Heterogeneous Grazing Research Articles

The Influence of Fine-Scale Grazing Heterogeneity on Dung Beetle Assemblages: What Trait Analysis Teaches Us.

Livestock grazing puts major anthropogenic pressure on biological communities worldwide. Not all species are expected to be affected in the same way, and the impacts will depend on species' traits. Focusing on traits thus helps identify the mechanisms underlying changes in community composition under grazing pressures. We investigated how fine-scale grazing heterogeneity affects the trait composition and diversity of dung beetle assemblages in Western Europe. We sampled dung beetles in habitat patches differing in terms of grazing intensity within rangelands of two distinct biogeographical areas: a Mediterranean lowland steppe and Western alpine meadows. We measured five morphological traits expected to respond to the local-scale filtering pressure exerted by variations in grazing intensity. Using individual-based data, we assessed responses in terms of single-trait mean values in communities and complementary trait diversity indices. We found strong shifts in trait composition and diversity between the habitat patches. In both study areas, variations in habitat conditions are likely to have filtered the local occurrence and abundance of dung beetles by the mean of traits such as body mass (which have several functional implications), as well as traits linked to underground activity. We hypothesize that fine-scale variation in resource availability (i.e., droppings) and disturbance intensity (i.e., trampling) are key drivers of the observed patterns in species assemblages. Trait richness peaks at moderate grazing intensity in both study areas, suggesting that patches with an intermediated level of available resources and soil disturbance enable individuals with a greater range of autecological requirements to coexist.

Wild guanacos as scapegoat for continued overgrazing by livestock across southern Patagonia

Abstract In a recently published paper, Oliva et al. concluded that domestic grazing pressure across Patagonian rangelands approached carrying capacity due to decades of stock adjustment, but that guanaco overpopulation may have altered that balance. The authors argued that unless guanaco numbers are controlled, they will reduce forage available for domestic stock and will negatively affect rangelands. We consider that the herbivore‐stock analysis presented is inaccurate and deserves revision, and that the stated conclusions lack empirical support. When the spatial distribution of herbivores is accounted for in the Oliva et al. analysis, domestic stock is far above carrying capacity, indicating that domestic overgrazing continues. Theoretical and empirical evidence on bottom‐up regulation and competitive exclusion challenges the supposed guanaco overpopulation and the hypothetical reduction of forage available for livestock. Even if guanaco numbers are reduced, grassland degradation and production losses will continue because their main drivers, domestic overstock and heterogeneous grazing, are still operating. Synthesis and applications. Oversimplified models with poor ecological insight can lead to erroneous conclusions and misguide management decisions. The incorrect inference that Patagonian domestic stock is adjusted to carrying capacity could help to consolidate current domestic overgrazing by reducing incentives to improve livestock management practices. Regarding guanacos, a controversial species in an unfavourable context, control‐oriented harvest without a clear justification threatens populations’ viability and genuine attempts of productive diversification. Addressing relevant ecological processes, such as niche partitioning, competitive exclusion and population regulation, is essential to correctly assess joint carrying capacity in multi‐herbivore systems, as well as to identify the true factors driving degradation processes and to optimize rangeland use on a sustainable basis.

El cuadro de pastoreo en los sistemas extensivos de Patagonia Sur. Percepción y manejo de la heterogeneidad

Fil: Ormaechea, Sebastian Gabriel. Instituto Nacional de Tecnologia Agropecuaria. Centro Regional Cordoba. Estacion Experimental Agropecuaria Manfredi; Argentina

