Grazer Treatments Research Articles

Evaluating Riparian Plant Communities After Restoration of Plains Bison in the Northern Great Plains of Montana

Restoration of plains bison (Bison bison bison) in the northern Great Plains has been controversial for a variety of reasons, including the public concern that bison will supplant cattle on public rangelands. Riparian zones are among the most biologically rich areas within rangelands, but they are highly sensitive to disturbances such as grazing, leading to the public perception that riparian vegetation communities will be negatively impacted by year-round bison grazing compared with the norm of seasonal cattle grazing. Our objective was to evaluate the vegetation community and soil compaction conditions of areas in the northern Great Plains of Montana, where bison have been restored 5−10 yr as year-round grazers, compared with adjacent sites where cattle have been retained as seasonal grazers. Out of the 24 variables we assessed, only 2 significantly differed between the grazer treatments within our study's time frame and stocking rates. Native species diversity was significantly higher in the bison-grazed treatment (F[1,1,5.98] = 8.16, P = 0.03), and woody height heterogeneity was twice as high in the bison-grazed treatment than the cattle-grazed treatment (F[1,5.97]= 5.81, P = 0.05). Although longer-term studies are required, our findings indicate that year-round bison grazing under the current stocking rates did not degrade riparian vegetation communities compared with seasonal cattle grazing and rather benefitted some aspects of the plant community.

Trait changes induced by species interactions in two phenotypically distinct strains of a marine dinoflagellate.

Populations of the toxigenic marine dinoflagellate Alexandrium are composed of multiple genotypes that display phenotypic variation for traits known to influence top-down processes, such as the ability to lyse co-occurring competitors and prospective grazers. We performed a detailed molecular analysis of species interactions to determine how different genotypes perceive and respond to other species. In a controlled laboratory culture study, we exposed two A. fundyense strains that differ in their capacity to produce lytic compounds to the dinoflagellate grazer Polykrikos kofoidii, and analyzed transcriptomic changes during this interaction. Approximately 5% of all analyzed genes were differentially expressed between the two Alexandrium strains under control conditions (without grazer presence) with fold-change differences that were proportionally higher than those observed in grazer treatments. Species interactions led to the genotype-specific expression of genes involved in endocytotic processes, cell cycle control and outer membrane properties, and signal transduction and gene expression regulatory processes followed similar patterns for both genotypes. The genotype-specific trait changes observed in this study exemplify the complex responses to chemically mediated species interactions within the plankton and their regulation at the gene level.

Grazers improve health of coral juveniles in ex situ mariculture

Scleractinian coral mariculture plays an increasingly important role in reef rehabilitation and the aquarium trade, but its efficiency can be impeded by fouling macroalgal blooms, thus compromising coral health and increasing maintenance costs. In this study, the sea urchin Salmacis sphaeroides and the gastropod Trochus maculatus were introduced into ex situ coral mariculture tanks as biological controls of fouling macroalgae. We examined the effects of both grazers on the overall health of Pocillopora damicornis juveniles by quantifying the changes in growth, colour saturation and extent of tissue lesion of the corals for 24weeks. The juvenile corals exposed to S. sphaeroides and T. maculatus grew seven to eleven times faster than the controls, and attained final ecological volumes of 14104±2955mm3 (25.1±0.9mm diameter), 11913±2644mm3 (24±0.7mm diameter) and 1924±620mm3 (12.2±1mm diameter) respectively. The juvenile corals in the grazer treatments were also significantly darker and showed fewer lesions than that in the control. In the tanks with (S. sphaeroides, T. maculatus) and without (control) grazers, the mean algal cover was 37.5%, 57.4% and 78.8%, respectively, and the final algal biomass was 13.9±3.9mg, 27.3±8.1mg and 71.9±7mg, respectively. Taken together, the results showed that S. sphaeroides and T. maculatus are effective biological controls of fouling macroalgae in ex situ coral culture.

Current velocity influences the facilitation and removal of algae by stream grazers

The modification of flows in lotic ecosystems can have dramatic effects on abiotic and biotic processes and change the structure of basal trophic levels. In high-gradient streams, most of the biota are benthic, and decreased flow may homogenize and reduce benthic current velocity, potentially changing stream ecosystem function. Grazing by macroinvertebrates is an important component of stream function because grazers regulate energy flow from primary producers to higher trophic levels. We conducted an experiment to examine how macroinvertebrate grazers facilitated or removed algal biomass across a gradient of benthic current velocity (0–40 cm s−1). We chose three grazers (Drunella coloradensis, Cinygmula spp., and Epeorus deceptivus) from a montane stream and conducted our experiment using 24 artificial stream channels that had three treatments: no grazers (control), single-grazer, and combined-grazer treatments. In the absence of grazers, algal biomass increased with benthic current velocity. Grazer treatments differed from the control in that more algal biomass was removed at higher velocities, whereas algal accrual was largely facilitated at low velocities. The transition from facilitation to removal ranged from 4.5 to 5.9 cm s−1 for individual grazer treatments and occurred at 11.7 cm s−1 for the combined-grazer treatment. Our data suggest that velocity plays a significant role in the facilitation and removal of algae by macroinvertebrate grazers. Additionally, the patterns revealed here could have general implications for algal accrual in systems where flow is reduced.

