Selective Grazing Research Articles

Phytoplankton community from Lake Taihu, China, has dissimilar responses to inorganic and organic nutrients

To evaluate the response of phytoplankton from Lake Taihu to different types of nutrients, the phytoplankton responses were measured after adding inorganic nitrogen (N) and phosphorus (P) or decomposed algal scum ( Microcystis spp.) into the lake water. Both types of nutrients promoted an increase in phytoplankton biomass as determined by chlorophyll a and algal wet weight. The addition of decomposed algal scum resulted in a significantly greater phytoplankton response than the addition of inorganic N and P alone. The dissolved inorganic N and P in the inorganic nutrient treatment were found not limit phytoplankton growth. The higher algal biomass obtained in the treatment with decomposed algal scum indicated the importance of other organic nutrients besides N and P such as trace elements, as well as the importance of the form of N since the levels of ammonia nitrogen (NH 4 +-N) from the decomposed algal treatment were actually higher than that of the inorganic N and P addition. Microcystis spp. (Cyanobacteria), Scenedesmus spp. (Chlorophyta) and Synechocystis spp. (Cyanobacteria) were the dominant taxa in the control, inorganic N and P treatment, and the decomposed algal scum treatment, respectively. Microcystis never bloomed in response to both types of nutrient additions indicating that the bloom propagation is not solely related to nutrient additions, but may be related to the absence of selective grazing from zooplankton.

Recovery of canopy-forming macroalgae following removal of the enigmatic grazing sea urchin Heliocidaris erythrogramma

The sea urchin Heliocidaris erythrogramma (Valenciennes, 1846) is considered an ecologically important member of shallow sub-tidal reef assemblages across temperate Australia. However, defining its ecological role has remained elusive due to a paucity of evidence demonstrating the ability of this species to graze and maintain sea urchin barrens. Here we present critical evidence for a grazing effect of H. erythrogramma at typically observable densities (~ 4–6 individuals m − 2 ) by demonstrating recovery of canopy-forming brown algae following two controlled removals of the sea urchin from ostensibly barrens reef at sheltered locations in eastern Tasmania. In experimental plots where H. erythrogramma was removed, canopy-forming algae gradually recovered to demonstrate an average 5 times (ranging nil to 10 times) increase in percentage cover at 24 months post manipulation (chiefly driven by the habitat-formers Cystophora spp., Macrocystis pyrifera, Acrocarpia paniculata, and Sargassum spp.). While divergence in the overall algal community was indicated by both experiments, a statistically significant shift, based on percentage cover community data, was observed for only one location at 24 months post sea urchin removal, suggesting that a complete canopy-driven shift in community structure will be gradual and will be contingent on urchin density remaining below approximately 1.5 m − 2 in the longer term. The relatively slow and highly variable rates of algal recovery across plots suggests that seasonal coexistence of annual (chiefly filamentous algae) and some perennial macroalgae on H. erythrogramma barrens is likely due to a relatively low and variable intensity of selective grazing. We conclude that H. erythrogramma, at sufficient densities, is capable of grazing and maintaining a sea urchin barrens state however this phenomenon is observed to manifest in sheltered waters only. In contrast, H. erythrogramma occurring on exposed Tasmanian reefs achieves similar densities but here barrens are not observed and the sea urchin occurs cryptically within crevices and, consistent with experiments elsewhere in Australia, the sea urchin under these conditions appears to feed exclusively on an abundance of detached ‘drift’ macroalgae. Top–down and bottom–up factors leading to increased abundance of H. erythrogramma will be important in determining the potential influence of this enigmatic grazer on rocky reef communities, however critical tests of mechanisms controlling the switch between foraging modes are needed. Taken together, correlative patterns and results of experimental removals across the range of H. erythrogramma demonstrate that models of temperate Australian reef dynamics must account for the highly context-dependent effects of sea urchin foraging.

A hierarchical model for analysing the stability of vegetation patterns created by grazing in temperate pastures

Questions: Does vegetation structure display any stability over the grazing season and in two successive years, and is there any correlation between the stability of these spatial patterns and local sward composition? Location: An upland grassland in the French Massif Central. Method: The mosaic of short and tall vegetation stands considered as grazed and ungrazed patches respectively is modeled as the realization of a Boolean process. This method does not require any arbitrarily set sward-height thresholds to discriminate between grazed and ungrazed areas, or the use of additional variables such as defoliation indexes. The model was validated by comparing empirical and simulated sward-height distributions and semi-variograms. Results: The model discriminated between grazed and ungrazed patches at both a fine (1 m2) and a larger (500 m2) scale. Selective grazing on legumes and forbs and avoidance of reproductive grass could partly explain the stability of fine-scale grazing patterns in lightly grazed plots. In these plots, the model revealed an inter-annual stability of large-scale grazing patterns at the time peak biomass occurred. At the end of the grazing season, lightly grazed plots showed fluctuating patch boundaries while heavily grazed plots showed a certain degree of patch stability. Conclusion: The model presented here reveals that selective grazing at the bite scale could lead to the creation of relatively stable patches within the pasture. Locally maintaining short cover heights would result in divergent within-plot vegetation dynamics, and thus favor the functional diversity of vegetation.

