Interactive Effects Of Grazing Research Articles

Integrated response of carbon sequestration along co-varying gradients of climate and grazing intensity in a desert steppe ecosystem

The interactive effects of grazing and climate on carbon (C) storage are important for the global C budget and regional C management. In a regional-scale field experiment based on multiple grazing gradients, we examined how changes in grazing intensity and climate affected the C sequestration rate and C storage at five desert steppe sites. The mean total ecosystem C storage at each site ranged from 1502 to 3728 g C m−2 and averaged 2341 g C m−2. The peak value of the C sequestration rate (290 g C m−2 yr−1) occurred in the light grazing intensity. The light grazing and fencing management were more beneficial to C sequestration. The rising temperature would decrease C storage in desert steppe, while rising precipitation would promote C sequestration. Soil nitrogen content plays an important role in the driving mechanism of C sequestration in desert steppe. The soil C pool, which dominated the C dynamics in desert steppe, was influenced by the changes of precipitation and grazing intensity. Grazing affects soil nitrogen content by changing the aboveground C pool. Precipitation changes may change soil C storage by influencing the aboveground C pool to change soil nitrogen content and microbial biomass. Such information improves understanding of the dynamics of C storage and sequestration in desert steppe, and is critical for improving the sustainability of grassland ecosystem services and C management.

Grazing and global change factors differentially affect biodiversity-ecosystem functioning relationships in grassland ecosystems.

Grazing and global change (e.g., warming, nitrogen deposition, and altered precipitation) both contribute to biodiversity loss and alter ecosystem structure and functioning. However, how grazing and global change interactively influence plant diversity and ecosystem productivity, and their relationship remains unclear at the global scale. Here, we synthesized 73 field studies to quantify the individual and/or interactive effects of grazing and global change factors on biodiversity-productivity relationship in grasslands. Our results showed that grazing significantly reduced plant richness by 3.7% and aboveground net primary productivity (ANPP) by 29.1%, but increased belowground net primary productivity (BNPP) by 9.3%. Global change factors, however, decreased richness by 8.0% but increased ANPP and BNPP by 13.4% and 14.9%, respectively. Interestingly, the strength of the change in biodiversity in response to grazing was positively correlated with the strength of the change in BNPP. Yet, global change flipped these relationships from positive to negative even when combined with grazing. These results indicate that the impacts of global change factors are more dominant than grazing on the belowground biodiversity-productivity relationship, which is contrary to the pattern of aboveground one. Therefore, incorporating global change factors with herbivore grazing into Earth system models is necessary to accurately predict climate-grassland carbon cycle feedbacks in the Anthropocene.

Effects of Rust on Plant Growth and Stoichiometry of Leymuschinensis under Different Grazing Intensities in Hulunber Grassland

Grazing is the main utilization of native grassland, and forage fungal disease is one of the limiting factors of grassland productivity. The present research in the Hulunber meadow steppe grassland was conducted to investigate the responses of the dominant plant Leymus chinensis (Trin.) to beef cattle grazing, rust, and their interaction influence. Six grazing intensity treatments with three replicates were established. The response of L. chinensis to grazing and rust was systematically studied for two consecutive years. The main findings were that grazing and rust had significant effects (p < 0.05) on the growth and nutrient elements content of L. chinensis. Compared with the 0 cattle ha−1 treatment, the dry matter of L. chinensis in the 0.42, 0.63, and 1.67 cattle ha−1 treatments decreased by 42.2%, 90.5%, and 339.5%, respectively. Compared with non-infected plants, dry matter of rust-infected L. chinensis plants decreased by 45.6%. The N:C and P:C ratios of rust-infected plants were lower than in non-infected plants, and positively correlated with their relative growth rates. Therefore, we concluded that the growth rate hypothesis still applied in L. chinensis under the interactive effects of grazing and disease. Additionally, grazing can alleviate the loss of dry matter caused by disease.

