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בית מבנה על רעייה השפעת-מדברי גידול הצומח חברת בדגש דו"ח מסכם לשנים
Abstract
הדו"ח מציג מחקר ארוך טווח שערכנו בשטחי הנגב ואשר בחן השפעות של רעיית צאן על חברת הצומח. המחקר נעשה בשיתוף ובמימון רשות הטבע והגנים
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The positive impact of grazing herbivores on plant diversity has been frequently reported in fertile grasslands. Grazing-induced heterogeneity is often evoked as an explanation for this influence. However, the relative importance of plant community heterogeneity induced by herbivores compared to other mechanisms linked to grazing remains unclear. We investigated this question by looking at 3 plant communities found in wet grasslands situated along the French Atlantic coast and traditionally grazed by horses and cattle. An experimental design set up in 1995 allowed us to compare the consequences of cattle-grazing, horse-grazing, and grazing abandonment on plant community diversity and heterogeneity. Floristic measurements made in 2007 showed that cattle and horses both had a positive impact on species richness and Shannon diversity index and that patchiness only occurred in grazed situations. The relative importance of grazing-induced patchiness within the overall positive effect of grazing on plant diversity was assessed by partitioning the diversity gain due to grazing into its additive within- and among-patch components. Grazing-induced patchiness entirely explained the increase in plant richness, whereas it accounted for only a small part of Shannon diversity. Grazing-related processes operating at the patch scale explained the main part of the increase in Shannon diversity. These processes make only a limited contribution to species recruitment, but they result in a more even species abundance distribution.
Grazing history alone is not a good predictor of plant-herbivore interactions. Interactions occur along gradients of convergent to divergent selection pressures with increasing environmental moisture and of intolerance to tolerance of grazing with increasingly long evolutionary histories of grazing. Feedback mechanisms between plants and grazing animals are well developed in grasslands with long evolutionary histories of grazing. Feedback mechanisms are manifest in the rapid switching capabilities (of plant species and modes of competition) of subhumid grasslands with long evolutionary histories of grazing and divergent selection pressures. Switching capabilities do not exist in semiarid grasslands with long evolutionary histories of grazing and convergent selection pressures. Rather, for heavily grazed dominant species dominance increases. Feedback mechanisms are not well developed in systems with short evolutionary histories of grazing. Here, differences in response to grazing by semiarid and subhumid situations arise primarily from differences in the grazing tolerance of plants adapted to semiaridity or of plants adapted to competition for light and from the different effects of grazing on canopy structure. -from Authors
We synthesized published and unpublished data from long-term grazing treatments in North American shortgrass steppe on diversity and abundance of plants, lagomorphs, rodents, birds, aboveground and belowground macroarthropods, microarthropods, and nematodes. The relatively small response of the plant community to grazing provides an opportunity to address some broad questions concerning relationships among responses of structural and functional aspects of systems in general. Are there consistencies in diversity and abundance responses to prating among groups of organisms? Are some groups more sensitive than others, or do responses mirror that of vegetation? Are the responses in terms of biodiversity related to ecosystem-level functional responses? Diversity, abundance, dominance, and dissimilarity responses to long-term grazing were highly variable across classes of organisms. Some groups of consumers displayed large differences among treatments even though differences in plant community attributes were relatively minor. Some responses were large even when comparing ungrazed to lightly or lightly to moderately grazed treatments. Birds appear to be particularly responsive to grazing. Differences among treatments in richness within groups other than plants and birds were relatively minor, especially when compared to large declines in abundance of some groups with increasing grazing intensity. For the well-studied groups (plants and birds), shifts in species in terms of 'quality' factors, such as exotic, endemic, and rare, generally suggest that livestock grazing may be more similar to conditions in recent evolutionary time in this particular system than would conditions resulting from the removal of the exotic, domestic grazers that appear to functionally serve as a surrogate to bison. Trophic structure composition did not vary greatly across grazing treatments. Further, large effects of grazing on some consumer groups did not translate into similarly large effects on ecosystem processes such as primary production or soil nutrient pools and cycling rates.