Paddock Size Mediates the Heterogeneity of Grazing Impacts on Vegetation

Domestic herbivores’ effect on vegetation is spatially heterogeneous, being one of the major causes of forage resources degradation. It has been proposed that paddock size controls grazing impact’s heterogeneity because as size decreases, herbivores’ utilization is spatially more even. However, this has not been critically evaluated in commercial-scale paddocks isolating paddock size effects from other factors influencing the interaction between herbivores and vegetation. Here we assessed how paddock size mediates the heterogeneity of continuous sheep grazing effects on vegetation, at constant stocking rate in Patagonian steppes. We selected three small (ca. 110 ha) and three large (ca. 1 100 ha) paddocks dominated by the same plant community. All paddocks contained a single watering point and presented similar shape. Total and specific plant cover, vegetation patchiness, population size distribution of dominant grass species, plant morphology, and sheep feces density were estimated at increasing distances from watering points. Relationships between vegetation variables and distance from the watering point were in most cases asymptotic exponential, although responses generally differed between small and large paddocks. In small paddocks, vegetation variables mostly reached a plateau at a short distance from the watering point (~200 m). Instead, in large paddocks, the changes in vegetation variables were larger and more gradual, and reached a plateau at much greater distances (~2 000 m). Vegetation heterogeneity throughout the paddock was lower in small than large paddocks. Our findings suggest that paddock size mediates the spatial pattern of grazing effects on vegetation. Reducing paddock size decreases grazing impacts spatial heterogeneity, which makes plant-animal interactions more predictable and might improve forage utilization efficiency.

Flora and vegetation of dry grasslands of Northeastern Ukraine, and problems of diversity conservation

Abstract The aim of this study was to describe the flora and vegetation of the grasslands of Northeastern Ukraine and to analyse how the steppe vegetation responds to grazing or its abandonment. We studied two gully systems in the east of the Kharkiv Region: the Regional Landscape Park “The Velykyi Burluk-Steppe” (steppe grasslands on chernozem soils; 10 sites) and the National Nature Park “Dvorichanskyi” (steppe grasslands on chalky outcrops; 5 sites). Long-term monitoring data exist for both these sites starting in 1991, shortly after grazing intensity reduced. We recorded the major grassland plant communities (reflecting their successional status) as well as their dominant species. Tree and scrub encroachment increased after management ceased. We conclude that (i) heterogeneous grazing (including ungrazed patches) in space and time is necessary in order to preserve grassland biodiversity in our study system; (ii) erosion of chalky outcrops (natural erosion as well as driven by cattle grazing) is a key factor promoting the richness of cretaceous species in steppe grassland.

Morphological and physiological responses of seagrasses (Alismatales) to grazers (Testudines: Cheloniidae) and the role of these responses as grazing patch abandonment cues.

Green sea turtles, Chelonia mydas, are grazers influencing the distribution of seagrass within shallow coastal ecosystems, yet the drivers behind C. mydas patch use within seagrass beds are largely unknown. Current theories center on food quality (nutrient content) as the plant responds to grazing disturbances; however, no study has monitored these parameters in a natural setting without grazer manipulation. To determine the morphological and physiological responses potentially influencing seagrass recovery from grazing disturbances, seagrasses were monitored for one year under three different grazing scenarios (turtle grazed, fish grazed and ungrazed) in a tropical ecosystem in Akumal Bay, Quintana Roo, Mexico. Significantly less soluble carbohydrates and increased nitrogen and phosphorus content in Thalassia testudinum were indicative of the stresses placed on seagrasses during herbivory. To determine if these physiological responses were the drivers of the heterogeneous grazing behavior by C. mydas recorded in Akumal Bay, patches were mapped and monitored over a six-month interval. The abandoned patches had the lowest standing crop rather than leaf nutrient or rhi- zome soluble carbohydrate content. This suggests a modified Giving Up Density (GUD) behavior: the critical threshold where cost of continued grazing does not provide minimum nutrients, therefore, new patches must be utilized, explains resource abandonment and mechanism behind C. mydas grazing. This study is the first to apply GUD theory, often applied in terrestrial literature, to explain marine herbivore grazing behavior.

