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 (similar to 100 kg/km(2) of metabolic biomass) resulted in a similar to 53 kg/ha reduction in standing herbaceous biomass and a similar to 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 Semi-arid rangelands cover 40% of the earth's land surface, but their ecosystem services have declined due to, among other factors, increasingly sedentary livestock husbandry by pastoralists. Such degradation might be reversed by adopting frequent, large-scale livestock movements similar to those associated with traditional nomadic pastoralist habits but data to support this hypothesis is lacking. We report here the consequences of implementing a program to increase the mobility of livestock grazing across 2 million ha in northern Kenya from 2014-2021, as compared to conditions prior to the program, 2001-2013. Despite increased human populations and livestock numbers on the study area during the program, nearly 60% of 213 sampled locations experienced the single major grazing events per year intended by the grazing program in at least 6 out of 7 years, with accompanying rainfall-corrected increases in forage biomass and soil carbon sequestration. Locations where mobile grazing was less consistently applied experienced neither of these responses. We demonstrate that increasing mobility in grazing practices within large-scale pastoralist systems can be implemented to improve range condition and soils and consequently sequester substantial CO 2 in soil organic carbon.

Many plant species are widely distributed and consequently are exposed to multiple abiotic factors and diverse herbivores, each of which may distinctly affect the magnitude of different defense traits. Alternative theories for optimal allocation to plant defense traits predict both positive and negative associations between magnitude of defense and resource availability. These predictions may apply even within species. This suggests potential for a single species' patterns of association of defense traits and resources to vary with both the type of defense and identity of resource, but relatively few studies have explored intraspecific variation in multiple defense traits along several resource gradients simultaneously. In order to address this gap, especially in an ecosytem dominated by large mammalian herbivores, we assessed relationships between multiple resources (rainfall, soil N, and soil P) and plant defense traits (prickle density, phenolics, and lignin content) using a widely distributed tropical savanna herb, Solanum incanum , growing in naturally occuring resource gradients within the Serengeti National Park. We found substantial intraspecific variation in all three defense traits across sites (n = 43). Variation in prickle density was positively associated with rainfall and soil P, but not soil N. In contrast to prickle density, phenolics and lignin were uncorrelated with all three resource gradients. This independent association of soil P with a carbon‐based defense, prickle density, suggests potential for resources that are not components of defenses to influence allocation to defense traits. Such influence may reflect association between resource and herbivore abundance and/or preference. These varied patterns in resource–defense associations further emphasize the tremendous variation in anti‐herbivore traits which may be influenced by different plant resources and highlight the need to consider multiple resource gradients in understanding evolution of plant traits.

Metabolic scaling is one of the most important patterns in biology. Theory explaining the 3/4-power size-scaling of biological metabolic rate does not predict the non-linear scaling observed for smaller life forms. Here we present a new model for cells $<10^{-8}$ m$^{3}$ that maximizes power from the reaction-displacement dynamics of enzyme-catalyzed reactions. Maximum metabolic rate is achieved through an allocation of cell volume to optimize a ratio of reaction velocity to molecular movement. Small cells $< 10^{-17}$ m$^{3}$ generate power under diffusion by diluting enzyme concentration as cell volume increases. Larger cells require bulk flow of cytoplasm generated by molecular motors. These outcomes predict curves with literature-reported parameters that match the observed scaling of metabolic rates for unicells, and predicts the volume at which Prokaryotes transition to Eukaryotes. We thus reveal multiple size-dependent physical constraints for microbes in a model that extends prior work to provide a parsimonious hypothesis for how metabolism scales across small life.

Introduction: Allocation to plant defense traits likely depends on resource supply, herbivory, and other plant functional traits such as the leaf economic spectrum (LES) traits. Yet, attempts to integrate defense and resource acquisitive traits remain elusive. Methods: We assessed intraspecific covariation between different defense and LES traits in a widely distributed tropical savanna herb, Solanum incanum, a unique model species for studying allocations to physical, chemical, and structural defenses to mammalian herbivory. Results: We found that in a multivariate trait space, the structural defenses - lignin and cellulose - were positively related to the resource conservative traits - low SLA and low leaf N. Phenolic content, a chemical defense, was positively associated with resource acquisitive traits - high SLA and high leaf N - while also being associated with an independent third component axis. Both principal components 1 and 3 were not associated with resource supply and herbivory intensity. In contrast, spine density - a physical defense - was orthogonal to the LES axis and positively associated with soil P and herbivory intensity. Discussion: These results suggest a hypothesized "pyramid" of trade-offs in allocation to defense along the LES and herbivory intensity axes. Therefore, future attempts to integrate defense traits with the broader plant functional trait framework, such as the LES, needs a multifaceted approach that accounts for unique influences of resource acquisitive traits and herbivory risk.

