Grazing Response Research Articles

A simple plankton model for the Oregon upwelling ecosystem: Sensitivity and validation against time-series ocean data

Simple plankton models serve as useful platforms for testing our understanding of the mechanisms underlying ecosystem dynamics. A simple, one-dimensional plankton model was developed to describe the dynamics of nitrate, ammonium, two phytoplankton size-classes, meso-zooplankton, and detritus in the Oregon upwelling ecosystem. Computational simplicity was maintained by linking the biological model to a one-dimensional, cross-shelf physical model driven by the daily coastal upwelling index. The model sacrificed resolution of regional-scale and along-shore (north to south) processes and assumed that seasonal productivity is primarily driven by local cross-shelf Ekman transport of surface waters and upwelling of nutrient-rich water from depth. Our goals were to see how well a simple plankton model could capture the general temporal and spatial dynamics of the system, test system sensitivity to alternate parameter set values, and observe system response to the effective scale of potential retention mechanisms. Model performance across the central Oregon shelf was evaluated against two years (2000–2001) of chlorophyll and copepod time-series observations. While the modeled meso-zooplankton biomass was close in scale to the observed copepod biomass, phytoplankton was overestimated relative to that inferred from the observed surface chlorophyll concentration. Inshore, the system was most sensitive to the nutrient uptake kinetics of diatom-size phytoplankton and to the functional grazing response of meso-zooplankton. Meso-zooplankton was more sensitive to alternate parameter values than was phytoplankton. Reduction of meso-zooplankton cross-shelf advection rates (crudely representing behavioral retention mechanisms) reduced the scale of model error relative to the observed seasonal mean inshore copepod biomass but had little effect of the modeled meso-zooplankton biomass offshore nor upon phytoplankton biomass across the entire shelf.

Grazing response in the vegetation communities of the Kamiesberg, South Africa: Adopting a plant functional type approach

Potential grazing impacts associated with the sustained heavy grazing of a communal rangeland are explored through a plant functional type approach. An informative typology is derived that shows grazing responses. An examination of plant functional types within growth forms does not provide any additional insights, whereas looking at traits alone does demonstrate a grazing response with increases in the traits of large basal and erect leafy growth forms and losses in the traits of medium height, simple leaves and few flowers. There is also evidence of a biogeographic shift, with an increase in a succulent-dominated plant functional type at higher altitudes on the heavily grazed communal rangeland. This is attributed to the fact that in adopting the proposed universal language advocated by plant functional type practitioners some of the subtleties of this particular vegetation type are lost. For example growth form height classes are too coarse for the shorter vegetation of the Succulent Karoo. In the resultant ‘fuzzy’ groups, responses are lost. While the method adopted is informative a stronger case is made for a more simple growth form analysis traditionally adopted in rangeland analysis.

Microzooplankton Grazing in Green Water—Results from Two Contrasting Estuaries

We measured seasonal variations in microzooplankton grazing in Long Island Sound (LIS) and San Francisco Bay (SFB). There was consistent evidence of nutrient limitation in LIS, but not SFB. We found higher chlorophyll a concentrations in LIS compared with SFB. In spite of differences in phytoplankton, there were no differences in microzooplankton abundance (summer: LIS, 12.4 ± 1.8 × 103 indiv. L−1; SFB, 14.1 ± 3.0 × 103 indiv. L−1), biomass (summer: LIS, 30.4 ± 5.0 μg C L−1; SFB, 26.3 ± 5.9 μg C L−1), or grazing rates (summer: LIS, 0.66 ± 0.19 day−1; SFB, 0.65 ± 0.18 day−1) between the two estuaries. In common with many other investigators, we found many instances of saturated as well as insignificant grazing. We suggest that saturation in some cases may result from high particle loads in turbid estuarine systems and that insignificant grazing may result from extreme saturation of the grazing response due to the need to process non-food particles.

