Domestic livestock have the potential to function as ecosystem engineers in semiarid rangelands, but prevailing management practices largely emphasize livestock production and uniform use of vegetation. As a result, variation in vegetation structure might not occur at appropriate spatial and temporal scales to achieve some contemporary conservation objectives. Here, we introduce the utility of livestock as ecosystem engineers and address potential benefits and consequences associated with heterogeneity-based management practices for conservation grazing in the semiarid rangelands of the western North American Great Plains. To illustrate the potential value of this approach, we provide specific examples where engineering effects of livestock could alter vegetation heterogeneity at within-pasture (< 100 ha) and among-pasture (∼100 ha to thousands of hectares) scales to improve habitat for declining native grassland birds. Experimental evaluations of the efficacy of livestock to achieve desired modifications to vegetation structure are needed, along with the economic aspects associated with implementing heterogeneity-based management practices. Using livestock as ecosystem engineers to alter vegetation structure for grassland bird habitat is feasible in terms of application by land managers within the context of current livestock operations, and provides land managers important tools to achieve desired contemporary objectives and outcomes in semiarid rangelands of the western North American Great Plains.
One of the clearest, but least acknowledged, roles of domestic livestock in rangeland ecosystems is as ecosystem engineers (Jones et al. 1997), where livestock directly and indirectly influence the availability of resources to a wide range of organisms by inducing changes in vegetation structure. Prevailing rangeland management practices emphasizing even distribution of livestock use have decreased both temporal and spatial heterogeneity (hereafter referred to as only heterogeneity) of vegetation (Fuhlendorf and Engle 2001). Consequently, conflicts have increased between goals associated with traditional livestock production and contemporary conservation uses of semiarid rangelands (Knight et al. 2002). Scientists and managers in mesic rangeland ecosystems have evaluated land management practices that mimic historical disturbance regimes such as interactive effects of grazing and fire (Fuhlendorf and Engle 2001; du Toit et al. 2003) to produce a mosaic pattern of vegetation that provided habitat for a suite of species, while still maintaining livestock production (Fuhlendorf and Engle 2004; Fuhlendorf et al. 2006). Limited consideration has been given to management for vegetation heterogeneity in semiarid rangelands of the western North American Great Plains, and the ecological and economic aspects of modifying vegetation heterogeneity remain little-studied (Hunt 2003).
Grazing has been demonstrated to influence vegetation structure in the Great Plains (Milchunas et al. 1988, 1989; Knopf 1996) as well as in other world rangelands (Sala et al. 1986; Fuls 1992; Hiernaux 1998; James et al. 1999; Hunt et al. 2007) and effects of grazing on spatial heterogeneity of vegetation have been recently reviewed (Adler et al. 2001; Parsons and Dumont 2003). Rangeland ecologists have recognized that livestock can create and maintain habitat for big game species in semiarid rangelands (Severson and Urness 1994) but livestock influences on habitat for grassland birds are not well understood. Widespread declines have occurred for many grassland birds of North America (Knopf 1992; Brennan and Kuvlesky 2005), including those in the western Great Plains (e.g., mountain plover [Charadrius montanus; Wiens and McIntyre 2008] and lesser prairie chicken [Tympanuchus pallidicinctus; US Fish and Wildlife Service 1998]). These declines have been hypothesized to be related to conversion of prairie to agriculture-dominated landscapes and prairie fragmentation (Knopf 1994), historic livestock grazing (Saab et al. 1995), and rangeland deterioration, including overgrazing, drought, lack of fire, and woody plant and exotic plant invasions (Brennan and Kuvlesky 2005). Although conversion of rangeland to cropland has likely been one factor driving grassland bird declines, this further emphasizes the need to effectively manage remaining rangelands in a manner that will sustain native species. Declining grassland bird populations might also be associated with livestock management methods that reduce variability in vegetation structure (Fig. 1), and hence reduce habitat for both grazing-intolerant and grazing-dependent bird species (Saab et al. 1995). Historical interactions of large grazers, fire, drought, and prairie dogs (Cynomys spp.) created and maintained distinctly different plant communities in the western Great Plains resulting in a mosaic of vegetation structure and composition that sustained grassland bird populations (Brennan and Kuvlesky 2005). These interactions largely have been replaced by management practices that attempt to improve livestock distribution because uneven use of rangeland by livestock is a major problem for rangeland managers (Holechek et al. 1998). As a result, there is reason to believe that management practices that increase vegetation heterogeneity will be positive for grassland birds by increasing the variability in vegetation structure and/or composition. For example, the community of over 30 extant bird species that evolved within the western Great Plains (Knopf and Sampson 1996) requires a gradient of vegetation structure from relatively undisturbed, taller-structured vegetation to very short structure associated with fire or heavy use by large ungulates or black-tailed prairie dogs (Cynomys ludovicianus; Knopf 1996). Concerning the recent petition to list the lesser prairie chicken as threatened under the Endangered Species Act, the US Fish and Wildlife Service noted that “the Service believes that areas of heavily, moderately, and lightly grazed areas are necessary on a landscape scale” (US Fish and Wildlife Service 1998). Here, we examine how heterogeneity-based management practices, through the use of livestock as ecosystem engineers, can result in a mosaic of habitats differing in their structural complexity to address contemporary conservation goals for grassland birds in the western Great Plains.