Incorporating farmed goats into sustainable rangeland grazing systems in southern Australia: a review

The recent trend to farming or re-domestication of feral goats poses serious questions for the ecological sustainability of the semiarid and arid rangelands of southern Australia. This paper reviews aspects of the biology and grazing habits of goats, and aspects of the Australian goat industry, relevant to their sustainable management in livestock enterprises. Key factors identified include high fertility and fecundity even under low seasonal rainfall conditions and a generalist feeding strategy. Adverse consequences for rangeland condition can be expected if seasonal or market conditions result in an imbalance between population growth and turnoff, resulting in high grazing pressures. Given the limited control of the reproductive process at the current stage of the development of goat farming, strategies aimed at ensuring continuity of sale of goats (e.g. on-property feed lots or supply chain and market development) will be important in ensuring that imbalances are avoided. Conservative stocking rates and use of seasonal risk management tools are also particularly relevant. The major research and development needs identified by this review, from a resource management perspective, concern the appropriate dry sheep equivalent rating for goat classes based on age, sex and reproductive status, the extent and consequences of heterogeneity of grazing in space, time, and across forage species, and development of means of establishing sustainable stocking rates based on the use of all available forage sources, including browse.

Different Grazing Strategies Are Necessary to Conserve Endangered Grassland Birds in Short and Tall Salty Grasslands of the Flooding Pampas

We evaluated how grazing affects bird assemblages in short and tall coastal salty grasslands at Samborombón Bay, Argentina. We studied ranches with three patterns of grazing (spatial rotation, continuous, and winter grazing) and a control site, at each censusing birds and surveying vegetation monthly. Results allowed us to compare the various treatments and their effects on endangered birds of short and tall grassland. Three species of special conservation concern showed significant responses to grazing. In short grasslands, the Buffbreasted Sandpiper (Tryngites subruficollis) is supported by continuous grazing. In tall grasslands, the Bay-capped Wren-Spinetail (Spartonoica maluroides) uses only grasslands with extensive cover of tall grass, whereas Hudson's Canastero (Asthenes hudsoni) uses those with high levels of cattle grazing. The diverse responses of grassland birds to grazing of different types of grassland leads us to propose a system of heterogeneous grazing that optimizes both production and conservation of grassland birds.

Large‐scale heterogeneous cattle grazing affects plant diversity and forage value of Alpine species‐rich Nardus pastures

AbstractIn Europe, the decline in agriculture in traditional mountain farming areas has caused abandonment, or irregular grazing, of less accessible pastures. In the Italian Alps, we studied plant diversity, forage value and trophic traits in six large‐scale heterogeneously grazed Nardus pastures, in their central and peripheral parts, and across the montane, lower and upper subalpine zones. Seventy cover‐abundance floristic relevés were performed in randomly located permanent plots, half at the pasture edge (M‐plots) and half in their central part (C‐plots). Plant diversity varied significantly with respect to the distance from the edge, being higher in M‐plots because of shrub invasion as a consequence of reduced cattle pressure. Nardus cover was slightly higher in C‐plots and negatively related to plant diversity. Forage value was lower in M‐plots and negatively correlated with the cover of Nardus and woody species, implying that different grazing pressure was negatively influencing sward productivity. Differences in trophic traits of swards between C‐ and M‐plots existed mostly as trends. Forage value significantly decreased in the lower subalpine and montane pastures, which are further grazed in late summer. Heterogeneous grazing management was therefore exerting positive effects on plant diversity and negative effects on forage value. Results are discussed in the context of future management recommendations.

