Changes In Patch Size Research Articles

Effect of fluid patch clustering on the P-wave velocity-saturation relation: a critical saturation model

Quantitative analysis of the relationship between seismic wave velocities and fluid saturation in porous media is of great significance for any fluid injection and extraction operation in subsurface rock formations. However, seismic velocities are not only dependent on the amount of saturation, but also on the distribution of fluid patches and their size. The patch size variation during changes in saturation is oftentimes ignored in modeling studies, even though it is natural to assume that with increasing saturation, fluid patches will form larger and, at some critical saturation, percolating clusters. To capture the evolution of patch size with saturation implied in the velocity-saturation relations, we are inspired by percolation theory. By incorporating the connectivity of water-filled patches in the continuous random medium model, we develop a critical saturation model. We apply this critical saturation model to examine recently reported experimental measurements, specifically analyzing the patch size changes. For measurements of drainage or imbibition processes in four sandstone samples, we indeed find a clear indication of growing patch size with water saturation. The predictions of the critical saturation model are in reasonable agreement with observations. Our approach improves the accuracy of the interpretation of the velocity-saturation relations in partially saturated rocks and forms a basis for exploring its underlying mechanisms.

Sea level rise-induced habitat loss does not alter effective migration rate for the salt marsh insect Tumidagena minuta due to large genetic effective population size

IntroductionAs anthropogenic change alters and fragments habitats, it is apparent that evolutionary change can co-occur with ecological change, though the scale and consequences of this contemporary evolution remain unclear. In coastal salt marshes of eastern North America, the flood tolerant low elevation marsh grass (Spartina alterniflora), is displacing Spartina patens, the flood intolerant high elevation marsh grass. Rising seas restrict S. patens, once occupying large areas of many hectares, to increasingly small patches, some as small as a few square meters. MethodsUsing nine microsatellite loci, we examined the genetic diversity and population structure of Tumidagena minuta, a minute, flightless planthopper and specialist herbivore of S. patens. We sampled T. minuta from S. patens habitat patches of varying radius (3–82 meters) and distances (54–1,100 meters) to test how landscape variation affects population genetic parameters associated with microevolutionary processes. We sampled and genotyped 142 T. minuta individuals across six S. patens patches in a single marsh in New Jersey, USA. ResultsWe observed high polymorphism, observing between 7 and 28 alleles per locus and an average of 13.3 alleles per locus. We observed no genetic differentiation among sampled patches (RST = −0.0109). The contemporary genetic effective population size (Ne) was estimated at approximately 360 (95% confidence interval: 208–1325) based on two-locus linkage disequilibrium. Based on an estimate of Nem = 32.4 in the finite island model, the estimated gene flow rate among these patches was 0.09 migrants per generation. DiscussionThese estimates, which are rarely produced for non-model insects, suggest that, despite rapid and precipitous decreases in habitat size and connectivity, T. minuta populations have remained large and have experienced little genetic differentiation due to drift. Ecological changes in patch size and isolation at this scale have not influenced population genetic processes like effective migration rate for T. minuta, consistent with our expectations for an insect with a large population size.

THE EFFECT OF DAMAGE LINE AND PATCH DIMENSIONS ON TENSILE STRESS BEHAVIOR IN REPAIR OF ALUMINUM PLATES

In this study, in the repair of angled channel aluminum plates; The effects of channel angle and patch sizes on tensile stress behavior were investigated experimentally. For this purpose, aluminum 2024 T3 sheets with angled channels with varying channel angles were prepared. These plates were subjected to tensile loads and their failure loads were investigated experimentally. Then, using DP460 adhesive material, angled channel aluminum 2024 T3 sheets were repaired using double-sided lap joints with aluminum 2024 T3 patches of varying sizes, and the repaired samples were subjected to tensile load, the effect of channel angle and patch size change on failure loads and stress behaviors in repair was also experimentally examined and the success of the repair was evaluated.