Reversed effects of grazing on plant diversity: the role of below‐ground competition and size symmetry

Grazing is known as one of the key factors for diversity and community composition in grassland ecosystems, but the response of plant communities towards grazing varies remarkably between sites with different environmental conditions. It is generally accepted that grazing increases plant diversity in productive environments, while it tends to reduce diversity in unproductive habitats (grazing reversal hypothesis). Despite empirical evidence for this pattern the mechanistic link between modes of plant–plant competition and grazing response at the community level still remains poorly understood. Root‐competition in particular has rarely been included in theoretical studies, although it has been hypothesized that variations in productivity and grazing regime can alter the relative importance of shoot‐ and root‐competition. We therefore developed an individual‐based model based on plant functional traits to investigate the response of a grassland community towards grazing. Models of different complexity, either incorporating only shoot competition or with distinct shoot‐ and root‐competition, were used to study the interactive effects of grazing, resource availability, and the mode of competition (size‐symmetric or asymmetric). The pattern predicted by the grazing reversal hypothesis (GRH) can only be explained by our model if shoot‐ and root‐competition are explicitly considered and if size asymmetry of above‐ and symmetry of below‐ground competition is assumed. For this scenario, the model additionally reproduced empirically observed plant trait responses: erect and large plant functional types (PFTs) dominated without grazing, while frequent grazing favoured small PFTs with a rosette growth form. We conclude that interactions between shoot‐ and root‐competition and size symmetry/asymmetry of plant–plant interactions are crucial in order to understand grazing response under different habitat productivities. Our results suggest that future empirical trait surveys in grassland communities should include root traits, which have been largely ignored in previous studies, in order to improve predictions of plants’ responses to grazing.

Linking grazing response of species abundance to functional traits in the Tibetan alpine meadow

Whether grazing response of species abundance can be predicted by plant functional trait remains a challenge untested in a specific ecosystem. We measured species abundance and relevant functional traits for 30 common component species in a moderately grazed and a control community over 3 years (2005, 2006 and 2007) in a Kobresia capillifolia dominated alpine meadow on the Tibetan Plateau. Our objective was to examine species response to grazing and to test the relationship between the response of species abundance and functional traits in grazed and control habitats. Our results showed: i) in terms of species relative abundance and saturated height (the maximum height of an adult individual), most component species significantly decrease in response to moderate grazing and the effect differed among species and between functional groups. ii) The response of species abundance was significantly negatively correlated with saturated height, but not correlated with specific leaf area or seed size. We concluded that the response direction of species abundance to grazing can be predicted by functional traits, whereas it is a weak predictor of the extent of grazing response in species abundance.

A community‐level test of the leaf‐height‐seed ecology strategy scheme in relation to grazing conditions

AbstractQuestion: Is the assumption of trait independence implied in Westoby's (1998) leaf‐height‐seed (LHS) ecology strategy scheme upheld in a Mediterranean grazing system dominated by annuals? Is the LHS approach applicable at the community level?Location: Northern Israel.Methods: LHS traits (specific leaf area [SLA], plant height and seed mass), and additional leaf traits (leaf dry matter content [LDMC], leaf area, and leaf content of nitrogen [LNC], carbon [LCC], and phosphorus [LPC]), were analyzed at the species and community levels. Treatments included manipulations of grazing intensity (moderate and heavy) and protection from grazing. We focused on species comprising 80% of biomass over all treatments, assuming that these species drive trait relationships and ecosystem processes.Results: At the species level, SLA and seed mass were negatively correlated, and plant height was positively correlated to LCC. SLA, seed mass, and LPC increased with protection from grazing. At the community level, redundancy analysis revealed one principal gradient of variation: SLA, correlated to grazing, versus seed mass and plant height, associated with protection from grazing. We divided community functional parameters into two groups according to grazing response: (1) plant height, seed mass, LDMC, and LCC, associated with protection from grazing, and (2) SLA, associated with grazing.Conclusions: The assumption of independence between LHS traits was not upheld at the species level in this Mediterranean grazing system. At the community level, the LHS approach captured most of the variation associated with protection from grazing, reflecting changes in dominance within the plant community.

