Evolutionary History Of Grazing Research Articles

The effect of livestock grazing on plant diversity and productivity of mountainous grasslands in South America - A meta-analysis.

Mountainous grasslands in South America, characterized by their high diversity, provide a wide range of contributions to people, including water regulation, soil erosion prevention, livestock feed provision, and preservation of cultural heritage. Prior research has highlighted the significant role of grazing in shaping the diversity and productivity of grassland ecosystems, especially in highly productive, eutrophic systems. In such environments, grazing has been demonstrated to restore grassland plant diversity by reducing primary productivity. However, it remains unclear whether these findings are applicable to South American mountainous grasslands, where plants are adapted to different environmental conditions. To address this uncertainty, we conducted a meta-analysis of experiments excluding livestock grazing to assess its impact on plant diversity and productivity across mountainous grasslands in South America. In alignment with studies in temperate grasslands, our findings indicated that herbivore exclusion resulted in increased aboveground biomass but reduced species richness and Shannon diversity. The effects of grazing exclusion became more pronounced with longer durations of exclusion; nevertheless, they remained resilient to various climatic conditions, including mean annual precipitation and mean annual temperature, as well as the evolutionary history of grazing. In contrast to results observed in temperate grasslands, the reduction in species richness due to herbivore exclusion was not associated with increased aboveground biomass. This suggests that the processes governing (sub)tropical grassland plant diversity may differ from those in temperate grasslands. Consequently, further research is necessary to better understand the specific factors influencing plant diversity and productivity in South American montane grasslands and to elucidate the ecological implications of herbivore exclusion in these unique ecosystems.

Evolutionary history of grazing and resources determine herbivore exclusion effects on plant diversity.

Ecological models predict that the effects of mammalian herbivore exclusion on plant diversity depend on resource availability and plant exposure to ungulate grazing over evolutionary time. Using an experiment replicated in 57 grasslands on six continents, with contrasting evolutionary history of grazing, we tested how resources (mean annual precipitation and soil nutrients) determine herbivore exclusion effects on plant diversity, richness and evenness. Here we show that at sites with a long history of ungulate grazing, herbivore exclusion reduced plant diversity by reducing both richness and evenness and the responses of richness and diversity to herbivore exclusion decreased with mean annual precipitation. At sites with a short history of grazing, the effects of herbivore exclusion were not related to precipitation but differed for native and exotic plant richness. Thus, plant species' evolutionary history of grazing continues to shape the response of the world's grasslands to changing mammalian herbivory.

A meta‐analysis of the effects of habitat aridity, evolutionary history of grazing and grazing intensity on bee and butterfly communities worldwide

Abstract A variety of habitat‐associated factors moderate effects of grazing on insect biodiversity. Here, we examine how aridity, evolutionary history of grazing and grazing intensity individually and interactively mediate the effect of livestock grazing on pollinator biodiversity (native bees and butterflies). Using a meta‐analysis of 59 studies published in the primary literature, we characterized the response of pollinator communities to grazing across several continents. In very humid habitats, high grazing intensities generally had negative impacts on pollinator abundance and richness, but these effects were not found in semi‐arid habitats, where livestock grazing intensity did not interact with aridity to impact pollinator abundance or richness. However, within semi‐arid habitats livestock grazing was associated with reduced pollinator richness in areas with short evolutionary histories grazing. Pollinator life history mediated effects of livestock grazing on pollinator communities: livestock grazing had negative impacts on richness of social bees and butterflies but not solitary bees, though abundances of all three pollinator categories were consistently reduced under livestock grazing. Our synthesis suggests that effects of cattle on pollinators may be driven by impacts on nesting habitats (e.g. soil compaction), rather than consumption or alteration of forb cover. Our collective findings have importance for coordinating grazing management and pollinator conservation efforts and help to distinguish how grazing practices could impact pollinator biodiversity across ecoclimatic regions.

