Papers by David Augustine
Ecological Applications, 2015
Annals of the New York Academy of Sciences, Jan 11, 2018
African savannas support an iconic fauna, but they are undergoing large-scale population declines... more African savannas support an iconic fauna, but they are undergoing large-scale population declines and extinctions of large (>5 kg) mammals. Long-term, controlled, replicated experiments that explore the consequences of this defaunation (and its replacement with livestock) are rare. The Mpala Research Centre in Laikipia County, Kenya, hosts three such experiments, spanning two adjacent ecosystems and environmental gradients within them: the Kenya Long-Term Exclosure Experiment (KLEE; since 1995), the Glade Legacies and Defaunation Experiment (GLADE; since 1999), and the Ungulate Herbivory Under Rainfall Uncertainty experiment (UHURU; since 2008). Common themes unifying these experiments are (1) evidence of profound effects of large mammalian herbivores on herbaceous and woody plant communities; (2) competition and compensation across herbivore guilds, including rodents; and (3) trophic cascades and other indirect effects. We synthesize findings from the past two decades to highlig...
Remote Sensing
Monitoring of aboveground net primary production (ANPP) is critical for effective management of r... more Monitoring of aboveground net primary production (ANPP) is critical for effective management of rangeland ecosystems but is problematic due to the vast extent of rangelands globally, and the high costs of ground-based measurements. Remote sensing of absorbed photosynthetically active radiation (APAR) can be used to predict ANPP, potentially offering an alternative means of quantifying ANPP at both high temporal and spatial resolution across broad spatial extents. The relationship between ANPP and APAR has often been quantified based on either spatial variation across a broad region or temporal variation at a location over time, but rarely both. Here we assess: (i) if the relationship between ANPP and APAR is consistent when evaluated across time and space; (ii) potential factors driving differences between temporal versus spatial models, and (iii) the magnitude of potential errors relating to space for time transformations in quantifying productivity. Using two complimentary ANPP da...
Rangeland Ecology & Management
Rangeland Ecology & Management
The distribution of livestock across heterogeneous landscapes is often uneven, which has importan... more The distribution of livestock across heterogeneous landscapes is often uneven, which has important implications for vegetation dynamics and how rangeland managers achieve desired outcomes from these landscapes. Here, we use data from widely available digital elevation models to classify a landscape in the shortgrass steppe with subtle topographic variation using two different approaches: topographic wetness index (TWI) and topographic position classes (TPCs) derived from topographic position indices. We used global positioning system collars to track the grazing locations of cattle within replicate pastures and fit generalized linear mixed models to their locations to quantify the influence of topography on grazing distribution. In addition, we examine the influence of the presence of saline vegetation communities on cattle use of lowlands. The resulting models indicate that TPC more effectively predicts grazing distribution than TWI and that the patterns are strongest in the second half of the growing season (August−October). Model performance was improved with the inclusion of saline vegetation communities, although the magnitude of cattle grazing time in these communities was not consistent across multiple pastures. These models, in combination with local knowledge, can be used by managers to predict and manage livestock distribution even in landscapes with relatively subtle topographic variability.
Journal of Vegetation Science
Questions: How does long-term exclusion of cattle grazing influence plant community composition i... more Questions: How does long-term exclusion of cattle grazing influence plant community composition in a semi-arid grassland? Can spatial variation in the effects of cattle grazing exclusion be explained by variation in soil texture? Location: Shortgrass steppe, northeast Colorado, USA, in the North American Great Plains. Method: We used 16 long-term (72 yr) cattle exclosures to examine the effects of grazers on plant communities and evaluate whether grazer effects interact with soil texture. Results: Although shortgrass steppe communities are relatively unaffected by grazing in the short-term (one to two decades), exclusion of cattle grazing for seven decades caused a compositional shift from dominance by a C 4 shortgrass (Bouteloua gracilis) to co-dominance by a C 3 midgrass (Pascopyrum smithii) and B. gracilis. The strength of this shift was highly variable across sites. Soil texture was correlated with the abundance of certain plant species, but did not explain variation in the magnitude of grazer effects. Grazed communities contained perennial and annual growth forms with a diversity of strategies to co-exist with grazers and B. gracilis. Ungrazed communities included increased abundance of annual, ruderal forbs and three woody plant species. Grazing effects occurred against a backdrop of changing plant communities: during the past seven decades, C 3 perennial graminoids and sub-shrubs have increased in relative abundance in both grazed and ungrazed communities. Conclusions: Our long-term experiment shows that community responses to grazing in this semi-arid grassland occur very slowly, but are predictable, with C 4 shortgrasses eventually giving way to taller C 3 grasses and ruderal forbs. Spatial variation in grazing effects across sites (and lack of a relationship with soil texture) may reflect the importance of fine-scale heterogeneity in C 3 grass abundance, and the slow rate at which taller C 3 grasses can coalesce into mono-dominant patches that outcompete C 4 shortgrasses. Increased abundance of C 3 species over the past seven decades, both in the presence and absence of grazing, may be related to recovery from the severe drought and dust storms of the 1930s as well as enhanced growth of C 3 plants under increasing atmospheric [CO 2 ].
