Long-term grazing reduces soil fungal network complexity but enhances plant-soil microbe network connectivity in a semi-arid grassland

Grazing plays a significant role in shaping both aboveground vegetation and belowground microbial communities in arid and semi-arid grasslands, which in turn affects ecosystem functions and sustainability. Therefore, it was essential to implement effective grazing management practices to preserve ecological balance and support sustainable development in these delicate environments. To optimize the traditional continuous grazing policy, we conducted a 10-year seasonal grazing experiment with five treatments in a typical grassland in northern China: no grazing (NG), continuous summer grazing (CG), and three seasonal grazing treatments (G57 in May and July, G68 in June and August, and G79 in July and September). Our study found that although grazing reduced plant community biomass, G68 treatment maintained high plant height and community diversity (P < 0.05). Grazing did not affect soil bacterial and archaeal alpha diversity, but CG treatment reduced soil fungal diversity (P < 0.05). CG reduced the archaeal network's vertices (which represent microbial taxa, OTUs) and connections (ecological interactions between taxa), but seasonal grazing increased its complexity. Furthermore, grazing did not change bacterial networks but enhanced cross-domain interactions (relationships between different biological groups) of plant-soil fungi and plant-soil archaea. Overall, we used the Mantel test to find that soil microbial diversity was positively correlated with soil physicochemical properties rather than plant community characteristics after grazing. These findings are beneficial for the optimization of sustainable grassland management policies and the protection of plant and soil biodiversity.

Meta-analysis shows that wild large herbivores shape ecosystem properties and promote spatial heterogeneity

Megafauna (animals ≥45 kg) have probably shaped the Earth's terrestrial ecosystems for millions of years with pronounced impacts on biogeochemistry, vegetation, ecological communities and evolutionary processes. However, a quantitative global synthesis on the generality of megafauna effects on ecosystems is lacking. Here we conducted a meta-analysis of 297 studies and 5,990 individual observations across six continents to determine how wild herbivorous megafauna influence ecosystem structure, ecological processes and spatial heterogeneity, and whether these impacts depend on body size and environmental factors. Despite large variability in megafauna effects, we show that megafauna significantly alter soil nutrient availability, promote open vegetation structure and reduce the abundance of smaller animals. Other responses (14 out of 26), including, for example, soil carbon, were not significantly affected. Further, megafauna significantly increase ecosystem heterogeneity by affecting spatial heterogeneity in vegetation structure and the abundance and diversity of smaller animals. Given that spatial heterogeneity is considered an important driver of biodiversity across taxonomic groups and scales, these results support the hypothesis that megafauna may promote biodiversity at large scales. Megafauna declined precipitously in diversity and abundance since the late Pleistocene, and our results indicate that their restoration would substantially influence Earth's terrestrial ecosystems.

Functional traits-not nativeness-shape the effects of large mammalian herbivores on plant communities

Large mammalian herbivores (megafauna) have experienced extinctions and declines since prehistory. Introduced megafauna have partly counteracted these losses yet are thought to have unusually negative effects on plants compared with native megafauna. Using a meta-analysis of 3995 plot-scale plant abundance and diversity responses from 221 studies, we found no evidence that megafauna impacts were shaped by nativeness, "invasiveness," "feralness," coevolutionary history, or functional and phylogenetic novelty. Nor was there evidence that introduced megafauna facilitate introduced plants more than native megafauna. Instead, we found strong evidence that functional traits shaped megafauna impacts, with larger-bodied and bulk-feeding megafauna promoting plant diversity. Our work suggests that trait-based ecology provides better insight into interactions between megafauna and plants than do concepts of nativeness.

Alteration of microbial carbon and nitrogen metabolism within the soil metagenome with grazing intensity at semiarid steppe

Grazing causes changes in microbiome metabolic pathways affecting plant growth and soil physicochemical properties. However, how grazing intensity affects microbial processes is poorly understood. In semiarid steppe grassland in northern China, shotgun metagenome sequencing was used to investigate variations in soil carbon (C) and nitrogen (N) cycling-related genes after six years of the following grazing intensities: G0, control, no grazing; G1, 170 sheep days ha-1 year-1; G2, 340 sheep days ha-1 year-1; and G3, 510 sheep days ha-1 year-1. Taxa and functions of the soil microbiome associated with the C cycle decreased with increasing grazing intensity. Abundances of genes involved in C fixation and organic matter decomposition were altered in grazed sites, which could effects on vegetation decomposition and soil dissolved organic carbon (DOC) content. Compared with the control, the abundances of nitrification genes were higher in G1, but the abundances of N reduction and denitrification genes were lower, suggesting that light grazing promoted nitrification, inhibited denitrification, and increased soil NO3- content. Q-PCR further revealed that the copies of genes responsible for carbon fixation (cbbL) and denitrification (norB) decreased with increasing grazing intensity. The highest copy numbers of the nitrification genes AOA and AOB were in G1, whereas copy numbers of the denitrification gene nirK were the lowest. A multivariate regression tree indicated that changes in C fixation genes were linked to changes in soil DOC content, whereas soil NO3- content was linked with nitrification and denitrification under grazing. Thus, genes associated with C fixation and the N cycle affected how C fixation and N storage influenced soil physicochemical properties under grazing. The findings indicate that grazing intensity affected C and N metabolism. Proper grassland management regimes (e.g., G1) are beneficial to the balances between ecological protection of grasslands and plant production in the semiarid steppe.