Different responses of soil C:N:P stoichiometry to stocking rate and nitrogen addition level in an alpine meadow on the Qinghai-Tibetan Plateau

Livestock grazing and nitrogen (N) deposition influenced most grasslands in the world by affecting the element contents and their stoichiometry in soil. Although the individual effects of grazing or N deposition on soil nutrients are well understood, how their interaction effects on soil stoichiometric characteristics are unclear. To reveal the response of soil carbon:nitrogen:phosphorus (C:N:P) stoichiometry to grazing and N deposition, we conducted an experiment in an alpine meadow on the Qinghai-Tibetan Plateau (QTP), which included three stocking rates (0, 8 and 16 sheep ha−1) and four N addition levels (0, 50, 100 and 200 kg N ha−1 yr−1). The results showed that grazing had no significant impact on soil C:N:P stoichiometry. Nitrogen addition significantly increased soil total N and N:P ratio at the stocking rate of 8 sheep ha−1 in the surface soil. The interactive effects of grazing and N addition did not significantly affect soil C:N:P stoichiometry, however, the interplay between grazing and N addition caused vegetation changes, which had indirect effects on soil C:N:P stoichiometry. The differential responses of plants and soil indicated that soil was less sensitive to grazing and N addition than plants. Our findings indicated that N was still a major limit factor for plant growth in the alpine meadow of QTP, and hopefully, provided that stocking rate at 8 sheep ha−1 and N addition at 50 kg N ha−1 yr−1 could be a suitable grassland management technique for soil fertility sequestration in the QTP.

Interactive effects of grazing and global change factors on soil and ecosystem respiration in grassland ecosystems: A global synthesis

AbstractAs the key carbon (C) fluxes between biosphere and atmosphere, soil respiration (Rs) and ecosystem respiration (Re) play vital roles in regulating global C balance and climate‐biosphere feedback in the Earth system. Despite the fact that numerous manipulative studies and a few meta‐analyses have been conducted to examine the responses ofRsand its components (i.e. autotrophic [Ra] and heterotrophic respiration [Rh]) as well asReto grazing (G) or global change factors, the interactive effects between grazing and global change factors remain poorly understood.Here, we performed a comprehensive meta‐analysis of manipulative experiments with both grazing and global change factors to quantify their individual and interactive effects onRsand its components as well asRe.Our results showed that grazing and drought significantly decreasedRsby 12.35% and 20.95%, respectively, whereas warming (W), nitrogen addition (N) and increased precipitation (P) stimulated it by 2.12%, 5.49% and 13.44%, respectively. Similarly, grazing, warming, nitrogen addition and increased precipitation increasedReby 7.21%, 4.94%, 48.45% and 21.57%, respectively, while drought decreased it by 16.86%. However, the combinations of grazing with warming (GW), nitrogen addition (GN) and increased precipitation (GP) exhibited non‐significant effects onRs.More importantly, additive interactions between grazing and global change factors exhibited a substantial predominance onRs,Ra,RhandRerather than synergistic and antagonistic ones.Synthesis and applications. Our findings highlight the crucial importance of the interactive effects between grazing and global change factors on soil respiration (Rs) and ecosystem respiration (Re). Therefore, incorporating this key influence on ecosystem processes into Earth system models (ESMs) could better improve the prediction of climate‐biosphere feedbacks and develop sustainable strategies for grassland management in the Anthropocene.

The impacts of nutrient addition and livestock exclosure on the soil nematode community in a degraded grassland

AbstractNutrient addition and livestock exclosure are two strategies for restoring degraded grassland. Although their impacts on plant community properties are well understood, the effects of fertilization and livestock exclosure on belowground fauna community in degraded grassland remain largely unknown. We examined the main and interactive effects of nutrient addition and livestock exclosure on soil nematode community in a severely degraded grassland of northern China, by recording soil nematode community composition in 0‐ to 10‐cm soil layer in the summer of 2017. Neither nutrient addition nor livestock exclosure had significant effects on the biodiversity of nematode community in this degraded grassland. Livestock exclosure significantly increased the abundances of total nematodes and the maturity index of the nematode community, mainly due to its positive effect on omnivorous + carnivorous nematodes. Nutrient addition weakened the positive effects of livestock exclosure on soil nematode community by reducing soil pH and enhancing soil ammonium concentration. Our results highlight that the positive effects of livestock exclosure on soil nematode community would be reassessed under the scenarios of nutrient enrichment. Given that most grasslands are used for livestock production, the interactive effects of grazing and nutrient enrichment on soil nematode community deserve more attention in further research.

