Grazing Response Research Articles

Grazing alters the soil nematode communities in grasslands: A meta-analysis

Grazing causes great disturbances in grassland ecosystems and may change the abundance, diversity, and ecological function of soil biota. Because of their important role in nutrient cycling and as good environmental indicators, nematodes are very representative soil organisms. However, the mechanisms by which grazing intensity, livestock type, duration, and environmental factors (e.g., climate and edaphic factors) affect soil nematodes remain poorly understood. In this study, we collected 1964 paired observations all over the world from 53 studies to clarify the grazing response patterns of soil nematodes and their potential mechanisms. Overall, grazing significantly decreased the abundance of bacterial-feeding (BF) nematodes (−16.54%) and omnivorous-predatory (OP) nematodes (−36.81%), and decreased nematode community diversity indices (Shannon–Weiner index: −4.33%, evenness index: −9.22%, species richness: −5.35%), but had no effect on ecological indices under a global regional scale. The response of soil nematodes to grazing varied by grazing intensity, animals, and duration. Heavy grazing decreased OP nematode abundance, but had no effect on the abundance of other trophic groups, or on diversity or ecological indices. Grazing by small animals had stronger effects than that by large animals and mixed-size animals on BF, fungal-feeding (FF), plant-feeding (PF) and OP nematodes, the Shannon–Wiener index, and the species richness index. The abundance of FF and OP nematodes influenced significantly under short-term grazing. The evenness index decreased significantly under long-term grazing (>10 years). Climate and edaphic factors impacted the effects of grazing on nematode abundance, diversity, and ecological indices. When resources (i.e., rain, heat, and soil nutrients) were abundant, the negative effects of grazing on nematodes were reduced; under sufficiently abundant resources, grazing even had positive effects on soil nematode communities. Thus, the influence of grazing on soil nematode communities is resource-dependent. Our study provides decision makers with grazing strategies based on the resource abundance. Resource-poor areas should have less grazing, while resource-rich areas should have more grazing to conserve soil biodiversity and maintain soil health.

Role of herbivory in shaping the dryland vegetation ecosystem: Linking spiral vegetation patterns and nonlinear, nonlocal grazing.

Self-organized vegetation patterns are an amazing aspect of dryland ecosystems; in addition to being visually appealing, patterns control how these water-deprived systems react to escalating environmental stress. Although there is a wide variety of vegetation patterns, little is known about the mechanisms behind spiral patterns. The well-known models that explain other vegetation patterns such stripes, rings, and fairy circles cannot account for these spirals. Here we have adopted a modeling approach in which the interplay between herbivore grazing and vegetation is found to be the reason why spirals form. To comprehend the nonlinear dependence of grazing on the availability vegetation, we have introduced a grazing term that gets saturated when forage is abundant. To account for the impact of the spatial nonhomogeneity in vegetation layout, it is thought that grazing is dependent on mean vegetation density rather than density at a single site. Results show how the system dynamics is changed fundamentally depending on the different types of grazing response. Incorporation of nonlocality into the herbivore grazing leads to spiral-shaped vegetation patterns only in natural grazing scenarios; however, no patterning is seen in human controlled herbivory. Overall, our research points to the nonlocal, nonlinear grazing behavior of herbivores as one of the major driving forces for the development of spiral patterns.

Variable effects of long‐term livestock grazing across the western United States suggest diverse approaches are needed to meet global change challenges

AbstractAimsLivestock production is the most widespread land use globally and occurs across a diverse set of ecosystems. Variability in long‐term livestock grazing impacts across ecosystems is poorly characterized, particularly at larger spatial scales, despite strong relationships with various ecosystem services related to soil fertility and stabilization and vegetation productivity. Here we examine the effects of grazing on vegetation and the implications for resistance and resilience to global change.MethodsWe use six long‐term research stations in the western United States, spanning two ecoregions, multiple ecosystems, and 311 total site‐years of research. Across these sites we evaluate convergence and divergence of vegetation response to grazing vs grazing removal, focusing on interactions with drivers of global change.ResultsWe found that at long time scales (multiple decades), grazing has numerous convergent and divergent effects across ecoregions and ecosystems. Similarity among precipitation patterns and plant traits linked to grazing and production timing were key elements explaining convergence or divergence in long‐term patterns of livestock grazing response. Ecosystem differences across western US rangelands are also associated with variable effects of grazing on resistance and resilience to invasive species and climate change.ConclusionsThese results suggest that unique ecosystem or ecoregion responses to future global change may result from complex interactions between grazing and environmental factors, such as precipitation timing and plant traits. Adapting livestock and grazing management to specific ecosystem vegetation and climate variability is needed to manage for the myriad global changes affecting rangeland production and diversity.