Plant community composition and structure under short-term grazing exclusion in steppic arid rangelands

Grazing exclusion has been proved to be an effective approach for naturally restoring degraded rangelands. Still, the effect of this management practice on plant community composition and structure is ambiguous, especially under prolonged and repeated drought events. Here, we investigated the responses of community composition (i.e. incidence-based) and structure (i.e. abundance-based) to short-term sheep grazing exclusion under severe drought episode in the arid steppes of Alfa-grass (Stipa tenacissima L.) with a long evolutionary history of grazing. Individual species responses were tested based on species occurrence and abundance in either grazed or grazing-excluded steppes. Besides, indicator species analysis was used to identify species indicative of grazing-excluded steppes. Likewise, incidence-based and abundance-based α-diversity, β-diversity and functional groups’ diversities were quantified using Hill Numbers and compared between the two steppe managements types. Under severe drought conditions, sheep grazing exclusion allowed the apparition of a large number of increasers, colonizers, and native indicator plant species. It also improved the size of regional species pool and increased overall incidence-based and abundance-based α-diversity. Moreover, grazing exclusion decreased the abundance-based β-diversity at local scale but increased it at landscape scale. The incidence-based β-diversity significantly decreased at the landscape scale. Grazing exclusion enabled a significant spatial structuration of abundance-based β-diversity components by maintaining high balanced variation in species abundance at large spatial scale and greater abundance-gradient at fine-scale. Our results suggest that the implementation of short-term grazing exclusion in arid steppes would be the appropriate management practice for vegetation and habitat during prolonged droughts since it permits the recovery of native plant species and affects positively the size of the regional species pool, the overall incidence- and abundance-based α-diversity as well as the abundance-based β-diversity (chiefly at landscape scale).

A global meta-analysis of grazing effects on plant richness

A major challenge in rangeland ecology is understanding diverse responses of diversity to grazing, what factors determine these responses and what their relative importance is. Numerous site-level studies have explored grazing-diversity relations, but meta-analyses of these empirical results at a global scale are scarce. Our goals were to estimate the mean effect of grazing on plant richness and to assess the relative importance of different variables at a global scale. We conducted a global meta-analysis of 259 comparisons of plant richness in grazed vs. ungrazed sites from 96 published studies. Then, we performed analyses of subgroup, meta-regression and correlation for testing the relative roles of regional and local variables (e.g. evolutionary history of grazing, aridity, stocking rate, etc.).Globally, grazing significantly increased plant richness compared to grazing exclusion, especially in wet grasslands. The effect of evolutionary history of grazing was insignificant, which cannot support the major perception behind Milchunas-Sala-Lauenroth (MSL) model. Aridity, vegetation type, and stocking rate were three important variables that together explained 41 % of the global variation in plant richness. The roles of stocking rate and duration of grazing exclusion were revealed only when aridity and vegetation type were added to the analyses, which showed a stronger effect of stocking rate in wet areas but a stronger effect of the duration of grazing exclusion in dry areas. Changes in plant richness were significantly correlated with changes in soil carbon and nitrogen, both showing hump-shaped patterns.Our findings substantiate that scholars may improve existing theories (e.g. the MSL model, the intermediate disturbance hypothesis) through further expounding the interactions between grazing variables and aridity and vegetation type. Rangeland managers need to devise local-scale grazing strategies for conserving plant diversity according to site-specific conditions, rather than adopting a one-size-fits-all solution.

Livestock grazing abandonment reduces soil microbial activity and carbon storage in a Mediterranean Dehesa

Extensive livestock grazing is a major determinant of local biodiversity and ecosystem functioning in many regio1ns of the world. In sub-humid Mediterranean areas with long history of extensive grazing, abandonment of grazing has been linked to changes in the composition and structure of vegetation communities and decreases in soil multifunctionality, with potential cascading consequences for ecosystem services such as soil fertility and potential for soil carbon storage. Herein, we assessed the effects of livestock grazing abandonment on soil fertility and the ability of soil microbial communities to process organic matter (enzymatic activity and decomposition rate using the Tea Bag Index) in twenty-four study sites in a Quercus ilex dehesa from Central Spain. Sites were distributed across four levels of grazing (none, very low, low and moderate) and two levels of resource availability in terms of water and nutrients (upper slope and lower slope zones). Grazing abandonment and upper slope zones were associated with a lower accumulation of organic matter and availability of nutrients such as phosphorus and potassium in the soil. Grazing abandonment also had a negative effect on microbial enzyme activity, which was directly related to litter decomposition and the ability of microbial communities to stabilize soil carbon. Microbial effects were particularly marked in the lower zones, indicating the modulating role of topography on the consequences of land use changes. Environmental management policies should consider the positive ecological effects of traditional grazing in regions with long evolutionary history of grazing by domestic livestock, and how the abandonment of these practices could negatively affect soil ecosystem functions and the provision of soil-related ecosystem services.