Variation in defense traits likely depends on access to different resources and risk from herbivory. Plant defense theories have predicted both positive and negative associations between defense traits and access to resources, but relatively few studies have explored intraspecific variation in defense traits along multiple resource and mammalian herbivory risk gradients. We assessed relationships between herbivory intensity, multiple resources, and plant defense traits using a widely distributed tropical savanna herb, Solanum incanum . As independent measures of risk from large mammal herbivores are rare, we used a satellite-based vegetation index to predict herbivory intensity at the landscape scale. We found that the satellite-based estimate of herbivory intensity was positively associated with browser abundance and total soil P, but negatively associated with rainfall. Intraspecific defense traits too varied substantially across sites (n=43) but only variation in spine density was associated with herbivory intensity and plant resources, such that spine density was positively associated with both rainfall and soil P, but bimodally associated with herbivory intensity. Taken together, it suggests that defenses maybe favored either where resources for defense are abundant under low but still present risk (i.e, at high rainfall sites) or where resource-expensive plant tissue is at high risk (i.e, at high soil P sites). This hints at the possibility of a shift from a resource-associated (bottom-up) to an herbivory-associated (top-down) control of allocation to defenses along an environmental gradient. Additionally, the independent effect of soil P on a carbon-based defense, spine density, suggests potential for resources that are not components of defenses to also influence allocation to defense traits. Thus, our study provides evidence for the influence of multiple drivers, resources, and herbivory intensity, on anti-herbivore defenses and their shifting relative importance on allocation to defenses along an environmental gradient.

On the second day of the 2021 United Nations Climate Change Conference (COP26), 130 nations announced a decision to halt global deforestation by 2030 (1). This is a welcome move and a political success, but ecologically it falls short. The plan needs to be expanded to include savannas, which cover an area of 20 million km2—more than the 17 million km2 covered by tropical forests (2)—and are potentially more important carbon sinks than forests.

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. 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.

While large herbivores are critically important components of terrestrial ecosystems and can have pronounced top-down effects on plants, our understanding of the underlying mechanisms driving these effects remains incomplete. Large herbivores can alter plant growth, reproduction and abundance through direct effects (predominantly consumption) and through indirect effects via altered interactions with abiotic factors and other species. We know considerably less about these indirect effects than the direct effects. Here, we integrate medium- and small-scale field experiments to investigate how a large vertebrate herbivore, cattle (Bos taurus), affects the aboveground biomass of a dominant forb species, Artemisia scoparia, via diverse direct and indirect pathways in a temperate grassland in northeast China. Although cattle consumed this forb, its biomass increased significantly in response to grazing, due to multiple indirect positive effects that outweighed the direct negative effects of consumption. Cattle preferentially consumed the competing grass Leymus chinensis, and altered Artemisia microhabitats by reducing total plant cover and litter biomass and by increasing the abundance of co-occurring ant species (e.g., Formica spp. and Lasius spp.). This led to additional indirect positive effects on A. scoparia likely due to 1) increased light availability in understory layers and other limiting resources (e.g., soil nutrients and moisture) caused by removal of competitors and plant litter at the soil surface and 2) the changes in resource availability (e.g., soil nutrients and moisture) associated with ant colonies. Our results show that large herbivores can affect plant growth not only via direct consumption, but also via multiple indirect effects. Focusing on the causes and consequences of herbivore-induced indirect effects will not only help us to better understand the influence of these animals in ecological systems, but will also lead to more effective land management and conservation practices in the regions they inhabit.

Many savanna-dependent species in Africa including large herbivores and apex predators are at increasing risk of extinction. Achieving effective management of protected areas (PAs) in Africa where lions live will cost an estimated USD >$1-2 B/year in new funding. We explored the potential for fire management-based carbon-financing programs to fill this funding gap and benefit degrading savanna ecosystems. We demonstrated how introducing early dry season fire management programs could produce potential carbon revenues (PCR) from either a single carbon-financing method (avoided emissions) or from multiple sequestration methods ranging from USD $59.6-$655.9 M/year (at USD $5/ton) or USD $155.0 M–$1.7 B/year (at USD $13/ton). We highlighted variable but significant PCR for savanna PAs from USD $1.5–$44.4 M/year per PA. We suggest investing in fire management programs to jump-start the United Nations Decade of Ecological Restoration to help restore degraded African savannas and conserve imperiled keystone herbivores and apex predators. Open Access article: https://doi.org/10.1016/j.oneear.2021.11.013

Lions (Panthera leo) in Africa have lost nearly half their population in just the last two decades, and effective management of the protected areas (PAs) where lions live will cost an estimated USD >$1 B/year in new funding. We explore the potential for launching a fire management and habitat restoration carbon-financing program to help fill this PA management funding gap. We demonstrate how introducing early dry season fire management programs could produce potential carbon revenues (PCR) from either a single carbon-financing method (avoided emissions) or from multiple sequestration methods of USD $59.6-$655.9 M/year (at USD $5/ton) or USD $155.0 M–$1.7 B/year (at USD $13/ton). We highlight variable but significant PCR for PAs with the greatest potential for restoring lion numbers between USD $1.5–$44.4 M per PA. We suggest investing in lion-centric fire management programs to jump-start the United Nations Decade of Ecological Restoration and help preserve African lions across their range.