Intraspecific trait variation of grassland species in response to grazing depends on resource acquisition strategy

AbstractQuestionGrazing is an important disturbance for maintenance of structure and functioning of the plant communities in grassy ecosystems. Functional characteristics of plants, which are responsible for their response to grazing, vary at inter‐ and intraspecific levels. Here, we assessed: (1) the extent of intraspecific trait variation when compared to interspecific variability; and (2) how grazing regime affects intraspecific trait variation in leaf functional traits of species with different leaf economics strategies.LocationA multisite grazing experiment in subtropical grasslands, southern Brazil.MethodsFor 30 dominant grassland plant species we measured functional traits related to resource acquisition strategy and grazing response (SLA, specific leaf area; LDMC, leaf dry matter content; LA, leaf area; and FT, leaf resistance to traction) under three grazing treatments (grazing exclusion, traditional, or deferred grazing) arranged in a randomized block design. We quantified the overall proportion of intra‐ and interspecific variation and, for each species, we tested for the grazing treatment effects on intraspecific trait variation through Bayesian analysis of variance.ResultsThe overall contribution of intraspecific variation was up to 37% of the total variation. For most grass species, grazing exclusion led to lower values of SLA and higher values of LA, while no effect of grazing treatments was detected on intraspecific variation of LDMC and FT. Altogether, the effect of grazing management on intraspecific trait variation was more prevalent in species presenting a conservative strategy than in species with an acquisitive strategy.ConclusionThe expression of leaf traits is variable within species, especially for those species with a conservative strategy of resource acquisition. The extent of intraspecific trait variation in response to grazing depends on the considered traits and the resource acquisition strategy of the studied species.

Grazing intensity in grassland ecosystems: implications for carbon storage and functional properties

Abstract Grazing lands cover more than one-third of the terrestrial land surface and are still expanding. The intensity of grazing is also growing with time due to the increase in the number of animals per unit of land or the duration of herbivory. Changes in grazing intensity exert strong pressure on ecosystem functioning, including carbon (C) sequestration. Grazing response to ecosystem C retention can be explained by the changes in biogeochemical processes that regulate C fluxes. Here, we summarized the ecosystem functional properties that affect C storage in grasslands as a result of changing grazing intensities. We synthesized 151 published studies globally and discussed the changes in different ecosystem functional property responses to grazing. High-intensity grazing mostly decreased C storage while light- to moderate-intensity grazing optimized the trade-offs between C sequestration and animal production. The synthesis showed that 86% of the studies reported either an increase or no change in SOC stocks under light grazing while under heavy grazing, 74% of the studies reported a significant decrease in SOC stocks. We also discussed the changes in ecosystem functional properties whereby grazing affects C sequestration. Some of them include: the changes in net primary productivity, soil organisms, enzymatic activity, soil respiration, and the complex interactions of these processes with climate change. Grassland management practices should be ecosystem-specific to ensure optimum production and enhance C sequestration. It is also important to understand the time factor (hysteresis), especially on how long it takes to sequester a certain amount of C from grassland management practices.