The spatial dimension of pastoral herding: A case study from the northern Negev

Sustainable management of heterogeneous grazing systems requires better understanding of the spatial dimension of grazing ecology. We deployed animal-borne GPS devices to map the daily foraging excursions of a shepherded flock of 200 goats under traditional Bedouin management, on a study site in the semiarid region of the northern Negev, Israel. A total of 88 daily foraging excursions were mapped during the herbaceous growing seasons of 2002 and 2003, and the spatial dimension was analyzed with GIS tools. A typical foraging excursion lasted 5.5 h, during which the flock moved across the landscape at an average speed of 0.3 m/s (1.1 km/h) and traveled 5.4 km. The foraging route was highly elongated in shape and reached a distance of 1.5 km from the night corral. Flock movement speed was affected significantly by distance from the night corral, being greatest at shortest and longest distances. Speed decreased with increasing slope angle and differed between aspects. The frequency of flock presence across the study site deviated significantly from random with respect to aspect but not to slope angle and distance from the corral. The effect of aspect changed slightly between the study years. The product of daily distance traveled and flock path width yielded a rate of area coverage of 0.122 km2per day. The average amount of herbage dry matter removed on any one pass of the flock is therefore in the order of just a few grams per square meter. The use of the area available to the flock was highly non-uniform, which suggests that better planning of grazing management could mitigate negative effects of grazing.

Paddock Size and Stocking Density Affect Spatial Heterogeneity of Grazing

The claim that intensive rotational grazing (IRG) can sustain higher stocking rates can be partially explained by more even spatial distribution of grazing such that livestock consume forage from a greater proportion of a pasture. To test the hypothesis that utilization is more even at the higher stocking densities of smaller paddocks, mean absolute deviation (heterogeneity) of utilization estimates by plot was compared in paddocks of sizes and stocking densities representing increasing subdivision from two-paddock deferred rotation grazing (DRG) to 16-, 32-, and 64-paddock, two-cycle IRG. These 70-, 4-, 2-, and 1-ha paddocks were grazed for 7 wk, 4 d, 2 d, and 1 d, respectively, at 32 animal unit days (AUD)·ha−1 during 2000 and 34 AUD·ha−1 during 2001. Within IRG there was no response to the treatment gradient. After one cycle in the IRG paddocks, heterogeneity of use was generally lower than in the DRG paddocks, in both 2000 (3–11% [outlier 18%] vs. 14–19%) and 2001 (9–17% vs. 24–28%). After a second cycle in 2001, heterogeneity in half of the IRG paddocks (17–21%) was nearly as high as the early-grazed (24%), but not the late-grazed (28%), of the DRG paddocks. This lack of a stronger difference between systems was probably due to the fixed two-cycle IRG schedule and lack of plant growth during the nongrazing interval. Across both systems heterogeneity of utilization was strongly positively correlated with paddock size. Because utilization was not severely patchy in the largest treatment, the difference between systems would likely be greater in commercial-scale paddocks. Thus grazing distribution can be more even under intensive than extensive management, but this depends on how adaptively the system, particularly the aspects of timing and frequency, is managed. La afirmación de que el pastoreo rotacional intensivo (PRI) logra mantener una mayor carga animal se puede explicar parcialmente a que existe una distribución más uniforme del pastoreo de tal manera de que el ganado consume forraje de la mayoría del potrero. Para probar la hipótesis que la utilización es más homogénea en potreros pequeños con una mayor densidad animal, la absoluta desviación de la media (heterogeneidad) de la utilización estimada, se comparó en potreros de diferentes tamaños y densidades de cargas representando un incremento en las sub-divisiones utilizando un sistema de pastoreo de rotación diferida (PRD). Se utilizaron 2 potreros en el PRD y 16, 32 y 64 potreros, en el pastoreo rotacional intensivo (PRI) de 2 ciclos. Estos potreros de 70, 4, 2, y 1 hectárea fueron pastoreados durante 7 semanas, 4 días, 2 días y 1 día respectivamente con 23 unidades animal-días (UAD)·ha−1 durante 2000 y 34 UAD·ha−1 durante 2001. Dentro del PRI no se encontró respuesta entre tratamientos. Después de un ciclo en los potreros de PRI, la heterogeneidad de uso fue generalmente menor que en los potreros del PRD, tanto en 2000 (3–11% [afuera 18%] vs. 14–19%) y en 2001 (9–17% vs. 24–28%). Después del segundo ciclo en 2001, la heterogeneidad (17–21%) en la mitad de los potreros de PRI fue muy similar a la presentada al inicio del pastoreo (24%), pero diferente al final del pastoreo (28%), de los potreros del PRD. El que no se hayan encontrado diferencias entre sistemas de pastoreo fue posiblemente debido a los 2 ciclos fijos programados del PRI y a la falta de crecimiento de las plantas durante el intervalo de descanso. A través de ambos sistemas, la heterogeneidad de la utilización mantuvo una estrecha y positiva correlación con el tamaño del potrero. Debido a que la utilización no fue totalmente heterogénea en el tratamiento con el potrero más grande, las diferencias entre sistemas posiblemente podrían ser mayores en sistemas a escala comercial. Por lo tanto, la distribución del pastoreo puede ser más uniforme bajo condiciones intensivas que la que pueda darse en condiciones extensivas, pero esto depende de la adaptabilidad del sistema, en particular si los aspectos de tiempo y frecuencia se manejan adecuadamente.