Transcriptional responses of Folsomia candida upon exposure to Aspergillus nidulans secondary metabolites in single and mixed diets

Genetic and molecular evidence to support the hypothesis that fungal secondary metabolites play a significant role in protecting the fungi against fungivory is scarce. We investigated the impact of fungal secondary metabolites on transcript regulation in the fungivorous springtail Folsomia candida feeding on mixed vs. single diets. Aspergillus nidulans, which is able to produce secondary metabolites including sterigmatocystin, and a knockout mutant with reduced secondary metabolism (A. nidulans ΔLaeA) were combined with the high quality fungus Cladosporium cladosporioides as mixed diets or offered as single diets. We hypothesized that (i) A. nidulans WT triggers more genes associated with stress responses compared to the A. nidulans ΔLaeA strain with suppressed secondary metabolism, (ii) C. cladosporioides causes significantly different transcript regulation than the A. nidulans strains ΔLaeA and WT, and (iii) mixed diets will cause significantly different transcript expression levels than single diets. All three hypotheses are generally supported regardless of the fact that functions of many of the differentially transcribed genes are still enigmatic and that a relatively subtle response, devoid of characteristic stress response genes, was observed towards the toxic fungi in the diet. The wild type A. nidulans strain elicited 30% more differentially expressed genes than the ΔLaeA mutant strain. Differentially expressed genes were not observed in the mixture diet of the C. cladosporoides × A. nidulans WT diet, indicating selective grazing on C. cladosporoides. The results bring molecular evidence for the existence of a link between fungal secondary metabolites and responses in springtails supporting the hypothesis that fungal secondary metabolites act as a shield against fungivory.

Identification of Compounds with Bioactivity against the Nematode Caenorhabditis elegans by a Screen Based on the Functional Genomics of the Marine Bacterium Pseudoalteromonas tunicata D2

Marine bacteria are a rich, yet underexplored, resource of compounds with inhibitory bioactivity against a range of eukaryotic target organisms. Identification of those inhibitors, however, requires a culturable or genetically tractable producer strain, a prerequisite that is not often fulfilled. This study describes a novel functional genomic screen that is based on expression of inhibitors in a heterogeneous recombinant host (i.e., Escherichia coli). Functional libraries were screened by selective grazing by the nematode Caenorhabditis elegans, in a simple, rapid, high-throughput manner. We applied our approach to discover inhibitors of C. elegans produced by the marine bacterium Pseudoalteromonas tunicata D2, a model organism for exploring a range of antagonistic activities between bacteria and eukaryotes and a known producer of several toxic compounds. Expression of P. tunicata DNA in E. coli and grazing selection by the nematode Caenorhabditis elegans identified two clones, with slow- and fast-killing modes of action. Genomic analysis of the slow-killing clone revealed that the activity was due to a small molecule, tambjamine, while the fast-killing activity involved a gene encoding for a novel protein. Microscopic analysis showed substantial colonization of the intestinal lumen, or rapid death of the nematode without colonization, for the two activities, respectively. The novel functional genomic screen presented here therefore detects new eukaryotic inhibitors with different chemical structures, kinetics, and predicted modes of actions.

Palatability of macrophytes to the invasive freshwater snail Pomacea canaliculata: differential effects of multiple plant traits

Summary1. By selective grazing, invasive grazers can alter macrophyte‐herbivore relationships in shallow freshwater bodies. Evaluating the palatability of macrophytes and understanding the determinants of plant palatability can help predict grazing impact. In no‐choice feeding assays, we tested the palatability of 21 species of freshwater macrophytes to the invasive freshwater apple snail Pomacea canaliculata.2. Daily feeding rate varied greatly with plant species, ranging from 1.1 to 22% of snail body mass. We assessed six plant properties and examined their correlation with feeding rate. Total nitrogen content was positively related, and C:N ratio and dry matter content (DMC) negatively related, to snail feeding rate. There was no significant correlation between snail feeding rate and plant phenolic content, but the feeding rate on Myriophyllum aquaticum (the plant with the highest phenolic content) was very low.3. We repeated the feeding assays for 15 species that were not palatable as fresh leaves with reconstituted plant tissues formed by mixing ground up dried leaves with agar. The feeding rate still differed greatly among macrophyte species. Phragmites australis and Vallisneria natans (two species with the highest DMC) were eaten much more as reconstituted plant than as fresh leaves, indicating that structure (i.e. DMC) may be important in their defence against snail herbivory. For two plants (M. aquaticum and Alternanthera philoxeroides) that had moderate amounts of nitrogen/phosphorus but were consumed very little as fresh and reconstituted tissues, we incorporated their extracts into a palatable agar‐based food. The extracts from both species greatly reduced snail feeding rate, indicating the presence of chemical defences in these two species.4. These results indicated that feeding was affected by several plant traits. The snail favoured plants with a high nitrogen content and avoided plants with a high DMC. Only a few plants possessed chemical feeding deterrents that were effective against this snail. Given the invasive spread of P. canaliculata in Asia, ecologists and managers should consider plant palatability when selecting plants for use in wetland restoration and when predicting the impact of further invasion by this species.