The interactive effect of grazing and small‐scale variation in primary productivity on the diversity of an annual plant community was studied in a semiarid Mediterranean rangeland in Israel over 4 years. The response of the community to protection from sheep grazing by fenced exclosures was compared in four neighbouring topographic sites (south‐ and north‐facing slopes, hilltop and wadi (dry stream) shoulders), differing in vegetation, physical characteristics and soil resources. The herbaceous annual vegetation was highly diverse, including 128 species. Average small‐scale species richness of annuals ranged between 5 and 16 species within a 20 × 20 cm quadrat, and was strongly affected by year and site.
Above‐ground potential productivity at peak season (i.e. in fenced subplots) was typical of semiarid ecosystems (10–200 g m ⁻² ), except on wadi shoulders (up to 700 g m ⁻² ), where it reached the range of subhumid grassland ecosystems. Grazing increased richness in the high productivity site (i.e. wadi), but did not affect, or reduced, it in the low productivity sites (south‐ and north‐facing slopes, hilltop). Under grazing, species richness was positively and linearly related to potential productivity along the whole range of productivity. Without grazing, this relationship was observed only at low productivity (< 200 g m ⁻² ).
The effect of grazing along the productivity gradient on different components of richness was analysed. At low productivity, number of abundant, common and rare species all tended to increase with productivity, both with and without grazing. Rare species increased three times compared with common and abundant species. At high productivity, only rare species continued to increase with productivity under grazing, while in the absence of grazing species number in the different abundance groups was not related to productivity.
In this semiarid Mediterranean rangeland, diversity of the annual plant community is determined by the interaction between grazing and small‐scale spatial and temporal variation in primary productivity, operating mainly on the less abundant species in the community.
A synthesis of a long-term (19 years) study assessing the effects of cattle grazing on the structure and composition of a Mediterranean grassland in north-eastern Israel is presented, with new insights on the response of the vegetation to grazing management and rainfall. We hypothesized that the plant community studied would be resistant to high grazing intensities and rainfall variability considering the combined long history of land-use and unpredictable climatic conditions where this community evolved. Treatments included manipulations of stocking densities (moderate, heavy and very heavy) and of grazing regimes (continuous vs. seasonal), in a factorial design. The effect of inter-annual rainfall variation on the expression of grazing impacts on the plant community was minor. The main effects of grazing on relative cover of plant functional groups were related to early vs. late seasonal grazing. Species diversity and equitability were remarkably stable across all grazing treatments. A reduction in tall grass cover at higher stocking densities was correlated with increased cover of less palatable groups such as annual and perennial thistles, as well as shorter and prostrate groups such as short annual grasses. This long-term study shows that inter-annual fluctuations in plant functional group composition could be partly accounted for by grazing pressure and timing, but not by the measured rainfall variables. Grazing affected the dominance of tall annual grasses. However, the persistence of tall grasses and more palatable species over time, despite large differences in grazing pressure and timing, supports the idea that Mediterranean grasslands are highly resistant to prolonged grazing. Indeed, even under the most extreme grazing conditions applied there were no signs of deterioration or collapse of the ecosystem. This high resistance to grazing intensity and inter-annual fluctuation in climatic conditions should favour the persistence of the plant community under forecasted increasing unpredictability due to climate change.This article is protected by copyright. All rights reserved.
In this article, a dynamic simulation model is presented, designed to add to the understanding of commercial livestock–grass–soil systems and, especially, the processes of desertification due to overgrazing. The model has two key innovative features: (i) the joint dynamic treatment of the three mentioned stock variables and (ii) the consideration of livestock dynamics determined by economic variables. The main possible behaviours of the resulting three-dimensional system are presented and it is shown that there are significant differences with respect to the subsystem constrained to livestock and grasses as considered in the literature to date. After analysing the model qualitatively, we are able to propose some early warning indicators of the risk of long-term desertification due to overgrazing. In this article, we illustrate the use of those indicators in three hypothetical, yet likely extensive livestock farming scenarios in Spain.