Degradation and recovery processes in arid grazing lands of central Australia. Part 1: soil and land resources

The distribution and quality of soil and land resources in heterogeneous grazing lands of central Australia were changed by grazing. Sites located at increasing distances from livestock watering points showed greater degrees of landscape organization and soil productive potential. The depositional strata, where resources tended to accumulate, occupied a larger proportion of the landscape as distance increased. Physical and nutrient cycling soil properties improved. All soil chemistry variables except pH and electrical conductivity increased and the trend was most apparent in the top 1 cm of the soil. Increasing erosion closer to water was a key degrading process. We showed degradation to be a systematic decline in regulation of scarce resources, which had implications for potential productivity.

Heterogeneous grazing causes local extinction of edible perennial shrubs: a matrix analysis

Summary Population modelling and field measurements of births, growth and deaths were used to investigate the long‐term change in abundance of Atriplex vesicaria (Chenopodiaceae), a long‐lived, palatable, perennial shrub, under sheep grazing. Of particular interest was whether A. vesicaria is at risk of being eliminated throughout grazed paddocks when the recommended practice of continuous grazing at conservative stocking rates is employed. Time‐invariant matrix population models indicated that the A. vesicaria population was in decline over much of the study paddock, but the rate of decline was greatest nearer to the water point (population growth rate λ ≈ 0·8). Time‐varying stochastic matrix models projected that the A. vesicaria population would become locally extinct at most sites up to approximately 2200 m from water, occurring first closer to water (within 12–29 years). The population was stable (i.e. λ ≥ 1) at sites greater than 2200 m from water over the projection period of 100 years. Decreases in adult survival and recruitment made the largest contributions to reductions in the population growth rate. However, there were spatial patterns centred on the water point in the degree to which particular demographic processes contributed to these reductions, because of a grazing gradient and the differential sensitivity of demographic processes to grazing. Thus decreases in recruitment contributed to reductions in the population growth rate at greater distances. Such responses, together with the sensitivity of the population growth rate to these processes, determined the spatial pattern in population growth. The results suggest that piospheres (i.e. the zone of impact) continue to expand over many years under set‐stocking so that the area around the water point that is devoid of A. vesicaria becomes larger. The process of expansion appears to first involve the inhibition of recruitment, followed by eventual mortality of established shrubs. The large contribution of adult survival to the population growth rate in A. vesicaria suggests that minimizing the mortality of established adults should be a priority for management. This is likely to involve resting from grazing at critical times such as during extended dry periods. This may also permit increased levels of recruitment during subsequent moister periods.