ROLE OF AYURVEDA DRUGS IN TREATMENT OF SHVITRA: A CASE STUDY

The term Kushtha is used in Ayurveda to describe the majority of skin diseases. In the Ayurvedic Samhita, Shvitra is one of the kinds of Kushtha that occurs due to vitiation of Tridoshas and Dushya such as Rakta, Mamsa, and Meda.1 Shvitra is mentioned in every classic, including the Bruhatrayees and Laghutrayees. Vitiligo, an acquired pigmentary illness of unknown aetiology, is the most common cause of depigmentation in the globe, with a preva- lence of 1%2. Vitiligo is characterized by the appearance of white patches in the skin or hair as a result of the loss of functional melanocytes in the skin or hair, or both known as Shvitra in Ayurveda. A female patient, aged 19, who had been suffering from white spots on her face for 11 years was treated with Shaman Chikitsa over a period of 24 months, with follow-up every 30 days in between. Relief from the complaints was measured by changes in patch size gradually. The patch's size was almost cured after 24 months of treatment. The results of this case study showed that Ayurveda therapy, both internal and external, was effective in the treatment of Shvitra over a period of 24 months. The efficacy of Ayurvedic therapy was demonstrated in this case study. Keyword: Shvitra, Leukoderma, Bakuchi, Trivang Bhasm

Effects of pore water distributions on P-wave velocity–water saturation relations in partially saturated sandstones

SUMMARY We have developed a new method for introducing the effects of pore water distribution into the interpretation of P-wave velocity (VP) dispersion in a partially saturated rock. Pore water in an unsaturated rock is distributed as patches, and mesoscopic-scale wave-induced fluid flow (WIFF) from saturated patches to empty patches is known as a key mechanism for interpreting P-wave dispersion. Mesoscopic WIFF depends on permeability and the scale of the saturated patch, thus evaluating their influences is important for estimating the relationship between water saturation (${S_{\rm{w}}}$) and VP. The simplest and conventional law for simulating the ${S_{\rm{w}}} - {V_{{P}}}$ relation considering the mesoscopic WIFF is White's model, which describes the patch size changes proportional to the saturation. However, White's model assumes homogeneity in the size and distribution of pores, and does not consider the effects of pore water distribution in a partially saturated rock. In the rock, pore water is distributed heterogeneously according to the pore radius, causing non-linear changes in the permeability and size of the saturated patches against the water saturation. In this study, we modified White's model by introducing relative permeability and a new coefficient describing the nonlinear change of the patch size into the conventional version, and applied the modified model to better interpret our experimental ${S_{\rm{w}}} - {V_{{P}}}$ relations, which were measured by an ultrasonic technique at 200 and 500 kHz for two types of Berea sandstone samples with different permeabilities as their water saturation was decreased by evaporative drying. The relative permeabilities were determined by applying the Mualem–van Genuchten model into the capillary pressure curves from mercury intrusion porosimetry. Furthermore, we proposed a calculation method for the new coefficient using tortuosity, which corresponds to the pore water connection. The modified White's model could reproduce the experimental ${S_{\rm{w}}} - {V_{{P}}}$ relations better than other conventional models. Consequently, our modification, considering the effect of pore water distribution, would be useful for more quantitative interpretation of P-wave velocity and attenuation under coexisting multifluid conditions.

Parallel responses of species diversity and functional diversity to changes in patch size are driven by distinct processes

Abstract Do species and functional trait diversity respond similarly to deterministic and random processes? Theory predicts that the contributions of random and deterministic processes to species diversity depend on patch size. Smaller patches are more strongly influenced by random sampling effects, by having fewer individuals, as well as ecological drift, which propagates the effects of small samples through stochastic birth and death processes. These random processes decrease species richness and increase compositional differences among small patches. Larger patches are predicted to be more deterministically assembled, with greater species richness and greater predictability of composition for a particular environment. The consequences of patch size for the diversity of functional traits, however, are poorly understood. Species diversity may be a poor proxy for functional diversity due to trait redundancies among species, making it unclear how random and deterministic processes alter functional diversity within patches of differing size, and how these differences scale up to determine among‐patch functional diversity. We report a novel experimental study of species and functional diversity across spatial scales. We manipulated patch area in an experimental plant metacommunity and used a nested sampling design to distinguish the effects of deterministic processes, ecological drift and sampling effects on species and functional trait diversity. Our study revealed a pervasive influence of ecological drift and sampling effects on diversity, with distinct influences on functional traits and species composition within and among patches. Overall, drift and sampling effects caused a two‐ to threefold decrease in the importance of deterministic processes in small fragments. Species and functional diversity showed similar patterns with patch size; larger patches had greater within‐patch (alpha) diversity and lower among‐patch (beta) diversity, consistent with theory. However, our nested sampling design revealed that sampling effects (i.e. the size of the sample area) largely determined alpha species diversity and beta functional diversity, while ecological drift had a stronger influence on alpha functional diversity and beta species diversity. Synthesis. The compositions of species and functional diversity in a community are influenced by distinct processes, resulting in divergent spatial scaling of species and their traits within patches and across landscapes.