Goat grazing reduces diversity and leads to functional, taxonomic, and phylogenetic homogenization in an arid shrubland

AbstractLivestock production is an important activity in drylands. However, lack of adequate regulation of ranching activities can lead to the degradation of plant communities, which in turn can impact ecosystem functioning. In the arid ecosystems of north‐central Chile, unregulated goat grazing is widespread. Because the vegetation has a relatively short evolutionary history of grazing, it is expected to be highly susceptible to this activity. In this study, we evaluated the effects of goat grazing on plant taxonomic, functional, and phylogenetic community structure by comparing 39‐year‐old grazing exclusion plots and unprotected plots in an arid shrubland in north‐central Chile. By integrating analyses of the impact of goat grazing on functional and phylogenetic diversity and dispersion, we studied the mechanisms behind goat impact and the potential consequences. Loss of functional and/or phylogenetic diversity can result in important losses in ecosystem function. As a measure of functional diversity, we recorded plant growth form, life span, and life form. We also reconstructed a phylogeny of all plant species found at the study site and determined the phylogenetic structure of the plant community in ungrazed and grazed areas. We found that goat grazing affected diversity and community composition, leading to taxonomic, functional, and phylogenetic biotic homogenization and causing overall community impoverishment. Goats acted as a habitat filter, increasing functional convergence and promoting the establishment of exotics plants, which can lead to further losses of biodiversity, decreased ecosystem function and overall lower ecosystem stability. Our results indicate that sustainable management strategies are necessary to prevent the further degradation of these ecosystems.

Thresholds and gradients in a semi‐arid grassland: long‐term grazing treatments induce slow, continuous and reversible vegetation change

Summary Temporal changes in semi‐arid ecosystems can include transitions between alternative stable states, involving thresholds and multiple domains of attraction, but can also include relatively continuous, symmetric and reversible shifts within a single stable state. Conceptual state‐and‐transition models (STMs) describe both types of ecosystem dynamics by including state transitions (plant community changes difficult‐to‐reverse without substantial input or effort) and phase shifts (easily reversible community changes) as consequences of management practices and environmental variability. Grazing management is purported to be the primary driver of state transitions in current STMs for North American grasslands, but there is limited empirical evidence from these grasslands showing that grazing can cause difficult‐to‐reverse transitions between alternate stable states. In a northern mixed‐grass prairie in Wyoming, USA, we examined plant community responses to (i) long‐term (33‐year) grazing intensity treatments (none, light, moderate and heavy stocking rates) and (ii) 8 years of light or no grazing in pastures that were grazed heavily for the previous 25 years. Long‐term grazing treatments were associated with distinct, but not stable, plant communities. From year 22 to 33, heavier stocking rates decreased cover of dominant C3 grasses and increased cover of the dominant C4 grass Bouteloua gracilis. Reversing stocking rates from heavy to light or no grazing resulted in reversal of changes induced by prior heavy stocking for dominant C3 grasses, but not for B. gracilis. For both groups, rates of change following grazing treatment reversals were consistent with rates of change during the initial years of the experiment (1982–1990). Synthesis and applications. In a semi‐arid rangeland with a long evolutionary history of grazing, different long‐term grazing intensity treatments caused slow, continuous and directional changes with important management implications, but did not appear to induce alternative stable states. For this and similar ecosystems, quantifying the time‐scales and compositional gradients associated with key phase shifts may be more important than identifying thresholds between alternative stable states.