Selectivity and grazing impact of microzooplankton on phytoplankton in two subtropical semi-enclosed bays with different chlorophyll concentrations

Microzooplankton grazing rates were compared between two sites (S1 and S2) in the coastal seas of eastern Hong Kong with similar physio-chemical parameters, but different chlorophyll concentrations. During the period from March 2007 to January 2008, six sets of dilution experiments, combined with high performance liquid chromatography and phytoplankton size fractionation (<200μm, <20μm and <5μm), were carried out to study the microzooplankton grazing rate on phytoplankton of different taxonomic groups and sizes. Although total chlorophyll a concentrations were much higher in S1 (4.98–18.42μgl−1) than in S2 (0.29–1.68μgl−1), size composition of phytoplankton was relatively similar between the two sites. Measured as chlorophyll a, phytoplankton growth rates (−0.84–1.91d−1 in S1; 0.03–2.85d−1 in S2) and microzooplankton grazing rates (0.00–2.26d−1 in S1; 0.00–1.49d−1 in S2) for all three size fractions were similar between the two bays. Phytoplankton growth rates and microzooplankton grazing rates measured as other pigments for phytoplankton of different size fractions did not show strong variations. Microzooplankton grazing impact, expressed as the ratio of microzooplankton grazing rate to phytoplankton growth rate, was generally higher in S1 than in S2, although the difference was not statistically significant. High microzooplankton grazing impact on alloxanthin (1.00–45.85) suggested strong selection toward cryptophytes. Our results provided no evidence for size selective grazing on phytoplankton by microzooplankton.

Altered Herbivore Distribution Associated With Focal Disturbance

Natural disturbances historically created structurally diverse patterns across the landscape, and large herbivores concentrated herbivory in areas where disturbance decreased standing senesced biomass that acted as a grazing deterrent through decreased palatability and overall forage quality. However, following European settlement, many natural large-scale disturbance regimes that influence vegetation and herbivore grazing selection were altered or removed and replaced with fine-scale anthropogenic disturbances. It is unclear how fine-scale focal disturbance and alteration of vegetation structure influences livestock distribution and grazing. Therefore we used a tracked vehicle as a disturbance agent in a mesic mixed-grass prairie to assess the influence of focal anthropogenic disturbance on livestock distribution and grazing. Track vehicle disturbance decreased the height of vegetation (P < 0.05) but did not alter plant species composition (P > 0.05). Cattle fecal pat density was greater (P ≤ 0.05) in locations with track vehicle disturbance. Little bluestem tiller height was shorter (P ≤ 0.05) in tracked locations than nontracked locations in grazed treatments, but was not different in nongrazed locations the first growing season following disturbance. Fecal pat density and tiller height were not different (P > 0.05) between tracked and nontracked locations following the second growing season. Therefore, we concluded that fine-scale focal anthropogenic disturbance alters herbivore distribution and defoliation and can maintain structural heterogeneity, but the effect is ephemeral and does not create long-lasting grazing lawns. Los disturbios naturales crearon históricamente patrones de diversidad estructural en el paisaje, y los grandes herbívoros concentraron herbivoría en áreas en las que los disturbios provocaron disminución de la biomasa senescente que actuaba como factor de antiherbivoria a través de la baja palatabilidad y valor nutritivo. No obstante, luego de la colonización Europea, muchos de los regímenes naturales de disturbio de gran escala que ejercen una influencia sobre la vegetación y la selección de pastoreo por parte de los herbívoros fueron alterados o removidos y reemplazados por disturbios antropogénicos de escala fina. No está claro como los disturbios focales de escala fina y la alteración de la estructura de la vegetación alteran la distribución del pastoreo del ganado. De modo que utilizamos un vehículo con orugas como agente de disturbio en una pradera mésica mixta para evaluar la influencia de disturbios focales antropogénicos sobre la distribución y el pastoreo del ganado. El disturbio generado con el vehículo con orugas disminuyó la altura de la vegetación (P < 0.05), pero no alteró la composición de especies (P > 0.05). La densidad de fecas bovinas fue mayor (P ≤ 0.05) en lugares disturbados por los vehículos con oruga. La altura de los macollos de Schizachyrium scoparium (Michx.) Nash disminuyó (P ≤ 0.05) en lugares afectados por los disturbios comparado con lugares que no fueron afectados por los vehículos con oruga en tratamientos pastoreados, pero dichas alturas no fueron diferentes en lugares no pastoreados una temporada después del disturbio. No hubo diferencias en la densidad de fecas y la altura de macollos (P > 0.05) entre lugares disturbados y no disturbados luego de la segunda temporada de crecimiento después de la aplicación del disturbio. De modo que concluimos que los disturbios focales antropogénicos de escala fina altera la distribución de herbívoros y que la defoliación puede mantener la diversidad estructural, pero el efecto es efímero y no genera céspedes de pastoreo de largo plazo.