We first provide background on the prior emphasis of uniform use of vegetation and subsequent reduction in heterogeneity of vegetation. Next, we address the biology for four grassland birds native to western Great Plains rangelands: 1) mountain plover, 2) long-billed curlew (Numenius americanus), 3) upland sandpiper (Bartramia longicauda), and 4) lesser prairie chicken. Habitat requirements of these four bird species for nesting and brood rearing are relatively well-known, and they collectively inhabit a gradient of vegetation structure. In addition, all four species are listed as “Birds of Conservation Concern” in the western Great Plains (US Fish and Wildlife Service 2002), and hence as priority species for conservation actions. Thirdly, we address potential benefits and consequences associated with heterogeneity-based management approaches for conservation grazing, and provide specific examples of where livestock might function as ecosystem engineers to alter heterogeneity of vegetation at within-pasture (< 100 ha) and among-pasture (∼100 ha to thousands of hectares) scales.
Prior Management Emphasis on Vegetation Homogeneity
Herbivores naturally select among plants in a pasture by eating preferred plants and ignoring others (Van Soest 1996), resulting in differential patterns of use of individual species within pastures (i.e., management units) when stocking rates are not excessive and pastures are of sufficient size (Launchbaugh and Howery 2005). Although the details of both forage plant and landscape selectivity have received considerable recent attention (Launchbaugh and Howery 2005), the view of livestock as ecosystem engineers that can alter the heterogeneity of vegetation is relatively new to rangeland managers. Most semiarid rangelands, however, have traditionally been managed for uniform use of vegetation and livestock production through implementation of growing-season grazing at a moderate intensity designed to utilize aboveground forage down to a particular amount, which maximizes livestock gains under the constraints of maintaining individual animal performance and preventing long-term ecosystem degradation (Bement 1969). This management practice has been very effective and sustainable from the standpoint of livestock and forage production (e.g., Hart and Ashby 1998), but the emphasis on homogeneity of use is counter to historic disturbance regimes (Fuhlendorf and Engle 2001). Widespread use of moderate grazing intensities has reduced availability of suitable habitat structure for many grassland birds at the extremes of the vegetation structure gradient (Fig. 1; Knopf 1996).
Role of Livestock Management in Promoting Desirable Bird Habitat
Conflicts between interests of livestock producers and conservationists can be reduced if regionally appropriate grazing management strategies can be identified that use the engineering abilities of livestock to enhance grassland bird habitat. Below, we summarize the nesting and brood rearing habitats for each of four grassland bird species native to the western Great Plains (Table 1), as well as present potential management practices that incorporate livestock as ecosystem engineers to meet these habitat requirements. We use a simple approach to spatial scale by limiting our discussion to two scales: within-pasture (< 100 ha) and among-pasture (∼100 ha to thousands of hectares), because these spatial scales have relevance to land managers. We recognize that these two scales can be used individually or in combination, depending on the size of individual pastures and desired outputs.
Summary of nesting and brood rearing habitat and estimate of the scale of habitat heterogeneity for four birds native to the western Great Plains.
The mountain plover winters in the Central and Imperial valleys of California, and migrates to the western Great Plains, arriving from mid-March to mid-April to nest (Dechant et al. 2003). Mountain plovers represent one extreme of the vegetation structure gradient (Fig. 1; Knopf 1996). Preferred habitats include flat topography on very heavily grazed sites, cultivated fields, recent burns, and prairie dog colonies, all of which have short vegetation and substantial amounts of bare ground (Knopf and Wunder 2006). They tend to shun sites with high plant cover or biomass (Table 1). Habitat requirements of mountain plovers are potentially in opposition to what has traditionally been considered good rangeland management practices in the shortgrass steppe (Bement 1969). Mountain plovers can both nest and rear broods in heavily disturbed rangeland, with estimates of home-range size for adults with broods of approximately 30–90 ha (Knopf and Rupert 1996).