A conceptual model for changes in floristic diversity under grazing in semi-arid Patagonia using the State and Transition framework

Effect of grazing on plant diversity was studied based in 92 sites of the semiarid Magellan Steppe, South Patagonia, Argentina. 85 species were registered, 96% of them perennials and mostly native and endemic, with only 7.6% exotics. Cover of tussock grass Festuca gracillima was used as a Proxy variable for grazing intensity. Maximum likelihood analysis of diversity vs. tussock cover selected a quadratic function with a maximum at intermediate grazing intensities, closer to predictions of equilibrium models (MSL) for sub humid environments with a long history of grazing than those for semi arid-short evolutionary grazing history. State and Transition (ST) models indicate that the most diverse areas have probably lost tussock cover by intense use in their grazing history but are at present under lower grazing pressure, as this induces gain of diversity of short grasses without reestablishing tussock cover. A new model is proposed combining previous MSL and ST models. It shows a smaller increase in diversity with intermediate/high grazing, loops in the trajectory and multiple alternate states under low grazing depending on the management history. It may be an example of non-linear models that are supposed to be common in semiarid areas but have few published examples.

Impacts of Differing Grazing Rates on Canopy Structure and Species Composition in Hulunber Meadow Steppe

In this study, the impacts of cattle grazing with differing grazing rates on species composition, canopy structural traits, standing crop of canopy biomass, and plant species diversity were examined in a meadow steppe of the Hulunber grasslands, Northeastern China. Six stocking-rate treatments (0, 0.23, 0.34, 0.46, 0.69, and 0.92 AU.ha-1) with three replicates were established, and observations were conducted from 2009 to 2011. Our findings demonstrate that short-term grazing substantially altered the species composition and relative dominance, standing crop of aboveground biomass, and canopy structural traits, whereas no significant changes in species diversity and evenness occurred in response to different-rated grazing in this meadow steppe, which has a long-term evolutionary grazing history and high-resources availabilities. We found that perennial graminoid significantly decreased, while forbs and annuals increased at the same time, with increasing grazing intensity and duration; canopy height and coverage decreased substantially with increasing stocking rates, whereas significant changes in plant density occurred only at heavy grazing in the second and third years; and significant negative linear relations were found between the standing crop of biomass and grazing intensity in each individual year or for 3 years on average. Significantly highest species richness and canopy dominance occurred only at the intermediate grazing rate in the third year, and intermediate grazing intensity also maintained a highly constant standing crop of canopy biomass in the 3 years, all being in accordance with the intermediate disturbance hypothesis. Our findings imply that monitoring changes in species composition, canopy traits, and standing crop of biomass in grassland communities can provide important references for assessing current grazing management scenarios and conducting timely adaptive practices to maintain the long-term ability of grassland systems to perform their ecological functions.

Endozoochorous seed dispersal and germination strategies of Serengeti plants

AbstractQuestionsHow do rates of endozoochory change across gradients of environmental variation, grazing intensity and herbivore body size? Is there evidence that plants experience a trade‐off in successful germination in dung vs soil, and if so, are opposing strategies associated with traits indicative of evolutionary grazing history?LocationSerengeti National Park, Tanzania and a greenhouse in Birmensdorf, Switzerland.MethodsWe compared rates of seedling emergence from the dung of six grazing herbivore species and soil collected across an environmental gradient in the Serengeti; we experimentally exposed both dung and soil to smoke to test for the effects of breaking seed dormancy. We analysed overlap in plant composition of seedlings germinated from dung and soil and asked whether species found in dung (but not soil) or soil (but not dung) differed in traits that are associated with evolutionary grazing history, such as leaf height and seed head height.ResultsSpatially, dung samples collected in the heavily grazed Serengeti plains contained more seeds than more northern sites. Median seedling emergence from dung varied among herbivores: hartebeest and wildebeest had the most, topi and zebra had intermediate numbers and Thomson's and Grant's gazelle the fewest, with Grant's gazelle dung being composed almost entirely of herbaceous dicots. Smoke failed to induce a germination response when applied to dung, but the composition of plant species germinating in ‘dung only’ or ‘soil only’ was surprisingly distinct. Moreover, seed head heights and leaf heights were shorter for plant species that germinated from dung compared to those that germinated from soil. Finally, the abundance of seeds germinating in dung and soil showed opposite patterns across the transition from short‐grass, historically grazed sites in the south to tall‐grass, fire‐prone sites in to the north.ConclusionsTogether, our results suggest that some Serengeti plant species may be specialized to disperse via the dung of large herbivores. Consequently, endozoochory may be an important, and thus far overlooked, ecological process in Serengeti and in tropical savannas dominated by large herbivores more generally.