Spatial distributions of multiple plant species affect herbivore foraging selectivity

Spatial distribution of food resources is an important factor determining herbivore foraging. Previous studies have demonstrated that clumped distribution of preferred species increases its consumption by herbivores in single‐ or two‐species systems. However, the potential impact of distribution pattern of less preferred species on foraging was ignored. In natural grasslands with high species diversity and complexity, the spatial distribution of preferred species impacts on herbivore foraging may be strongly correlated with the distribution of less preferred species.Our aims were to determine the effect of distribution of both preferred and other plant species on herbivore foraging under conditions close to a native, multi‐species foraging environment, and conceptualize the relationships between spatial distribution of food resources and herbivore consumption. We hypothesized that random distribution of non‐preferred species reduces herbivore consumption of preferred species because the dispersion of less preferred species likely disturbs herbivore foraging. We conducted an experiment using three species with five combinations of clumped and random distribution patterns. Three species Lathyrus quinquenervius, Phragmites australis and Leymus chinensis, were of high, intermediate and low preferences by sheep, respectively. Results showed that distribution of low preferred species, but not that of high preferred one, affected the consumption of preferred species. Sheep obtained higher consumption of high preferred species when low preferred species followed a clumped distribution than a random distribution. Distance between aggregations of high and low preferred species did not affect sheep foraging. It was concluded that the effects of spatial distribution of preferred species on its consumption are dependent on herbivore foraging strategy, and sheep can consume more preferred species when there is a consistent spatial pattern between preferred species and the entire food resource, and that the random dispersion of low preferred species in grassland may reduce herbivore consumption of high preferred species, thus minimizing selective grazing.

Do grazer hair and faeces reflect the carbon isotope composition of semi-arid C3/C4 grassland?

The carbon isotope composition (δ13C) of C3/C4 mixed grassland is reflected in the δ13C of diet, hair or faeces of grazers, if 13C discrimination (13Δ) between grassland vegetation and these tissues is known and constant. However, these relationships could be modified by selective grazing or differential digestibility of the C3 and C4 components, potentially creating a bias between grassland and grazer tissue δ13C. Importantly, these factors have never been studied in detail. We investigated the relation between δ13C of C3/C4 grassland vegetation and that of faeces and hair of sheep in a 3-year (2005–2007) experiment in the Inner Mongolian semi-arid steppe. The experiment employed six stocking rates (0.375–2.25 sheep ha−1 year−1; four replications), which allowed for a large variation in species composition, digestibility, and diet selection. Faecal-nitrogen content, a proxy for digestibility, decreased from 1.9% to 1.5% during the grazing period due to aging of the herbage. At the same time, the C3/C4 ratio decreased due to the later growth initiation of C4 species. 13Δ between diet and faeces (13ΔDF; 0.6‰) and between diet and hair (13ΔDH; −3.9‰) were not influenced by stocking rate, period in the season or C3/C4 ratio. Moreover, faeces–hair discrimination (13ΔFH; −4.3‰), which reflects differences between digestibility of the C3 and C4 components, did not vary along the different gradients. The δ13C of grassland vegetation can be estimated from the δ13C of sheep faeces and hair, provided that 13Δ was accounted for. This is useful for landscape- or regional-scale investigations or reconstruction of C3/C4 vegetation distribution from faeces and hair, which provide different temporal and spatial integration of grassland isotope signals. Die Kohlenstoff-Isotopenzusammensetzung (δ13C) von C3/C4 Grasland widerspiegelt sich im δ13C des Futters, der Haare sowie des Kots von Weidetieren, sofern die 13C-Diskriminierung (13Δ) zwischen der Vegetation und dem jeweiligen Tierparameter bekannt und konstant ist. Unterschiedliche Verdaulichkeiten der C3 und C4 Komponenten, oder selektiver Fraß können jedoch 13Δ verändern und somit zu Fehlern bei der Schätzung des δ13C von Grasland führen. Diese Zusammenhänge wurden bisher noch nicht systematisch untersucht. In der vorliegenden Studie untersuchten wir deshalb die Beziehung zwischen dem δ13C von gemischtem C3/C4 Grasland und dem Kot und den Haaren von Schafen. Die Studie wurde in einem mehrjährigen Beweidungsexperiment (2005–2007) mit sechs verschiedenen Besatzdichten (0.375 bis 2.25 Schafe pro ha, vier Wiederholungen) in der semi-ariden Steppe der Inneren Mongolei durchgeführt. Dieses Versuchsdesign ermöglichte eine große Variation der Artenzusammensetzung, der Verdaulichkeit und der Futterselektion. Der Kotstickstoffgehalt (ein Maß für die Verdaulichkeit) sank während der Weideperiode von 1.9% auf 1.5%. Dieser Effekt beruhte auf der Alterung des Pflanzenbestandes. Gleichzeitig nahm das C3/C4-Verhältnis durch den späteren Austrieb der C4 Arten ab. Die 13Δ zwischen Futter und Kot (13ΔDF; 0.6‰) und zwischen Futter und Haar (13ΔDH; −3.9‰) war unabhängig von der Besatzdichte, dem Zeitpunkt innerhalb der Weideperiode und dem C3/C4-Verhältnis. Des Weiteren wurde die Kot-Haar Diskriminierung (13ΔFH; −4.3‰) – ein Maß für Verdaulichkeitsunterschiede zwischen C3 und C4 Arten – ebenfalls nicht von den verschiedenen Gradienten beeinflusst. Diese Untersuchungen zeigen, dass das δ13C von C3/C4 Grasland in der Inneren Mongolei aus dem δ13C von Kot oder Haaren bestimmt werden kann, sofern die (konstante) 13Δ berücksichtigt wird. Diese Ergebnisse sind besonders für lokale und regionale Untersuchungen, bzw. die Rekonstruktionen des C3/C4-Verhältnisses nützlich, da Kot und Haare Isotopensignale über (unterschiedlich lange) räumlich-zeitliche Skalen integrieren.