Teasing Apart Silvopasture System Components Using Machine Learning for Optimization

Silvopasture systems combine tree and livestock production to minimize market risk and enhance ecological services. Our objective was to explore and develop a method for identifying driving factors linked to productivity in a silvopastoral system using machine learning. A multi-variable approach was used to detect factors that affect system-level output (i.e., plant production (tree and forage), soil factors, and animal response based on grazing preference). Variables from a three-year (2017–2019) grazing study, including forage, tree, soil, and terrain attribute parameters, were analyzed. Hierarchical variable clustering and random forest model selected 10 important variables for each of four major clusters. A stepwise multiple linear regression and regression tree approach was used to predict cattle grazing hours per animal unit (h ha−1 AU−1) using 40 variables (10 per cluster) selected from 130 total variables. Overall, the variable ranking method selected more weighted variables for systems-level analysis. The regression tree performed better than stepwise linear regression for interpreting factor-level effects on animal grazing preference. Cattle were more likely to graze forage on soils with Cd levels <0.04 mg kg−1 (126% greater grazing hours per AU), soil Cr <0.098 mg kg−1 (108%), and a SAGA wetness index of <2.7 (57%). Cattle also preferred grazing (88%) native grasses compared to orchardgrass (Dactylis glomerata L.). The result shows water flow within the landscape position (wetness index), and associated metals distribution may be used as an indicator of animal grazing preference. Overall, soil nutrient distribution patterns drove grazing response, although animal grazing preference was also influenced by aboveground (forage and tree), soil, and landscape attributes. Machine learning approaches helped explain pasture use and overall drivers of grazing preference in a multifunctional system.

Evaluating grazing response strategies in winter annuals: A multi‐trait approach

Abstract Plants minimize fitness losses through grazing by three fundamental strategies: tolerance, avoidance and escape. Annual species have been traditionally assumed to escape grazing through their short life cycle and seed dormancy; however, their grazing response strategies remain almost unexplored. How traits and their coordination affect species' grazing responses, and whether the generalized grazing model, which posits convergent filtering by grazing and drought, is applicable to this ecologically and economically important species group thus remain unclear. We used a trait‐based approach to evaluate grazing response strategies of winter annuals from the Middle East. Across 23 species, we examined the coordination of 16 traits hypothesized to be relevant for grazing responses, and linked them to species' fecundity responses, as proxy for fitness responses, to simulated grazing in controlled conditions, to species' abundance responses to grazing in the field and to species' distribution along a large‐scale rainfall gradient. Winter annuals exhibited both grazing escape and to a lesser extent tolerance indicated by (a) independent coordination of escape and tolerance traits, and (b) maintenance of higher fecundity in species with more pronounced escape or tolerance traits under simulated grazing. In the natural habitat, species with a more pronounced escape but not tolerance strategy maintained higher abundance under grazing in dry habitats, indicating convergent favouring of escape by both grazing and drought. However, this finding at the local scale was not mirrored by a strategy shift along a large‐scale rainfall gradient. Synthesis. The convergent favouring of escape traits by grazing and drought in annuals is consistent with the generalized grazing model. This model, which has been developed for perennials based on the avoidance strategy, can thus be extended to annuals based on escape, a finding that should facilitate projecting consequences of global change in drylands dominated by annuals.

Different Roles of Top-Down and Bottom-Up Processes in the Distribution of Size-Fractionated Phytoplankton in Gwangyang Bay

The interactive roles of zooplankton grazing (top-down) and nutrient (bottom-up) processes on phytoplankton distribution in a temperate estuary were investigated via dilution and nutrient addition experiments. The responses of size-fractionated phytoplankton and major phytoplankton groups, as determined by flow cytometry, were examined in association with zooplankton grazing and nutrient availability. The summer bloom was attributed to nanoplankton, and microplankton was largely responsible for the winter bloom, whereas the picoplankton biomass was relatively consistent throughout the sampling periods, except for the fall. The nutrient addition experiments illustrated that nanoplankton responded more quickly to phosphate than the other groups in the summer, whereas microplankton had a faster response to most nutrients in the winter. The dilution experiments ascribed that the grazing mortality rates of eukaryotes were low compared to those of the other groups, whereas autotrophic cyanobacteria were more palatable to zooplankton than cryptophytes and eukaryotes. Our experimental results indicate that efficient escape from zooplankton grazing and fast response to nutrient availability synergistically caused the microplankton to bloom in the winter, whereas the bottom-up process (i.e., the phosphate effect) largely governed the nanoplankton bloom in the summer.