Écoscience
DJ Augustine, D deCalesta Ecoscience 10:44, 472-486, 2003. ... We apply this analysis to Trillium... more DJ Augustine, D deCalesta Ecoscience 10:44, 472-486, 2003. ... We apply this analysis to Trillium populations from forests in Minnesota (highly fragmented agricultural landscape with varying deer densities), Pennsylvania (forested and fragmented landscape with long-term high ...
Remote Sensing in Ecology and Conservation, 2017
Small-bodied terrestrial animals such as songbirds (Order Passeriformes) are especially vulnerabl... more Small-bodied terrestrial animals such as songbirds (Order Passeriformes) are especially vulnerable to hail-induced mortality; yet, hail events are challenging to predict, and they often occur in locations where populations are not being studied. Focusing on nesting grassland songbirds, we demonstrate a novel approach to estimate hail-induced mortality. We quantify the relationship between the probability of nests destroyed by hail and measured Level-III Next Generation Radar (NEXRAD) data, including atmospheric base reflectivity, maximum estimated size of hail and maximum estimated azimuthal wind shear. On 22 June 2014, a hailstorm in northern Colorado destroyed 102 out of 203 known nests within our research site. Lark bunting (Calamospiza melanocorys) nests comprised most of the sample (n = 186). Destroyed nests were more likely to be found in areas of higher storm intensity, and distributions of NEX-RAD variables differed between failed and surviving nests. For 133 ground nests where nest-site vegetation was measured, we examined the ameliorative influence of woody vegetation, nest cover and vegetation density by comparing results for 13 different logistic regression models incorporating the independent and additive effects of weather and vegetation variables. The most parsimonious model used only the interactive effect of hail size and wind shear to predict the probability of nest survival, and the data provided no support for any of the models without this predictor. We conclude that vegetation structure may not mitigate mortality from severe hailstorms and that weather radar products can be used remotely to estimate potential for hail mortality of nesting grassland birds. These insights will improve the efficacy of grassland bird population models under predicted climate change scenarios.
Large herbivores can both positively and negatively affect primary productivity and rates of nutr... more Large herbivores can both positively and negatively affect primary productivity and rates of nutrient cycling in different ecosystems. Positive effects of grazers in grasslands have been attributed to migratory behavior of the dominant ungulate species and soil fertility. We studied the effects of grazers on aboveground net primary productivity (ANPP) and N cycling on central Kenyan rangeland characterized by intense, chronic grazing by a mixed community of cattle and resident native ungulates. Exclosure studies conducted at high and low levels of soil fertility showed that both soil fertility and annual rainfall patterns mediate the effects of grazers on ANPP and N cycling. In a lowrainfall year with short (1 month) growing seasons, grazers reduced aboveground productivity regardless of soil nutrient availability. However, in a highrainfall year with a 5-month growing season, grazers increased ANPP on nutrient-rich glades and suppressed ANPP on nutrient-poor bushland sites. Concomitant studies of grazer effects on N cycling revealed complex interactions with the seasonal pattern of N-mineralization and inorganic N availability. Grazers increased the size of the inorganic N pool available to plants at the onset of the growing season, particularly in nutrient-rich glades. However, grazers also decreased N mineralization rates at all sites early in the growing season. Measures of N availability via ion-exchange resin bags suggested that the combined effects of grazers on inorganic N pool fluctuations and N-mineralization rates resulted in a net increase in N availability at glade sites and a net decrease in N availability at bushland sites. The net effect of grazers on soil N availability mirrored grazer effects on ANPP in the high-rainfall year. Overall, our results suggest that grazer effects on N dynamics are closely linked to effects on productivity and resilience to drought. Furthermore, even under optimal conditions of high soil fertility and above-average rainfall, grazer promotion of ANPP in this chronically grazed system dominated by resident ungulates was small compared to systems dominated by migratory ungulates.