Diagnostic Challenge

The fire-grazing interaction is well studied in mesic grasslands worldwide, but research is limited in semiarid systems. We examined the principal drivers and feedbacks of the fire-grazing interaction on the strength of cattle grazing selection, herbaceous biomass, crude protein, and vegetation structure and composition in two pastures in the Northern Great Plains. Cattle showed significant preference, use, and grazing utilization in recently burned patches that declined as time since fire increased. Cattle selection was driven by significantly increased crude protein in recent burns. Grazing utilization of 70% in patches with < 1 yr after fire established low herbaceous biomass, but the extent to which it was maintained varied with precipitation. Herbaceous biomass increased to nonburned levels 2 yr after fire, and crude protein decreased to nonburned levels 120 d after fire. Species composition was influenced primarily by site and year, though bare ground and litter were influenced by the fire-grazing interaction. Our data indicate that mixed-grass prairies of the Northern Great Plains are resilient to the fire-grazing interaction and that rest from grazing following fire is likely ecologically unnecessary. The use of the fire-grazing interaction is an alternative management strategy suitable for the Northern Great Plains, effectively increasing heterogeneity of grassland habitat.

Phytoplankton community structure and stocks in the Southern Ocean (30–80°E) determined by CHEMTAX analysis of HPLC pigment signatures

Abstract The geographic distribution, stocks and vertical profiles of phytoplankton of the seasonal ice zone off east Antarctica were determined during the 2005–2006 austral summer during the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) survey. CHEMTAX analysis of HPLC pigment samples, coupled with microscopy, permitted a detailed survey along seven transects covering an extensive area between 30°E and 80°E, from 62°S to the coast. Significant differences were found in the composition and stocks of populations separated by the Southern Boundary of the Antarctic Circumpolar Current (SB), as well as a small influence of the Weddell Gyre in the western sector of the ‘zone south of the Antarctic Circumpolar Current’ (SACCZ). Within the SACCZ, we identified a primary bloom under the ice, a secondary bloom near the ice edge, and an open-ocean deep population. The similarity of distribution patterns across all transects allowed us to generalise a hypothesized sequence for the season. The primary phytoplankton bloom, with stocks of Chl a up to 239 mg m−2, occurred about 35 days before complete disappearance of the sea ice, and contained both cells from the water column and those released from melting ice. These blooms were composed of haptophytes (in particular, colonies and gametes of Phaeocystis antarctica), diatoms and cryptophytes (or the cryptophyte symbiont-containing ciliate Myrionecta rubrum). Aggregates released by melting ice quickly sank from the upper water column and Chl a stocks declined to 56–92 mg m−2, but the bloom of diatoms and, to a lesser extent, cryptophytes continued until about 20 days after ice melt. The disappearance of sea ice coincided with a sharp increase in P. antarctica and grazing, as indicated by increasing phaeophytin a and phaeophorbide a. Chlorophyllide content suggests that the diatom bloom then senesced, probably due to iron exhaustion. Stocks rapidly declined, apparently due to grazing krill that moved southward following the retreating sea ice. We suggest that grazing of the bloom and export of faecal pellets stripped the upper water column of iron (as suggested by low Fv/Fm ratios and CHEMTAX pigment ratios in Haptophytes – iron was not measured). Thus, export of iron by grazing, and possibly sedimentation, created a southward migrating iron gradient, limiting growth in the upper water column. North of the postulated iron gradient, a nanoflagellate community developed at depth, with Chl a stocks from 36–49 mg m−2. This community was probably based on regenerated production, sustained by residual and/or upwelling iron, as indicated by a close correspondence between distributions of Chl a and profiles of Fv/Fm. The community consisted of haptophytes (chiefly Phaeocystis gametes), dinoflagellates, prasinophytes, cryptophytes, and some small diatoms. Selective grazing by krill may have fashioned and maintained the community. North of the SB, Chl a ranged from 40–67 mg m−2 and was found predominantly in the mixed layer, but Fv/Fm ratios remained low, suggesting the community of P. antarctica and diatoms was iron-limited. These interpretations provide a cogent explanation for the composition and structure of late summer microbial populations in the marginal ice zone.