Grazing responses in herbs in relation to herbivore selectivity and plant traits in an alpine ecosystem

Herbivores shape plant communities through selective foraging. However, both herbivore selectivity and the plant's ability to tolerate or resist herbivory may depend on the density of herbivores. In an alpine ecosystem with a long history of grazing, plants are expected to respond to both enhanced and reduced grazing pressures, and the interaction between plant traits and changes in species abundance are expected to differ between the two types of alteration of grazing regime. To understand the mechanisms behind species response, we investigated the relationship between sheep selectivity (measured in situ), plant traits and experimentally derived measures of change in species abundance as a response to the enhancement (from low to high density) or cessation (from low to zero density) of sheep grazing pressure over a six-year time period for 22 abundant herb species in an alpine habitat in south Norway. Sheep selected large, late-flowering herbs with a low leaf C/N ratio. Species that increased in abundance in response to enhanced grazing pressure were generally small and had high root/shoot ratios, thus exhibiting traits that reflect both resistance (through avoidance) and tolerance (through regrowth capacity) strategies. The abundance of selected species remained stable during the study period, and also under the enhanced grazing pressure treatment. There was, however, a tendency for selected species to respond positively to cessation of grazing, although overall responses to cessation of grazing were much less pronounced than responses to enhanced grazing. Avoidance through short stature (probably associated with increased light availability through the removal of tall competitors) as well as a certain amount of regrowth capacity appear to be the main mechanisms behind a positive response to enhanced grazing pressure in this study. The plant trait perspective clearly improves our insight into the mechanisms behind observed changes in species abundance when the disturbance regime is altered.

Animal and Pasture Productivity of ‘Coastal’ and ‘Tifton 44’ Bermudagrass at Three Nitrogen Rates and Associated Soil Nitrogen Status

‘Coastal’ and ‘Tifton 44’ (T44) bermudagrass [Cynodon dactylon (L.) Pers.] are well adapted across the lower southern United States, but the grazing response of (T44) to N application in the Piedmont of the upper South warrants further evaluation. This 3‐yr experiment compared animal and pasture productivity of Coastal and T44 with three annual N rates of 101, 202, and 303 kg of N ha−1 on a Cecil clay loam (fine, kaolinitic thermic Typic Kanhapludult) soil typical of the Piedmont. Herbage mass differed for Coastal and T44 (3.5 and 3.0 Mg ha−1 respectively, P < 0.01), but not among N rates. The canopy of T44 was leafier (20.6 vs. 14.5% of dry matter) than Coastal and greater for in vitro true organic matter disappearance (IVTOD) (522 vs. 498 g kg−1) and CP (107 vs. 84 g kg−1) and lesser in NDF (596 vs. 605 g kg−1). The diet selected from T44 was greater in IVTOD (764 vs. 743 g kg−1) and lesser in NDF (596 vs. 605 g kg−1) giving greater steer average daily gain (0.63 kg vs. 0.57 kg; P < 0.01) which increased (P = 0.05) with N rate. Weight gain ha−1 (884 kg) and effective feed units (EFU) (4735 kg ha−1) were similar, and N rate linearly increased gain from 723 to 1073 kg ha−1 and EFU from 3978 to 5523 kg ha−1. Soil inorganic N was similar between cultivars but differed among soil depths. Tifton 44 pasture was greater in nutritive value, hence steer performance, and as productive as Coastal in the Piedmont.

Indicator patches: exploiting spatial heterogeneity to improve monitoring systems

When choosing which environmental attributes to monitor in assessing disturbance, it is important to consider not only which metric will provide the most sensitive indicator of disturbance, but also the spatial considerations of where in the landscape that metric will be most responsive to change. Degradation in landscapes is often unevenly expressed because: (i) disturbance is spatially localised, (ii) landscape elements differ in their sensitivity to disturbance, and (iii) degradation following localised disturbance is spatially contagious. The spatial heterogeneity of degradation has proven to be a key obstacle to rangeland monitoring (e.g. where the initiating processes of broad-scale degradation are concentrated in landscape locations that are not detected by surveys) but can also provide opportunities to focus monitoring efforts. We propose that the effectiveness of monitoring could be enhanced by identifying and selectively monitoring ‘indicator patches’, i.e. specific landscape locations that provide the most management-relevant and timely information about the consequences of a monitored disturbance. We tested and demonstrated the utility of the ‘indicator patch’ concept in the rangelands of the Succulent Karoo in southern Africa. We contrasted the grazing response of dominant ‘representative’ vegetation, with responses of interspersed patches of distinct vegetation associated with zoogenic mounds. Since mound vegetation is more palatable and preferentially grazed by sheep, we tested whether mounds could serve as ‘indicator patches’ in providing a sensitive measure of grazing disturbance. Percentage canopy cover measurements in dominant off-mound vegetation provided a poor indicator of grazing disturbance (although more intensive plant size measurements did reveal grazing impacts on plant population dynamics). In contrast, vegetation on mounds displayed patterns of changes in species abundances that were easier to detect and useful for interpreting and quantifying the effects of grazing. Mound vegetation could, therefore, be used as ‘indicator patches’ and targeted for exclusive sampling as a sensitive method for monitoring rangeland condition and detecting early warnings of vegetation change. This approach could be widely employed to better harness the extensive knowledge base regarding the patchy, spatially localised nature of degradation-initiating processes in numerous other landscapes. Routinely incorporating this understanding into the design of monitoring programs could improve the effectiveness of sampling effort, allow detection of more subtle trends (changes), and provide earlier warning of impending degradation so remedial action can be taken before degradation becomes severe and widespread.