Evaluating extinction debt in fragmented forests: the rapid recovery of a critically endangered primate

AbstractFragmented tropical forests can be highly dynamic, with the spatial configuration of forest patches changing through time. Yet, the lack of longitudinal studies limits our understanding of how patch dynamics affect biodiversity, especially when there is a time lag in species extinctions (extinction debt). We assessed how temporal changes in patch size, shape complexity and isolation affect populations of the Mexican howler monkey (Alouatta palliata mexicana), hypothesizing that we would find an extinction debt in this relatively long‐lived species. We assessed patch occupancy, subpopulation size and immature‐to‐female ratio in 39 forest patches from Los Tuxtlas, Mexico, in both 2001 and 2013. To identify time‐lag responses to habitat disturbance, we related demographic attributes in 2013 to patch metrics in 2001 and 2013 and tested whether primate subpopulations were better predicted by current or historical patch characteristics. We also assessed how changes in patch metrics affected each demographic attribute between 2001 and 2013. Patch size and shape complexity increased over time, whereas isolation decreased. These positive spatial changes were accompanied by a 1.6‐fold increase in mean subpopulation size over the same period. In addition, occupancy and immature‐to‐female ratio were similarly related to patch attributes in both years, suggesting that there is no extinction debt. Our findings are ‘good news’, suggesting that forest recovery over a relatively short period can promote the recovery of this Critically Endangered taxon. They also highlight the importance of preventing forest loss and promoting forest regeneration in human‐modified tropical landscapes.

Effects of patch size changes on the time-lapse seismic reflections of highly attenuating reservoirs

Reservoir fluid depletion and injection inevitably induce changes in pore pressure and saturation, leading to a varying fluid patch size and hence perturbations in seismic velocities and observable time-lapse seismic attributes. The patch size, which may exhibit spatial variations, can possess a characteristic size at each scenario, and therefore another variable impacting time-lapse seismic signatures. In this study, we extend previous research efforts of time-lapse seismic interpretation by considering the fluid patch size effect on time-lapse seismic signals to investigate uncertainties in time-lapse seismic observations. We predicted and analyzed the influence of reservoir permeability and gas saturation on seismic acoustic properties, i.e., attenuation and velocity dispersion, reflectivity and phase, on the basis of a generalized White's spherical model for varied patch sizes on millimeters to centimeters. A stratified model consisting of a fluid-saturated sandstone reservoir showing viscoelastic properties sandwiched between two elastic half-spaces is employed, so as to give insights into the impacts of fluid extraction induced changes of velocity dispersion and attenuation, as well as acoustic impedance contrast, on time-lapse seismic reflections. Analysis of time-lapse seismic travel-time and reflection amplitude differences indicates that ignoring the time-dependent patch size behaviour can give rise to underestimation or overestimation of the reservoir fluid saturation and permeability change.

La Sal Daisy,Erigeron mancus,Density and Associated Species from Treeline Ecotone and Alpine Habitats

The La Sal daisy, Erigeron mancus, is endemic to treeline ecotone and alpine meadow habitats of the La Sal Mountains in Utah, an insular, laccolithic mountain range on the Colorado Plateau in southeastern Utah. From 2009 to 2011 we established elevational transects through upper spruce-fir forest, treeline ecotone, and alpine meadow habitats on Laurel Ridge in the Middle Mountain group of the La Sal Mountains to measure (1) periodic changes in E. mancus population density, (2) changes in the elevation of the E. mancus population centroid, (3) changes in patch size occupied by E. mancus, and (4) changes in frequency of occurrence in herbaceous plant species associated with E. mancus along these transects. We measured both E. mancus density and vascular plant species composition within 1-m × 1-m square frames in mid-July, near peak alpine plant flowering time. The E. mancus population density on Mt. Laurel ridge did not significantly change from 2009 to 2015, but the species was most abundant in alpine meadow habitat for both years. Changes in patch width, centroid elevation, and frequency of occurrence of 30 associated plant species were also not statistically significant. Like E. mancus, most species show changes in frequency of occurrence between upper spruce-fir forest, treeline ecotone, and alpine meadow habitats. Individual plants of E. mancus are probably long-lived perennials, so changes in population density and distribution due to global warming are expected to be gradual and/or have an unknown lag time. Although there is no evidence of ongoing change in the Laurel Ridge population of E. mancus, having this information will provide a solid statistical basis for determining significant future changes.