The Journal of Wildlife Management, 2015
Anthropogenic changes to disturbance regimes in grasslands, and associated homogenization of vege... more Anthropogenic changes to disturbance regimes in grasslands, and associated homogenization of vegetation structure, have been implicated as factors contributing to declines in populations of grassland birds in North America. We examined the influence of patch-burn grazing management, which employs spatiotemporal interactions between fire and livestock grazing guided by historical disturbance patterns, on vegetation structure and bird abundance in shortgrass steppe in northeastern Colorado, USA. All study pastures were grazed by cattle at moderate stocking rates from May to October each year. In the patch-burn treatment, we burned 25% of each pasture in autumn (Oct or Nov) each year during 2007-2010; control pastures were not burned. Patch-burn grazing management increased vegetation heterogeneity by generating short (<4 cm), sparse vegetation on recent burns. Although cattle selectively grazed recent burns, this did not alter vegetation structure in unburned portions of patch-burned pastures relative to controls. Of the 6 grassland bird species we examined, mountain plovers (Charadrius montanus) occurred exclusively on recent burns, whereas grasshopper sparrows (Ammodramus savannarum) occurred exclusively in grassland not burned for !3 years. Two species (lark bunting [Calamospiza melanocorys] and western meadowlark [Sturnella neglecta]) were 2-3 times less abundant on recent burns compared to controls, whereas densities of horned larks (Eremophila alpestris) and McCown's longspur (Rhynchophanes mccownii) were unaffected by burning. Lark bunting, western meadowlark, and grasshopper sparrow densities varied substantially among years. In the years when they were abundant, all 3 species increased in density across the time-since-burning gradient. Consistent with this pattern, patch-burn grazing management reduced the abundance of all 3 species at the whole-pasture scale. We found no evidence that unburned patches within the patch-burned pastures differed from unburned pastures in terms of the abundance of any bird species. Patch-burn grazing management was an effective strategy to create breeding habitat for mountain plovers. However, our findings suggest that in the shortgrass steppe, additional strategies that generate taller, more dense vegetation than occurs under moderate cattle grazing need to be considered in combination with patch-burn grazing management to sustain breeding habitat for the full suite of native grassland birds. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
Ecology, 2015
Increasingly, the restoration of large carnivores is proposed as a means through which to restore... more Increasingly, the restoration of large carnivores is proposed as a means through which to restore community structure and ecosystem function via trophic cascades. After a decades-long absence, African wild dogs (Lycaon pictus) recolonized the Laikipia Plateau in central Kenya, which we hypothesized would trigger a trophic cascade via suppression of their primary prey (dik-dik, Madoqua guentheri) and the subsequent relaxation of browsing pressure on trees. We tested the trophic-cascade hypothesis using (1) a 14-year time series of wild dog abundance; (2) surveys of dik-dik population densities conducted before and after wild dog recovery; and (3) two separate, replicated, herbivore-exclusion experiments initiated before and after wild dog recovery. The dik-dik population declined by 33% following wild dog recovery, which is best explained by wild dog predation. Dik-dik browsing suppressed tree abundance, but the strength of suppression did not differ between before and after wild dog recovery. Despite strong, top-down limitation between adjacent trophic levels (carnivoreherbivore and herbivore-plant), a trophic cascade did not occur, possibly because of a time lag in indirect effects, variation in rainfall, and foraging by herbivores other than dik-dik. Our ability to reject the trophic-cascade hypothesis required two important approaches: (1) temporally replicated herbivore exclusions, separately established before and after wild dog recovery; and (2) evaluating multiple drivers of variation in the abundance of dik-dik and trees. While the restoration of large carnivores is often a conservation priority, our results suggest that indirect effects are mediated by ecological context, and that trophic cascades are not a foregone conclusion of such recoveries.
