire, New York, Maine and Massachusetts) ROW, with many high priority conservation species occurring in ROW variably managed with herbicides, mowing, and prescribed fire. On the Cumberland Plateau in Tennessee, Bulluck and Buehler (2006) found that among three early successional habitat types (regenerating clearcuts, reclaimed surface mines, ROW), ROW were intermediate in bird diversity and harbored higher bird species richness than regenerating clearcuts. Of particular note, ROW sites were more likely to harbor the Kentucky Warbler (Oporonis formosus), a high conservation priority forest interior species across much of its range. Early successional species, including Chestnut-sided Warblers, Eastern Towhees, and Song Sparrows (Melospiza melodia) were more closely allied with ROW than either clearcuts or reclaimed mines.

12.2.2 Ecological Benefits and Costs of Rights-of-Way Occupancy

Creation of early successional habitats, whether by forest management or maintenance of ROW, is often controversial. Aesthetically, the dense, jungle-like vegetation in a regenerating clearcut or a power line ROW may not be appealing. Beyond appearances, edges between forested and early successional habitats can have both positive and negative impacts on songbirds. Avian species diversity and density are greater along edges (MacArthur and MacArthur 1961; Odum 1971; Roth 1976; Hansson 1983); ecotones between forests and ROW could contain more species and more individuals than pure communities if species requiring both ecosystems occur together with more specialized species from each ecosystem (Odum 1971). Additionally, edges can act as boundaries of individual breeding territories and concentrate birds (Anderson et al. 1977). Increased primary productivity, insect species richness and density (Hansson 1983), vegetation density and structure, and light intensity (Strelke and Dickson 1980) are other factors thought to attract birds to edges. Hanowski et al. (1993) found differences in composition of bird communities over 200 m into the forest interior from a ROW edge. Additionally, Small and Hunter (1989) contrasted two permanent ecotones (transmission-line corridors and river edges) and found that edges bordering transmission-line corridors supported more avian species than river edges – probably due to abundant brushy cover, which is required by many edge species. Although use of ROW in the breeding season by early successional obligates and edge species might be expected, the number of forest interior birds occurring in these areas is noteworthy. Forest interior neotropical migrants like the Rose-breasted Grosbeak (Pheuticus ludovicianus), Scarlet Tanager (Piranga rubra), Red-eyed Vireo (Vireo olivaceous), Eastern Wood-Pewee (Contupus virens), Ovenbird (Seirus aurocapillus) and Wood Thrush (Hylocichla mustelina) were recorded in upland hardwood forest ROW in Pennsylvania (Bramble et al. 1992; Yahner et al. 2004). Several studies (Pagen et al. 2000; Marshall et al. 2003; Yahner 2004; Vitz and Rodewald 2006) have revealed that early successional habitats provide important cover and foraging for mature forest birds in the post-fledging period. Thus, the appearance of mature forest birds might be explained by the high levels of vegetative productivity in ROW attracting a diverse arthropod assemblage that provides potential prey items such as adult and larval Lepidoptera (see Greenberg et al., Chap. 8).

Transmission-line ROW may also serve as movement corridors and provide important stopover habitats for migrating land birds, (as passerines are demonstrated to use some early successional habitats) during migration (Moore et al. 1990; Mabey et al. 1992). Moore et al. (1990) found that scrub-shrub areas were selected as stopover habitats by migratory birds on an island in the Gulf of Mexico. Relative to four additional habitat types available to the migrating songbirds, scrub/shrub areas harbored both the greatest species diversity and number of individuals. The “value-added” facets of early successional habitats as areas used for fledgling foraging, migration stopover, and species associated with other habitat types (e.g., forest interior species) may place utility-line corridor management for songbirds in a new and more fully informed light.