Fragment size affects plant herbivory via predator loss

Fragmentation and resultant changes in patch size are predicted to alter species diversity and community composition, yet the consequences of these differences for species interactions are poorly understood. Theory predicts that predators are more sensitive to fragmentation than their prey, resulting in greater predator loss in small patches. Predator loss, in turn, is predicted to 1) increase herbivory rates overall, and 2) cause herbivores to shift feeding from plants that act as refugia to those that are preferred forage. We tested these predictions in an old‐field community using two experiments. The first was a large‐scale experiment that included hundreds of arthropod species in fragments of various sizes, and used goldenrod and switchgrass to assess herbivory. Our second experiment manipulated densities of a focal predator species and a focal prey species to determine if changes in densities, rather than other characteristics of fragments, were sufficient to cause the trends observed in the first experiment. We found that predator densities declined in small fragments whereas herbivore densities showed the opposite trend. Total herbivory mirrored herbivore densities by increasing in small patches, and this mean increase was driven by large increases in goldenrod herbivory but declines in switchgrass herbivory. Experimental manipulation of densities confirmed that herbivores preferentially feed on goldenrod, and that predators depress herbivory on goldenrod but have a negligible effect on switchgrass. Our results suggest that fragmentation alters trophic interactions by causing declines in predator densities and increases in herbivore densities, but that feeding preferences of herbivores may generate unequal impacts among plant species.

Vegetation patch dynamics and tree diversity in a diverse conifer and oak forest in central Mexico

Mexican conifer and oak forests are very diverse, and represent the vegetation type with the highest timber production in Mexico. Yet, there is little knowledge about the relationship of forest recovery and tree diversity in this type of vegetation. In this study, we analyzed the dynamics of vegetation patches over three time periods covering five decades, within an area in central Mexico dominated by conifer and oak forest. We identified vegetation patches, and evaluated their number and changes in patch size and vegetation cover. We also evaluated the composition and structure of the plant canopy in vegetation plots representing different canopy cover classes. Over the three periods studied, the number and total area of patches remained relatively constant, but an increase in canopy vegetation cover occurred in most vegetation patches. Patch canopies were dominated by the long-lived pioneer tree species Juniperus deppeana and the shrub Arctostaphylos pungens . We suggest that forest management in our study region should focus on facilitating the dominance of pines and oaks in sites that have recovered their canopy cover, and to identify the factors inhibiting succession in patches with less than 10 % vegetation cover.

Velocity‐saturation relation in partially saturated rocks: Modelling the effect of injection rate changes

ABSTRACTThe relationship between P‐wave velocity and fluid saturation in a porous medium is of importance for reservoir rock characterization. Forced imbibition experiments in the laboratory reveal rather complicated velocity–saturation relations, including rollover‐like patterns induced by injection rate changes. Poroelasticity theory‐based patchy saturation models using a constant fluid patch size are not able to describe these velocity–saturation relations. Therefore, we incorporate a saturation‐dependent patch size function into two models for patchy saturation. This recipe allows us to model observed velocity–saturation relations obtained for different and variable injection rates. The results reveal an increase in patch size with fluid saturation and show a reduction in the patch size for decreasing injection rate. This indicates that there can exist a distinct relation between patch size and injection rate. We assess the relative importance of capillarity on velocity–saturation relations and find that capillarity stiffening impairs the impact of patch size changes. Capillarity stiffening appears to be a plausible explanation when a decrease in the injection rate is expected to boost the importance of capillarity.

Habitat loss, not fragmentation, drives occurrence patterns of Canada lynx at the southern range periphery.