Cessation of grazing causes biodiversity loss and homogenization of soil food webs

There is widespread concern that cessation of grazing in historically grazed ecosystems is causing biotic homogenization and biodiversity loss. We used 12 montane grassland sites along an 800 km north-south gradient across the UK, to test whether cessation of grazing affects local α- and β-diversity of below-ground food webs. We show cessation of grazing leads to strongly decreased α-diversity of most groups of soil microbes and fauna, particularly of relatively rare taxa. By contrast, the β-diversity varied between groups of soil organisms. While most soil microbial communities exhibited increased homogenization after cessation of grazing, we observed decreased homogenization for soil fauna after cessation of grazing. Overall, our results indicate that exclusion of domesticated herbivores from historically grazed montane grasslands has far-ranging negative consequences for diversity of below-ground food webs. This underscores the importance of grazers for maintaining the diversity of below-ground communities, which play a central role in ecosystem functioning.

Herbivory limits success of vegetation restoration globally

Restoring vegetation in degraded ecosystems is an increasingly common practice for promoting biodiversity and ecological function, but successful implementation is hampered by an incomplete understanding of the processes that limit restoration success. By synthesizing terrestrial and aquatic studies globally (2594 experimental tests from 610 articles), we reveal substantial herbivore control of vegetation under restoration. Herbivores at restoration sites reduced vegetation abundance more strongly (by 89%, on average) than those at relatively undegraded sites and suppressed, rather than fostered, plant diversity. These effects were particularly pronounced in regions with higher temperatures and lower precipitation. Excluding targeted herbivores temporarily or introducing their predators improved restoration by magnitudes similar to or greater than those achieved by managing plant competition or facilitation. Thus, managing herbivory is a promising strategy for enhancing vegetation restoration efforts.

Impacts of large herbivores on terrestrial ecosystems

Large herbivores play unique ecological roles and are disproportionately imperiled by human activity. As many wild populations dwindle towards extinction, and as interest grows in restoring lost biodiversity, research on large herbivores and their ecological impacts has intensified. Yet, results are often conflicting or contingent on local conditions, and new findings have challenged conventional wisdom, making it hard to discern general principles. Here, we review what is known about the ecosystem impacts of large herbivores globally, identify key uncertainties, and suggest priorities to guide research. Many findings are generalizable across ecosystems: large herbivores consistently exert top-down control of plant demography, species composition, and biomass, thereby suppressing fires and the abundance of smaller animals. Other general patterns do not have clearly defined impacts: large herbivores respond to predation risk but the strength of trophic cascades is variable; large herbivores move vast quantities of seeds and nutrients but with poorly understood effects on vegetation and biogeochemistry. Questions of the greatest relevance for conservation and management are among the least certain, including effects on carbon storage and other ecosystem functions and the ability to predict outcomes of extinctions and reintroductions. A unifying theme is the role of body size in regulating ecological impact. Small herbivores cannot fully substitute for large ones, and large-herbivore species are not functionally redundant - losing any, especially the largest, will alter net impact, helping to explain why livestock are poor surrogates for wild species. We advocate leveraging a broad spectrum of techniques to mechanistically explain how large-herbivore traits and environmental context interactively govern the ecological impacts of these animals.

Resource-dependent foraging behaviour of grazers enhances effects of nutrient enrichment on algal biomass

Both the quantity and nutritional quality of food resources can strongly influence the foraging movements of herbivores, which in turn determine the strength of top-down control on primary producer biomass. Nutrient enrichment can alter the biomass and nutritional quality of primary producers, but the consequences for the foraging of herbivores and hence for top-down control are still poorly understood. In this study, we combined a two-factorial experiment (two nutrient levels × grazing by the freshwater gastropod Ancylus fluviatilis) with video analyses tracking grazers' movements to investigate nutrient enrichment effects on spatial ranges of grazing activity and algal biomass removal. Natural stream biofilms were grown in phosphorus-enriched (P+) and phosphorus-poor flumes (P-) for two weeks before A. fluviatilis were added to the flumes and allowed to graze on biofilm for an additional 2 weeks. Total periphyton biomass was enhanced by P+ and reduced by grazer presence. However, the total grazer effect depended on the nutrient level: at the end of the experiment, on average 95% of algal cover were removed by grazing in the P- flumes versus 26% in the P+ flumes. Fast movements of A. fluviatilis were detected significantly more often in the P- treatment, whereas grazers were detected resting more often in the P+ treatment. Our results demonstrate that nutrient enrichment can increase primary producer biomass both directly and indirectly by limiting the foraging ranges of herbivores. The resulting feedback loop between reduced grazing activity and increased plant biomass might in turn exacerbate eutrophication effects on habitat structure.