Predicting spatial variation in heather utilization by sheep and red deer within heather/grass mosaics

Summary 1. Recent work has shown that the dynamics of woody and herbaceous vegetation mosaics under grazing or browsing can be strongly affected by the spatial pattern of the vegetation. A suite of models is described here to predict spatial variation in the utilization of an internationally important woody species, heather Calluna vulgaris, within heather/grass mosaics when grazed by sheep or red deer Cervus elaphus. This type of information is needed for the successful management of free‐ranging herbivores grazing these extensive vegetation types. 2. The models predict: (i) heather utilization at the edge of grass patches; (ii) utilization distant from the grass edge; and (iii) the rate of change of utilization with distance from the edge of grass patches as functions of (a) the proportion by area of grass in the mosaic, (b) the length of heather/grass edge per unit area, and (c) the overall heather utilization on a site. Differences between species of grazers and seasons are also taken into account. 3. The models were fitted to data from two series of grazing experiments conducted on natural and artificial heather/grass mosaics in north‐east Scotland. 4. Overall heather utilization in the experiments was estimated using a geographical information system (GIS), taking into account higher levels of utilization in proximity to grass patches. 5. A modelling approach for predicting how the heather/grass mosaic pattern will change through time under heavy grazing is proposed. As no field data are yet available on this process, this model was developed by simulation within a GIS. 6. This work demonstrates how incorporation of spatial information can improve the accuracy of utilization predictions, and thus the design of appropriate management practices. As with many other similar systems, the paucity of data on spatial variation in heather utilization by grazing ungulates, and the effects of utilization at the heather/grass edge, particularly under high grazing pressures, is highlighted. This is considered to be a major limitation to the efficacy of currently available decision support tools to aid the management of heterogeneous grazing systems.

Benthic microalgal diversity enhanced by spatial heterogeneity of grazing.

This study presents model experiments on the effect of the spatial pattern of herbivory on primary producer diversity. Microalgal biofilms (periphyton) were exposed to different mixtures of two benthic herbivores, the isopod Idothea chelipes and the gastropod Littorina littorea. The herbivores are similar in their feeding selectivity but differ strongly in the spatial pattern of grazing. Idothea did not increase the spatial heterogeneity of algal cell densities beyond the level of ungrazed controls (<1 order of magnitude between local minima and maxima at the 1 mm2 scale). Littorina grazing, in contrast, created a pronounced spatial heterogeneity with maximum:minmum ratios of almost 3 orders of magnitude. When algae were exposed to mixtures of both gazers, the spatial heterogeneity of microalgal cell densities increased with an increasing proportion of Littorina in the herbivore mixture. Algal species richness, diversity and evenness also increased with increasing proportions of Littorina, and was highly significantly correlated with the spatial heterogeneity of cell densities.

Simulated long-term vegetation response to alternative stocking strategies in savanna rangelands

Increasing cover by woody vegetation, prevalent in semiarid savanna rangelands throughout the world, is a degrading process attributed to the grazing impact as a major causal factor. We studied grazing effects on savanna vegetation dynamics under alternative stocking strategies with a spatially explicit grid-based simulation model grounded in Kalahari (southern Africa) ecology. Plant life histories were modeled for the three major life forms: perennial grasses, shrubs, annuals. We conducted simulation experiments over a range of livestock utilization intensities for three alternative scenarios of small scale grazing heterogeneity, and two alternative strategies: fixed stocking versus adaptive stocking tracking herbage production. Additionally, the impact of the duration of the management planning horizon was studied, by comparing community response and mean stocking rates after 20 and 50 years. Results confirmed a threshold behavior of shrub cover increase: at low, subcritical utilization intensity little change occurred; when utilization intensity exceeded a threshold, shrub cover increased drastically. For both stocking strategies, thresholds were highly sensitive to grazing heterogeneity. At a given critical utilization intensity, the long term effect of grazing depended on the level of grazing heterogeneity: whereas under low heterogeneity, shrub cover remained unchanged, a large increase occurred under highly heterogeneous grazing. Hence, information on spatial grazing heterogeneity is crucial for correct assessment of the impact of livestock grazing on vegetation dynamics, and thus for the assessment of management strategies. Except for the least heterogeneous grazing scenario, adaptive stocking allowed a more intensive utilization of the range without inflating the risk of shrub cover increase. A destabilizing feedback between rainfall and herbage utilization was identified as the major cause for the worse performance of fixed compared to adaptive stocking, which lacks this feedback. Given the usually high grazing heterogeneity in semiarid rangelands, adaptive stocking provides a management option for increasing herbage utilization and thus returns of livestock produce without increasing degradation risks.