Snail herbivory on submerged macrophytes and nutrient release: Implications for macrophyte management

Radix swinhoei (H. Adams) is a freshwater snail commonly found in shallow regions of Lake Taihu. This research estimated, based on experiments, the consumption rates of R. swinhoei on three young submerged plants ( Vallisneria spiralis, Hydrilla verticillata and Potamogeton malaianus) and its rates of nutrient release. Results showed that the snails consumed V. spiralis at the highest rate (23.34 mg g −1 d −1), P. malaianus at a lower rate (11.97 mg g −1 d −1), and H. verticillata at the lowest rate (7.04 mg g −1 d −1). The consumption rates on V. spiralis varied significantly, with snail size, ranging from 13.63 mg g −1 d −1 for large-size snails to 143.42 mg g −1 d −1 for small-size ones. The average nutrient release rates of snails grazing on different macrophytes were 45.93 μg PO 4-P and 0.58 mg NH 4-N g −1 d −1. The food species had a significant effect on NH 4-N release rates but not on PO 4-P. However, the snail size had a significant effect on PO 4-P release rates and not on NH 4-N. The present study indicates that through selective grazing and nutrient release, snails may impose a significant impact on the macrophyte community, which should be considered in managing the macrophytes of a lake.

Nutrient versus herbivore control of macroalgal community development and coral growth on a Caribbean reef

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 389:71-84 (2009) - DOI: https://doi.org/10.3354/meps08142 Nutrient versus herbivore control of macroalgal community development and coral growth on a Caribbean reef Deron E. Burkepile1,2,*, Mark E. Hay1 1Georgia Institute of Technology, School of Biology, 310 Ferst Drive, Atlanta, Georgia 30332, USA 2Present address: Marine Sciences Program, Department of Biological Sciences, Florida International University, 3000 NE 151st Street, North Miami, Florida 33181, USA *Email: deron.burkepile@fiu.edu ABSTRACT: Coral reefs are in global decline, with seaweeds replacing corals as spatial dominants. Overfishing of herbivores, anthropogenic eutrophication, and interactions between these factors have been postulated as causes, but long-term tests of these factors are uncommon. We factorially manipulated herbivorous fishes and nutrients at a depth of 16 to 18 m for 7 to 10 mo in 2 experiments over 2 yr. Herbivore exclusion increased algal cover by about 4 to 10×, algal biomass by 4 to 6×, and suppressed cover of crustose coralline algae by 80 to 100%. Nutrient enrichment had no effect on the cover or mass of upright algae, but altered species composition by suppressing cyanobacteria and facilitating red macroalgae in the absence of herbivores. Nutrient addition increased macroalgal species richness in the absence, but not the presence of herbivores. Feeding by herbivorous fishes increased by 3 to 13× on nutrient-enriched vs. control plots. This herbivory facilitated cover of crustose coralline algae, demonstrated that herbivores selectively target more nutritional prey, and suggested that herbivores could suppress macrophyte accumulation at sites with increased nutrient availability. Effects of fishes and nutrients on corals varied as a function of coral species. For the branching coral Porites porites, 56% of individuals exposed to fishes were completely consumed; however, individuals that survived grew 60 to 80% more in the presence of fishes. For Porites astreoides, exposure to fishes did not affect mortality, but increased net growth by 3 to 4 times. For this coral, nutrient addition decreased growth when exposed to fishes but not when protected from fishes, suggesting that fishes may have fed more on nutrient-enriched corals. KEY WORDS: Eutrophication · Overfishing · Parrotfish · Selective grazing · Surgeonfish · Plant–herbivore interactions Full text in pdf format PreviousNextCite this article as: Burkepile DE, Hay ME (2009) Nutrient versus herbivore control of macroalgal community development and coral growth on a Caribbean reef. Mar Ecol Prog Ser 389:71-84. https://doi.org/10.3354/meps08142Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 389. Online publication date: September 04, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Growth Phase and Elemental Stoichiometry of Bacterial Prey Influences Ciliate Grazing Selectivity

Protozoa are known to selectively graze bacteria and can differentiate prey based on size and viability, but less is known about the effects of prey cellular composition on predator selectivity. We measured the effect of growth phase and elemental stoichiometry of Escherichia coli on grazing by two ciliates, Euplotes vannus and Cyclidium glaucoma. Bacterial cells of a single strain were transformed with green and red fluorescent protein and harvested from culture at differing growth stages. Cells in exponential growth phase had low carbon:phosphorus (39) and nitrogen:phosphorus (9) ratios, while cells from stationary phase had high carbon:phosphorus of 104 and nitrogen:phosphorus of 26. When offered an equal mixture of both types of bacteria, Cyclidium grazed stationary phase, high carbon:phosphorus, high nitrogen:phosphorus cells to 22% of initial abundance within 135 min, while Euplotes reduced these cells to 33%. Neither ciliate species decreased the abundance of the exponential phase cells, lower carbon:phosphorus and nitrogen:phosphorus, relative to control treatments. Because protozoa have higher nitrogen:phosphorus and carbon:phosphorus ratios than their prokaryotic prey, this study raises the possibility that it may be advantageous for protozoa to preferentially consume more slowly growing bacteria.