Land use intensification alters the relative contributions of plant functional diversity and soil properties on grassland productivity

Understanding the mechanisms of grassland productivity variation is critical for global carbon cycling and climate change mitigation. Heretofore, it is unknown how different environmental factors drive small-scale spatial variation in productivity, and whether land use intensification, one of the most important global changes, can regulate the processes that drive productivity change. Here we performed an 18-year exclosure experiment across six sites with high-intensity mowing/grazing history in northern China to examine the effects of land use intensification on plant functional diversity, soil properties, and their relative contributions to above-ground net primary productivity (ANPP). We found that plant functional diversity and soil properties contributed to the variation in ANPP both independently and equally in enclosed grasslands (plant diversity: 20.6%; soil properties: 19.5%). Intensive land use significantly decreased the Rao's quadratic entropy (RaoQ) and community-weighted mean value (CWM) of plant height, and further suppressed the contributions of plant functional diversity to ANPP. In contrast, intensive land use increased soil available N, P, pH, electrical conductivity, and homogeneity of soil available P, and strengthened their contributions to ANPP (31.5%). Our results indicate that high-intensity land use practices in grasslands decrease the role of plant functional diversity, but strengthen the effects of soil properties on productivity. We, therefore, suggest that plant functional diversity can be used effectively to boost productivity in undisturbed grasslands, while soil properties might be a more critical consideration for grassland management in an areas with increased land use.

Structure and functioning of wild and agricultural grazing ecosystems: A comparative review

For more than 10 million years, large, herd forming ruminants have thrived as parts of sustainable grazing ecosystems. Conversely, since their domestication 8,000–11,000 years ago, cattle, sheep, and goats have often exhibited dysfunctional relationships with the ecosystems they inhabit. A considerable literature, developed over decades, documents the negative impacts of animal agriculture and associated activities (e.g., feed production) on grassland ecosystems. Coincident with the accumulating data documenting the impacts of “conventional” animal agriculture, has been a growing interest in restoring functionality to agricultural grazing ecosystems. These “regenerative” protocols often seek to mimic the structure and functions of wild grazing ecosystems. The objectives of this paper were two-fold: First to review the literature describing the structure and some key functional attributes of wild and agricultural grazing ecosystems; and second, to examine these attributes in conventionally and regeneratively managed grazing ecosystems and, assuming the wild condition to be the standard for sustainable grazer-environment relationships, to ascertain whether similar relationships exist in conventionally or regeneratively managed agricultural grazing ecosystems. Not unexpectedly our review revealed the complexity of both wild and agricultural grazing ecosystems and the interconnectedness of biological, chemical, and physical factors and processes within these systems. Grazers may increase or decrease system functionality, depending upon environmental conditions (e.g., moisture levels). Our review revealed that biodiversity, nitrogen cycling, and carbon storage in regenerative grazing systems more closely resemble wild grazing ecosystems than do conventional grazing systems. We also found multiple points of disagreement in the literature, particularly with respect to aboveground primary production (ANPP). Finally, we acknowledge that, while much has been accomplished in understanding grazing ecosystems, much remains to be done. In particular, some of the variability in the results of studies, especially of meta-analyses, might be reduced if datasets included greater detail on grazing protocols, and a common definition of the term, “grazing intensity.”

Reintroducing bison results in long-running and resilient increases in grassland diversity

The widespread extirpation of megafauna may have destabilized ecosystems and altered biodiversity globally. Most megafauna extinctions occurred before the modern record, leaving it unclear how their loss impacts current biodiversity. We report the long-term effects of reintroducing plains bison (Bison bison) in a tallgrass prairie versus two land uses that commonly occur in many North American grasslands: 1) no grazing and 2) intensive growing-season grazing by domesticated cattle (Bos taurus). Compared to ungrazed areas, reintroducing bison increased native plant species richness by 103% at local scales (10 m²) and 86% at the catchment scale. Gains in richness continued for 29 y and were resilient to the most extreme drought in four decades. These gains are now among the largest recorded increases in species richness due to grazing in grasslands globally. Grazing by domestic cattle also increased native plant species richness, but by less than half as much as bison. This study indicates that some ecosystems maintain a latent potential for increased native plant species richness following the reintroduction of native herbivores, which was unmatched by domesticated grazers. Native-grazer gains in richness were resilient to an extreme drought, a pressure likely to become more common under future global environmental change.

Heavy grazing reduced the spatial heterogeneity of Artemisia frigida in desert steppe

BACKGROUND

Grazing disturbance plays an important role in the desert steppe ecosystem in Inner Mongolia, China. Previous studies found that grazing affected the spatial distribution of species in a community, and showed patchiness characteristics of species under different grazing treatments. Artemisia frigida is the dominant species and semi-shrub in desert steppe, and whether grazing interference will affect the spatial distribution of A. frigida is studied. In this study, geo-statistical methods were mainly used to study the spatial distribution characteristics of A. frigida population in desert steppe of Inner Mongolia at two scales (quadrat size 2.5 m × 2.5 m, 5 m × 5 m) and four stocking rates (control, CK, 0 sheep·ha^-1·month^-1; light grazing, LG, 0.15 sheep·ha^-1·month^-1, moderate grazing, MG, 0.30 sheep·ha^-1·month^-1, heavy grazing, HG, 0.45 sheep·ha^-1·month^-1).