Livestock grazing behavior and inter- versus intraspecific disease risk via the fecal–oral route

Livestock herbivores are at risk of parasite/pathogen exposure from livestock and wild mammal feces during grazing. Livestock exposure to parasites/pathogens will be dependent on the behavioral contact processes between grazing livestock and host animal (both livestock and wild mammal) feces at the bite scale. Here we use 2 grazing experiments to determine the affect of feces from different species and in different defecation patterns on the grazing response of cattle. In experiment 1, there were 4 plots, each with 4 replicates of 5 patch treatments of different fecal contamination (240 g/m2 of Eurasian badger feces, cattle feces, fallow deer feces, Eurasian rabbit feces, and noncontaminated control patches). In experiment 2, there were 3 treatment patterns of badger fecal contamination (one 1-m2 circular patch contaminated with 960 g of badger feces; two 1-m2 circular patches each contaminated with 480 g of badger feces; and four 1-m2 circular patches each contaminated with 240 g of badger feces), divided into 2 plots per treatment. The cattle's grazing response was determined by measuring sward depletion at each of the treatment patches. In experiment 1, cattle-grazed control and rabbit fecal–contaminated patches the most, whereas badger-contaminated patches were grazed the least. In experiment 2, cattle grazed the treatment with the greatest number of fecal-contaminated patches the most. We conclude that cattle vary their grazing response to feces from different host species and to feces in different spatial patterns. Quantifying these behavioral responses to feces is a key step toward quantifying infection risk to herbivores via the fecal–oral route in grazing systems.

Functional responses and ecosystem dynamics: how clearance rates explain the influence of satiation, food-limitation and acclimation

Modellers have long been aware that the mathematical form of zooplankton mortality, or closure, significantly affects the dynamics of planktonic ecosystem models. Another important formulation is the functional response, i.e. how ingestion rates change with prey density. Here we explain why different grazing responses can have profoundly differing influences on modelled dynamics, and how common practices may limit models due to misguided characterization of feeding behaviours. Use of different ingestion functions in a Nutrient-Phytoplankton-Zooplankton (NPZ) model results in oscillating versus constant densities. Contrary to the conclusions of previous studies, it is shown that these results are not due to zooplankton satiation versus non-satiation. Analysis of a predator-prey model is used to derive the necessary condition for ecological stability, which is related to food-limited clearance rates. Sensitivity studies demonstrate that zooplankton clearance rates have a strong influence on the dynamics of more complex models. Moreover, it is shown that acclimation time lags can dramatically alter results from those where zooplankton instantly adapt to changing prey densities due to the corollary effect on clearance rates. These results are discussed in terms of practical advice to modellers who face uncertainty in choosing expressions for the functional response.