Regrowth strategies of Leymus chinensis in response to different grazing intensities.

In temperate grassland ecosystems, grazing can affect plant growth by foraging, trampling, and excretion. The ability of dominant plant species to regrow after grazing is critical, since it allows the regeneration of photosynthetic tissues to support growth. We conducted a field experiment to evaluate the effects of different grazing intensities (control, light, medium, and heavy) on the physiological and biochemical responses of Leymus chinensis and the carbon (C) sources utilized during regrowth. Light grazing promoted regrowth and photoassimilate storage of L.chinensis, by increasing the net photosynthetic rate (Pn ), photosynthetic quenching, light interception, sugar accumulation, sucrose synthase activities, and fructose supply from stems. At medium grazing intensity, L.chinensis had low Pn , light interception, and sugar accumulation, but higher expression of a sucrose transporter gene (LcSUT1) and water-use efficiency, which reflected a tendency to store C in belowground to promote survival. This strategy was associated with regulation by abscisic acid (ABA), jasmonate, and salicylic acid (SA) signaling. However, L.chinensis tolerated heavy grazing by increased ABA and jasmonate-induced promotion of C assimilation and osmotic adjustment, combined with photoprotection against photo-oxidation, suggesting a strategy based on regrowth. In addition, stems were the main C source organs and energy supply rather than roots. Simultaneously, SA represented a weaker defense than ABA and jasmonate. Therefore, L.chinensis adopted different strategies for regrowth under different grazing intensities, and light grazing promoted regrowth the most. Our results demonstrate the regulation of C reserves utilization by phytohormones, and this regulation provides an explanation for recent results about grazing responses.

Influence of Light Availability and Prey Type on the Growth and Photo-Physiological Rates of the Mixotroph Noctiluca scintillans

A strain of the mixotrophic green Noctiluca scintillans (Noctiluca) isolated from the Arabian Sea afforded us an opportunity to investigate the photosynthetic and feeding characteristics of this organism which has recently replaced the once diatom dominated food chain of winter blooms in the Arabian Sea. Here we present the first in a series of experiments undertaken to study the interactive effects of irradiance and grazing response of this mixotroph to four phytoplankton species provided as food. Noctiluca showed a distinct preference for the dinoflagellate Peridinium foliciaeum and the diatom Phaeodactylum tricornutum but not the chlorophyte Pyramimonas nor the chain forming diatom Thalassiosira weissflogii. However, irrespective of food provided, adequate light was required for Noctiluca to grow as evidenced by its maximum growth rates of 1.44 cells day-1 when fed the preferred Peridinium and exposed to optimal irradiance of 250 E m-2 s-1 versus growth rates of 0.18 cells day-1 with the same food but at a low irradiance of 10 E m-2 s-1. Measurements of Noctiluca’s electron transport rates (ETR) per PSII Reaction Center as a function of irradiance also indicated severe light limitation of photosynthesis at 10 E m-2 s-1. The active fluorescence derived ETR versus Irradiance curves revealed an interesting finding in that there was no significant difference in photosynthetic parameters such the maximum photosynthetic capacity (ETRmax) nor α, the rate of increase of photosynthesis with light between fed and unfed cells under optimal light conditions. These results suggest that feeding does not enhance the photosynthetic activity of the endosymbionts when nutrients are not limiting as was the case in these experiments. Measurements of Noctiluca’s intracellular ammonium concentrations under optimal light conditions, the first for this strain, show significant accumulation of NH4+ (0.003-0.012 M NH4+ cell-1) after 14 days for fed and unfed Noctiluca which was undetectable 4 days later. A similar 14-day increase but of significantly higher concentrations (0.005-0.08 M NH4+ cell-1) was obtained under low light conditions. For Phaeodactylum and Thalassiosira fed cultures under light limitation, NH4+ continued to increase past the 14-day period suggesting Noctiluca's strong and efficient mechanism for regulation of intracellular nutrients.

