David J. Augustine

Determinants of woody cover in African savannas

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than ∼650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of ∼650 mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.

Livestock as Ecosystem Engineers for Grassland Bird Habitat in the Western Great Plains of North America

Abstract 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.

Native ungulates of diverse body sizes collectively regulate long-term woody plant demography and structure of a semi-arid savanna

Summary 1. Large mammalian herbivores are well recognized to play important roles in regulating woody cover and biomass in savannas, but the extent to which browsing ungulates are capable of regulating woody populations in the absence of other disturbances such as fire is unclear. Moreover, the degree to which browser effects on savannas operate through effects on woody plant recruitment vs. mortality has rarely been examined. 2. We conducted a 10-year, replicated herbivore exclusion experiment in a semi-arid savanna in East Africa (mean annual rainfall = 514 mm), where fires have been actively suppressed for decades. Browsers dramatically influenced recruitment, growth and mortality of all size classes of woody vegetation. A decade of herbivore exclusion resulted in a sevenfold increase in recruitment, a 2.5-fold decrease in mortality and a threefold increase in woody biomass inside exclosures, while biomass outside exclosures remained relatively unchanged. 3. At the plant community level, extensive browsing of seedlings by small-bodied ungulates suppressed woody recruitment in this semi-arid system, generating a ‘browsing trap’ comparable to the ‘fire trap’ reported for mesic systems. Browsing by large- and medium-bodied ungulates reduced both growth and survival of individuals in larger size classes. 4. At the plant species level, browser impacts were variable. Although browsers negatively influenced recruitment of all species, they had little to no impact on the mortality of some dominant species, resulting in a longterm, browser-driven shift in woody species composition that was largely mediated via their differential effects on plant mortality rates rather than recruitment. 5. Synthesis. Our results demonstrate unequivocally that, even in the absence of fire, native browsing ungulates can exert dramatic ‘top-down’ controls in semi-arid savannas, influencing all aspects of woody plant demography. Besides suppressing woody plant recruitment, browsers can also have substantial cumulative long-term impacts on growth and mortality rates of woody plants, including adults, which can differ between species in ways that fundamentally alter the structure and function of woody vegetation. In semi-arid rangelands, intact communities of native browsing ungulates thus provide a critical ecosystem service by regulating woody cover, and their removal (or extinction) from these systems can lead to rapid woody encroachment.

Functional response of U.S. grasslands to the early 21st-century drought

Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the prolonged warm droughts and more intense rainfall events predicted with climate change. The early 21st-century drought in the southwestern United States resulted in hydroclimatic conditions that are similar to those expected with future climate change. We investigated the impact of the early 21st-century drought on aboveground net primary production (ANPP) of six desert and plains grasslands dominated by C4 (warm season) grasses in terms of significant deviations between observed and expected ANPP. In desert grasslands, drought-induced grass mortality led to shifts in the functional response to annual total precipitation (P(T)), and in some cases, new species assemblages occurred that included invasive species. In contrast, the ANPP in plains grasslands exhibited a strong linear function of the current-year P(T) and the previous-year ANPP, despite prolonged warm drought. We used these results to disentangle the impacts of interannual total precipitation, intra-annual precipitation patterns, and grassland abundance on ANPP, and thus generalize the functional response of C4 grasslands to predicted climate change. This will allow managers to plan for predictable shifts in resources associated with climate change related to fire risk, loss of forage, and ecosystem services.

Controls over the strength and timing of fire-grazer interactions in a semi-arid rangeland