RESULTS

The results showed that the spatial distribution of A. frigida tended to be simplified with the increase of stocking rate, and tended to be banded with increased spatial scale. The density and height of A. frigida increased with increasing scale. With increased stocking rate, the density of A. frigida population decreased linearly, while its height decreased in a step-wise fashion. The spatial distribution of A. frigida was mainly affected by structural factors at different scales and stocking rate. The density of A. frigida was more sensitive to change in stocking rate, and the patchiness distribution of A. frigida was more obvious with increase in scale.

CONCLUSIONS

Stocking rate has a strong regulatory effect on the spatial pattern of A. frigida population in the desert steppe. Heavy grazing reduced the spatial heterogeneity of A. frigida in the desert steppe. The smaller dominant populations are unfavourable for its survival in heavy grazing condition, and affects the stability and productivity of the grassland ecosystem.

Short duration overnight cattle kraaling in natural rangelands: Does time after kraal use affect their utilization by wildlife and above ground grass parameters?

In east and southern Africa some private ranch owners are corralling (hereafter kraaling) cattle overnight for short periods (for example, seven days) in natural rangelands to create nutrient enriched hotspots which are attractive to large herbivores. However, the effect of season and time after kraal use (alt. age of nutrient enriched hotspots) on large herbivore use of these sites has not been examined. We collated the number of large herbivore sightings per day from camera traps during wet, early and late dry season in nutrient enriched hotspots of varying ages (1, 2, 3 and 4 years) and surrounding vegetation. In addition, above ground grass biomass and height in nutrient enriched hotspots was compared to that of the surrounding vegetation. Furthermore, we tested if repeated grazing in nutrient enriched hotspots stimulated grass compensatory growth. Large herbivore use of nutrient enriched hotspots was similar during wet, early and late dry season. Time after kraal use had a significant effect on mixed feeders (impala and African savanna elephant) utilization of nutrient enriched hotspots but not grazers (zebra and warthog) and browsers (giraffe and greater kudu). Both impala and African savanna elephants mostly used nutrient enriched hotspots one year after kraal use. Aboveground grass biomass and height were higher in surrounding vegetation than in nutrient enriched hotspots. Repeated clipping (proxy for grazing) resulted in compensatory aboveground grass biomass in nutrient enriched hotspots, which declined with time after kraal use. We concluded that nutrient enriched hotspots created through short duration overnight kraaling were important foraging sites for large herbivores.

In defense of elemental currencies: can ecological stoichiometry stand as a framework for terrestrial herbivore nutritional ecology?

Nutritional ecologists aim to predict population or landscape-level effects of food availability, but the tools to extrapolate nutrition from small to large extents are often lacking. The appropriate nutritional ecology currencies should be able to represent consumer responses to food while simultaneously be simple enough to expand such responses to large spatial extents and link them to ecosystem functioning. Ecological stoichiometry (ES), a framework of nutritional ecology, can meet these demands, but it is typically associated with ecosystem ecology and nutrient cycling, and less often used to study wildlife nutrition. Despite the emerging zoogeochemical evidence that animals, and thus their diets, play critical roles in nutrient movement, wildlife nutritional ecology has not fully embraced ES, and ES has not incorporated nutrition in many wildlife studies. Here, we discuss how elemental currencies are "nutritionally, organismally, and ecologically explicit" in the context of terrestrial herbivore nutritional ecology. We add that ES and elemental currencies offer a means to measure resource quality across landscapes and compare nutrient availability among regions. Further, we discuss ES shortcomings and solutions, and list future directions to advance the field. As ecological studies increasingly grow in spatial extent, and attempt to link multiple levels of biological organization, integrating more simple and unifying currencies into nutritional studies, like elements, is necessary for nutritional ecology to predict herbivore occurrences and abundances across regions.

The relative and combined effects of herbivore assemblage and soil nitrogen on plant diversity

Plant diversity can be affected by both herbivore grazing and soil resources. However, it is unclear if the joint effects of herbivores and soil resources might vary with components of plant diversity. Here, we evaluated the relative and combined effects of herbivore assemblage and soil nitrogen (N) quantity and heterogeneity on the α and β components of plant diversity in a grassland that was subjected to four years of grazing under differing herbivore assemblages (no grazing, cattle grazing, sheep grazing, and mixed grazing). We found that herbivore assemblage combined with soil N quantity explained 41% of the variation in plant α-diversity, while herbivore assemblage combined with soil N heterogeneity explained 15% of the variation in plant β-diversity. The independent effects of herbivore assemblage explained more than those of soil N for both α- and β-diversity (α-diversity: 12% vs. 4%; β-diversity: 18% vs. 16%). We concluded that the effects of herbivores are stronger than those of soil N, and that grazing-induced changes in soil resources are important drivers of plant diversity change, especially α-diversity. Therefore, we suggest that managing herbivore species by accounting for the effects that their grazing can have on soil resources may be significant for plant diversity maintenance.