Nonlocal grazing in patterned ecosystems

Many ecosystems exhibit gapped, labyrinthine, striped or spotted patterns. Important examples are vegetation patterns in drylands: these patterns are viewed as precursors of a catastrophic transition to a degraded state. A possible source of degradation is overgrazing, but many current spatially extended models include grazing in a local linear way. In this article nonlocal grazing responses are derived, taking into account (1) how many consumers there are (demographic response) (2) where they are (aggregative response) and (3) how much they forage (functional response). Different assumptions lead to different grazing responses, the type of grazing has a large influence on how ecosystems adapt to changing environmental conditions. In dryland simulations the different types of grazing are shown to alter the desertification process driven by decreasing rainfall. A sufficiently strong aggregative response leads to the suppression of vegetation patterns, nuancing their role as generic early warning signals.

Vegetation gradients around cattleposts in the eastern mountains of Lesotho

The grasslands of the mountains in eastern Lesotho support large numbers of sheep, angora goats and other livestock during the summer, during which they are corralled nightly by herders at cattleposts. Few studies, however, have quantified the impact of grazing on the composition and structure of the mountain grasslands. Gradients of grazing and trampling intensity related to distance from 23 cattleposts in the subalpine and alpine belts (2 650–3 175 m above sea level) were examined to describe the long-term effects of livestock on unpalatable dwarf shrubs, bare area and plant species composition. The potential for shrub invasion and bare areas (low herbaceous cover) to develop was highest near cattleposts. Species composition also changed significantly along the grazing gradient, accompanied by a shift in dominants from grazing-sensitive to grazing-resistant grasses and shrubs. Species either decreased or increased monotonically along the grazing gradient or had an intermediate, unimodal response. The relative positions of species’ centroids along the grazing gradient were used to quantify their grazing sensitivity on a scale from 10 (most sensitive) to 0 (most resistant), which confirmed and contradicted previous classifications of their grazing responses. Basic ecological research on this important mountain grazing and catchment system is recommended.

Arboreality increases reptile community resistance to disturbance from livestock grazing

Abstract Domestic livestock grazing directly alters ground‐level habitat but its effects on arboreal habitat are poorly known. Similarly, the response to grazing of ground‐dwelling fauna has been examined, but there are few studies of arboreal fauna. Globally, grazing has been implicated in the decline of vertebrate fauna species, but some species appear resistant to the effects of grazing, either benefiting from the structural changes at ground level or avoiding them, as may be the case with arboreal species. Here, we examine arboreal and terrestrial habitat responses and reptile community responses to grazing, to determine whether arboreal reptile species are more resistant than terrestrial reptile species. We conducted arboreal and terrestrial reptile surveys on four different grazing treatments, at a 19‐year experimental grazing trial in northern Australia. To compare the grazing response of arboreal and terrestrial reptile assemblages, we used community, functional group and individual species‐level analyses. Species responses were modelled in relation to landscape‐scale and microhabitat variables. Arboreal reptile species were resistant to the impact of grazing, whereas terrestrial reptiles were negatively affected by heavy grazing. Terrestrial reptiles were positively associated with complex ground structures, which were greatly reduced in heavily grazed areas. Arboreal lizards responded positively to microhabitat features such as tree hollows. Synthesis and applications. Arboreal and terrestrial reptiles have different responses to the impact of livestock grazing. This has implications for rangeland management, particularly if management objectives include goals relating to conserving certain species or functional groups. Arboreal reptiles showed resistance in a landscape that is grazed, but where trees have not been cleared. We highlight the importance of retaining trees in rangelands for both terrestrial and arboreal microhabitats.

