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Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one‐zebra increase in herbivore biomass density (~100 kg/km2 of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass. [ABSTRACT FROM AUTHOR]

Herbivores form an important link in the transfer of energy within a food web and are strongly influenced by bottom‐up trophic cascades. Current hypotheses suggest that herbivore consumption and impact on plants should scale positively with plant resource availability. However, depending on the effect of resources on plant quantity and quality, herbivore impact may vary with different types of resources.We test four alternative hypotheses for the relationship between plant biomass, herbivore impact on plant biomass and plant resource gradients, each based on how resources might affect plant abundance and quality to herbivores. We measured plant biomass for four non‐consecutive years in a long‐term grazing exclosure experiment in the Serengeti National Park that includes seven sites that vary substantially in rainfall and soil and plant nitrogen (N) and phosphorus (P).Our data supported the hypothesis that herbivore impact is controlled by plant quality, in this case driven by plant P, as herbivore effects on biomass decreased with higher rainfall but increased with greater plant P, but not N content. To our knowledge, this is the first experimental study to indicate that wild mammalian herbivory is associated with P availability rather than N.Synthesis. Our results suggest that P, in addition to water and N, may play a more important role in driving trophic interactions in terrestrial systems than previously realized. Given the uncertainties in rainfall due to climate change and increasing anthropogenic manipulations of global N and P cycles, our findings emphasize the need to consider multiple resources for understanding how trophic interactions might be influenced by environmental variables. [ABSTRACT FROM AUTHOR]

Herbivory is a major energy transfer within ecosystems; an open question is under what circumstances it can stimulate aboveground seasonal primary production. Despite multiple field demonstrations, past theory considered herbivory as a continuous process and found stimulation of seasonal production to be unlikely. Here, we report a new theoretical model that explores the consequences of discrete herbivory events, or episodes, separated in time. We discovered that negative density (biomass) dependence of plant growth, such as might be expected from resource limitation of plant growth, favors stimulation of seasonal production by infrequent herbivory events under a wide range of herbivory intensities and maximum plant relative growth rates. Results converge to those of previous models under repeated, short‐interval herbivory, which generally reduces seasonal production. Model parameters were estimated with new and previous data from the Serengeti ecosystem. Patterns of observed frequent and large magnitude stimulated production in these data agreed generally with those predicted by the episodic herbivory model. The model thus may provide a new framework for evaluating the sustainability and impact of herbivory. [ABSTRACT FROM AUTHOR]

Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestrial ecosystems worldwide, and both types of herbivory have been implicated in major ecosystem state changes. Despite the ecological consequences of differences in diets and feeding habits among herbivores, studies that experimentally distinguish effects of grazing from spatially co-occurring, but temporally segregated browsing are extremely rare. Here we use a set of long-term exclosures in northern Utah, USA, to determine how domestic grazers vs. wild ungulate herbivores (including browsers and mixed feeders) affect sagebrush-dominated plant communities that historically covered ~62 million ha in North America. We sampled plant community properties and found that after 22 years grazing and browsing elicited perceptible changes in overall plant community composition and distinct responses by individual plant species. In the woody layer of the plant community, release from winter and spring wild ungulate herbivory increased densities of larger Wyoming big sagebrush (Artemisia tridentata, ssp. wyomingensis) at the expense of small sagebrush, while disturbance associated with either cattle or wild ungulate activity alone was sufficient to increase bare ground and reduce cover of biological soil crusts. The perennial bunchgrass, bottlebrush squirretail (Elymus elymoides), responded positively to release from summer cattle grazing, and in turn appeared to competitively suppress another more grazing tolerant perennial grass, Sandberg’s blue grass (Poa secunda). Grazing by domestic cattle also was associated with increased non-native species biomass. Together, these results illustrate that ungulate herbivory has not caused sagebrush plant communities to undergo dramatic state shifts; however clear, herbivore-driven shifts are evident. In a dry, perennial-dominated system where plant community changes can occur very slowly, our results provide insights into potential long-term trajectories of these plant communities under different large herbivore regimes. Our results can be used to guide long-term management strategies for sagebrush systems and improve habitat for endemic wildlife species such as sage-grouse (Centrocercus spp.). [ABSTRACT FROM AUTHOR]

Given the role of fire in shaping ecosystems, especially grasslands and savannas, it is important to understand its broader impact on these systems. Post-fire stimulation of plant nutrients is thought to benefit grazing mammals and explain their preference for burned areas. However, fire also reduces vegetation height and increases visibility, thereby potentially reducing predation risk. Consequently, fire may be more beneficial to smaller herbivores, with higher nutritional needs and greater risks of predation., We tested the impacts of burning on different sized herbivores' habitat preference in Serengeti National Park, as mediated by burning's effects on vegetation height, live : dead biomass ratio and leaf nutrients., Burning caused a less than 4 month increase in leaf nitrogen (N), and leaf non-N nutrients [copper (Cu), potassium (K), and magnesium (Mg)] and a decrease in vegetation height and live : dead biomass. During this period, total herbivore counts were higher on burned areas. Generally, smaller herbivores preferred burned areas more strongly than larger herbivores., Unfortunately, it was not possible to determine the vegetation characteristics that explained burned area preference for each of the herbivore species observed. However, total herbivore abundance and impala ( Aepyceros melampus) preference for burned areas was due to the increases in non-N nutrients caused by burning., These findings suggest that burned area attractiveness to herbivores is mainly driven by changes to forage quality and not potential decreases in predation risk caused by reductions in vegetation height. [ABSTRACT FROM AUTHOR]