Warming counteracts grazing effects on the functional structure of the soil microbial community in a Tibetan grassland

Grazing intensity and global warming are expected to increase in the forthcoming decades, with uncertain consequences for their interaction on grassland ecosystems and their functions. We investigated the effects of warming, grazing and their interaction in a factorial warming (+1.2–1.7 °C) and grazing (moderate intensity with ca. 50% vegetation consumption) experiment in a Tibetan alpine meadow on microbial communities by studying functional genes involved in soil carbon and nitrogen cycles, using GeoChip technology. Our results showed that microbial functional gene structure and abundances were largely affected by the interactive effect of grazing and warming, rather than the main effect of warming or grazing. Compared to the control, grazing alone significantly increased the functional gene alpha diversity, changed the overall functional community structure, and increased the abundances of C fixation, C degradation, N mineralization and denitrification genes, likely due to the stimulating impact of urine and dung deposition. Warming alone did not change these microbial properties, possibly related to the unchanged soil nutrient status. Despite an increase in soil NO3− concentrations and the deposition of urine and dung, the combined treatment did not change functional gene alpha diversity, community structure, or C/N cycling gene abundances, possibly resulting from the limiting effect of water depletion in the combined treatment. Our study revealed antagonistic interactions between warming and grazing on microbial functional gene structure and abundances, which remained stable under the moderate intensity of grazing in future warming scenario in the Tibetan alpine meadow, raising potentially important implications for predicting future soil carbon and nitrogen processes in these systems.

Grazing effect on grasslands escalated by abnormal precipitations in Inner Mongolia.

Grazing effects on arid and semi‐arid grasslands can be constrained by aridity. Plant functional groups (PFGs) are the most basic component of community structure (CS) and biodiversity & ecosystem function (BEF). They have been suggested as identity‐dependent in quantifying the response to grazing intensity and drought severity. Here, we examine how the relationships among PFGs, CS, BEF, and grazing intensity are driven by climatic drought. We conducted a manipulative experiment with three grazing intensities in 2012 (nondrought year) and 2013 (drought year). We classified 62 herbaceous plants into four functional groups based on their life forms. We used the relative species abundance of PFGs to quantify the effects of grazing and drought, and to explore the mechanisms for the pathway correlations using structural equation models (SEM) among PFGs, CS, and BEF directly or indirectly. Grazers consistently favored the perennial forbs (e.g., palatable or nutritious plants), decreasing the plants’ relative abundance by 23%–38%. Drought decreased the relative abundance of ephemeral plants by 42 ± 13%; and increased perennial forbs by 20 ± 7% and graminoids by 80 ± 31%. SEM confirmed that annuals and biennials had negative correlations with the other three PFGs, with perennial bunchgrasses facilitated by perennial rhizome grass. Moreover, the contributions of grazing to community structure (i.e., canopy height) were 1.6–6.1 times those from drought, whereas drought effect on community species richness was 3.6 times of the grazing treatment. Lastly, the interactive effects of grazing and drought on BEF were greater than either alone; particularly, drought escalated grazing damage on primary production. Synthesis. The responses of PFGs, CS, and BEF to grazing and drought were identity‐dependent, suggesting that grazing and drought regulation of plant functional groups might be a way to shape ecosystem structure and function in grasslands.

Effects of Grazing and Fire Frequency on Floristic Quality and its Relationship to Indicators of Soil Quality in Tallgrass Prairie.

Fire and grazing are widely used to manage grasslands for conservation purposes, but few studies have evaluated the effects of these drivers on the conservation value of plant communities measured by the floristic quality index (FQI). Further, the influence of fire and grazing on soil properties and functions are difficult for land managers and restoration practitioners to assess. The objectives of this study were to: (1) quantify the independent and interactive effects of grazing and fire frequency on floristic quality in native tallgrass prairie to provide potential benchmarks for community assessment, and (2) to explore whether floristic quality can serve as an indicator of soil structure and function for more holistic ecosystem assessments. A factorial combination of fire frequencies (1-2, 4, and 20 years return intervals) and grazing (by bison or ungrazed) treatments were sampled for plant species composition, and for several indicators of soil quality in lowland tallgrass prairie. Floristic quality, diversity, and richness were higher in grazed than ungrazed prairie over all fire frequencies (P < 0.05). Available inorganic N, microbial biomass N, total N, and soil bulk density were also higher in grazed prairie soil over all fire frequencies (P < 0.05). Microbial biomass C, total organic C, and total soil N were positively correlated with FQI (P < 0.05). This study shows that floristic quality and soil N pools are more strongly influenced by grazing than fire and that floristic quality can be an indicator of total soil C and N stocks in never cultivated lowland prairie.