Peripheral populations often experience more extreme environmental conditions than those in the centre of a species' range. Such extreme conditions include habitat loss, defined as a reduction in the amount of suitable habitat, as well as habitat fragmentation, which involves the breaking apart of habitat independent of habitat loss. The ‘threshold hypothesis’ predicts that organisms will be more affected by habitat fragmentation when the amount of habitat on the landscape is scarce (i.e., less than 30%) than when habitat is abundant, implying that habitat fragmentation may compound habitat loss through changes in patch size and configuration. Alternatively, the ‘flexibility hypothesis’ predicts that individuals may respond to increased habitat disturbance by altering their selection patterns and thereby reducing sensitivity to habitat loss and fragmentation. While the range of Canada lynx (Lynx canadensis) has contracted during recent decades, the relative importance of habitat loss and habitat fragmentation on this phenomenon is poorly understood. We used a habitat suitability model for lynx to identify suitable land cover in Ontario, and contrasted occupancy patterns across landscapes differing in cover, to test the ‘threshold hypothesis’ and ‘flexibility hypothesis’. When suitable land cover was widely available, lynx avoided areas with less than 30% habitat and were unaffected by habitat fragmentation. However, on landscapes with minimal suitable land cover, lynx occurrence was not related to either habitat loss or habitat fragmentation, indicating support for the ‘flexibility hypothesis’. We conclude that lynx are broadly affected by habitat loss, and not specifically by habitat fragmentation, although occurrence patterns are flexible and dependent on landscape condition. We suggest that lynx may alter their habitat selection patterns depending on local conditions, thereby reducing their sensitivity to anthropogenically-driven habitat alteration.

Reflectance of sexually dichromatic UV-blue patches varies during the breeding season and between two subspecies ofGallotia galloti(Squamata: Lacertidae)

Body coloration is sexually dimorphic in many vertebrate species, including lizards, in which males are often more conspicuous than females. A detailed analysis of the relative size of coloured patches and their reflectance, including the ultraviolet (UV) range, has rarely been performed. In the present work we quantified sexual dimorphism in body traits and surface area of all lateral patches from adult females and males of two subspecies of Gallotia galloti (G. g. galloti and G. g. eisentrauti). We also analysed the magnitude of sexual dichromatism in the UV-visible reflectance of such patches and the changes in patch size and brightness during the reproductive season (April–July). Males had significantly larger patch areas (relative to their snout-vent length) and higher brightness (mainly in the UV-blue range) than did females in both subspecies. The comparison of relative patch areas among months did not reach statistical significance. However, patch brightness significantly changed during the breeding season: that of the UV-blue (300–495 nm) range from lizards of the two subspecies was significantly larger in June than in April, while brightness in the 495–700 nm range in G. g. galloti was larger in May, June, and July than in April. A different pattern of dichromatism was also detected in the two populations, with G. g. eisentrauti being more sexually dichromatic than G. g. galloti. We discuss the results in terms of possible evolutionary causes for the sexual dichromatism related to different ecological characteristics of the habitats where each subspecies live. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 556–569.

Mapping large-scale distribution and changes of aquatic vegetation in Honghu Lake, China, using multitemporal satellite imagery

Honghu Lake, one of the seven largest fresh-water lakes in China, is well known for its ecological and economic importance, as well as its rapid changes in recent years. This study investigates the potential of using remote sensing to map and monitor aquatic vegetation changes in Honghu Lake on a large scale. Landsat TM/ETM+ images dated July 27, 2000, July 9, 2002, and July 17, 2008, and CBERS image dated August 12, 2005, are employed to map the aquatic vegetation distribution in the lake. A hybrid classification method, combining the power of the decision tree classifier, naive Bayes classifier, and supporting vector machine classifier is used to distinguish different wetland types. A novel polar coordinate map method is proposed to map the changes of aquatic vegetation on a large scale. The map demonstrates vegetation patch size changes and percentage changes in the whole lake directions during four periods. Validation using in situ surveys and historical ancillary data suggests that this approach could map the distribution and monitor the changes of aquatic vegetation on a large scale efficiently.

The effects of forest fragmentation on species richness on the Obudu Plateau, south‐eastern Nigeria

AbstractThis study examines how fragmentation through changes in patch size and increasing distances between forest patches affects bird species richness. Numbers of bird species were higher in large forest patches and decrease with decreasing patch size. Bird species richness also decreased with increased isolation of forest patches. Suitable trees should be planted in the gaps between the forests that will create wildlife corridors to maintain connectivity.

The relative influence of habitat loss and fragmentation: Do tropical mammals meet the temperate paradigm?