Grazing effects on aboveground primary production and root biomass of early-seral, mid-seral, and undisturbed semiarid grassland

Annual/perennial and tall/short plant species differentially dominate early to late successional shortgrass steppe communities. Plant species can have different ratios of above-/below-ground biomass distributions and this can be modified by precipitation and grazing. We compared grazing effects on aboveground production and root biomass in early- and mid-seral fields and undisturbed shortgrass steppe. Production averaged across four years and grazed and ungrazed treatments were 246, 134, and 102 g m−2 yr−1 for the early-, mid-seral, and native sites, respectively, while root biomass averaged 358, 560, and 981 g m−2, respectively. Early- and mid-seral communities provided complimentary forage supplies but at the cost of root biomass. Grazing increased, decreased, or had no effect on aboveground production in early-, mid-seral, and native communities, and had no effect on roots in any. Grazing had some negative effects on early spring forage species, but not in the annual dominated early-seral community. Dominant species increased with grazing in native communities with a long evolutionary history of grazing by large herbivores, but had no effects on the same species in mid-seral communities. Effects of grazing in native communities in a region cannot necessarily be used to predict effects at other seral stages.

Grazing effects on plant community succession of early‐ and mid‐seral seeded grassland compared to shortgrass steppe

AbstractQuestionsGrazing may speed or slow secondary succession, and the direction may depend on seral stage and relative tolerance of native perennial grasses compared with annual invasive species. How does grazing affect succession where undisturbed communities have a long evolutionary history of grazing by native herbivores and are tolerant to livestock grazing?LocationShortgrass steppe, North American Great Plains, Colorado (40°49′N, 104°46′W), USA.MethodsUngrazed and grazed early‐seral (4–6 yr) and mid‐seral (18–20 yr) seeded fields (Conservation Reserve Program) and traditionally grazed native steppe were compared for effects on plant composition in relation to changes expected from regional succession models.ResultsRecovery towards undisturbed native shortgrass steppe for early‐ and mid‐seral communities, respectively, was 19% and 36% for total vegetation cover, 5% and 21% for planted native species, 10% and 88% for non‐planted native perennial grasses, only 0.2% and 13% for short grasses, and overall dissimilarity in community species compositions was 97% and 68%. In general, grazing effects were neutral or most often not significant in all years and/or were small in overall community magnitude. The early‐seral community displayed more changes indicative of a slowing of succession with grazing (total vegetative and grass basal cover) rather than reducing invasive species (species targeted by timing of grazing), although drought had limited the establishment of grazing‐tolerant short grasses. The mid‐seral community showed more changes consistent with advancing successional recovery with grazing (overall community dissimilarity, forbs, planted native perennial grasses, tall grasses and target species). However, non‐planted native perennial grasses responded negatively to grazing in the mid‐seral community and positively in native shortgrass steppe where outside seed would originate.ConclusionsGrazing effects on particular functional groups and species were not the same across seral stages, were mixed in terms of speeding or slowing succession, and were generally not large at the community level. Evolutionary history of grazing may serve as a general guide but decisions on whether to graze successional grasslands may best be made after assessing whether tolerant perennial short grass species are significant components. Monitoring may then be necessary to determine species responses in particular community matrixes and effects on subsequent immigration of non‐seeded native perennial species.

A test of two mechanisms proposed to optimize grassland aboveground primary productivity in response to grazing

Aims mesic grasslands have a long evolutionary history of grazing by large herbivores and as a consequence, grassland species have numerous adaptations allowing them to respond favourably to grazing. although empirical evidence has been equivocal, theory predicts that such adaptations combined with alterations in resources can lead to grazing-induced overcompensation in aboveground net primary production (aNPP; grazed aNPP > ungrazed aNPP) under certain conditions. We tested two specific predictions from theory. First, overcompensation is more likely to occur in annually burned grasslands because limiting nutrients that would be lost with frequent fires are recycled through grazers and stimulate aNPP. second, overcompensation of biomass lost to grazers is more likely to occur in unburned sites where grazing has the greatest effect on increasing light availability through alterations in canopy structure. Methods

Is livestock grazing compatible with amphibian diversity in the High Mountains of Córdoba, Argentina?