Management practices of supplemental feed sites and patch burning can incorporate livestock as ecosystem engineers to alter vegetation structure at within-pasture scales for the mountain plover. Supplemental feed sites can be located strategically on flat topographical sites preferred by this species, and the combination of intense localized grazing and physical disturbance can increase bare ground and reduce vegetation cover (Fig. 2), as well as alter vegetation structure, but not alter arthropod and small mammal abundances (T. A. S. Newbold, unpublished data, 2004–2006). These supplemental sites can be moved to different locations within and between years to prevent long-term degradation of the area. At the pasture scale, very heavy grazing intensities can be implemented in an attempt to reduce forage residue to very low levels and decrease vegetation structure, but these intensities result in substantially lowered animal performance (Bement 1969). Patch burning can be used successfully to create habitat conditions conducive to nesting mountain plovers (Fig. 2), with subsequent higher proportional use of these recently burned areas by livestock maintaining low vegetation structure (e.g., Vermeire et al. 2004). Heavy grazing and soil disturbance associated with the combination of black-tailed prairie dogs and livestock also provide breeding habitat for mountain plovers (Dinsmore et al. 2005; Tipton et al. 2008).
The long-billed curlew is unique among the large-bodied avian species in its use of low-stature and recently disturbed rangeland for nesting (reviewed by Paton and Dalton 1994) and its reliance on resemblance to bovid fecal piles for concealment (King 1978). In the Great Plains, the breeding distribution of long-billed curlews is strongly clustered in northwestern Nebraska, and in the region immediately surrounding the confluence of Colorado, New Mexico, Oklahoma, and Texas (Sauer et al. 2005; Sparks and Hanni 2006). These areas contain include a patchwork of lightly to heavily grazed rangelands, irrigated and dryland agricultural fields, and tall-structure vegetation in formerly cropped lands that have been restored. Long-billed curlews in shortgrass steppe of southeastern Colorado can nest in extremely short vegetation (Fig. 2) associated with recent burns, heavy grazing, or prairie dog colonies. In contrast, adults with broods increase use of midheight (20–50 cm) rangeland and areas with a mosaic of short (< 10 cm) to midheight (10–40 cm) grasses (Fig. 2; King 1978). Similarly, long-billed curlews with chicks in northern mixed-grass prairie were reported in grass that was 18 cm tall (Spomer 1981 as cited in Dechant et al. 2002). The limited available information on spatial ecology of this species suggests an association with vegetation heterogeneity both at the within- and among-pasture scales. For example, within-pasture scale heterogeneity can result from variation across a gradient from heavily grazed areas near water to more distant, lightly grazed areas. Similarly, among-pasture scale heterogeneity can encompass vegetative boundaries found at pasture edges that can be attributable to different timing and/or intensity of grazing in adjacent pastures.
Management practices at both the within-pasture and among-pasture scales can incorporate livestock as ecosystem engineers to alter vegetation structure for the long-billed curlew. As described previously, both supplemental feed and patch burning with subsequent grazing can create low structure and recently disturbed areas for nesting. In addition, both practices can provide areas of taller structure within the same pasture by proportionally increasing grazing pressure at the area of interest, and reducing grazing pressure at other locations in the pasture to increase within-pasture heterogeneity (Fuhlendorf et al. 2006). If pastures are of sufficient size such that areas in the pasture are > 2 km from a water source, a gradient of very heavy use (around water) to light or no use (area furthest from water) can result (e.g., Adler and Hall 2005), commonly called the piosphere effect (Lange 1969), and provide a suite of needed habitats for this species. At the among-pasture scale, varying intensities and/or seasons of use, including deferment and/or rest might be sufficient to create differences in vegetation structure between pastures, thereby increasing edge effects.