Resource heterogeneity and foraging behaviour of cattle across spatial scales

BackgroundUnderstanding the mechanisms that influence grazing selectivity in patchy environments is vital to promote sustainable production and conservation of cultivated and natural grasslands. To better understand how patch size and spatial dynamics influence selectivity in cattle, we examined grazing selectivity under 9 different treatments by offering alfalfa and fescue in patches of 3 sizes spaced with 1, 4, and 8 m between patches along an alley. We hypothesized that (1) selectivity is driven by preference for the forage species that maximizes forage intake over feeding scales ranging from single bites to patches along grazing paths, (2) that increasing patch size enhances selectivity for the preferred species, and that (3) increasing distances between patches restricts selectivity because of the aggregation of scale-specific behaviours across foraging scales.ResultsCows preferred and selected alfalfa, the species that yielded greater short-term intake rates (P < 0.0001) and greater daily intake potential. Selectivity was not affected by patch arrangement, but it was scale dependent. Selectivity tended to emerge at the scale of feeding stations and became strongly significant at the bite scale, because of differences in bite mass between plant species. Greater distance between patches resulted in longer patch residence time and faster speed of travel but lower overall intake rate, consistent with maximization of intake rate. Larger patches resulted in greater residence time and higher intake rate.ConclusionWe conclude that patch size and spacing affect components of intake rate and, to a lesser extent, the selectivity of livestock at lower hierarchies of the grazing process, particularly by enticing livestock to make more even use of the available species as patches are spaced further apart. Thus, modifications in the spatial pattern of plant patches along with reductions in the temporal and spatial allocation of grazing may offer opportunities to improve uniformity of grazing by livestock and help sustain biodiversity and stability of plant communities.

Biogeochemical responses to late-winter storms in the Sargasso Sea. IV. Rapid succession of major phytoplankton groups

In this paper, we present multi-parameter data on phytoplankton community composition, and its response to storm events in the Sargasso Sea in late February and early March of 2 years (2004 and 2005). Observed physical conditions spanned a continuum from pulsed destratification/stratification to continuous mixing, with a corresponding range of phytoplankton growth responses. The pulsed destratification/stratification condition resulted in a rapid (1–2 d) doubling of euphotic zone chlorophyll (Chl-a) along with a rapid succession, days timescale, from diatoms to haptophytes and then to cyanobacteria. Deep (>300 m) continuous mixing led to a slow (8–9 d) doubling of autotrophic biomass with no observed succession in the phytoplankton community. These different temporal responses appear to be due to differences between nutrient-limited and light-limited phytoplankton growth, although differences in grazing rates or selective grazing cannot be ruled out. Unexpectedly, we found that flow cytometrically enumerated picoeukaryotes were not accounted for in HPLC-pigment derived phytoplankton classifications and did not covary with any of the pigments quantified. Yet, the picoeukaryotes were positively related to increases in total Chl-a and increased carbon export, suggesting an important but as yet unknown role in the Sargasso Sea carbon cycle.

Effects of nutrient and insecticide treatments on invertebrate numbers and predation on slugs in an upland grassland: A monoclonal antibody-based approach

Slugs can have profound effects upon plant community structure and, by selective grazing, act as ecosystem engineers. Experimental treatments on upland grasslands that included nutrient addition and use of an insecticide were shown to have significant effects on invertebrate communities, estimated by pitfall traps and Tullgren funnel extractions, including potential slug predators. Nutrient enhancement (nitrogen and lime) significantly increased the depth of soil surface thatch, but this did not affect slug numbers or biomass. However, nutrient enhancement did increase numbers of some slug predators. Invertebrates that fed on slugs in the vicinity were first identified by screening a wide range of potential predators captured by pitfall trapping and hand searching. We then tested the invertebrates for slug remains in their guts using a mollusc-specific monoclonal antibody and enzyme-linked immunosorbent assays. The primary slug predators (those that tested positive for slug antigen) were shown to be species of carabid and staphylinid beetle, some of which had not been recorded to eat slugs before. Other slug predators included Silphidae, Araneae and Opiliones. When time was included as a factor, there was no relationship between slug predator numbers and slug numbers, nor were numbers of other invertebrates a significant co-variable. It was predicted that slug numbers should be greatest where there was more surface organic matter (nutrient enhancement treatment), and lowest where slug predators were reduced (insecticide treatment). The lack of any overall effect of treatment on slug numbers possibly suggests that the greater numbers of potential slug predators in the nutrient enhancement treatment suppressed any increase in the slug population, emphasizing the care needed in interpreting such field data.