Plant responses to agricultural intensification

Summary 1. A large proportion of the world's land surface is extensively managed for livestock production. In areas where livestock systems are becoming more intensive, a major challenge is to predict those plant species likely to decline, persist or increase as a result of agricultural intensification. 2. Most analyses develop inferences for frequent or abundant species, or rely on intensive studies of single species. A promising approach is to identify plant traits related to disturbance to enable inference to be made about changes in plant community composition. We used a Bayesian hierarchical model to analyse the response to agricultural intensification of 494 plant species of pastures and woodlands in southern Australia, and to identify how simple species' traits (life form, growth form and species origin) influence those responses. 3. The probability of occurrence of most species declined along the two intensification gradients, grazing intensity and soil phosphorous concentration, although the occurrence of a greater proportion of species was negatively correlated with soil phosphorous. Responses could be broadly predicted from both plant origin and plant traits, in particular growth form. 4. Native perennial geophytes, ferns and shrubs were most negatively affected by both gradients, while exotic annual grasses and forbs were more tolerant. Along the phosphorous gradient, 24 of the 30 most negatively affected plant species were native geophytes. Mean within-group responses masked considerable within- and between-species variation, particularly for the exotic species group which included species that responded both negatively and positively to intensification. 5. Synthesis and applications. The hierarchical model described here provides a powerful method for estimating individual plant responses and identifying how species' traits influence those responses. Plant species native to southern Australia are sensitive to grazing and phosphorous apparently due to a shared evolutionary history of low grazing intensity and low phosphorous soils. Invading exotic plants have faced strongly contrasting ecological filters, leading to a greater diversity of responses. Where grazing systems have been most intense, a small suite of exotics dominate. Maintaining native and functional plant diversity will necessitate limits being placed on intensive livestock management systems.

Growth and grazing responses of the mixotrophic dinoflagellate Dinophysis acuminata as functions of light intensity and prey concentration

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 51:301-310 (2008) - DOI: https://doi.org/10.3354/ame01203 Growth and grazing responses of the mixotrophic dinoflagellate Dinophysis acuminata as functions of light intensity and prey concentration Sunju Kim1,5, Yi Gu Kang1, Hyung Seop Kim2, Wonho Yih1, D. Wayne Coats3, Myung Gil Park4,* 1Department of Oceanography, Kunsan National University, Gunsan, 573-701, Republic of Korea 2Gunsan Fisheries Office, NFRDI, Gunsan 573-882, Republic of Korea 3Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA 4Laboratory of HAB Ecophysiology (LOHABE), Department of Oceanography, Chonnam National University, Gwangju, 500-757, Republic of Korea 5Present address: Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA *Corresponding author. Email: mpark@chonnam.ac.kr ABSTRACT: Dinophysis acuminata, a photosynthetic marine dinoflagellate, possesses plastids of cryptophyte origin and causes diarrhetic shellfish poisoning (DSP). Recent work has shown D. acuminata to be a mixotroph that grows well when feeding on the photosynthetic ciliate Myrionecta rubra. Using established cultures, we examined the effects of light intensity and prey (M. rubra) concentration on growth and ingestion rates of D. acuminata. Growth rates increased with increasing prey concentration under continuous illumination of 60 µE m–2 s–1, with maximum mixotrophic growth (0.91 d–1) almost 5 times higher than growth in the absence of prey (0.19 d–1). The maximum ingestion rate of D. acuminata was 1296 pg C Dinophysis–1 d–1 (3.2 M. rubra cells Dinophysis–1 d–1) for data fitted to a Michaelis-Menten equation. Growth rate also increased with increasing light intensity, an effect even stronger when prey was supplied. Increased growth with increasing irradiance was accompanied by a corresponding increase in ingestion. While D. acuminata continued to grow in semi-continuous food-replete cultures at high (200 µE m–2 s–1) and low (10 µE m–2 s–1) light intensity, it failed to grow in darkness, despite the presence of prey. Our results suggest that D. acuminata is an obligate mixotroph that requires both light and prey for long-term survival. Results indicate that Dinophysis species are typically prey-limited in the field. KEY WORDS: Dinophysis acuminata · Myrionecta rubra · Growth · Grazing · Light · Mixotrophy · Phagotrophy Full text in pdf format PreviousCite this article as: Kim S, Kang YG, Kim HS, Yih W, Coats DW, Park MG (2008) Growth and grazing responses of the mixotrophic dinoflagellate Dinophysis acuminata as functions of light intensity and prey concentration. Aquat Microb Ecol 51:301-310. https://doi.org/10.3354/ame01203 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 51, No. 3. Online publication date: June 16, 2008 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2008 Inter-Research.