Influence of Acartia cf. bifilosa (Copepoda) on morphology and toxicity of Nodularia spumigena (Cyanophyceae)

Nodularia spumigena was exposed directly and indirectly (grazer cages) to increasing densities of Acartia cf. bifilosa to investigate if the presence of copepods influenced the morphology and/or the toxicity of the cyanobacterium. Monocultures with only N. spumigena and mixed cultures, containing N. spumigena and the non-toxic Dunaliella tertiolecta, were included in each experiment. Following 6 days of incubation, the morphology and toxicity in grazer treatments were compared with grazer-free controls. We observed no effects of A. cf. bifilosa on either morphology or toxicity of N. spumigena. The lack of grazer induced nodularin production and morphological alterations suggest that these two potential defense strategies either has evolved as constitutive defenses or never evolved as grazer defenses. The mortality of copepods was higher in the monoculture than in the mixed culture treatments. Gut content observations indicated a low level of grazing in monoculture treatments and a higher level of grazing in mixed culture treatments. This higher level of grazing most likely occurred on the alternative food D. tertiolecta. Given the indications of low grazing and the concentrations of dissolved nodularin observed, we postulate that the higher mortality was not related to toxic effects, but to starvation. This in turn may have resulted from bad taste, production of unknown grazer deterrents or morphological constraints; although the size of the filaments would not have imposed an absolute limit for ingestion by A. cf. bifilosa. The higher copepod mortality observed on monocultures of N. spumigena may contribute to the success and maintenance of N. spumigena blooms.

Grazer diversity affects resistance to multiple stressors in an experimental seagrass ecosystem

When multiple stressors act simultaneously, their effects on ecosystems become more difficult to predict. In the face of multiple stressors, diverse ecosystems may be more stable if species respond differently to stressors or if functionally similar species can compensate for stressor effects on focal species. Many habitats around the globe are threatened by multiple stressors, including highly productive seagrass habitats. For example, in Chesapeake Bay, USA, regional climate change predictions suggest that elevated temperature and freshwater inputs are likely to be increasingly important stressors. Using seagrass mesocosms as a model system, we tested whether species richness of crustacean grazers buffers ecosystem properties against the impacts of elevated temperature and freshwater pulse stressors in a fully factorial experiment. Grazer species responded to pulsed salinity changes differently; abundance of Elasmopus levis responded negatively to freshwater pulses, whereas abundance of Gammarus mucronatus and Erichsonella attenuata responded positively or neutrally. Consistent with the hypothesis that biodiversity provides resistance stability, biomass of epiphytic algae that form the base of the food web was less affected by stressors in species-rich grazer treatments than in single-species grazer treatments. Stochastic (among-replicate) variation of sessile invertebrate biomass within treatments was also reduced in more diverse grazer treatments. Therefore, grazer species richness tended to increase the resistance stability of both major components of the seagrass fouling community, algae and invertebrates, in the face of environmental stressors. Finally, in our model system, multi-stressor impacts suggested a pattern of antagonism contrary to previous assumptions of synergistic stressor effects. Overall, our results confirm that invertebrate grazer species are functionally diverse in their response to environmental stressors, but are largely functionally redundant in their grazing effects leading to greater resistance stability of certain ecosystem properties in diverse grazer assemblages even when influenced by multiple environmental stressors.

Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows

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 406:33-45 (2010) - DOI: https://doi.org/10.3354/meps08533 Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows Lesley P. Baggett1,2,*, Kenneth L. Heck Jr.1,2, Thomas A. Frankovich3, Anna R. Armitage3,4, James W. Fourqurean3 1Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA 2Department of Marine Science, University of South Alabama, Mobile, Alabama 36688, USA 3Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International University, 3000 NE 151st Street, North Miami, Florida 33181, USA 4Department of Marine Biology, Texas A&M University at Galveston, PO Box 1675, Galveston, Texas 77553, USA *Email: lbaggett@disl.org ABSTRACT: We tested the relative importance of top-down and bottom-up effects by experimentally evaluating the combined and separate effects of nutrient availability and grazer species composition on epiphyte communities and seagrass condition in Florida Bay. Although we succeeded in substantially enriching our experimental cylinders, as indicated by elevated nitrogen concentrations in epiphytes and seagrass leaves, we did not observe any major increases in epiphyte biomass or major loss of Thalassia testudinum by algal overgrowth. Additionally, we did not detect any strong grazer effects and found very few significant nutrient-grazer interactions. While this might suggest that there was no important differential response to nutrients by individual grazer species or by various combinations of grazers, our results were complicated by the lack of significant differences between control and grazer treatments, and as such, these results are best explained by the presence of unwanted amphipod grazers (mean = 471 ind. m–2) in the control cylinders. Our estimates of grazing rates and epiphyte productivities indicate that amphipods in the control cylinders could have lowered epiphyte biomass to the same level that the experimental grazers did, thus effectively transforming the control treatments into grazer treatments. If so, our experiments suggest that the effects of invertebrate grazing (and those of amphipods alone) were stronger than the effects of nutrient enrichment on epiphytic algae, and that it does not require a large density of grazers to control epiphyte biomass even when nutrient loading rates are substantially elevated. KEY WORDS: Nutrient enrichment · Grazing · Seagrass · Epiphytes · Gastropods · Caridean shrimp · Hermit crab Full text in pdf format PreviousNextCite this article as: Baggett LP, Heck KL Jr, Frankovich TA, Armitage AR, Fourqurean JW (2010) Nutrient enrichment, grazer identity, and their effects on epiphytic algal assemblages: field experiments in subtropical turtlegrass Thalassia testudinum meadows. Mar Ecol Prog Ser 406:33-45. https://doi.org/10.3354/meps08533 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 406. Online publication date: May 10, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Impact of Arenicola marina (Polychaeta) on the microbial assemblages and meiobenthos in a marine intertidal flat

The benthic microbial food web can be responsible for a large proportion of benthic carbon cycling yet there are few data on the trophic interactions between this food web and macrobenthos. A large-scale field experiment was conducted to investigate effects of eliminating the polychaete Arenicola marina on benthic microbes (prokaryotes, heterotrophic and autotrophic protists) and metazoan meiofauna in a marine intertidal flat of the North Sea, Germany. Over a period of 2 years, quantity and composition of micro- and meiobenthos from unmanipulated sites were compared to those from sites deplete of lugworms. These grazer treatments were cross-classified with different sediment characteristics (low- and mid-intertidal areas). Lugworm removal resulted in an initial increase in abundance of prokaryotes and nanoflagellates, which became less pronounced in the second year. Ciliates were not affected quantitatively, but in the absence of lugworms, diversity and the proportion of carnivorous forms increased. Meiobenthos (nematodes, ostracods and copepods) were affected only moderately. The observed changes are probably due to a combination of release from grazing/predation pressure, changes in the species composition of higher trophic levels (namely large polychaetes) and altered environmental conditions (such as depth of the oxygenated layer and sediment grain size). Spatial differences between sites of different tidal exposure/grain size appeared to be as large as temporal differences during the 2 years following the manipulation of the system. We conclude that in intertidal sediments, indirect effects due to habitat transformation are as important as direct biological interactions (grazing pressure and competition) for the dynamics of the benthic microbial food web.

Spatial heterogeneity increases the importance of species richness for an ecosystem process

The role of biodiversity in mediating ecosystem processes has been the subject of focused theoretical and empirical attention since the mid‐1990s. Theory predicts that the balance between species richness and identity effects will critically depend on the degree of environmental heterogeneity, which dictates the extent to which differences between species in patterns of resource use can be expressed. We conducted a mesocosm experiment to explicitly test this hypothesis. We manipulated the richness and identity of intertidal molluscan grazers, as well as the spatial heterogeneity of the substrate upon which they grazed. The magnitude of algal consumption was used as our focal ecosystem process. The grazer treatments consisted of three monocultures and a single polyculture including all three species; heterogeneity was represented as the proportion of topographically complex and flat substrate. Species identity had strong effects on homogeneous substrates, with the identity of the best‐performing species dependent on the substrate. On the heterogeneous substrate, suitable conditions for all three species were represented, allowing the expression of spatial complementarity of resource use and the enhancement of total algal consumption. Our findings provide the first explicit experimental evidence that spatial heterogeneity of the physical environment can play a key role in mediating effects of species diversity.