Although the potential benefits of shrubby vegetation in ROW and elsewhere to songbirds are known, creation of early successional vegetation structure and edges as practiced in traditional wildlife management (Leopold 1933) is controversial. Studies of forest fragmentation and its impacts on forest interior species reveal some potential negatives of creating too much edge that may outweigh the benefits (Harris 1984). For example, nest productivity may decrease near habitat edges. Chasko and Gates (1982) showed that higher fledging success was associated with increased distance from ROW edge. Gates and Gysel (1978) suggested that forest species’ breeding habitat suitability decreases as the number of nests increases toward narrow field-forest edges. The isolation of mature forests and impacts of nest predation and brood parasitism on edges are well documented and have remained an issue of concern among some researchers (Ratti and Reese 1988; Robinson 1988). Suarez et al. (1997) found that nest predation rates were higher along exterior agricultural edges than forest-interior edges and were also higher along abrupt, permanently-maintained edges than along gradual edges. Rights-of-way edges may also attract potential nest predators due to their greater prey density and natural travel lanes created by the change in structure of vegetation (Gottfried and Thompson 1978; Ratti and Reese 1988). By attracting songbirds as well as brood parasites and nest predators, edges may act as “ecological traps” for some disturbance-dependent species while adversely affecting forest-interior songbirds in adjacent forests (Wilcove 1985; Pulliam 1988; Ratti and Reese 1988; Robinson 1988).

The impacts of ROW on brood parasitism in Brown-headed Cowbirds (Molothrus ater) exemplify the variable effects of ROW on parasitism and nest predation. Confer and Pascoe (2003) reported Cowbird parasitism was relatively low (5.3%) in some northeastern USA study sites and ROW did not reduce forest bird productivity. Although Suarez et al. (1997) found that cowbird parasitism rates in southern Illinois did not differ among several types of edge habitats (agricultural, forest interior, stream, wildlife openings, and treefall gaps), predation rates were higher and nest productivity was lower next to agricultural edges. Chasko and Gates (1982) and Gates and Griffen (1991) found Brown-headed Cowbird parasitism rates were higher near power-line corridors than within adjacent forest. These contradictory results illustrate the complex influences within variably managed areas and landscape context. For example, managers might consider impacts of an agricultural landscape on parasitism when creating habitats such as wildlife food plots for game species. Rights-of-way embedded within different matrices (e.g., forest versus agriculture versus riparian zones) may have different effects on parasitism and predation.

There are often strong associations between mature forest bird species and forest fragmentation (see Gates and Gysel 1978; Wilcove 1985; Yahner and Wright 1985; Angelstam 1986; Robinson 1988; Ratti and Reese 1988; Keyser et al. 1998); yet, some area-sensitive species require large, contiguous patches of early successional habitats (Robbins et al. 1989; Robinson et al. 1992). In contrast, most early successional species in the northeastern USA breed in a wide range of habitat patch sizes, and patch size appears not to be a limiting factor for their conservation (Dettmers 2003). However, an increasing number of researchers (Askins 1998; Confer 1992; Lanham 1997; Lanham and Guynn 1998; Kremmentz and Christie 2000; Rodewald and Vitz 2005; Schlossberg and King 2008) provide strong evidence that early successional species may also be sensitive to variations in habitat area. For example, Yellow-breasted Chats (Icteria virens) were absent from shrubby patches smaller than 2 ha (Dennis 1958). Similarly, Golden-winged Warblers are probably an area-sensitive species with 10 ha early successional patches offering the minimum area for nesting in New York (Confer and Knapp 1981; Confer 1992). Lanham (1997) and Lanham and Guynn (1998) described Prairie Warblers and Yellow-breasted Chats as area-sensitive shrub-scrub species in because neither species was ever observed in small (<2 ha), regenerating, clearcut patches in the South Carolina mountains and upper piedmont. Although linear in configuration, some ROW may provide adequate area for area–sensitive, disturbance-dependent species. Powerline corridors vary in width by the voltage the lines they carry, with 500-kv transmission-lines typically among the widest at 45.7 m (150 ft) or greater (American Transmission Company; http://www.atcllc.com/IT6.shtml, April 5, 2010). Over sufficient lengths, ROW may provide adequate area as evidenced by the occurrence of Prairie Warblers and Yellow-breasted Chats in 500-kv transmission corridors in Pennsylvania (Yahner et al. 2004) and Prairie Warblers, Yellow-breasted Chats, and Golden-winged Warblers in 230-kv corridors in that same region (Bramble et al. 1992).