Recovering growth and grazing rates from nonlinear dilution experiments

AbstractBiological rate measurements provide critical information for understanding key processes and modeling future states of marine ecosystems. Experimentally derived rates can be challenging to interpret when methodological assumptions are untested or potentially violated under variable natural conditions, such as the assumed linear grazing response of the dilution technique for estimating rates of phytoplankton growth and microzooplankton grazing impact. Here, we show that grazing nonlinearity can be related to the ratio of initial phytoplankton biomass to the half‐saturation parameter in the Holling II model, while not being affected by varying grazer biomass during dilution experiments. From this, we present theory to recover growth and grazing rates from multi‐treatment dilution experiments with nonlinear grazing results. We test our analyses with data collected during the California Current Ecosystem‐Long‐Term Ecological Research (CCE‐LTER) program. We show that using a linear regression in 2‐treatment dilution experiments may lead to underestimates of microzooplankton grazing rates, particularly in high‐phytoplankton‐biomass coastal regions where grazing can be saturated. Using the Holling II grazing model and a correction factor, growth and grazing rates from 2‐treatment experiments can also be estimated, as illustrated by application to Lagrangian water‐tracking studies of growth and grazing dynamics in the CCE.

The adaptive value of grass traits in response to grazing

Grazing is associated with several plant traits that may confer resistance to herbivores. However, cross-species analyses do not allow for the differentiation between adaptive evolution and common ancestry. In this study, we evaluated the effect of grazing on 5 morphological traits in 41 native grasses growing in natural grasslands of Uruguay and investigated whether such effects are independent of phylogeny. We used data of grass species from 17 paired, grazed and ungrazed plots located in different regions of natural grasslands of Uruguay. For each species, we calculated the Grazing Response Index (GRI) and estimated the culm length, blade length, blade width, blade length/width ratio and caryopsis length. Trait values were calculated as the mean of the maximum and minimum values reported in a public database. We assessed the relationship between the GRI and the morphological traits using cross-species correlations, and we re-examined the correlations using phylogenetically controlled comparative analysis. Culm length and blade length were significantly correlated with the GRI. Species with higher culms and longer blades diminished their cover under grazing. This association remained significant after statistical control of phylogenetic relatedness among species. By contrast, blade width, blade length/width ratio and caryopsis length did not show any significant relationship with the GRI. Many studies in temperate grasslands recognized that several plant traits respond to grazing but were rarely evaluated in a phylogenetic context. Our results are consistent with the idea that grazing is a selective force with a clear effect on the evolution of grass stature, selecting smaller plants with shorter blades.

Is the grazing tolerance of mesic decreaser and increaser grasses altered by soil nutrients and competition?

An ability to tolerate recurrent defoliation likely plays a role in the compositional shift from decreaser to increaser species with overgrazing of mesic grassland, but the grazing tolerance of local species has not been extensively studied. The growth response of two decreasers, three Increaser II grasses, and an Increaser III species to frequent, severe defoliation under three levels of competition from neighbours and two levels of soil nutrients was examined in a pot trial. The effects of competition and especially nutrients markedly modified the defoliation tolerance of different species, and grazing response groups varied in the manner in which their defoliation responses were mediated by these interactions. Contrary to expectation, defoliation constrained potential productivity most in nutrient-rich, competition-free environments, probably because it exacerbates a carbon limitation on growth. However, nutrient enrichment did enable Increaser II grasses, but not other species, to better tolerate intense defoliation, explaining why they can persist on intensively grazed fertile patches and in heavily stocked grassland, such as in communal areas, where nutrients are more available. It is concluded that grazing tolerance is not a fixed property of species or grazing response groups but depends on biotic and abiotic environmental factors.

Differential responses of three grasses to defoliation, water and light availability

Grazing not only modifies the structure and functioning of grasslands, it also changes micro-environmental conditions that alter the availability of resources. The aim of this study was to analyze the response of grasses with different photosynthetic pathways (C3/C4), growth forms (prostrate/erect), and grazing responses (increaser/decreaser) to defoliation and resource availability. In a greenhouse, we performed a factorial experiment with three factors: defoliation, light, and water and three species: Axonopus affinis (C4 prostrate, increaser), Coelorachis selloana (C4 erect, decreaser), and Bromus auleticus (C3 erect, decreaser). We measured the relative growth rate (RGR), biomass assignment, and specific leaf area. The RGR of both C4 species was affected by light availability, while the decreaser C3 did not respond to any factor. Biomass allocation to leaves and stolons changed with the interaction between light and water in the C4 prostrate species (increaser). In the C4 erect grass (decreaser), biomass allocation was more affected by defoliation under low levels of light and water. Low light availability and defoliation reduced the assignment to leaves, while the allocation to rhizomes increased. Species-specific responses to resources availability that are modified by grazing were related to photosynthetic pathway, growth form, and grazing responses. Biomass allocation was related to strategies to avoid and/or tolerate grazing. The investment to leaves was limited by light and water availability in prostrate species, while in erect grasses it was controlled by defoliation and water availability. Our results highlight the importance of species responses to changes in resource availability associated to grazing regimes.