RELATIVE AND INTERACTIVE EFFECTS OF PLANT AND GRAZER RICHNESS IN A BENTHIC MARINE COMMUNITY

The interactive effects of changing biodiversity of consumers and their prey are poorly understood but are likely to be important under realistic scenarios of biodiversity loss and gain. We performed two factorial manipulations of macroalgal group (greens, reds, and browns) and herbivore species (amphipods, sea urchin, and fish) composition and richness in outdoor mesocosms simulating a subtidal, hard-substratum estuarine community in North Carolina, U.S.A. In the experiment where grazer richness treatments were substitutive, there were no significant effects of algal or herbivore richness on final algal biomass. However, in the experiment in which grazer treatments were additive (i.e., species-specific densities were held constant across richness treatments), we found strong independent and interactive effects of algal and herbivore richness. Herbivore polycultures reduced algal biomass to a greater degree than the sum of the three herbivore monocultures, indicating that the measured grazer richness effects were not due solely to increased herbivore density in the polycultures. Taking grazer density into account also revealed that increasing algal richness dampened grazer richness effects. Additionally, the effect of algal richness on algal biomass accumulation was far stronger when herbivores were absent, suggesting that grazers can utilize the increased productivity and mask the positive effects of plant biodiversity on primary production. Our results highlight the complex independent and interactive effects of biodiversity between adjacent trophic levels and emphasize the importance of performing biodiversity-ecosystem functioning experiments in a realistic multi-trophic context.

Top‐down and bottom‐up control in an eelgrass–epiphyte system

Nutrient supply and the presence of grazers can control primary producers in aquatic ecosystems, but the relative importance of bottom‐up and top‐down effects remains inconclusive. We conducted a mesocosm experiment and a field study to investigate the independent and interactive effects of nutrient enrichment and grazing on primary producers in an eelgrass bed Zostera marina. Nutrient treatments consisted of ambient or enriched (2× and 4× ambient) concentrations of inorganic nitrogen and phosphate. Grazer treatments consisted of presence or absence of field densities of the common isopod Idotea baltica. We found strong and interacting effects of nutrients and grazing on epiphytes. Epiphyte biomass and productivity were enhanced by nutrient enrichment and decreased in the presence of grazers. The absolute amount of epiphyte biomass consumed by grazers increased under high nutrient supply, and thus, nutrient effects were stronger in the absence of grazing. The effects of grazers and fertilisation on epiphyte composition were antagonistic: chain‐forming diatoms and filamentous algae profited from nutrient enrichment, but their proportions were reduced by grazing. Eelgrass growth was positively affected by grazing and by nutrient enrichment at moderate nutrient concentrations. High nutrient supply reduced eelgrass productivity compared to moderate nutrient conditions. The monthly measured field data showed a nitrogen limitation for epiphytes and eelgrass in summer, which may explain the positive effect of nutrient enrichment on both primary producers. Generally, the field data suggested the possibility of seasonally varying importance of bottom‐up and top‐down control on primary producers in this eelgrass system.

Differential response of stream periphyton and invertebrate grazers to habitat modification by the emergent macrophyte Justicia americana

An instream experiment was conducted using artificial substrate tiles during summer to examine the relative importance of shading by the emergent macrophyte Justicia americana and top-down control by stream grazers on summer periphyton accrual. Macrophyte treatments included removal of above-ground stems, removal of stems and rhizomes and an unmanipulated control, whereas grazer treatments included snail-accessible and snail-exclusion tiles. Above-ground Justicia structures reduced both sunlight to the stream bed and velocity by almost 50%. Abundance of the dominant snail, Elimia ucheensis (Pleuroceridae), on snail-exclusion tiles was significantly lower than on snail-accessible tiles only during the first week of the experiment; therefore, the barriers were ineffective over the entire experiment. Despite ineffective reductions of Elimia abundance over the entire experiment, periphyton accrual was higher on snail-exclusion than snail-accessible tiles irrespective of macrophyte treatment. Among the macrophyte treatments, periphyton biomass (as ash-free dry mass) was significantly lower on tiles in the unmanipulated control than treatments where Justicia stems were removed. Higher mean differences and relative magnitude of effects were associated with the macrophyte treatments when compared with the grazer treatments. Periphyton appeared to be primarily light-limited by Justicia and secondarily controlled by grazing invertebrates; however, the effects of grazing may have been underestimated because snail-exclusion barriers were ineffective at reducing grazer abundance over the experiment. Removal of Justicia canopy did not result in higher abundance of any invertebrate grazers on tiles; however, the abundance of the freshwater limpet (Ferrissia mcneili) was twice as high on tiles within the more shaded Justicia control sections compared with either of the two open-canopy treatments. Habitat modification by Justicia can negatively affect benthic primary producers and influence the distribution of some, but not all, primary consumers.