Substantial evidence suggests plants and herbivores are limited by multiple resources but their role in driving plant-herbivore interactions is still poorly understood. Here we model multiple resource limitation of plants and herbivores and derive analytical solutions for steady-state plant biomass, herbivore biomass and herbivore impact. The model predicts “apparent” limitation of herbivore biomass by resources that otherwise do not limit herbivore growth. Consequently, higher supply of plant-growth limiting resources allows herbivores to persist at lower supplies of herbivore growth-limiting resources. Likewise, increased supply of these non-limiting resources to herbivores can dramatically increase herbivore impacts on plants. Additionally, the outcomes of herbivore exclusion experiments should differ along different resource gradients, depending on herbivore response to plant resource concentrations. Analysis of existing and new data from marine, freshwater and terrestrial systems supports several of these predictions. Our analysis expands ecologists’ understanding of plant-herbivore dynamics to accommodate multiple limiting resources.

Increasing pressure for communities to conserve wildlife in mixtures with livestock faces scepticism about whether such management is sustainable. The study by Sitters et al. shows that wildlife–livestock coexistence may be sustainable, but only if megaherbivores are included.

Questions Grasslands support ecosystem services, promote diversity, and assist in carbon sequestration. However, grasslands worldwide are diminishing in area, and understanding the drivers shaping the remaining grasslands is critical for their maintenance. The North American tallgrass prairie covers approximately 13% of its historical range and is shaped by fire and herbivory. Fire frequency negatively correlates with plant species richness, while bison (Bos bison) - the historical grazers - offset this effect. However, bison populations have declined, and large browsers are increasing in density. Few studies though have examined the role of large browsers - particularly white-tailed deer (Odocoileus virginianus) - and their interaction with fire frequency in tallgrass prairies. Here, we addressed two questions: (a) What are the impacts of deer on plant diversity, species identities, and relative abundances; and (b) is there an interactive effect between the pressures exerted by deer and the well-documented effects of fire in driving plant community responses? Location This study took place at the Konza Prairie Biological Station in northeastern Kansas, USA. Methods Using a 22-year deer exclosure experiment, we examined differences in plant species richness, evenness, and plant community composition between plots that were either accessible or inaccessible to deer, in areas burned annually or once every four years. Results We did not find significant effects of deer or interactive effects between deer and burning frequency on any metric of the plant community measured, including plant species richness, evenness, and plant community composition. Conclusions Contrary to the impact that deer have in other ecosystems (e.g. forests), our results indicate that deer do not affect the plant community of herbaceous-dominated tallgrass prairies. These results indicate that while the loss of bison-grazers has shifted tallgrass prairie plant communities to C(4)grass-dominated systems, the shift to browsing-dominated herbivore pressure from deer has a minimal effect on the plant community.

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.

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.

Protected areas provide major benefits for humans in the form of ecosystem services, but landscape degradation by human activity at their edges may compromise their ecological functioning. Using multiple lines of evidence from 40 years of research in the Serengeti-Mara ecosystem, we find that such edge degradation has effectively "squeezed" wildlife into the core protected area and has altered the ecosystem's dynamics even within this 40,000-square-kilometer ecosystem. This spatial cascade reduced resilience in the core and was mediated by the movement of grazers, which reduced grass fuel and fires, weakened the capacity of soils to sequester nutrients and carbon, and decreased the responsiveness of primary production to rainfall. Similar effects in other protected ecosystems worldwide may require rethinking of natural resource management outside protected areas.

Ubiquitous declines in biochemical reaction rates above optimal temperatures ( T opt ) are normally attributed to enzyme state changes, but such mechanisms appear inadequate to explain pervasive T op t well below enzyme deactivation temperatures ( T den ). Here, a meta-analysis of 92 experimental studies shows that product formation responds twice as strongly to increased temperature than diffusion or transport. This response difference has multiple consequences for biochemical reactions, such as potential shifts in the factors limiting reactions as temperature increases and reaction-diffusion dynamics that predict potential product inhibition and limitation of the reaction by entropy production at temperatures below T den . Maximizing entropy production by the reaction predicts T opt that depend on enzyme concentration and efficiency as well as reaction favorability, which are patterns not predicted by mechanisms of enzyme state change. However, these predictions are strongly supported by patterns in a meta-analysis of 121 enzyme kinetic studies. Consequently, reaction-diffusion thermodynamics and entropy production may constrain organism performance at higher temperatures, yielding temperature optima of life that may depend on reaction characteristics and environmental features rather than just enzyme state changes.

•Protected wildlife grazed areas had nearly double AM spore densities.•AM fungal species composition was highly sensitive to land use.•AM fungal spore densities were positively correlated with soil N, organic matter and soil moisture levels.•Lower abundance and diversity of spores in livestock grazed soils may have been driven by low carbon availability.