12.2.3 Songbird Responses to Transmission-Line Maintenance

Mowing, cut/stump treatments, and broadcast or selectively applied herbicides are some common methods used to manage ROW. In Tennessee, densities of several bird species were positively associated with presence of blackberry (Rubus spp.), which often increases in managed ROW. The numbers of Yellow-breasted Chats, White-eyed Vireos (Vireo griseus), Common Yellowthroat (Gymnothylpis trichas) and Prairie Warbler all increased with density of blackberry patches. Field Sparrows showed the opposite trends as the grass-dominated habitats the species prefers were apparently not abundant (Kroodsma 1982). In New York, bird diversity and density were highest in ROWs where trees received cut and stump or basal treatments with herbicides (Malefyt 1984). Bird density and diversity also were lowest on brush-mowed ROWs in the year after the mowing. Similarly, in central Pennsylvania, songbird species diversity decreased immediately after hand-cutting and herbicide treatments were applied to ROWs (Bramble et al. 1986). However, Bramble et al. (1992) observed higher bird densities on those areas sprayed with herbicides (basal, stem foliage and foliage sprayed) relative to those that received mechanical treatments and herbicide treatments.

In a comparison of two transmission line corridors, one maintained in an early successional stage (grass and forbs) by annual mowing and the other maintained in a later seral stage (small trees and shrubs) by selective herbicides, Chasko and Gates (1982) found that bird use of a grassland stage corridor was minimal (limited to two mixed-habitat species) and did not include grassland birds characteristic of the region. Songbird diversity and breeding success were highest in the shrubland-stage corridor. Maintenance by selective herbicide application may create more vertical structure and habitat heterogeneity (Bramble and Byrnes 1979a; Lawson and Gates 1981) than annual mowing. Yahner et al. (2004) assessed the trends in bird usage of Pennsylvania ROW managed by the “wire/border zone method” (Bramble et al. 1992) that creates lower strata of grass, forbs and shrubs in the central wire zone and higher shrub zones at the forest edge. During 15 years of mowing and selective herbicide management, bird community composition remained relatively stable throughout the study units, with highest densities (birds/100 ha/day) observed in mowed and herbicide-treated units. Somewhat in contrast to other research, Kroodsma (1982, 1984) found bird density was lower in mowed, grass-dominated corridors than in forb-Rubus-dominated transmission-line corridors in Tennessee.

12.3 Butterfly Diversity in Rights-of-Way: A Case Study of an Ecological Benefit

Butterfly watching and gardening have become popular outdoor activities. Because many butterfly species are brightly colored, have interesting life histories, and even allow close observation, the interest in conserving them and their habitats has increased. Beyond their aesthetic appeal to nature enthusiasts, butterflies are key components of terrestrial ecosystems. They pollinate many species of flowering plants (Webb and Bawa 1983) and are prey for a diverse array of vertebrates, including many species of birds. The lives of many bird species are closely linked to the presence and abundance of arthropods (Rodenhouse and Holmes 1992; Marra and Holberton 1998; Marshall et al. 2002) including butterfly larvae (caterpillars) that are important food resources for both nestlings and adults. Because of the important role that they play as insect prey, the abundance and diversity of butterflies may impact local bird populations dramatically (Holmes and Sherry 1988; Sherry and Holmes 1995).

Many nectar-producing flowering plants and other species important in the life cycles of butterflies occur in early successional habitats. For example, Monarch butterflies (Danaus plexippus) prefer milkweeds (Asclepias spp.), which grow primarily in disturbed, open sites as an egg-laying substrate and larval food source. From this and multiple other examples of early-successional plants that proliferate in ROW, it should follow that these areas may have great potential as butterfly conservation areas. However, relatively little work has been done to determine the suitability of ROW for butterflies (see Yahner 2004). Nichols and Lanham (2002) surveyed butterflies and skippers in six South Carolina ROW and found 101 species occupying those sites (Table 12.2). They also recorded a diverse community of nectar and larval host plants as well as an abundance of other butterfly habitat requisites (e.g. open areas, bare ground, and moist puddling areas; see Nichols and Lanham 2002). Their work showed that the ROW were botanically diverse, structurally heterogeneous, early successional habitats that harbored an impressive diversity of butterflies and skippers. The high butterfly/skipper habitat suitability was in part due to the combination of selective herbicide application and mowing that resulted in a rich mosaic of shrubs, grasses, and bare ground.