Developing decision support tools for rangeland management by combining state and transition models and Bayesian belief networks

State and transition models provide a simple and versatile way of describing vegetation dynamics in rangelands. However, state and transition models are traditionally descriptive, which has limited their practical application to rangeland management decision support. This paper demonstrates an approach to rangeland management decision support that combines a state and transition model with a Bayesian belief network to provide a relatively simple and updatable rangeland dynamics model that can accommodate uncertainty and be used for scenario, diagnostic, and sensitivity analysis. A state and transition model, developed by the authors for subtropical grassland in south-east Queensland, Australia, is used as an example. From the state and transition model, an influence diagram was built to show the possible transitions among states and the factors influencing each transition. The influence diagram was populated with probabilities to produce a predictive model in the form of a Bayesian belief network. The behaviour of the model was tested using scenario and sensitivity analysis, revealing that selective grazing, grazing pressure, and soil nutrition were believed to influence most transitions, while fire frequency and the frequency of good wet seasons were also important in some transitions. Overall, the integration of a state and transition model with a Bayesian belief network provided a useful way to utilise the knowledge embedded in a state and transition model for predictive purposes. Using a Bayesian belief network in the modelling approach allowed uncertainty and variability to be explicitly accommodated in the modelling process, and expert knowledge to be utilised in model development. The methods used also supported learning from monitoring data, thereby supporting adaptive rangeland management.

Selective feeding behaviour of key free-living protists: avenues for continued study

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 53:83-98 (2008) - DOI: https://doi.org/10.3354/ame01229 AME Special: 'Progress and Perspectives in Aquatic Microbial Ecology: Highlights of the SAME 10, Faro, Portugal, September 2007' Selective feeding behaviour of key free-living protists: avenues for continued study David J. S. Montagnes1,*, Ana B. Barbosa2, Jens Boenigk3, Keith Davidson4, Klaus Jürgens5, Miroslav Macek6,9, Jacqueline D. Parry7, Emily C. Roberts8, Karel Simek9 1School of Biological Sciences, Biosciences Building, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK 2CIMA, Center for Marine and Environmental Research, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal 3Institute for Limnology, Austrian Academy of Sciences, Mondseestr. 9, 5310 Mondsee, Austria 4Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll PA37 1QA, UK 5Leibniz Institute for Baltic Sea Research, Department of Biological Oceanography, Seest. 15, 18119 Rostock-Warnemünde, Germany 6National Autonomous University of Mexico, FES Iztacala, Department of Tropical Limnology, Av. de los Barrios 1, 54090 Tlalnepantla, Edo. México, Mexico 7Division of Biomedical and Life Sciences, School of Health and Medicine, Lancaster University, Lancaster LA1 4YQ, UK 8Biological Sciences, Swansea University, Singleton Park, Swansea SA2 8PP, UK 9Biology Centre of the Academy of Sciences of the Czech Republic, v.v.i., Institute of Hydrobiology, Na Sádkách 7, 37005 Ceské Budéjovice, Czech Republic *Email: dmontag@liv.ac.uk ABSTRACT: Phagotrophic protists are diverse and abundant in aquatic and terrestrial environments, making them fundamental to the transfer of matter/energy within their respective food webs. Recognising their grazing impact is essential to evaluate the role of protists in ecosystems, and this includes appreciating prey selectivity. Efforts have been made by groups and individuals to understand selective grazing behaviour by protists: many approaches and perspectives have been pursued, not all of which are compatible. This article, which is not a review, is the product of our discourse on this subject at the SAME 10 meeting. It is the work of individuals, assembled for their breadth of backgrounds, approaches, views, and expertise. Firstly, to communicate ideas and approaches, we develop a framework for selective feeding processes and suggest 6 steps: searching, contact, capture, processing, ingestion, digestion. We then separate study approaches into 2 categories: (1) those examining whole organisms at the community, population, and individual levels, and (2) those examining physiology and molecular attributes. Finally, we explore general problems associated with the field of protistan selective feeding (e.g. linking food selection into food webs and modeling). We do not present all views on any one topic, nor do we cover all topics; instead, we offer opinions and suggest avenues for continued study. Overall, this paper should stimulate further discourse on the subject and provide a roadmap for the future. KEY WORDS: Amoeba · Ciliate · Flagellate · Grazing · Ingestion · Phagotrophic · Protozoa Full text in pdf format PreviousNextCite this article as: Montagnes DJS, Barbosa AB, Boenigk J, Davidson K and others (2008) Selective feeding behaviour of key free-living protists: avenues for continued study. Aquat Microb Ecol 53:83-98. https://doi.org/10.3354/ame01229Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 53, No. 1. Online publication date: September 18, 2008 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2008 Inter-Research.