Lesser Prairie Chicken and Upland Sandpiper
Both the lesser prairie chicken and the upland sandpiper rely on concealment in taller structure vegetation for nesting in semiarid rangelands (Table 1). The lesser prairie chicken primarily occurred in the semiarid, southern Great Plains prior to European settlement (Johnsgard and Wood 1968) and is associated with plant communities on sandy soils with an unpalatable shrub component such as sand sagebrush (Artemisia filifolia Torr.) or shinnery oak (Quercus havardii Rydb.; Robb and Schroeder 2005). In contrast, the upland sandpiper has a broad distribution that extends beyond central North America, but historically occurred throughout the semiarid northern Great Plains (Houston and Bowen 2001). In semiarid rangeland, upland sandpipers preferentially nest in areas with light or no cattle grazing during the spring and early summer, even though they often forage, raise broods, and are observed in moderately to heavily grazed pastures (Kirsch and Higgins 1976; Bowen and Kruse 1993). Lesser prairie chickens often preferentially nest in the tallest and most dense vegetation available in the landscape (Giesen 1991; Riley et al. 1992; Pitman et al. 2005), but optimal brood-rearing habitat contains less dense vegetation with lower shrub and grass cover, and greater invertebrate biomass (Jones 1963; Riley and Davis 1993; Hagen et al. 2005).
The dependence of lesser prairie chickens and upland sandpipers on tall-structure vegetation for nesting indicates a need for patches that have either been ungrazed or lightly grazed during the preceding growing season. Yet, Jones (1963) concluded that lesser prairie chicken habitat generally consists of small patches of short grass interspersed with large patches of shrub or half-shrub vegetation. In addition, chick growth of prairie chickens and upland sandpipers is dependent upon the arthropod supply (Houston and Bowen 2001), which is less abundant in undisturbed vegetation, and increases with recent grazing or fire disturbance (Joern 2004). Thus, these native grassland birds appear to depend upon a shifting mosaic of recently disturbed and undisturbed patches across the landscape. Large, uniform areas of taller-structure vegetation might prevent broods from reaching patches with vegetation structure conducive to their movement (Jones 1963; Hagen et al. 2004). Rest-rotation grazing systems, where at least one pasture remains ungrazed each year, can be an effective among-pasture scale approach to sufficiently modify vegetation structure and provide necessary nesting habitat for these grassland bird species (Bowen and Kruse 1993; Hagen et al. 2004). The same accumulation of residual vegetation that provides nesting concealment the following spring can begin to reduce brood-rearing habitat quality and insect availability in subsequent years. As a result, long-term grazing exclusion can be as detrimental to habitat quality as continuous grazing pressure. For a multipasture rotation with moderate stocking rates, shifting from a system where each pasture is utilized equally over the grazing season to a system where some pastures are grazed more intensively and other pastures are either rested or grazed lightly could increase both within-pasture and among-pasture scale variability in vegetation structure. Rotation of grazing intensity among pastures over annual time scales can prevent any one patch or pasture within the landscape from experiencing long-term degradation.
Utility of Livestock as Ecosystem Engineers: Potential Benefits and Consequences
Livestock grazing behavior can be modified through location of supplemental feed, water, and herding (e.g., Bailey 2005). As a result, creating mosaics of vegetation patches and increasing structural diversity of vegetation can be accomplished (Vavra 2005). Livestock have the potential to function as ecosystem engineers, but there remain unanswered questions regarding potential benefits and consequences associated with heterogeneity-based management practices.
The within-pasture scale approach minimizes negative ecological impacts to the entire pasture. Localized disturbances (i.e., patches) are created in which vegetation structure is altered in an appropriate time frame for the desired goal. Three within-pasture scale strategies with potential for widespread application in semiarid rangelands of the western Great Plains are use of supplemental feed, the implementation of patch burns, and the manipulation of multiple water sources within a pasture (Bailey 2005). Salt placement is also often discussed as a way to alter livestock distribution (Williams 1954), but salt locations only have a minor influence of grazing distribution over a growing season (Ganskopp 2001). Strategic placement of supplemental feed (Bailey and Welling 1999; Bailey et al. 2001) can reduce vegetation structure in a given patch (Fig. 2) in a pasture, and this disturbance can be shifted temporally and spatially to achieve desired goals and prevent long-term ecosystem degradation. Manipulation of water sources can be used in a similar manner as supplemental feed (Williams 1954; Ganskopp 2001) but this strategy requires availability or installation of multiple tank locations and can be less flexible for rotating areas of intensive grazing disturbance over time. In the case of patch burning, a portion of a pasture can be burned in an effort to restore a shifting mosaic of vegetation patterns (e.g., Fuhlendorf and Engle 2004; Fig. 2). Burned areas of a pasture receive greater grazing pressure because cattle prefer green nutritious regrowth, and unburned areas receive proportionately less use, resulting in an accumulation of standing crop in some areas (Fuhlendorf and Engle 2001, 2004).