The High Mountains of Cordoba, Argentina have a long evolutionary history of grazing by large herbivores. However, about 400 years ago, European livestock were introduced and gradually replaced native herbivores. Since the 1920s, domestic herbivores have been the only large herbivores present in the area, causing severe soil erosion and a threat to the system diversity. The endemic fauna of the region includes four amphibian species. We evaluated the effect of livestock rearing on amphibian diversity and water bodies in woodlands and grasslands of the High Mountains of Cordoba. The work was conducted on stream stretches and ponds in two contrasting grazing situations: an area with livestock presence and another area where livestock was excluded 14 years ago. The following variables were recorded at each sampling site: amphibian richness and abundance, percentage of emergent, submerged and peripheral vegetation in areas surrounding the water bodies, water pH, and water dissolved O2. No significant differences were detected in amphibian diversity between streams of both grazing situations, whereas a greater diversity (p < 0.01) was observed in ponds in grazed grasslands. Our results suggest that livestock rearing, qualitatively measured as grazing and 14 years of livestock exclusion, in the study area would not have negative effects on amphibian diversity. This finding might be due to the long evolutionary grazing history of the area, large-scale livestock exclusion exhibiting a novel scenario. The ongoing reintroduction of native grazers may provide the benefits of grazing without the consequent soil erosion and habitat degradation associated with domestic livestock.

Plant diversity consequences of a herbivore‐driven biome switch from Grassland to Nama‐Karoo shrub steppe in South Africa

AbstractQuestionsHow does heavy grazing change plant community structure, composition and species richness and diversity in an ecotone between grassland and semi‐arid shrub steppe‐type vegetation? Does grazing favour plants with arid affinity over those with less arid affinity? Does the grazing‐induced transformation constitute a switch to the equivalent of a shrub‐dominated biome?LocationCentral South Africa.MethodsUsing systematic scanning of SPOT 5 imagery and ground‐truthing, a grazing treatment area was selected that met criteria of intensity of grazing, sampling requirements, and biogeographical position within a broad ecotonal zone. Differential vegetation responses to heavy grazing were tested for significant differences in plant traits, vegetation structure, and species diversity, richness and evenness. Gamma diversity was calculated for the whole study site, whereas, independent beta diversity was calculated across the treatments assuming the additive partitioning of diversity. In addition, the biogeographical association of grazing‐induced species shifts was determined using a range of available databases.ResultsCanopy cover and height of woody shrubs increased significantly with heavy grazing whereas that of graminoid plants declined. The resultant species turnover was modest, apparent extinctions of local species were minimal, species richness was maintained and species diversity was significantly enhanced. There was a significant increase in species evenness, through possible suppression of dominant species. Significant increases in species cover were those associated with mainly the Nama‐Karoo biome indicating that species from more arid areas are more resistant to grazing as would be expected by the convergence model of aridity and grazing resistance.ConclusionsThe significant increase in shrub cover in heavily grazed semi‐arid grassland followed general global expectations. The study confirmed that the supposed former large shift of grassland to shrubby Nama‐Karoo in the eastern upper Karoo can indeed be readily affected by heavy grazing. The negative connotations for biodiversity that have often been associated with intense grazing seem, in terms of the positive responses of plant species diversity in this study, to perhaps be exaggerated. The elevated species diversity with grazing of vegetation with a long evolutionary grazing history in a low resource area may require a reappraisal of the application of certain grazing hypotheses.

Plant species richness responses to grazing protection and degradation history in a low productivity landscape

Questions: Does species richness and abundance accumulate with grazing protection in low productivity ecosystems with a short evolutionary history of grazing, as predicted by emerging theory? How do responses to grazing protection inform degradation history?

Impact of grazing on species composition: Adding complexity to a generalized model