The relative influence of habitat loss vs. habitat fragmentation per se (the breaking apart of habitat) on species distribution and abundance is a topic of debate. Although some theoretical studies predict a strong negative effect of fragmentation, consensus from empirical studies is that habitat fragmentation has weak effects compared with habitat loss and that these effects are as likely to be positive as negative. However, few empirical investigations of this issue have been conducted on tropical or wide-ranging species that may be strongly influenced by changes in patch size and edge that occur with increasing fragmentation. We tested the relative influence of habitat loss and fragmentation by examining occupancy of forest patches by 20 mid- and large-sized Neotropical mammal species in a fragmented landscape of northern Guatemala. We related patch occupancy of mammals to measures of habitat loss and fragmentation and compared the influence of these two factors while controlling for patch-level variables. Species responded strongly to both fragmentation and loss, and response to fragmentation generally was negative. Our findings support previous assumptions that conservation of large mammals in the tropics will require conservation strategies that go beyond prevention of habitat loss to also consider forest cohesion or other aspects of landscape configuration.

Phenotypic plasticity in a conspicuous female plumage trait: information content and mating patterns

Explaining sexual ornamentation in the limiting sex, usually females, requires information on the proximate background of ornaments and their consequences for sexual selection. Phenotypic variation within individuals has received little attention in either of these research directions. We used 6 years of data to examine the information content and potential role of white wing patch size in female collared flycatchers, Ficedula albicollis. Female wing patch size differed among years. Yearling females had smaller wing patches than older females. The negative effect of original patch size on intraindividual patch size change was stronger in yearling than in older birds, which may reflect an age-dependent trade-off. Change in wing patch size was strongly positively related to the summer North Atlantic Oscillation (NAO) index. Clutch size laid in the previous breeding season had a negative effect on patch size change, but only in high-NAO years. Thus, the immediate effect of poor climate during moult apparently overrode the more indirect influence of reproductive effort. Two sexually selected ornaments of mates were unrelated to female wing patch size at the population level, but intraindividual changes of female wing patch size significantly predicted differences in wing patch size between mates obtained in the 2 years. Our data suggest that significant mating advantages to more ornamented females may not be detected from population-level mating patterns. Research on potential female ornaments should also pay more attention to age-dependent phenotypic plasticity, the trade-off between current and future ornament size and the costs of reproduction to apparent future attractiveness.

Evaluation of five landscape-level metrics for measuring the effects of urbanization on landscape structure: the case of Tucson, Arizona, USA

Landscape-level metrics can be used to measure changes in landscape structure over time. Four landscape-level metrics and rank-size distributions were used to describe changes in landscape structure caused by urbanization in a portion of the Tucson, Arizona metropolitan area between 1984 and 1998. This analysis describes what each metric conveys regarding how urbanization affects landscape structure. It also compares the efficacy of rank-size distributions with the other metrics. Results indicate all five metrics provided information about a specific aspect of landscape structure including patch size, shape, or dispersion. Results indicate that rank-size distributions and their scaling exponents are useful because they provide information not available from the other metrics. Rank-size distributions describe the patch-size scaling properties of specific land-cover types, the relative effect of large and small patches on the distribution of patch sizes, the magnitude of change in patch size, where patch sizes differ from what is expected by the regression model, and the range of patch sizes displaying fractal patterns.

Moderate sheep grazing in semiarid shrubland alters small-scale soil surface structure and patch properties

We investigated whether long-term moderate livestock grazing by sheep (ca. 10 animal unit days/ha) in a semiarid shrubland with a long-term average annual rainfall of 200 mm causes changes in soil surface structure and dimensions of shrub and intershrub patches. We examined grazing-induced changes in landscape patchiness, patch structure, and soil moisture in three grazed and three ungrazed plots of 4 m × 4 m on a south-facing slope and the opposite, more productive north-facing slope. The measurements were done in early spring 2001 and 2002 before grazing started, in two surveys, one using two parallel transects and one using sample quadrats of 20 cm × 30 cm under three shrubs per plot and on an adjacent part of the intershrub matrix. On the north-facing slope, the sheep reduced shrub patch size as they trampled the soil mounds under the shrubs and browsed the shrub canopy. Reduced shrub patch size decreases the area and resources available for plant production. On the south-facing slope, the sheep mainly disrupted the soil crust in the intershrub area. This may increase soil erosion, but also seedling establishment. The contrast in impacts on the two slopes is due to the interaction between environment (productivity, exposure and vegetation) and sheep behavior (herbivory and trampling). On both slopes, the changes due to grazing are significant, though small. Changes in patch size and properties induced by moderate grazing can have positive and negative effects on productivity and diversity, but may also be viewed as early signs of landscape degradation as is often caused by heavy grazing.