Species-environment interactions changed by introduced herbivores in an oceanic high-mountain ecosystem.

Summit areas of oceanic islands constitute some of the most isolated ecosystems on earth, highly vulnerable to climate change and introduced species. Within the unique high-elevation communities of Tenerife (Canary Islands), reproductive success and thus long-term survival of species may depend on environmental suitability as well as threat by introduced herbivores. By experimentally modifying the endemic and vulnerable species Viola cheiranthifolia along its entire altitudinal occurrence range, we studied plant performance, autofertility, pollen limitation and visitation rate and the interactive effect of grazing by non-native rabbits on them. We assessed the grazing effects by recording (1) the proportion of consumed plants and flowers along the gradient, (2) comparing fitness traits of herbivore-excluded plants along the gradient, and (3) comparing fitness traits, autofertility and pollen limitation between plants excluded from herbivores with unexcluded plants at the same locality. Our results showed that V. cheiranthifolia performance is mainly affected by inter-annual and microhabitat variability along the gradient, especially in the lowest edge. Despite the increasingly adverse environmental conditions, the plant showed no pollen limitation with elevation, which is attributed to the increase in autofertility levels (≥ 50% of reproductive output) and decrease in competition for pollinators at higher elevations. Plant fitness is, however, extremely reduced owing to the presence of non-native rabbits in the area (consuming more than 75% of the individuals in some localities), which in turn change plant trait-environment interactions along the gradient. Taken together, these findings indicate that the elevational variation found on plant performance results from the combined action of non-native rabbits with the microhabitat variability, exerting intricate ecological influences that threaten the survival of this violet species.

Interactive effects of grazing and drought on desert rangelands of Tunisia

Arid rangelands worldwide regularly confront drought and overgrazing. Land degradation is mostly apparent in desert rangelands, where drought events are important and grazing influences and intrinsic vegetation change act intermittently. The objectives of this study were to determine the effects of grazing and drought stress on diversity, species richness and primary production in desert rangeland of southern Tunisia. This assessment was conducted in March 2008 (wet year) and March 2009 (dry year) both on grazed and ungrazed sites. Diversity, species richness and biomass production exhibited bigger variation between years than among sites within two years. Although protection from grazing could slightly increase rangeland production in favorable years, the additive effects of heavy grazing and drought could result in restoration failure. This study suggests that climatic variations, particularly droughts, control major trends in plant species composition, diversity and biomass. Under grazing, plants of desert regions are able to respond very rapidly to small drought. The latter is the most debilitating risk in desert areas.

Grazing and the absence of fire promote the dominance of an unpalatable shrub in a patch mosaic cyclic successional system

We experimentally determined how an apparently unpalatable and fire-sensitive shrub, Richea acerosa (Lindley) F.Muell., responded to the single and interactive effects of grazing and burning over 3–24 years at two subalpine sites in Tasmania. At the Middlesex Plains site, a low-intensity burn killed most individuals of the species 3 years after fire and fencing. At the Lake Botsford site, total grazing exclusion for 24 years resulted in a strong reduction of the number of small shrubs of the species, and in a lower proportion of dead material in plants of any given size. An open shrub layer results from the absence of R. acerosa seedlings in the open centres of senescing plants and their preferential location along the margins of the foliage of mature and senescing plants. Thus, the persistence of R. acerosa appears to require an absence of frequent fire and the presence of sufficient grazing pressure from vertebrate herbivores to produce lawn between shrubs. Richea acerosa is, thus, an unpalatable shrub that depends on grazing.