The development of land for modern agriculture has resulted in losses of native prairie habitat. The small, isolated patches of prairie habitat that remain are threatened by fire suppression, overgrazing, and invasion by non-native species. We evaluated the effects of three restoration practices (grazing only, burning only, and burning and grazing) on the vegetation characteristics and butterfly communities of remnant prairies. Total butterfly abundance was highest on prairies that were managed with burning and grazing and lowest on those that were only burned. Butterfly species richness did not differ among any of the restoration practices. Butterfly species diversity was highest on sites that were only burned. Responses of individual butterfly species to restoration practices were highly variable. In the best predictive regression model, total butterfly abundance was negatively associated with the percent cover of bare ground and positively associated with the percent cover of forbs. Canonical correspondence analysis revealed that sites with burned only and grazed only practices could be separated based on their butterfly community composition. Butterfly communities in each of the three restoration practices are equally species rich but different practices yield compositionally different butterfly communities. Because of this variation in butterfly species responses to different restoration practices, there is no single practice that will benefit all species or even all species within habitat-specialist or habitat-generalist habitat guilds.
The relative effects of tree clearing, increased livestock densities and nutrient enrichment have rarely been compared across markedly different organism types, but negative effects are generally predicted. In contrast, adoption of rotational grazing is thought to benefit biodiversity in pastures but there are few supporting data. We examined the response of native plants, birds and reptiles to livestock management in south-eastern Australia. We selected 12 pairs of rotationally and continuously grazed farms. Two 1-ha plots were established in native pastures on each farm, one cleared and the second still retaining woodland tree cover. Stocking rates, fertilizer histories and landscape tree cover varied among farms. The abundance and richness of all taxa was lower in cleared pastures. The less mobile organisms (reptiles and plants) were positively correlated with tree cover at landscape scales, but only when trees were present at the plot scale. This pattern was driven by a few observations in landscapes with approximately 50% tree canopy cover. Neither bird abundance nor richness was correlated with stocking rates or nutrient enrichment, but plant richness responded negatively to both. The response of reptiles varied, declining with nutrient enrichment but positively correlated with livestock densities. These responses may be partly interpreted within the context of prior filtering of species pools through long-term grazing pressure. No taxa responded positively to rotational grazing management. We predict that reductions in livestock density and soil nutrients will directly benefit plants and less so reptiles, but not birds. Indirect benefits are predicted for birds and reptiles if management increases persistence of trees within paddocks. Although some forms of rotational grazing can increase woodland tree recruitment, rotational grazing in itself is unlikely to enhance diversity.
Aims: (1) Understanding how the relationship between species richness and its determinants depends on the interaction between scales at which the response and explanatory variables are measured. (2) Quantifying the relative contributions of local, intermediate and large-scale determinants of species richness in a fragmented agro-ecosystem. (3) Testing the hypothesis that the relative contribution of these determinants varies with the grain size at which species richness is measured.
Location: A fragmented agro-ecosystem in the Southern Judea Lowland, Israel, within a desert–Mediterranean transition zone.
Methods: Plant species richness was estimated using hierarchical nested sampling in 81 plots, positioned in 38 natural vegetation patches within an agricultural matrix (mainly wheat fields) among three land units along a sharp precipitation gradient. Explanatory variables included position along that gradient, patch area, patch isolation, habitat heterogeneity and overall plant density. We used general linear models and hierarchical partitioning of variance to test and quantify the effect of each explanatory variable on species richness at four grain sizes (0.0625, 1, 25 and 225 m2).
Results: Species richness was mainly affected by position along a precipitation gradient and overall plant density, and to a lesser extent by habitat heterogeneity. It was also significantly affected by patch area and patch isolation, but only for small grain sizes. The contribution of each explanatory variable to explained variance in species richness varied with grain size, i.e. scale-dependent. The influence of geographic position and habitat heterogeneity on species richness increased with grain size, while the influence of plant density decreased with grain size.
Main conclusions: Species richness is determined by the combined effect of several scale-dependent determinants. Ability to detect an effect and effect size of each determinant varies with the scale (grain size) at which it is measured. The combination of a multi-factorial approach and multi-scale sampling reveals that conclusions drawn from studies that ignore these dimensions are restricted and potentially misleading.