Summary 1. The degree to which large herbivores select and forage within recently burned areas is a key driver of vegetation heterogeneity in rangeland ecosystems. However, few studies have quantified the strength and timing of herbivore selection for burned areas or examined how selection strength varies among ecosystems differing in precipitation and primary productivity. 2. We conducted a 4-year patch-burning experiment in semi-arid rangeland of Colorado, USA, where 25% of the area available to cattle was burned each year and burned patches were shifted annually. We used GPS collars with activity sensors to quantify the distribution of free-ranging cattle at a high temporal resolution (5-min intervals) during the growing season each year. We used a classification tree model to discriminate between cattle grazing vs. non-grazing locations, which significantly increased precision in quantifying burn selection strength. We fit generalized linear models predicting the frequency of cattle use of a given location within each study area and month, enabling comparisons between the relative influence of burns and topography on grazing distribution. 3. Across multiple growing seasons, cattle selectively spent 31% of grazing time on recently burnt areas, which comprised 25% of the landscape; this selection strength was half as strong as that documented in mesic rangeland. 4. At a monthly temporal scale, strong cattle selection for burned areas occurred during periods of rapid vegetation growth regardless of when during the growing season this greening occurred. Outside these intervals, burn selection strength was inconsistent and cattle grazing distribution was primarily influenced by topography. Thus, the relative importance of fire and topography in controlling grazer distribution was temporally contingent upon the timing and size of precipitation pulses. 5. Synthesis and applications. Spatiotemporal interactions between fire and herbivores are a consistent feature of both semi-arid and mesic rangelands, with interaction strength varying across gradients of precipitation and primary productivity. Management of semi-arid ecosystems to sustain ecological processes should include strategies that allow ungulate herbivores to shift their grazing distribution seasonally in response to fire, topoedaphic variation and precipitation patterns. Combined management of fire and grazing for conservation objectives can be consistent with, and even complementary to, livestock production goals.

Managing for Biodiversity and Livestock: A scale-dependent approach for promoting vegetation heterogeneity in western Great Plains grasslands

In the western Great Plains (semiarid rangelands west of the 100th meridian and east of the Rocky Mountains), millions of acres of native grasslands remain intact. With benefi cial management, these grasslands have great potential to revive populations of grassland birds1 (such as the Mountain Plover, Lark Bunting, Upland Sandpiper, Long-billed Curlew, and McCown’s Longspur), which have declined over the last 30 years. To help reverse this decline, land managers can implement strategies to improve grassland bird habitat while maintaining the productivity of their livestock operations. Wildlife conservationists and agencies recommend managing grasslands for vegetation heterogeneity to improve grassland bird habitat and recover declining populations.2 Vegetation heterogeneity refers to variability in the structure and composition of plant communities over space and time. Grassland plant communities are inherently heterogeneous because of plant species diversity, which results from differences in physical characteristics (climate, soils, topography) as well as disturbance processes (e.g., livestock grazing, prescribed fi re, burrowing mammals, and ant hills). Grasslands with more vegetation heterogeneity support a greater number of plant and animal species because they contain additional structural complexity and/or diverse plant communities, which provide added spatial and temporal niches.3 For example, western Great Plains grasslands grazed at a range of intensities harbor more bird species than purely ungrazed or heavily grazed grasslands. This occurs because grasslands managed for a gradient of grazing intensities contain a wider variety of vegetation heights, which provide a diverse suite of nesting and feeding habitats4 (Fig. 1). Compared to other ecosystems, western Great Plains grasslands are relatively simple structurally and compositionally, making the maintenance and improvement of vegetation heterogeneity even more important. To increase habitat diversity for wildlife, management strategies should maintain or maximize inherent vegetation heterogeneity. However, many management practices tend to emphasize livestock production through more homogeneous use of plant communities, which results in decreasing vegetation heterogeneity and wildlife diversity.5 In contrast, grassland management practices that promote vegetation heterogeneity involve the application of techniques