The effects of grazing and watering on ecosystem CO2 fluxes vary by community phenology

Grazing profoundly influences vegetation and the subsequent carbon fluxes in various ecosystems. However, little effort has been made to explore the underlying mechanisms for phenological changes and their consequences on carbon fluxes at ecosystem level, especially under the coupled influences of human disturbances and climate change. Here, a manipulative experiment (2012–2013) was conducted to examine both the independent and interactive effects of grazing and watering on carbon fluxes across phenological phases in a desert steppe. Grazing advanced or delayed phenological timing, leading to a shortened green-up phase (GrP: 23.60 days) in 2013 and browning phase (BrP: 12.48 days) in 2012 from high grazing, and insignificant effects on the reproductive phase (ReP) in either year. High grazing significantly enhance carbon uptake, while light grazing reduce carbon uptake in ReP. Watering only delayed the browning time by 5.01 days in 2013, producing no significant effects on any phenophase. Watering promoted the net ecosystem exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP) only in the GrP. When calculating the yearly differences in phenophases and the corresponding carbon fluxes, we found that an extended GrP greatly enhanced NEE, but a prolonged ReP distinctly reduced it. The extended GrP also significantly promote GEP. Increases in growing season length appeared promoting ER, regardless of any phenophase. Additionally, the shifts in NEE appeared dependent of the variations in leaf area index (LAI).

Interactive effects of grazing and environmental stress on macroalgal biomass in subtropical rocky shores: Modulation of bottom-up inputs by wave action

In contrast to what is observed in most temperate regions, perennial macroalgae are rare at the mid intertidal level of tropical and subtropical shores, and energy transfer through benthic herbivores largely relies on the consumption of periphyton and ephemeral algae. In this study, we evaluated the interactive effects of environmental stress and mesoherbivore grazing in the regulation of ephemeral macroalgal standing stock along subtropical shores moderately exposed and sheltered from waves in southeastern Brazil. Our results show that grazers can prevent ephemeral algal blooms at the most sheltered shores, and that amelioration of environmental stress, through provision of shade, has no consistent effect on overall biomass or temporal persistence of the algal blooms in these shores. At nearby shores exposed to waves, grazers had no measurable effect on algal biomass and shading rock areas from direct solar radiation can have positive effects on some years, but not on others, probably associated to variation in the species comprising the assemblage. Because nitrate concentration in nearshore waters is remarkably low, we suggest that increased water motion may enhance nutrient flux to the midshore and thus algal blooming. At more exposed sites, algae develop faster and reach a canopy size no longer controlled by grazers. Higher biomass of herbivores at exposed rocky shores is thus best explained as a bottom-up effect of increased plant productivity, without a coupled top-down effect on algae. Thus, besides the well documented effect of waves on temperature and desiccation stresses, wave modulation of nutrient supply may be a very important factor controlling abundance of midshore intertidal macroalgae, and deserves more attention in typically nutrient-depleted tropical and subtropical shores.

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

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

Interactive effects of grazing and burning on wind- and water-driven sediment fluxes: rangeland management implications

Rangelands are globally extensive, provide fundamental ecosystem services, and are tightly coupled human-ecological systems. Rangeland sustainability depends largely on the implementation and utilization of various grazing and burning practices optimized to protect against soil erosion and transport. In many cases, however, land management practices lead to increased soil erosion and sediment fluxes for reasons that are poorly understood. Because few studies have directly measured both wind and water erosion and transport, an assessment of how they may differentially respond to grazing and burning practices is lacking. Here, we report simultaneous, co-located estimates of wind- and water-driven sediment transport in a semiarid grassland in Arizona, USA, over three years for four land management treatments: control, grazed, burned, and burned + grazed. For all treatments and most years, annual rates of wind-driven sediment transport exceeded that of water due to a combination of ongoing small but nontrivial wind events and larger, less frequent, wind events that generally preceded the monsoon season. Sediment fluxes by both wind and water differed consistently by treatment: burned + grazed > burned >> grazed > or = control, with effects immediately apparent after burning but delayed after grazing until the following growing season. Notably, the wind:water sediment transport ratio decreased following burning but increased following grazing. Our results show how rangeland practices disproportionally alter sediment fluxes driven by wind and water, differences that could potentially help explain divergence between rangeland sustainability and degradation.