Grazing affects vegetation diversity and heterogeneity in California vernal pools

Disturbance often increases local-scale (α) diversity by suppressing dominant competitors. However, widespread disturbances may also reduce biotic heterogeneity (β diversity) by making the identities and abundances of species more similar among patches. Landscape-scale (γ) diversity may also decline if disturbance-sensitive species are lost. California's vernal pool plant communities are species rich, in part because of two scales of β diversity: (1) within pools, as species composition changes with depth (referred to here as vertical β diversity), and (2) between pools, in response to dispersal limitation and variation in pool attributes (referred to here as horizontal β diversity). We asked how grazing by livestock, a common management practice, affects vernal pool plant diversity at multiple hierarchical spatial scales. In terms of abundance-weighted diversity, grazing increased α both within local pool habitat zones and at the whole-pool scale, as well as γ at the pasture scale without influencing horizontal or vertical β diversity. In terms of species richness, increases in α diversity within habitat zones and within whole pools led to small decreases in horizontal β diversity as species occupancy increased. This had a dampened effect on species richness at the γ (pasture) scale without any loss of disturbance-sensitive species. We conclude that grazing increases species richness and evenness (α) by reducing competitive dominance, without large disruptions to the critical spatial heterogeneity (β) that generates high landscape-level diversity (γ).

Effects of Fire and Large Herbivores on Canopy Nitrogen in a Tallgrass Prairie

This study analyzed the spatial heterogeneity of grassland canopy nitrogen in a tallgrass prairie with different treatments of fire and ungulate grazing (long-term bison grazing vs. recent cattle grazing). Variogram analysis was applied to continuous remotely sensed canopy nitrogen images to examine the spatial variability in grassland canopies. Heterogeneity metrics (e.g., the interspersion/juxtaposition index) were calculated from the categorical canopy nitrogen maps and compared among fire and grazing treatments. Results showed that watersheds burned within one year had higher canopy nitrogen content and lower interspersions of high-nitrogen content patches than watersheds with longer fire intervals, suggesting an immediate and transient fire effect on grassland vegetation. In watersheds burned within one year, high-intensity grazing reduced vegetation density, but promoted grassland heterogeneity, as indicated by lower canopy nitrogen concentrations and greater interspersions of high-nitrogen content patches at the grazed sites than at the ungrazed sites. Variogram analyses across watersheds with different grazing histories showed that long-term bison grazing created greater spatial variability of canopy nitrogen than recent grazing by cattle. This comparison between bison and cattle is novel, as few field experiments have evaluated the role of grazing history in driving grassland heterogeneity. Our analyses extend previous research of effects from pyric herbivory on grassland heterogeneity by highlighting the role of grazing history in modulating the spatial and temporal distribution of aboveground nitrogen content in tallgrass prairie vegetation using a remote sensing approach. The comparison of canopy nitrogen properties and the variogram analysis of canopy nitrogen distribution provided by our study are useful for further mapping grassland canopy features and modeling grassland dynamics involving interplays among fire, large grazers, and vegetation communities.

Interaction of livestock grazing and rainfall manipulation enhances herbaceous species diversity and aboveground biomass in a humid savanna

Understanding of the interaction of livestock grazing and rainfall variability may aid in predicting the patterns of herbaceous species diversity and biomass production. We manipulated the amount of ambient rainfall received in grazed and ungrazed savanna in Lambwe Valley-Kenya. The combined influence of livestock grazing and rainfall on soil moisture, herbaceous species diversity, and aboveground biomass patterns was assessed. We used the number of species (S), Margalef's richness index (D_mg), Shannon index of diversity (H), and Pileou's index of evenness (J) to analyze the herbaceous community structure. S, D_mg, H and J were higher under grazing whereas volumetric soil water contents (VWC) and aboveground biomass (AGB) decreased with grazing. Decreasing (50%) or increasing (150%) the ambient rainfall by 50% lowered species richness and diversity. Seasonality in rainfall influenced the variation in VWC, S, Dmg, H, and AGB but not J (p = 0.43). Overall, D_mg declined with increasing VWC. However, the AGB and D_mg mediated the response of H and J to the changes in VWC. The highest H occurred at AGB range of 400-800 g m^-2. We attribute the lower diversity in the ungrazed plots to the dominance (relative abundance > 70%) of Hyparrhenia fillipendulla (Hochst) Stapf. and Brachiaria decumbens Stapf. Grazing exclusion, which controls AGB, hindered the coexistence among species due to the competitive advantage in resource utilization by the more dominant species. Our findings highlight the implication of livestock grazing and rainfall variability in maintaining higher diversity and aboveground biomass production in the herbaceous layer community for sustainable ecosystem management.