The among-pasture scale heterogeneity approach differs from the within-pasture scale approach in that the disturbance is applied to the entire pasture, with the overriding goal of creating substantial differences in vegetation structure among pastures within a larger management unit, at the scale of hundreds to thousands of hectares or square kilometers (e.g., ranch, allotment). Thus, the approach would be to vary grazing intensities (none, light, moderate, heavy, very heavy), seasons (winter, spring, summer, fall), and/or grazing animals (sheep, goats, cattle, or some combination) among pastures to alter the structure of vegetation within a given pasture. Burning of an entire pasture will not increase vegetation heterogeneity within the pasture, but if the burned pasture is part of a larger management unit, then among-pasture differences can be enhanced. Pasture-scale heterogeneity approaches also can take advantage of naturally-occurring variability among pastures due to differences in topo-edaphic conditions (e.g., lowlands vs. uplands), presence/absence of prairie dogs, and season of use (e.g., spring vs. summer grazing).
A benefit of altering within-pasture scale heterogeneity is minimizing negative ecological consequences over the entire pasture by localizing impacts spatially to targeted locations (e.g., supplemental feed sites, patch burn areas) and temporally because these patches can be moved within a pasture each year. In contrast, among-pasture scale heterogeneity has the benefit that this is typically the management unit for land managers. As such, managers are likely more comfortable in modifying management practices within the confines of existing infrastructure limitations (e.g., fences) compared to intensifying management within pastures to create smaller patches of heterogeneity.
Traditional emphasis on homogeneous use of vegetation (i.e., “management to the middle”) at the pasture scale has resulted in the lack of suitable habitat for grassland birds at the extremes of the vegetation structure gradient in semiarid rangelands (Fig. 1). Therefore, use of livestock as ecosystem engineers has substantial potential to alter vegetation structure, primarily at the extremes of the structure gradient. Numerous studies have documented responses of grassland birds to varying grazing intensities (Saab et al. 1995), and the importance of vegetation heterogeneity for semiarid rangeland birds has been recognized for at least four decades (Jones 1963), but development and application of rangeland management practices to maintain or enhance vegetation heterogeneity remains a need. Incorporating livestock as ecosystem engineers to create an array of habitats, therefore, has utility for addressing the interface of production and conservation goals for land managers. Unfortunately, few experiments have quantified the economic costs of heterogeneity-based management in semiarid rangelands (Hunt 2003). There is an emergent need to determine these costs in relation to the traditional management approaches to determine the practicality of landowners modifying management practices, and if incentive programs might be needed to facilitate the implementation of heterogeneity-based management to achieve both production and conservation goals. Another limitation in the application of livestock as ecosystem engineers for enhancing habitats for grassland birds is the paucity of information on the optimal size, distribution, and juxtaposition of habitat patches for individual species across the landscape. Also, most land managers commonly apply rangeland management practices at the pasture scale, indicating that there might be some discomfort in applying heterogeneity-based management practices within pastures to create this scale of vegetation heterogeneity. The fact that many grassland birds require a mosaic of habitat patches to complete their breeding requirements (Table 1), however, suggests that within-pasture scale management might often be appropriate, and that mechanisms (e.g., incentives) should be considered to promote these approaches.
Approaches using livestock as ecosystem engineers to alter vegetation structure provide an alternative to complete cessation of livestock grazing on public lands as called for by Fleischner (1994) and Donahue (1999). The heterogeneity-based management practices are feasible in terms of application by land managers within the context of current livestock operations. The ecological and economic aspects of using livestock as ecosystem engineers to provide appropriate habitat through heterogeneity-based management for grassland birds such as the mountain plover merit additional research. Using livestock as ecosystem engineers to achieve conservation grazing objectives and outcomes in semiarid rangelands of the western Great Plains provides land managers with the opportunity to reduce conflicts between conservation and livestock production goals on these lands.
We thank the Shortgrass Steppe Long-Term Ecological Research project for support (National Science Foundation Grant DEB-0217631). We thank David Engle, Chad Boyd, Tom Thurow, and two anonymous reviewers, as well as Associate Editor Leigh Hunt for helpful comments on a previous version of the manuscript.