Parameter constraints of grazing response functions. Implications for phytoplankton bloom initiation

Phytoplankton blooms are events of production and accumulation of phytoplankton biomass that influence ecosystem dynamics and may also have effects on socio-economic activities. Among the biological factors that affect bloom dynamics, prey selection by zooplankton may play an important role. Here we consider the initial state of development of an algal bloom and analyse how a reduced grazing pressure can allow an algal species with a lower intrinsic growth rate than a competitor to become dominant. We use a simple model with two microalgal species and one zooplankton grazer to derive general relationships between phytoplankton growth and zooplankton grazing. These relationships are applied to two common grazing response functions in order to deduce the mathematical constraints that the parameters of these functions must obey to allow the dominance of the lower growth rate competitor. To assess the usefulness of the deduced relationships in a more general framework, the results are applied in the context of a multispecies ecosystem model (ERSEM).

Transitions and invasion along a grazing gradient in experimental California grasslands.

Resilience-based frameworks, founded upon the existence of multiple attractors and regime shifts, have long been applied to complex dynamics of semiarid systems. Utilizing seed addition tests in experimental plantings along grazing gradients, we applied an increase-when-rare criterion to identify bidirectional (states can invade each other) and directional (only one state can invade) transitions among vegetation states characteristic of California grasslands over fiveyears. Annual forage and medusahead grasslands were able to invade each other at all grazing intensities, indicating coexistence. Directional transitions involving invasion of native bunchgrass by other species occurred as grazing intensity increased; recovery (transitions to natives) did not occur at low grazing. While directional transitions at some grazing intensities were accompanied by state persistence at others, we found little evidence for persistence of alternative states at any grazing intensity. Our results suggest that grazing can affect resilience by causing hard-to-reverse transitions, but rarely produces alternative states. However, variation in precipitation seems to dominate grazing responses, supporting the applicability of the nonequilibrium concept in our study system.

Cultivation grazing response in seagrass may depend on phosphorus availability

In terrestrial systems, grazing can have lasting beneficial or damaging effects on plants based on nutrient availability, but it remains unclear how nutrients shape plant responses to grazing in marine systems, especially in resource-poor environments. We chose a phosphorus-limited subtropical turtlegrass Thalassia testudinum meadow in Bermuda to investigate short- and long-term plant responses to repeated herbivory. In a multiyear field experiment, we manipulated nitrogen (N) and phosphorus (P) availability and simulated green turtle Chelonia mydas grazing for 4 months and then tracked recovery through 2 years. In the short term, simulated grazing increased T. testudinum area-specific growth and both foliar N and P content in fertilized plots compared to the control. In unfertilized plots, simulated grazing (trimming) decreased short-term shoot-specific growth and foliar P content. By comparison, long-term data revealed sustained elevated T. testudinum growth after 2 years in fertilized plots in the absence of trimming. While trimming alone did not affect seagrass growth rates 2 years after the treatment application, these plants sustained depleted foliar P concentrations up to 1 year and displayed residual stress symptoms of fewer shoots with thinner leaves. Our data indicate the potential for N and P stoichiometry to influence “cultivation grazing” in marine carbonate environments. Seagrasses apparently responded positively to trimming when the foliar N:P balance did not exceed the “seagrass Redfield ratio” of 30:1. This P-sensitive outcome of herbivory should be evaluated at larger spatial scales and may have important implications for estimating C. mydas carrying capacity of seagrass meadows across resource gradients.