Soils of grasslands represent a large potential reservoir for storing CO2, but this potential likely depends on how grasslands are managed for large mammal grazing. Previous studies found both strong positive and negative grazing effects on soil organic carbon ( SOC) but explanations for this variation are poorly developed. Expanding on previous reviews, we performed a multifactorial meta-analysis of grazer effects on SOC density on 47 independent experimental contrasts from 17 studies. We explicitly tested hypotheses that grazer effects would shift from negative to positive with decreasing precipitation, increasing fineness of soil texture, transition from dominant grass species with C3 to C4 photosynthesis, and decreasing grazing intensity, after controlling for study duration and sampling depth. The six variables of soil texture, precipitation, grass type, grazing intensity, study duration, and sampling depth explained 85% of a large variation (±150 g m−2 yr−1) in grazing effects, and the best model included significant interactions between precipitation and soil texture ( P = 0.002), grass type, and grazing intensity ( P = 0.012), and study duration and soil sampling depth ( P = 0.020). Specifically, an increase in mean annual precipitation of 600 mm resulted in a 24% decrease in grazer effect size on finer textured soils, while on sandy soils the same increase in precipitation produced a 22% increase in grazer effect on SOC. Increasing grazing intensity increased SOC by 6-7% on C4-dominated and C4-C3 mixed grasslands, but decreased SOC by an average 18% in C3-dominated grasslands. We discovered these patterns despite a lack of studies in natural, wildlife-dominated ecosystems, and tropical grasslands. Our results, which suggest a future focus on why C3 vs. C4-dominated grasslands differ so strongly in their response of SOC to grazing, show that grazer effects on SOC are highly context-specific and imply that grazers in different regions might be managed differently to help mitigate greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

Ecology Letters (2010) 13: 959–968 Grazing occurs over a third of the earth’s land surface and may potentially influence the storage of 109 Mg year−1 of greenhouse gases as soil C. Displacement of native herbivores by high densities of livestock has often led to overgrazing and soil C loss. However, it remains unknown whether matching livestock densities to those of native herbivores can yield equivalent soil C sequestration. In the Trans-Himalayas we found that, despite comparable grazing intensities, watersheds converted to pastoralism had 49% lower soil C than watersheds which retain native herbivores. Experimental grazer-exclusion within each watershed type, show that this difference appears to be driven by indirect effects of livestock diet selection, leading to vegetation shifts that lower plant production and reduce likely soil C inputs from vegetation by c. 25 gC m−2 year−1. Our results suggest that while accounting for direct impacts (stocking density) is a major step, managing indirect impacts on vegetation composition are equally important in influencing soil C sequestration in grazing ecosystems. [ABSTRACT FROM AUTHOR]

Soil carbon pools remain a target for sequestering greenhouse gases, but appropriate land management options to achieve such sequestration remain uncertain. Livestock grazing can have profound positive or negative effects on soil carbon. Different models for assessing the influences of grazing are available, but few explicitly account for different management options on soil organic carbon (SOC). Here, I link a previous simple SOC dynamic model (SNAP) to a recent model of episodic grazing and its effects on primary production. The resulting combined model, called SNAPGRAZE, assesses the potential effects of grazing management on SOC across a range of climates with only eight climate, soil, and management input variables. SNAPGRAZE predicts that, at high stocking densities relative to those sustainable under continuous grazing and at higher mean annual temperature and precipitation, short-duration, high stocking density (SDHSD) grazing schemes can enhance forage production and increase stocks of soil organic carbon. Model predictions for current SOC, given a known 50 year grazing history, agrees well with data from nine private ranches in the North American Great Plains. SNAPGRAZE may provide a framework for exploring the consequences of grazing management for forage production and soil carbon dynamics. [ABSTRACT FROM AUTHOR]

Abstract: The high-altitude rangelands of the Trans-Himalaya represent a grazing ecosystem which has supported an indigenous pastoral community for millennia alongside a diverse assemblage of wild herbivores including burrowing mammals (pikas and voles). Pastoralists consider the small mammals to cause rangeland degradation and as competitors for their livestock, and actively eradicate them at many places. We present data on the ways in which small herbivores like pikas and voles mediate plant community dynamics. Vegetation cover and plant species richness were compared on and off both active and abandoned small mammal colonies. Plant species richness was higher inside colonies (about 4–5 species/plot) than outside (about 3 species/plot) whereas vegetation cover was only marginally lower (52% compared to 60%). Soil disturbance due to small mammals is seen to be associated with higher plant diversity without causing dramatic decline in overall vegetation cover. Such disturbance-mediated dynamics and vegetation mosaics produce a rich array of testable hypotheses that can highlight how small mammals influence assembly processes, succession, and dominance hierarchies in plant communities in this arid ecosystem. So, eradicating small mammals may lead to declining levels of diversity in this ecosystem, and compromise ecosystem-functioning. Changes in traditional pastoral practices and overstocking are more likely to be responsible for degradation. We emphasize that eradicating small mammals can lead to loss of diversity in this ecosystem and it is not a solution for the degradation problems. [Copyright &y& Elsevier]