Effects of the red tide dinoflagellate, Karenia brevis, on grazing and fecundity in the copepod Acartia tonsa

Among studies of copepod grazers fed harmful algae, decreased grazing and fecundity are the most common results. The causes of decreased grazing (physiological incapacitation, behavioral avoidance or lack of stimulation) and decreased fecundity (toxic versus nutritional effect) vary among studies. This study used a series of controlled laboratory experiments to investigate the cause of decreased grazing and fecundity in the copepod Acartia tonsa fed sole and mixed diets of the harmful alga, Karenia brevis. Copepods fed K. brevis mixed with the nutritionally viable dinoflagellate Peridinium foliaceum had higher ingestion rates and offspring production than copepods fed a sole diet of K. brevis (even when K. brevis was virtually nontoxic). Copepods fed mixtures did not discriminate between P. foliaceum and K. brevis while feeding. The results of this study suggest that K. brevis is not toxic to A. tonsa but lacks some chemical component responsible for stimulating a grazing response in A. tonsa as well as the nutritional requirements for normal offspring production.

Confronting complexity: reply to Le Quéré and Flynn

Complexity pervades ecology. The search for unifying laws therefore entails that ‘biologists work very close to the frontier between bewilderment and understanding’ (Medawar, 1969). One approach to developing ecological theory is to abandon simplicity at the outset and operate at the highest appropriate organizational level (Dunbar, 1980). On this basis, Le Quere (Le Quere, 2006) issues a plea for inclusion of plankton functional types (PFTs) in plankton models because ‘we will not understand ecology until we have built models that include the necessary processes’. From a purely a priori standpoint, my own view is quite the same: ‘The case for superseding; nutrient–phytoplankton–zooplankton–detritus (NPZD)type models with those that include PFT is clear: biogeochemical cycling in marine systems is intimately linked to particular plankton groups if not in some instances primarily to individual plankton genera or species’ (Anderson, 2005). The practical difficulties of representing PFTs as a series of interacting differential equations in models should not, however, be underestimated, the requirement being to demonstrate robustness of parameterization in the face of what is often inconclusive knowledge and scarcity of available data. In this context, Le Quere claims that PFT models perform better-informed tuning than do NPZD models, which ‘rely solely on the reproduction of biogeochemical fields as a test of their performance’. NPZD modellers are effectively labelled instrumentalists whereby their models are no more than empirically adequate calculating devices employing abstract parameters with little or no basis in reality. In contrast, PFT models are elevated to a realist status, that is, having a basis in true laws of nature, parameter values having been ‘narrowed down’ by extensive laboratory work. This supposed dichotomy is mistaken. The equations of NPZD models very often have a good basis in reality, such as the use of photosynthesis–irradiance curves or Holling-type grazing responses. Yes, the parameters that go into these equations have to take account of the aggregate properties of the whole phytoplankton or zooplankton communities. But the same problem, namely that of aggregation, is fundamental to PFT models too. No amount of laboratory studies on Emiliania huxleyi will, for example, be conclusive in setting parameter values for a state variable for calcifiers, which includes numerous other species of coccolithophores, as well as foraminiferans (Anderson, 2005). Putting aside the problem of aggregation for a moment, Flynn (Flynn, 2006) identifies difficulties at an even more fundamental level. It may not currently be possible, he suggests, to adequately parameterize a model of even a single algal species in laboratory culture, E. huxleyi surely being a case in point. Let us hope that this assessment is unduly pessimistic. Extending to the natural environment to include the maze of interactions such as competition between species and top-down control by grazers represents a formidable task. Providing examples such as the roles of allelopathy and food quality in trophic interactions, Flynn’s critique serves admirably to redouble my assertion that much of the devil is in the details of those interactions (Anderson, 2005), a worrying prospect for modellers. The proof of the pudding, so to speak, is in the eating, and so rigorous validation lies at the heart of future progress. Recognizing the limited success of PFT models to date in this regard, Le Quere tamely describes them as being ‘no worse’ than their NPZD counterparts. Given two empirically equivalent theories, scientists will usually deem the simpler one to be better constrained by available data and therefore more reliable. The challenge for PFT modelling is to demonstrate robustness in parameterization of the PFTs themselves, a mighty task indeed. The strategy of model building is all about balancing complexity, accuracy and generality (Levins, 1966). JOURNAL OF PLANKTON RESEARCH j VOLUME 28 j NUMBER 9 j PAGES 877–878 j 2006