Long-term impacts of livestock herbivory on herbaceous and woody vegetation in semiarid savannas

In savannas, degrading trends such as increasing woody cover are caused by livestock ranging. Hence, identifying sustainable utilisation intensities is crucial for range management. However, knowledge concerning herbivory effects on herbaceous vegetation and primary productivity, but also productivity of and foraging on woody vegetation (browsing) is incomplete. Consequences of these knowledge gaps for predictions of long-term herbivory impacts are analysed by exploring a set of herbivory models in the framework of a spatially explicit simulator of vegetation dynamics in a semiarid savanna. We distinguish herbaceous and woody vegetation, and two impacts of herbivory, namely on phytomass and on primary productivity. Herbaceous phytomass is consumed under all grazing models, which are distinct by effects on herbaceous productivity per unit ground area: none; direct effect on productivity; direct effects on productivity and survival. Browsing is either absent or present. Additionally, low and high spatial grazing heterogeneity are distinguished. For a time scale of 100 years, herbivory impacts were analysed in a simulation experiment over a range of low to high stocking rates. Without effects on primary productivity, herbivory showed no long-term impact on shrub cover. However, under higher stocking rates, mean herbage utilisation reached over 50% of herbaceous phytomass production, rendering the null hypothesis on productivity effects of herbivory unrealistic. When grass productivity was affected by herbivory, shrub cover showed a threshold response to utilisation intensity: at low subcritical intensities, shrub cover did not exceed the low initial values; above a threshold intensity, savanna vegetation was characterised by high shrub cover. The level of this threshold was affected by heterogeneity, browsing, and by the nature of the productivity effects, and also depended on the time span considered. Since sustainable livestock ranging is possible only at utilisation intensities below this threshold, its determinants are crucial for long-term assessments. Given the insufficient quantitative understanding of all three of the factors determining long-term herbivory impacts, our results suggest caution towards current predictions, and provide for a priority ranking in urgently needed further research.

Simulated long‐term vegetation response to grazing heterogeneity in semi‐arid rangelands

1. The long‐term effects of small‐scale spatial heterogeneity of livestock grazing on vegetation dynamics were studied with a grid‐based model of major life forms of savanna vegetation. Based on southern Kalahari ecology, the model includes stochastic life‐history variables, resource competition for soil water, and biomass production for annuals, perennial grasses and shrubs. 2. Grazed at individual severities, the model’s 25 m2 grid cells defined the spatial scale of heterogeneity. Different scenarios of grazing heterogeneity were generated by modifying distributional and behavioural features of the grazing model. Simulations were run over 50 years under moderate to high constant stocking rates. 3. Results confirmed a previously reported threshold response of shrub cover increase: under moderate grazing pressure, little change in shrub cover occurred; when grazing pressure exceeded a threshold, shrub cover increased drastically. 4. Under moderate or high stocking rates, grazing heterogeneity did not modify grazing effects. However, within an intermediate range of stocking rates, small‐scale heterogeneity determined the long‐term impact of grazing. In particular, utilization intensity at the threshold of shrub cover increase was 60% less under high compared to low local grazing heterogeneity. 5. Sensitivity of vegetation dynamics to local grazing heterogeneity was also exemplified under a landscape‐scale grazing gradient as observed at watering points: at a given utilization intensity, a wide zone of increased shrub cover occurred under large local grazing heterogeneity, while under the least heterogeneous grazing only a narrow zone of slightly increased shrub cover occurred. 6. Because of the slow progress of shrub cover increase, a mismatch of management and ecological time scales was diagnosed and its implications for management are discussed. 7. We conclude that knowledge of local grazing heterogeneity is crucial for correct assessment of livestock impact on vegetation dynamics. Consequently, management aiming at sustainable land use should account for spatial grazing aspects. These poorly understood aspects form a gap to be filled by both empirical and theoretical studies.