Wildlife Society Bulletin, 2012
Aerial surveys using line-intercept methods are one approach to estimate the extent of prairie do... more Aerial surveys using line-intercept methods are one approach to estimate the extent of prairie dog colonies in a large geographic area. Although black-tailed prairie dogs (Cynomys ludovicianus) construct conspicuous mounds at burrow openings, aerial observers have difficulty discriminating between areas with burrows occupied by prairie dogs (colonies) versus areas of uninhabited burrows (uninhabited colony sites). Consequently, aerial line-intercept surveys may overestimate prairie dog colony extent unless adjusted by an on-the-ground inspection of a sample of intercepts. We compared aerial line-intercept surveys conducted over 2 National Grasslands in Colorado, USA, with independent ground-mapping of known black-tailed prairie dog colonies. Aerial line-intercepts adjusted by ground surveys using a single activity category adjustment overestimated colonies by !94% on the Comanche National Grassland and !58% on the Pawnee National Grassland. We present a ground-survey technique that involves 1) visiting on the ground a subset of aerial intercepts classified as occupied colonies plus a subset of intercepts classified as uninhabited colony sites, and 2) based on these ground observations, recording the proportion of each aerial intercept that intersects a colony and the proportion that intersects an uninhabited colony site. Where line-intercept techniques are applied to aerial surveys or remotely sensed imagery, this method can provide more accurate estimates of black-tailed prairie dog abundance and trends. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.
Smithsonian Contributions to Zoology, 2011
Livestock-wildlife interactions in rangelands are often viewed in terms of competition, but lives... more Livestock-wildlife interactions in rangelands are often viewed in terms of competition, but livestock and native ungulates can also benefi t each other through longterm modifi cations of rangeland habitats. Here we synthesize research on rangelands in central Laikipia focusing on two types of cattle-wildlife interactions that have implications for their long-term coexistence. The fi rst interaction occurs via redistribution of soil nutrients within the ecosystem, which is a consequence of the use of bomas (temporary corrals) to manage livestock. Our studies on two different soil types show that rotational boma management creates hectare-scale patches in the landscape that are enriched in soil and plant nutrients and persist for decades to centuries. In both of the predominant soil types in Laikipia, forage phosphorus content is low relative to ungulate demands during peak lactation. Nutrient-rich boma sites (hereafter referred to as glades) provide a key wet-season forage resource of nutritional suffi ciency for lactation. Our studies further show that a wide range of native ungulates selectively use glades relative to surrounding nutrient-poor habitats. Impala (Aepyceros melampus) in particular show intensive use of glades on sandy soils and are rare in portions of the landscape lacking glades. A second important pathway for cattle-wildlife interaction occurs through the infl uence of native browsing ungulates on woody vegetation. Shrub and tree cover has been increasing in Laikipia over the past century, followed by increases in native browsers in recent decades on ranches where wildlife are allowed to coexist with cattle. Our exclosure experiments in central Laikipia indicate that native browsers suppress shrub encroachment on both dominant soil types. However, the strength of browser effects are three to seven times greater on sandy soils, where two browsers, dik-dik and elephants, are both abundant, compared to heavy clay soils, where elephants are the only dominant browser. In the clay soils, native browsers still exert a signifi cant infl uence on dynamics of the dominant tree, Acacia drepanolobium, and suppress encroachment by subdominant shrub species. Browser effects on woody vegetation likely enhance forage production for cattle and maintain open habitats favored by native grazers for predator avoidance. Taken together, our studies indicate that boma rotation and browser control of shrub encroachment are key interaction pathways that promote cattle-wildlife coexistence in the Ewaso ecosystem.