Effects of consumers and enrichment on abundance and diversity of benthic algae in a rocky intertidal community

Human alteration of nutrient cycling and the densities of important consumers have intensified the importance of understanding how nutrients and consumers influence the structure of ecological systems. We examined the effects of both grazing and nutrient enrichment on algal abundance and diversity in a high-intertidal limpet-macroalgal community on the South Island of New Zealand, a relatively nutrient-poor environment. We used a fully factorial design with three levels each of grazing (manipulations of limpet and snail densities) and nutrients (nutrient-diffusers attached to the rock). Top-down control by grazers appears to be the driving organizing mechanism for algal communities in this system, with strong negative effects of grazing on algal diversity and abundance across all levels of nutrient enrichment. However, in contrast to the conclusions drawn from the analysis of the whole algal community, there was an interactive effect of grazing and enrichment on foliose algae, an important component of the algal system. When herbivory was reduced to very low levels, enrichment generated increases in the abundance and biomass of foliose algae. As expected, top-down control was the primary determinant of algal community structure in this system, controlling abundance and diversity of macrophytes on the upper shore. Contrary to expectations, however, increased nutrients had no community-wide effects, although foliose algal abundance increases were greatest with high nutrients and reduced grazing. It seems likely that most of the corticated algal species have limited capacity to respond to nutrient pulses in this nutrient-poor environment.

Responses of soil respiration to simulated precipitation pulses in semiarid steppe under different grazing regimes

Aims Precipitation pulses and different land use practices (such as grazing) play important roles in regulating soil respiration and carbon balance of semiarid steppe ecosystems in Inner Mongolia. However, the interactive effects of grazing and rain event magnitude on soil respiration of steppe ecosystems are still unknown. We conducted a manipulative experiment with simulated precipitation pulses in Inner Mongolia steppe to study the possible responses of soil respiration to different precipitation pulse sizes and to examine how grazing may affect the responses of soil respiration to precipitation pulses. Methods Six water treatments with different precipitation pulse sizes (0, 5, 10, 25, 50 and 100 mm) were conducted in the ungrazed and grazed sites, respectively. Variation patterns of soil respiration of each treatment were determined continuously after the water addition treatments.

Interactive effects of grazing and enrichment on diversity; conceptual implications of a rocky intertidal experiment

Understanding the ways in which consumers and productivity act and interact to yield differences in diversity is of primary conceptual and pragmatic importance in a world in which humans are simultaneously changing ecological communities and substantially altering the availability of nutrients. Here, I used macroalgal communities on rocky reefs to examine the effects of both limpet grazing and nutrient enrichment on algal diversity throughout almost two years of succession. The experimental design included three levels each of grazing and nutrients, with unglazed terracotta pots attached to the rock as replicate plots in a high intertidal limpet‐macroalgal community. Grazing effects varied by year. During the first year, grazing effects were context‐dependent with limpets resulting in lower species richness, especially at the highest level of limpet density. However, at this highest level of limpet density, high enrichment counteracted the negative effect of limpets such that diversity was similar to that in treatments with lower limpet densities. In the second year, grazing generally decreased richness, regardless of enrichment. The results of this experiment are partially consistent with the grazer‐reversal hypothesis – grazing decreased richness in low nutrient conditions and this effect was neutralized (rather than reversed) under high enrichment. Inconsistencies with model predictions may be explained by the apparent unresponsiveness of algal productivity to experimental enrichment, the unique substrate‐scraping feeding mechanisms of limpets, and potentially limited propagule supply.

Land use and drought interactively affect interspecific competition and species diversity at the local scale in a semiarid steppe ecosystem

AbstractFew studies have considered interactive effects of grazing and drought on species composition and the relative contribution of species to total biomass, although it is important to understand the short‐term dynamics and community succession in grazed ecosystems. We monitored species diversity and relative biomass contribution at one site protected from grazing since 1979 (UG79), and at winter grazing (WG) and heavily grazed (HG) sites. Continuous heavy grazing resulted in lower plant height and more but small individuals (tillers or stolons). Drought significantly reduced total plant density on all sites. Grazing affected species diversity more than drought. Species richness at site UG79 was significantly higher than at sites WG and HG, while drought only tended to reduce species diversity. Drought stress and grazing disturbance interactively controlled species competition and functional groups. Both perennial grasses and forbs had greater contribution to total biomass at site UG79, and perennial grasses contributed more than 97% of total biomass at site WG. The contribution to total biomass of annual forbs and semi‐shrubs significantly increased at site HG after two dry years. The significant decrease in Potentilla acaulis and a substantial increase in annual species at this site indicate that the perennial vegetation of this ecosystem is in great danger of extinction under conditions of prolonged drought.