Island rewilding with giant tortoises in an era of climate change

Replacing recently extinct endemic giant tortoises with extant, functional analogues provide the perhaps best examples of island rewilding to date. Yet, an efficient future application of this conservation action is challenging in an era of climate change. We here present and discuss a conceptual framework that can serve as a roadmap for the study and application of tortoise rewilding in an uncertain future. We focus on three main ecological functions mediated by giant tortoises, namely herbivory, seed dispersal and nutrient cycling, and discuss how climate change is likely to impact these. We then propose and discuss mitigation strategies such as artificial constructed shade sites and water holes that can help drive and maintain the ecosystem functions provided by the tortoises on a landscape scale. The application of the framework and the mitigation strategies are illustrated with examples from both wild and rewilded populations of the Aldabra giant tortoise, Aldabrachelys gigantea, in the Western Indian Ocean.This article is part of the theme issue 'Trophic rewilding: consequences for ecosystems under global change'.

Nitrogen and carbon isotopic dynamics of subarctic soils and plants in southern Yukon Territory and its implications for paleoecological and paleodietary studies

We examine here the carbon and nitrogen isotopic compositions of bulk soils (8 topsoil and 7 subsoils, including two soil profiles) and five different plant parts of 79 C3 plants from two main functional groups: herbs and shrubs/subshrubs, from 18 different locations in grasslands of southern Yukon Territory, Canada (eastern shoreline of Kluane Lake and Whitehorse area). The Kluane Lake region in particular has been identified previously as an analogue for Late Pleistocene eastern Beringia. All topsoils have higher average total nitrogen δ15N and organic carbon δ13C than plants from the same sites with a positive shift occurring with depth in two soil profiles analyzed. All plants analyzed have an average whole plant δ13C of -27.5 ± 1.2 ‰ and foliar δ13C of -28.0 ± 1.3 ‰, and average whole plant δ15N of -0.3 ± 2.2 ‰ and foliar δ15N of -0.6 ± 2.7 ‰. Plants analyzed here showed relatively smaller variability in δ13C than δ15N. Their average δ13C after suitable corrections for the Suess effect should be suitable as baseline for interpreting diets of Late Pleistocene herbivores that lived in eastern Beringia. Water availability, nitrogen availability, spacial differences and intra-plant variability are important controls on δ15N of herbaceous plants in the study area. The wider range of δ15N, the more numerous factors that affect nitrogen isotopic composition and their likely differences in the past, however, limit use of the modern N isotopic baseline for vegetation in paleodietary models for such ecosystems. That said, the positive correlation between foliar δ15N and N content shown for the modern plants could support use of plant δ15N as an index for plant N content and therefore forage quality. The modern N isotopic baseline cannot be applied directly to the past, but it is prerequisite to future efforts to detect shifts in N cycling and forage quality since the Late Pleistocene through comparison with fossil plants from the same region.

Grassland productivity in response to nutrient additions and herbivory is scale-dependent

Vegetation response to nutrient addition can vary across space, yet studies that explicitly incorporate spatial pattern into experimental approaches are rare. To explore whether there are unique spatial scales (grains) at which grass response to nutrients and herbivory is best expressed, we imposed a large (∼3.75 ha) experiment in a South African coastal grassland ecosystem. In two of six 60 × 60 m grassland plots, we imposed a scaled sampling design in which fertilizer was added in replicated sub-plots (1 × 1 m, 2 × 2 m, and 4 × 4 m). The remaining plots either received no additions or were fertilized evenly across the entire area. Three of the six plots were fenced to exclude herbivory. We calculated empirical semivariograms for all plots one year following nutrient additions to determine whether the scale of grass response (biomass and nutrient concentrations) corresponded to the scale of the sub-plot additions and compared these results to reference plots (unfertilized or unscaled) and to plots with and without herbivory. We compared empirical semivariogram parameters to parameters from semivariograms derived from a set of simulated landscapes (neutral models). Empirical semivariograms showed spatial structure in plots that received multi-scaled nutrient additions, particularly at the 2 × 2 m grain. The level of biomass response was predicted by foliar P concentration and, to a lesser extent, N, with the treatment effect of herbivory having a minimal influence. Neutral models confirmed the length scale of the biomass response and indicated few differences due to herbivory. Overall, we conclude that interpretation of nutrient limitation in grasslands is dependent on the grain used to measure grass response and that herbivory had a secondary effect.

The impact of sheep grazing on the depth of spider burrows and of burrows selected by the pygmy bluetongue lizard (Tiliqua adelaidensis)