Western North American Naturalist, 2008
Swift foxes (Vulpes velox) are shortgrass specialists and as a result are heavily dependent upon ... more Swift foxes (Vulpes velox) are shortgrass specialists and as a result are heavily dependent upon grassland disturbance regimes to maintain high-quality habitat. To better understand this relationship, we monitored the movement and spatial ecology of resident swift foxes before and after a 2005 prescribed burn in southeastern Colorado. We hypothesized that foxes would shift home ranges into and increase foraging activity within the prescribed burn area. Foxes did appear to alter their space-use patterns in response to the burn, although the response was constrained by territoriality. Foxes whose core-use areas overlapped the burn increased their use of the burn area for foraging and denning, but we did not observe shifts of individual home ranges to encompass more of the burned area. Foxes whose coreuse areas did not overlap the burn did not alter their space use or change home range boundaries in response to the burn. Because we observed only positive or neutral responses to prescribed burning, we recommend this tool as an appropriate method to maintain high-quality swift fox habitat.
Rangeland Ecology & Management, 2009
Oikos, 2013
Th e response of semiarid grasslands to small, non-colonial herbivores has received little attent... more Th e response of semiarid grasslands to small, non-colonial herbivores has received little attention, focusing primarily on the eff ects of granivore assemblages on annual plant communities. We studied the long-term eff ects of both small and large herbivores on vegetation structure and species diversity of shortgrass steppe, a perennial semiarid grassland considered marginal habitat for small mammalian herbivores. We hypothesized that 1) large generalist herbivores would aff ect more abundant species and proportions of litter-bare ground-vegetation cover through non-selective herbivory, 2) small herbivores would aff ect less common species through selective but limited consumption, and 3) herbivore eff ects on plant richness would increase with increasing aboveground net primary production (ANPP). Plant community composition was assessed over a 14-year period in pastures grazed at moderate intensities by cattle and in exclosures for large (cattle) and large-plus-small herbivores (additional exclusion of rabbits and rodents). Exclusion of large herbivores aff ected litter and bare ground and basal cover of abundant, common and uncommon species. Additional exclusion of small herbivores did not aff ect uncommon components of the plant community, but had indirect eff ects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis and total vegetation, and increased litter and species diversity. Th ere was no relationship between ANPP and the intensity of eff ects of either herbivore body size on richness. Exclusion of herbivores of both body sizes had complementary and additive eff ects which promoted changes in vegetation composition and physiognomy that were linked to increased abundance of tall and decreased abundance of short species. Our fi ndings show that small mammalian herbivores had disproportionately large eff ects on plant communities relative to their small consumption of biomass. Even in smallseeded perennial grasslands with a long history of intensive grazing by large herbivores, non-colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.
Oecologia, 2011
Carbon allocation and N acquisition by plants following defoliation may be linked through plant-m... more Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Plant C allocation patterns and rhizosphere interactions can also be affected by rising atmospheric CO 2 concentrations, which in turn could influence plant and microbial responses to defoliation. We studied two widespread perennial grasses native to rangelands of western North America to test whether (1) defoliation-induced enhancement of rhizodeposition would stimulate rhizosphere N availability and plant N uptake, and (2) defoliation-induced enhancement of rhizodeposition, and associated effects on soil N availability, would increase under elevated CO 2. Both species were grown at ambient (400 lL L-1) and elevated (780 lL L-1) atmospheric [CO 2 ] under water-limiting conditions. Plant, soil and microbial responses were measured 1 and 8 days after a defoliation treatment. Contrary to our hypotheses, we found that defoliation and elevated CO 2 both reduced carbon inputs to the rhizosphere of Bouteloua gracilis (C 4) and Pascopyrum smithii (C 3). However, both species also increased N allocation to shoots of defoliated versus nondefoliated plants 8 days after treatment. This response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and reduced allocation of post-defoliation assimilate to roots. In contrast, B. gracilis increased allocation of post-defoliation assimilate to roots, and did not exhibit defoliation-induced reductions in root biomass or N content. Our findings highlight key differences between these species in how postdefoliation C allocation to roots versus shoots is linked to shoot N yield, but indicate that defoliation-induced enhancement of shoot N concentration and N yield is not mediated by increased C allocation to the rhizosphere. Keywords Global change Á Grazing tolerance Á Pulse dynamics Á Rhizodeposition Á Water relations Communicated by Christian Körner.
Landscape Ecology, 2008
Black-tailed prairie dogs (Cynomys ludovicianus) are a key component of the disturbance regime in... more 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 The U.S. Government's right to retain a non-exclusive, royaltyfree license in and to any copyright is acknowledged.
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Papers by David Augustine