that increase variability in vegetation structure and/or composition. Managing for vegetation heterogeneity can be a win– win approach for wildlife and livestock managers alike, because it can maintain livestock production levels while increasing biodiversity. We suggest using a simple, scale-dependent framework to promote vegetation heterogeneity on western Great Plains grasslands. Our approach is designed to facilitate land management decisions that maintain or improve vegetation heterogeneity and biodiversity on these grasslands. We intend to help land managers consider appropriate scales at which to address vegetation heterogeneity, suggesting important management considerations at each scale, and providing examples of land management practices that can improve vegetation heterogeneity at each scale. Our approach is most useful for grassland managers in the western Great Plains endeavoring to maintain or increase vegetation heterogeneity for grassland wildlife or overall grassland health in general, and not for those implementing species-specifi c management. In cases of species-specifi c management, the objective(s) will be narrower, heterogeneitypromoting management may or may not be appropriate, and the scale of management decisions may be defi ned specifi cally by habitat needs of the target species.In the western Great Plains (semiarid rangelands west of the 100th meridian and east of the Rocky Mountains), millions of acres of native grasslands remain intact. With benefi cial management, these grasslands have great potential to revive populations of grassland birds1 (such as the Mountain Plover, Lark Bunting, Upland Sandpiper, Long-billed Curlew, and McCown’s Longspur), which have declined over the last 30 years. To help reverse this decline, land managers can implement strategies to improve grassland bird habitat while maintaining the productivity of their livestock operations. Wildlife conservationists and agencies recommend managing grasslands for vegetation heterogeneity to improve grassland bird habitat and recover declining populations.2 Vegetation heterogeneity refers to variability in the structure and composition of plant communities over space and time. Grassland plant communities are inherently heterogeneous because of plant species diversity, which results from differences in physical characteristics (climate, soils, topography) as well as disturbance processes (e.g., livestock grazing, prescribed fi re, burrowing mammals, and ant hills). Grasslands with more vegetation heterogeneity support a greater number of plant and animal species because they contain additional structural complexity and/or diverse plant communities, which provide added spatial and temporal niches.3 For example, western Great Plains grasslands grazed at a range of intensities harbor more bird species than purely ungrazed or heavily grazed grasslands. This occurs because grasslands managed for a gradient of grazing intensities contain a wider variety of vegetation heights, which provide a diverse suite of nesting and feeding habitats4 (Fig. 1). Compared to other ecosystems, western Great Plains grasslands are relatively simple structurally and compositionally, making the maintenance and improvement of vegetation heterogeneity even more important. To increase habitat diversity for wildlife, management strategies should maintain or maximize inherent vegetation heterogeneity. However, many management practices tend to emphasize livestock production through more homogeneous use of plant communities, which results in decreasing vegetation heterogeneity and wildlife diversity.5 In contrast, grassland management practices that promote vegetation heterogeneity involve the application of techniques that increase variability in vegetation structure and/or composition. Managing for vegetation heterogeneity can be a win– win approach for wildlife and livestock managers alike, because it can maintain livestock production levels while increasing biodiversity. We suggest using a simple, scale-dependent framework to promote vegetation heterogeneity on western Great Plains grasslands. Our approach is designed to facilitate land management decisions that maintain or improve vegetation heterogeneity and biodiversity on these grasslands. We intend to help land managers consider appropriate scales at which to address vegetation heterogeneity, suggesting important management considerations at each scale, and providing examples of land management practices that can improve vegetation heterogeneity at each scale. Our approach is most useful for grassland managers in the western Great Plains endeavoring to maintain or increase vegetation heterogeneity for grassland wildlife or overall grassland health in general, and not for those implementing species-specifi c management. In cases of species-specifi c management, the objective(s) will be narrower, heterogeneitypromoting management may or may not be appropriate, and the scale of management decisions may be defi ned specifi cally by habitat needs of the target species.