Context Grazing by domestic stock can potentially influence ecosystems positively or negatively, depending on the grazing regime and intensity. The pygmy bluetongue lizard (Tiliqua adelaidensis) is an endangered species, restricted to fragmented grasslands in the Mid North region of South Australia, predominantly grazed by sheep. These lizards refuge in vertical burrows. They do not dig their own burrows, instead relying on co-existing spiders for this essential resource. Aims We investigated how sheep grazing influenced spider burrows, and predicted the following: that grazing sheep might trample and destroy shallow burrows; that burrows in grazed habitat with less vegetative cover would have higher temperatures in summer; and that, by way of compensation, spiders would dig deeper burrows in grazed habitat. Methods We monitored 12 30m×30m plots monthly, over two austral spring/summer periods (September–March 2012–14). We recorded the number, depth and occupant of all spider burrows in those plots in each survey. We also measured temperatures inside artificial burrows in grazed and ungrazed habitat over a 3-day period in September 2014. Key results Mean burrow depth increased with grazing. This was not because individual burrows became deeper, but because the shallowest burrows were lost where there were sheep. Burrows were significantly hotter in grazed habitat, but lizards consistently chose deeper, cooler burrows. In the subset of burrows that were occupied by lizards, there was no difference in depth between grazed and ungrazed treatments. We detected no difference in spider digging activity between grazed and ungrazed plots. Conclusions Sheep grazing can destroy shallow burrows, probably through trampling, and because sheep produce burrow filling debris. However, grazing did not affect deeper burrows occupied by pygmy bluetongue lizards. Implications This short study indicated that moderate levels of grazing have low impact on deeper burrows suitable for pygmy bluetongue lizards and for lizard persistence. It explains how sheep and lizards have co-occurred for so long, but leaves open the question of how the loss of shallow burrows affects populations of the burrow-digging spiders that are an essential component of the lizard environment.

Transgenerational effects of land use on offspring performance and growth in Trifolium repens

Central European grasslands vary widely in productivity and in mowing and grazing regimes. The resulting differences in competition and heterogeneity among grasslands might have direct effects on plants, but might also affect the growth and morphology of their offspring through maternal effects or adaptive evolution. To test for such transgenerational effects, we grew plants of the clonal herb Trifolium repens from seeds collected in 58 grassland sites differing in productivity and mowing and grazing intensities in different treatments: without competition, with homogeneous competition, and with heterogeneous competition. In the competition-free treatment, T. repens from more productive, less frequently mown, and less intensively grazed sites produced more vegetative offspring, but this was not the case in the other treatments. When grown among or in close proximity to competitors, T. repens plants did not show preferential growth towards open spaces (i.e., no horizontal foraging), but did show strong vertical foraging by petiole elongation. In the homogeneous competition treatment, petiole length increased with the productivity of the parental site, but this was not the case in the heterogeneous competition treatment. Moreover, petiole length increased with mowing frequency and grazing intensity of the parental site in all but the homogeneous competition treatment. In summary, although the expression of differences between plants from sites with different productivities and land-use intensities depended on the experimental treatment, our findings imply that there are transgenerational effects of land use on the morphology and performance of T. repens.

Contrasting effects of different mammalian herbivores on sagebrush plant communities

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.).

Effects of grazing on ecosystem CO₂ exchange in a meadow grassland on the Tibetan Plateau during the growing season

Effects of human activity on ecosystem carbon fluxes (e.g., net ecosystem exchange (NEE), ecosystem respiration (R(eco)), and gross ecosystem exchange (GEE)) are crucial for projecting future uptake of CO2 in terrestrial ecosystems. However, how ecosystem that carbon fluxes respond to grazing exclusion is still under debate. In this study, a field experiment was conducted to study the effects of grazing exclusion on R(eco), NEE, and GEE with three treatments (free-range grazing (FG) and grazing exclusion for 3 and 5 years (GE3 and GE5, respectively)) in a meadow grassland on the Tibetan Plateau. Our results show that grazing exclusion significantly increased NEE by 47.37 and 15.84%, and R eco by 33.14 and 4.29% under GE3 and GE5 plots, respectively, although carbon sinks occurred in all plots during the growing season, with values of 192.11, 283.12, and 222.54 g C m(-2) for FG, GE3, and GE5, respectively. Interestingly, grazing exclusion increased temperature sensitivity (Q10) of R eco with larger increases at the beginning and end of growing season (i.e., May and October, respectively). Soil temperature and soil moisture were key factors on controlling the diurnal and seasonal variations of R(eco), NEE, and GEE, with soil temperature having a stronger influence. Therefore, the combined effects of grazing and temperature suggest that grazing should be taken into consideration in assessing global warming effects on grassland ecosystem CO2 exchange.

Interactive effects of grazing, drought, and fire on grassland plant communities in North America and South Africa

Grazing, fire, and climate shape mesic grassland communities. With global change altering all three factors, understanding how grasslands respond to changes in these combined drivers may aid in projecting future changes in grassland ecosystems. We manipulated rainfall and simulated grazing (clipping) in two long-term fire experiments in mesic grasslands in North America (NA) and South Africa (SA). Despite their common drivers, grasslands in NA and SA differ in evolutionary history. Therefore, we expected community structure and production in NA and SA to respond differently to fire, grazing, and drought. Specifically, we hypothesized that NA plant community composition and production would be more responsive than the SA plant communities to changes in the drivers and their interactions, and that despite this expected stability of SA grasslands, drought would be the dominant factor controlling production, but grazing would play the primary role in determining community composition at both sites. Contrary to our hypothesis, NA and SA grasslands generally responded similarly to grazing, drought, and fire. Grazing increased diversity, decreased grass cover and production, and decreased belowground biomass at both sites. Drought alone minimally impacted plant community structure, and we saw similar treatment interactions at the two sites. Drought was not the primary driver of grassland productivity, but instead drought effects were similar to or less than grazing and fire. Even though these grasslands differed in evolutionary history, they responded similarly to our fire, grazing, and climate manipulations. Overall, we found community and ecosystem convergence in NA and SA grasslands. Grazing and fire are as important as climate in controlling mesic grassland ecosystems on both continents.