The impact of grazing widely differs among plant communities. A generalized model published in 1988 proposed that this variation could be accounted for by the interaction between primary productivity and the evolutionary history of grazing. As productivity increased, the model predicted larger changes of species composition. Evolutionary history of grazing interacted with productivity: the changes in low-production systems were smaller if evolutionary history was long, whereas the changes in high-production systems were independent of evolutionary history. In this paper, we focus on: (i) the difficulties of determining the evolutionary history of grazing of a community, and (ii) additional mechanisms, which, as a sequence of filters in the process of community assembly, could be operating across the gradient of primary production. Assigning a given evolutionary history of grazing to a site has been difficult due to the lack of information on the historical population and distribution of herbivores with an adequate spatial and temporal resolution, and the lack of agreement on the size of the relevant evolutionary time window. Regarding the variation through a gradient of primary production, we propose three additional mechanisms coherent with the prediction of the model. First, the regional pool of available species increases with primary production. Second, grazing intensity (consumption as a proportion of above-ground production) also increases with primary production. Third, the strength of interspecific positive biotic interactions that protect plants from herbivores decreases with primary production. We highlight an additional potential mechanism, seed dispersal, whose variation across the gradient of productivity is not yet sufficiently understood. By connecting the logic of environmental filters to explain community assemblage with the original proposition of the generalized model, we suggest a series of research lines that can lead to a better understanding of why different communities respond differently to grazing.

Pyric-Herbivory to Promote Rangeland Heterogeneity: Evidence From Small Mammal Communities

Management of rangelands has largely operated under the paradigm of minimizing spatially discrete disturbances, often under the objective of reducing inherent heterogeneity within managed ecosystems. This has led to a simplified understanding of rangelands and in many cases simplified rangelands. We argue that this type of management focus is incapable of maintaining biodiversity. An evolutionary model of disturbance (pyric-herbivory) suggests that grazing and fire interact through a series of feedbacks to cause a shifting mosaic of vegetation patterns across the landscape and has potential to serve as a model for management of grasslands with an evolutionary history of grazing. Our study demonstrates that the spatially controlled interaction of fire and grazing can be used to create heterogeneity in grassland ecosystems and the resulting heterogeneity in vegetation is expressed through other trophic levels, specifically small mammals in this study. Discrete fires were applied to patches, and patchy grazing by herbivores promoted a shifting vegetation mosaic across the landscape that created unique habitat structures for various small mammal species. Peromyscus maniculatus was about 10 times more abundant on recently burned patches (1–2 mo) than the uniform treatment or unburned patches within the shifting mosaic treatment. Chaetodipus hispidus was about 10 times greater in patches that were 15–20 mo post-fire in the shifting mosaic treatment than in the uniform treatment. Sigmodon hispidus, Microtus ochrogaster, and Reithrodontomys fluvescens became dominant in the shifting mosaic in patches that were more than 2 yr post-fire. This study, along with others, suggests that by managing transient focal patches, heterogeneity has the potential to be a new central paradigm for conservation of rangeland ecosystems and can enhance biological diversity and maintain livestock production across broad scales.

Predicting cover types in a mountain range with long evolutionary grazing history: a GIS approach

AbstractAim To determine how the distribution and cover of different vegetation types are affected by physical factors and livestock in a mountain range with a long evolutionary history of grazing.Location Upper vegetation belt of the Córdoba mountains (1700–2800 m a.s.l., 31º34′ S, 64º50′ W) in central Argentina.Methods Using GIS, we analysed the relationships of plant cover types to physical features (physiography and topography) and indicators of accumulated livestock pressure (distance to human settlements and roads) through multinomial logistic regression. We predicted a present vegetation map which was validated with a real map. We then constructed two maps simulating minimum and maximum values of accumulated livestock pressure for the whole area. Map comparisons allowed evaluation of the possible influence of livestock, both in extension and intensity.Results Both physical features and livestock pressure influenced the occurrence of vegetation units. The overall accuracy of the predicted map at the pixel level was low (26%) indicating low habitat specificity of the vegetation units. We suggest that some part of the unaccounted for variance was due to livestock pressure patterns that were not fully captured by our indicators. Our models proved adequate for predicting the total percentages of vegetation units at coarser scales. The extrapolations showed that under a history of low livestock pressure, such as in sites far away from human settlements and roads, the area would be dominated by woodlands, tussock grasslands and natural rock outcrops. Under a history of heavy livestock pressure, in turn, rock exposed by erosion, tussock grasslands and natural rock outcrops would dominate.Main conclusions Vegetation units showed low habitat specificity, and were associated with accumulated livestock pressure, indicating that livestock and its associated activities are important factors structuring the landscape and have important consequences for the integrity of the ecosystem. Results suggest that although this system evolved with large herbivores, it has experienced irreversible degradation processes, and intensification of current domestic livestock pressure is likely to lead to even more land degradation.