PLANT TRAITS AND ECOSYSTEM GRAZING EFFECTS: COMPARISON OF U.S. SAGEBRUSH STEPPE AND PATAGONIAN STEPPE

Plant functional traits provide one tool for predicting the effects of grazing on different ecosystems. To test this approach, we compared plant traits and grazing response across analogous climatic gradients in sagebrush steppe, USA (SGBR), known to have a short evolutionary history of grazing, and Patagonian steppe, Argentina (PAT), where generalist herbivores exerted stronger selective pressures. We measured grazing response by sampling vegetation and soils across distance‐from‐water gradients at arid, semiarid, and subhumid study areas in both regions. Based on a previous analysis of graminoid traits, we predicted that: (1) high forage quality in all three SGBR communities would lead to high utilization and large grazing effects, whereas low quality in arid PAT would constrain utilization and grazing impacts, with semiarid and subhumid PAT intermediate in quality and grazing response; and (2) grazing in arid PAT would cause shifts in relative abundance within the graminoid functional group, due to the large range of forage quality among graminoids, but in SGBR, where all graminoids are relatively palatable, shifts in abundance would occur between grasses and shrubs.Utilization in locations close to water was higher in SGBR than in PAT study areas. This utilization difference led to differences in grazing effects consistent with our first prediction. Abundance of graminoids increased with distance from water in all three SGBR communities and in subhumid PAT, but not in arid PAT. Shrub and total production decreased with distance in SGBR but not PAT study areas. Grazing variables explained less variation in species composition in arid PAT (43%) than in any other study area (59–74%). Grazing did not significantly influence species richness. Evidence for our second prediction was mixed. Grazing did alter the relative abundance of SGBR graminoids and shrubs, but abundance shifts among the graminoids in SGBR communities were larger than in PAT communities, counter to our prediction.This case study demonstrates how plant traits can explain relative effects of grazing on ecosystem structure and functioning, although predicting species‐specific responses remains a challenge. Regardless of their evolutionary origin, poor‐quality graminoids make the arid Patagonian steppe more resistant to overgrazing than communities dominated by more nutritious species.

Plant functional traits, herbivore selectivity and response to sheep grazing in Patagonian steppe grasslands

Summary In some ecosystems there is a positive feedback between forage quality and grazing intensity. This involves three components of plant tolerance to grazing: functional traits, herbivore selectivity and response to grazing. We analysed the relationships between these components at species and community levels in Patagonian steppe grasslands. We measured plant functional traits [height, specific leaf area (SLA) and foliar toughness] and estimated sheep selectivity and grazing response indices for 35 plant species. Sheep selectivity indices were obtained from microhistological and species’ availability data, and grazing response indices from species’ abundances in sites with contrasting grazing intensities. We performed correlations and multiple regressions among the three types of variables across the pool of 35 species. To analyse data at the community level, we computed weighted averages of traits and sheep selectivity indices for 34 floristic samples taken from each side of 17 fence lines with contrasting grazing intensities. Correlations between mean trait values and sheep selectivity across the 34 samples, and paired comparisons of those variables between sides of the fences, were performed. Taller plants had leaves with lower SLA and/or higher toughness. Short species of intermediate toughness were selected more often by sheep, while SLA was not related to sheep selectivity. Short species with high SLA increased with grazing, while toughness and sheep selectivity were unrelated to grazing response. At the community level, short swards with high average SLA had high selectivity indices and were more abundant on the most intensively grazed sides of fence lines. Leaf toughness was unrelated to other traits or to sheep selectivity, and showed no significant response to grazing. Synthesis and applications. Intensive grazing can increase the forage value of grasslands by the creation of lawns dominated by tolerant species. However, results from this study showed that some plant species that were avoided by grazers also increased, indicating a potential risk of a shift in composition of grazing lawns towards states of low forage value. This suggests that periodic resting of lawns could be a good management strategy to favour palatable species, thereby minimizing the risk of undesirable shifts in the overall species composition.