Vegetation Responses to Prescribed Burning of Grazed Shortgrass Steppe

Abstract Over the past century, fire has been widely suppressed in the western Great Plains, in part because of the potential negative effects on forage production for livestock. More recently, interest in the use of prescribed fire in shortgrass steppe has increased because of the potential applications for wildlife management, control of unpalatable plant species, and restoration of historic disturbance regimes. We studied the effects of prescribed burns conducted during late winter on herbaceous production, forage nitrogen content, and plant species composition of shortgrass steppe on the Pawnee National Grassland in northeastern Colorado. Late-winter burns conducted in moderately grazed sites under a wide range of precipitation conditions during 1997–2001 did not negatively affect herbaceous production in either the first or the second postburn growing season. Burning followed by a severe drought in 2002 reduced production by 19% in the second postburn growing season of 2003. Burns temporarily suppressed the abundance of broom snakeweed (Gutierrezia sarothrae) and prickly pear cactus (Opuntia polyacantha) and enhanced forage nitrogen content during May and June of the first postburn growing season. These findings suggest that, except following severe drought, prescribed burns conducted during late winter in grazed shortgrass steppe for objectives unrelated to livestock production can also have neutral or positive consequences for livestock.

Spatiotemporal dynamics of black-tailed prairie dog colonies affected by plague

Black-tailed prairie dogs (Cynomys ludovicianus) are a key component of the disturbance regime in semi-arid grasslands of central North America. Many studies have compared community and ecosystem characteristics on prairie dog colonies to grasslands without prairie dogs, but little is known about landscape-scale patterns of disturbance that prairie dog colony complexes may impose on grasslands over long time periods. We examined spatiotemporal dynamics in two prairie dog colony complexes in southeastern Colorado (Comanche) and northcentral Montana (Phillips County) that have been strongly influenced by plague, and compared them to a complex unaffected by plague in northwestern Nebraska (Oglala). Both plague-affected complexes exhibited substantial spatiotemporal variability in the area occupied during a decade, in contrast to the stability of colonies in the Oglala complex. However, the plague-affected complexes differed in spatial patterns of colony movement. Colonies in the Comanche complex in shortgrass steppe shifted locations over a decade. Only 10% of the area occupied in 1995 was still occupied by prairie dogs in 2006. In 2005 and 2006 respectively, 74 and 83% of the total area of the Comanche complex occurred in locations that were not occupied in 1995, and only 1% of the complex was occupied continuously over a decade. In contrast, prairie dogs in the Phillips County complex in mixed-grass prairie and sagebrush steppe primarily recolonized previously occupied areas after plague-induced colony declines. In Phillips County, 62% of the area occupied in 1993 was also occupied by prairie dogs in 2004, and 12% of the complex was occupied continuously over a decade. Our results indicate that plague accelerates spatiotemporal movement of prairie dog colonies, and have significant implications for landscape-scale effects of prairie dog disturbance on grassland composition and productivity. These findings highlight the need to combine landscape-scale measures of habitat suitability with long-term measures of colony locations to understand the role of plague-affected prairie dogs as a grassland disturbance process.

Strategic management of livestock to improve biodiversity conservation in African savannahs: a conceptual basis for wildlife–livestock coexistence

Summary 1. African savannas are complex socio-ecological systems with diverse wild and domestic herbivore assemblages, which adapt spatially to intra- and interannual variation in forage quantity and quality, predation and disease risks. 2. As African savannas become increasingly fragmented by growing human populations and their associated ecological impacts, adaptive foraging options for wild and domestic herbivore populations are correspondingly limited, resulting in declining wildlife populations and impoverished pastoral societies. In addition, competition for grazing by expanding domestic herbivore populations threatens the viability of wild herbivore populations occupying similar grazing niches. 3. Conservation initiatives are further impacted by conflicts between wildlife and local communities of people who often receive little benefit from adjacent protected areas, creating conflict between the livelihood-orientated goals of communities and the conservation-oriented goals of the international community and those with vested interests in wildlife. Conservation strategies facilitating the alignment of these opposing goals of communities and conservationists are needed. 4. Synthesis and applications. Key to understanding facilitative and competitive interactions between wild and domestic herbivores are the concepts of niche differentiation and functional resource heterogeneity. Uncontrolled incursions of burgeoning domestic herbivore populations into protected areas (PAs) threaten the conservation of wild herbivore biodiversity. However, domestic herbivores can be managed to minimize competition with wild herbivores and to enhance habitat by maximizing grassland structural heterogeneity (greater adaptive foraging options), creation of nutrient hotspots in the landscape and facilitation of high-quality grazing. Ecosystem service benefits to communities through controlled access to grazing resources in PAs, associated with appropriate disease management, can provide a conservation payment to promote communities’ support of conservation of key wildlife migratory ranges and corridors outside PAs.

Feature ArticlesManaging for Biodiversity and Livestock: A scale-dependent approach for promoting vegetation heterogeneity in western Great Plains grasslands