Spatial and seasonal dynamic of abundance and distribution of guanaco and livestock: insights from using density surface and null models

Monitoring species abundance and distribution is a prerequisite when assessing species status and population viability, a difficult task to achieve for large herbivores at ecologically meaningful scales. Co-occurrence patterns can be used to infer mechanisms of community organization (such as biotic interactions), although it has been traditionally applied to binary presence/absence data. Here, we combine density surface and null models of abundance data as a novel approach to analyze the spatial and seasonal dynamics of abundance and distribution of guanacos (Lama guanicoe) and domestic herbivores in northern Patagonia, in order to visually and analytically compare the dispersion and co-occurrence pattern of ungulates. We found a marked seasonal pattern in abundance and spatial distribution of L. guanicoe. The guanaco population reached its maximum annual size and spatial dispersion in spring-summer, decreasing up to 6.5 times in size and occupying few sites of the study area in fall-winter. These results are evidence of the seasonal migration process of guanaco populations, an increasingly rare event for terrestrial mammals worldwide. The maximum number of guanacos estimated for spring (25,951) is higher than the total population size (10,000) 20 years ago, probably due to both counting methodology and population growth. Livestock were mostly distributed near human settlements, as expected by the sedentary management practiced by local people. Herbivore distribution was non-random; i.e., guanaco and livestock abundances co-varied negatively in all seasons, more than expected by chance. Segregation degree of guanaco and small-livestock (goats and sheep) was comparatively stronger than that of guanaco and large-livestock, suggesting a competition mechanism between ecologically similar herbivores, although various environmental factors could also contribute to habitat segregation. The new and compelling combination of methods used here is highly useful for researchers who conduct counts of animals to simultaneously estimate population sizes, distributions, assess temporal trends and characterize multi-species spatial interactions.

Broadening the ecological context of ungulate-ecosystem interactions: the importance of space, seasonality, and nitrogen

Spatial heterogeneity of soil resources, particularly nitrogen availability, affects herbaceous-layer cover and diversity in temperate forest ecosystems. Current hypotheses predict that ungulate herbivores influence nitrogen availability at the stand scale, but how ungulates affect nitrogen availability at finer spatial scales that are relevant to the herb layer is less understood. We tested the hypothesis that ungulate exclusion reduces the spatial complexity of nitrogen availability at neighborhood scales (1-26 m) apart from mean stand scale effects. This outcome was expected due to a lack of ungulate nitrogenous waste deposition within exclosures and seasonally variable ungulate habitat use. To test this hypothesis we examined spatial patterning of ammonium and nitrate availability, herb-layer cover and diversity, and under-canopy solar radiation using geostatistical models. Our study sites included six stands of eastern hemlock (Tsuga canadensis) forest: three where white-tailed deer (Odocoileus virginianus) were excluded and three that were accessible to deer. Where deer were present, patch sizes of ammonium availability, cover, and diversity were smaller compared to deer exclosures, whereas mean site-level effects were not significant. Within deer exclosures cover and solar radiation were more similar in patch size than were cover and nitrogen availability. Our results suggest that browsing ungulates affect spatial patterns of herb-layer cover and diversity through the excretion of nitrogenous wastes in small, discrete patches. Ungulate-excreted nitrogen deposition and herbivory were concentrated in the dormant season, allowing herb-layer plants a greater opportunity to benefit from nitrogen additions. Therefore, the impact of ungulates on nitrogen cycling in forest ecosystems varies with spatial scale and the seasonal timing of ungulate impacts. In this way, ungulates may function as a seasonally dependent link between fine-scale and landscape-level ecological processes.

An overview of nitrogen cycling in a semiarid savanna: some implications for management and conservation in a large African park

Nitrogen (N) is a major control on primary productivity and hence on the productivity and diversity of secondary producers and consumers. As such, ecosystem structure and function cannot be understood without a comprehensive understanding of N cycling and dynamics. This overview describes the factors that govern N distribution and dynamics and the consequences that variable N dynamics have for structure, function and thresholds of potential concern (TPCs) for management of a semiarid southern African savanna. We focus on the Kruger National Park (KNP), a relatively intact savanna, noted for its wide array of animal and plant species and a prized tourist destination. KNP's large size ensures integrity of most ecosystem processes and much can be learned about drivers of ecosystem structure and function using this park as a baseline. Our overview shows that large scale variability in substrates exists, but do not necessarily have predictable consequences for N cycling. The impact of major drivers such as fire is complex; at a landscape scale little differences in stocks and cycling were found, though at a smaller scale changes in woody cover can lead to concomitant changes in total N. Contrasting impacts of browsers and grazers on N turnover has been recorded. Due to the complexity of this ecosystem, we conclude that it will be complicated to draw up TPCs for most transformations and pools involved with the N cycle. However, we highlight in which cases the development of TPCs will be possible.