Table 12.2 Butterflies and skippers on six South Carolina ROWa
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12.4 The Future for Songbirds, Butterflies and ROW Management

Permanently maintained openings on public lands (e.g., ROW, wildlife openings, reclaimed landfills, and strip mines) may become increasingly important to disturbance-dependent birds. Bulluck and Buehler (2006) provide some of the most compelling evidence for the value of ROW and reclaimed areas as early successional forest habitats relative to “traditional” regenerating forest habitats. Many surface (e.g., mine) reclamation efforts focus on grassland habitats (Bajema and Lima 2001; Bajema et al. 2001; Cox and Maehr 2004; Scott et al. 2002), and few appear to be focused on “restoring” shrub-scrub habitats. Thus, ROW may provide the best opportunities for managing early successional shrub-scrub habitats.

Rights-of-way have been largely ignored by conservationists and frequently derided as landscape scars. However, in light of the declines in early successional habitats and the concurrent declines in disturbance-dependent bird and butterfly species, there are numerous opportunities to maximize the benefits of a system that essentially arrests succession and provides habitats that would otherwise disappear. Again, the growing body of evidence linking early successional habitats to the important post-fledgling stage of some songbirds (Pagen et al. 2000; Marshall et al. 2003; Yahner 2004; Vitz and Rodewald 2006) provides strong evidence that managing early successional areas like ROW can benefit many disturbance-dependent species that include both predators (songbirds) and prey (caterpillars).

Because landscape-level habitat restoration or alteration (e.g., “ecosystem management”) is often regarded as the best management tool for maintenance of songbird communities, perhaps the ubiquitous nature of ROW offers opportunities to manage for some early successional songbirds on a large scale. Development and implementation of landscape-level management requires intimate knowledge of population-level requirements and limitations, as well as community-level interactions. Transmission-line position on the landscape, maintenance regimes, and successional stage may ultimately determine ROW use by shrub-scrub song-bird species.

Future research to evaluate transmission-line corridors as songbird habitats should continue to focus on: (1) impacts and benefits for both disturbance-dependent and forest interior species, (2) differences among power line corridors and other early successional areas (e.g., ephemeral timber harvest openings), and (3) effects of landscape level characteristics (e.g., shape, size, configuration, and context) and maintenance regimes. Perhaps most importantly, these studies should include some measure of fitness (e.g., nesting success, predation rates, or fledgling and adult survival) along with density and diversity estimates when addressing ROW quality as songbird habitats. Petit et al. (1995) claim that “the landscape context in which the habitat is imbedded can be an important influence on [songbird] demography and, hence, population health.” By understanding how transmission line ROWs and other forest openings, within the context of the landscape, impact songbird density, diversity, and reproductive rates, we can better devise management alternatives at a landscape level for the maintaining declining, disturbance-dependent songbird populations in ROW.

The ecological linkages and public appeal of birds and butterflies can provide opportunities for natural resource managers and conservationists to move advocacy for early successional habitats forward. Some utility companies have recognized the potential for enhancing their reputations on environmental issues, and have implemented programs to enhance the habitat suitability of their ROW. Likewise, as the use of selective herbicides has become the common means of managing ROW vegetation, those companies providing the chemicals have also taken advantage of the opportunities to “green” their image by promoting the highly selective nature of the herbicides and advancing their use in promoting or restoring wildlife habitats (Hurst 1997). Beyond the ecological benefits to early successional species and the marketing advantages to corporate utility and chemical entities, there are social benefits to be gained. Rights-of-way teeming with bird life and butterflies within forests that cross urban and suburban landscapes can provide opportunities for educating audiences heretofore uninformed or with negative impressions of early successional habitats - novel opportunities indeed for wildlife conservation.