In the western Great Plains (semiarid rangelands west of the 100th meridian and east of the Rocky Mountains), millions of acres of native grasslands remain intact. With benefi cial management, these grasslands have great potential to revive populations of grassland birds1 (such as the Mountain Plover, Lark Bunting, Upland Sandpiper, Long-billed Curlew, and McCown’s Longspur), which have declined over the last 30 years. To help reverse this decline, land managers can implement strategies to improve grassland bird habitat while maintaining the productivity of their livestock operations. Wildlife conservationists and agencies recommend managing grasslands for vegetation heterogeneity to improve grassland bird habitat and recover declining populations.2 Vegetation heterogeneity refers to variability in the structure and composition of plant communities over space and time. Grassland plant communities are inherently heterogeneous because of plant species diversity, which results from differences in physical characteristics (climate, soils, topography) as well as disturbance processes (e.g., livestock grazing, prescribed fi re, burrowing mammals, and ant hills). Grasslands with more vegetation heterogeneity support a greater number of plant and animal species because they contain additional structural complexity and/or diverse plant communities, which provide added spatial and temporal niches.3 For example, western Great Plains grasslands grazed at a range of intensities harbor more bird species than purely ungrazed or heavily grazed grasslands. This occurs because grasslands managed for a gradient of grazing intensities contain a wider variety of vegetation heights, which provide a diverse suite of nesting and feeding habitats4 (Fig. 1). Compared to other ecosystems, western Great Plains grasslands are relatively simple structurally and compositionally, making the maintenance and improvement of vegetation heterogeneity even more important. To increase habitat diversity for wildlife, management strategies should maintain or maximize inherent vegetation heterogeneity. However, many management practices tend to emphasize livestock production through more homogeneous use of plant communities, which results in decreasing vegetation heterogeneity and wildlife diversity.5 In contrast, grassland management practices that promote vegetation heterogeneity involve the application of techniques that increase variability in vegetation structure and/or composition. Managing for vegetation heterogeneity can be a win– win approach for wildlife and livestock managers alike, because it can maintain livestock production levels while increasing biodiversity. We suggest using a simple, scale-dependent framework to promote vegetation heterogeneity on western Great Plains grasslands. Our approach is designed to facilitate land management decisions that maintain or improve vegetation heterogeneity and biodiversity on these grasslands. We intend to help land managers consider appropriate scales at which to address vegetation heterogeneity, suggesting important management considerations at each scale, and providing examples of land management practices that can improve vegetation heterogeneity at each scale. Our approach is most useful for grassland managers in the western Great Plains endeavoring to maintain or increase vegetation heterogeneity for grassland wildlife or overall grassland health in general, and not for those implementing species-specifi c management. In cases of species-specifi c management, the objective(s) will be narrower, heterogeneitypromoting management may or may not be appropriate, and the scale of management decisions may be defi ned specifi cally by habitat needs of the target species.In the western Great Plains (semiarid rangelands west of the 100th meridian and east of the Rocky Mountains), millions of acres of native grasslands remain intact. With benefi cial management, these grasslands have great potential to revive populations of grassland birds1 (such as the Mountain Plover, Lark Bunting, Upland Sandpiper, Long-billed Curlew, and McCown’s Longspur), which have declined over the last 30 years. To help reverse this decline, land managers can implement strategies to improve grassland bird habitat while maintaining the productivity of their livestock operations. Wildlife conservationists and agencies recommend managing grasslands for vegetation heterogeneity to improve grassland bird habitat and recover declining populations.2 Vegetation heterogeneity refers to variability in the structure and composition of plant communities over space and time. Grassland plant communities are inherently heterogeneous because of plant species diversity, which results from differences in physical characteristics (climate, soils, topography) as well as disturbance processes (e.g., livestock grazing, prescribed fi re, burrowing mammals, and ant hills). Grasslands with more vegetation heterogeneity support a greater number of plant and animal species because they contain additional structural complexity and/or diverse plant communities, which provide added spatial and temporal niches.3 For example, western Great Plains grasslands grazed at a range of intensities harbor more bird species than purely ungrazed or heavily grazed grasslands. This occurs because grasslands managed for a gradient of grazing intensities contain a wider variety of vegetation heights, which provide a diverse suite of nesting and feeding habitats4 (Fig. 1). Compared to other ecosystems, western Great Plains grasslands are relatively simple structurally and compositionally, making the maintenance and improvement of vegetation heterogeneity even more important. To increase habitat diversity for wildlife, management strategies should maintain or maximize inherent vegetation heterogeneity. However, many management practices tend to emphasize livestock production through more homogeneous use of plant communities, which results in decreasing vegetation heterogeneity and wildlife diversity.5 In contrast, grassland management practices that promote vegetation heterogeneity involve the application of techniques that increase variability in vegetation structure and/or composition. Managing for vegetation heterogeneity can be a win– win approach for wildlife and livestock managers alike, because it can maintain livestock production levels while increasing biodiversity. We suggest using a simple, scale-dependent framework to promote vegetation heterogeneity on western Great Plains grasslands. Our approach is designed to facilitate land management decisions that maintain or improve vegetation heterogeneity and biodiversity on these grasslands. We intend to help land managers consider appropriate scales at which to address vegetation heterogeneity, suggesting important management considerations at each scale, and providing examples of land management practices that can improve vegetation heterogeneity at each scale. Our approach is most useful for grassland managers in the western Great Plains endeavoring to maintain or increase vegetation heterogeneity for grassland wildlife or overall grassland health in general, and not for those implementing species-specifi c management. In cases of species-specifi c management, the objective(s) will be narrower, heterogeneitypromoting management may or may not be appropriate, and the scale of management decisions may be defi ned specifi cally by habitat needs of the target species.