An assessment of variation in Idaho fescue [Festuca idahoensis (Elmer)] in southern Alberta

Idaho fescue (Festuca idahoensis Elmer) is a native grass species that has attracted interest for use in revegetation, reclamation and other applications. However, there is a serious shortage of commercially available seed and concern that germplasm used will not be adapted to the site. A component of adaptation is genetic variability that allows a species to occupy a greater range of environments. Field trials were conducted in southern Alberta to determine the genetic variability of phenotypic and performance characteristics among genotypes of Idaho fescue. Since grazing pressure may contribute to genetic selection and may therefore affect the variability within a population, we also compared genotypes taken from a heavily grazed paddock with those taken from a lightly grazed paddock. Forty-nine genotypes from three populations were randomly sampled in 1992, propagated vegetatively to produce plants for replicated trials and planted in four locations in 1993. Nine plant characteristics were observed from 1993 to 1997. Since the genotypes were grown in common environments, variation among them was assumed to be caused by genetic differences. All variables were affected (P < 0.05) by population, test location and year, while the effect of population was also influenced by test location and year for a few variables. While the Idaho fescue plants expressed differences (P < 0.05) among populations for all selected traits, they displayed considerable overlap in the range of values for all variables both within and among test locations. Therefore, while the populations may be different, individuals within populations exhibit common attributes over a large range. This suggests that sufficient genetic variability exists in all populations to allow successful establishment over a large range of environmental variability. Results from a secondary test suggest that selection pressure, induced by grazing, resulted in genotypes that were smaller in crown circumference, had less spring vigor, had shorter flowering tillers and produced less seed. However, this observation needs further validation with a more robust test. Key words: Morphology, genotypes, seed yield, winter kill, grazing response

Comparison of the numerical grazing response of two marine heterotrophic nanoflagellates fed with different bacteria

The numerical grazing responses of two marine heterotrophic nanoflagellates (HNF), Jakoba libera strain 5(2) and Cafeteria sp. strain 5(3) fed with three bacterial strains ( Alteromonas sp., Flavobacterium sp., and Pseudomonas sp.) and mixed bacterial communities (MBC) were compared. J. libera-5(2) had different numerical grazing responses to food bacteria. The net HNF cellular production, maximum specific growth rate (μ max) and maximum clearance rate (F max) values of J. libera-5(2) were highest with Pseudomonas sp., followed by Flavobacterium sp., MBC and Alteromonas sp. In contrast, Cafeteria sp.-5(3) showed the same numerical response to the food bacteria, and attained a 57-fold higher net stationary phase population, a 9-fold higher μ max, and a 5-fold higher F max than J. libera-5(2) with Alteromonas sp. as food bacteria. These differences in numerical grazing response of HNF may be important in structuring both the bacterial and protozoan species composition and succession in the marine environments.

Simple traits do not predict grazing response in Australian dry shrublands and woodlands

Summary Plant species composition and species response to grazing are fundamental to the management of grazing lands. Reliable prediction of grazing responses from species’ traits would be a major step forward in generalizing knowledge and management beyond the locality. Recently results have been presented showing that plant species’ responses to grazing in subhumid grasslands in Argentina and Israel could be predicted from simple traits: height, leaf size, life history and specific leaf area (SLA). This study assessed whether those relationships between traits and grazing responses hold for Australian semi‐arid and arid shrublands and woodlands. Eleven lists of grazing responses from five published grazing studies were matched with a plant trait data set. Trait distributions were compared between response groups pooled across studies. Relationships between traits and grazing response were also assessed, both within and across studies, using meta‐analysis. Overall there was little evidence for predictability of grazing responses with simple traits in the semi‐arid and arid shrublands and woodlands. There were relationships between grazing response and life history and growth form, and some weak evidence for grazing increasers having high SLA. Synthesis and applications. It was concluded that prediction of grazing responses with simple traits is less clear in semi‐arid and arid rangelands, which are characterized by openness at ground level and high diversity of growth forms, compared with subhumid grasslands that have structurally simple, continuous swards. The finding that species’ traits may have differing predictive capacity for grazing responses in different situations means that we need more empirical studies in different situations. The most important contrasts between situations for investigation are likely to be on axes of rainfall/productivity and evolutionary history of grazing.