Sediment on rocky intertidal reefs: Effects on early post-settlement stages of habitat-forming seaweeds

Modification of the coastal environment by human activities often leads to an increase in sedimentation of nearshore waters, with potential impacts on benthic marine assemblages. Here we assess the relationships between the levels of sedimentation, wave exposure and benthic organisms on rocky intertidal platforms around the Kaikoura Peninsula in southern New Zealand. We designed and tested five sediment traps to provide a tool for measuring the relative abundance of sediment across sites. Using field- and laboratory-based experiments, we tested hypotheses concerning whether different levels of sedimentation affected algal germling survival and algal zygote attachment, and whether the interactions of grazers and sediments affected germling survival. Levels of sediment and exposure were inversely related across seven sites. The fucoid alga Hormosira banksii characterized the more sedimented wave-sheltered and intermediately wave-exposed sites, with up to 80% cover in the lower mid-tidal zone, while the bull kelp Durvillaea antarctica characterized the three most wave-exposed sites. Grazing molluscs were found across all sites but species abundances varied by sediment and exposure levels. We did two 11-day trials testing the effects of different levels of sediment and different species of molluscan grazers on the survival of 1-week-old Hormosira germlings. Generally, there was no significant treatment effect of grazers, but mortality varied considerably among sites. In particular, one site had very high levels of sediment, which resulted in 100% mortality of germlings across all grazer treatments. Removing sediment at 1-week, 2-week and 4-week intervals made no difference to the survival of Hormosira germlings. In laboratory-based experiments, a light dusting of sediment reduced the percentage of zygotes of Hormosira by 34% and Durvillaea by 71% that attached to primary substratum, and a complete cover of sediment prevented attachment altogether. Overall, the effects of sediments and its interaction with molluscan grazing were highly variable but often large, particularly on the attachment of zygotes to primary substratum.

Grazer species effects on epilithon nutrient composition

Summary1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory.2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon.3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment.4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.

Selectivity and competitive interactions between two benthic invertebrate grazers (Asellus aquaticus and Potamopyrgus antipodarum): an experimental study using 13C‐ and 15N‐labelled diatoms

Summary1. Tracer experiments with two diatoms labelled with 13C (Nitzschia palea) and 15N (Fragilaria crotonensis), were conducted to investigate feeding selectivity and interspecific competition between the grazers Asellus aquaticus (Isopoda, Crustacea) and Potamopyrgus antipodarum (Hydrobiidae, Gastropoda). Conventional methods, such as cell counts and estimated biovolume, were used first to detect feeding preferences within the different grazer treatments.2. The results revealed a significant decline in algal biovolume in all grazer treatments and no indications of active selectivity were observed. In contrast to conventional methods, measurements based on isotope signatures showed strong differences in tracer uptake, thus indicating different degrees of assimilation and digestion by the two grazers.3. The selectivity index Q, which provides information on the uptake ratio of 13C to 15N, showed a significant time effect for both grazer species and a significant difference between single‐ and mixed‐grazer treatments for P. antipodarum. Thus, this technique enabled the direct quantification of the uptake by grazers and, therefore, served as an ideal tool for the detection of passive selectivity.4. Our results indicate a shift in feeding preferences related to between‐species competition and a potential divergence of trophic niches when species coexist.

How do grazers affect periphyton heterogeneity in streams?

The effects of grazing by stream invertebrates on algal biomass and spatial heterogeneity were tested experimentally in flow-through microcosms with natural substrates (rocks). One experiment tested the effects of fixed densities of three species of grazers (the caddisfly Allomyia sp. and two mayflies, Epeorus deceptivus and Baetis bicaudatus) on periphyton. Baetis was tested with and without chemical cues from fish predators, which reduced grazer foraging activity to levels similar to the less mobile mayfly (Epeorus). Mean algal biomass (chlorophyll a; chl a) was reduced in grazer treatments compared to ungrazed controls, but there were no differences among grazer treatments. Algal heterogeneity (Morisita index) increased with grazer mobility, with the highest heterogeneity occurring in the Baetis-no fish treatment (most mobile grazer) and the lowest in the caddisfly treatment (most sedentary grazer). A second experiment used a three factorial design, and tested whether initial resource distribution (homogeneous vs. heterogeneous), Baetis density (high vs. low) and fish odor (present vs. absent) affected grazer impact on algal resources. Abundances of Baetis and chl a on individual rocks were recorded to explore the mechanisms responsible for the observed distributions of algae. Initial resource heterogeneity was maintained despite being subjected to grazing. Mean chl a was highest in controls, as in experiment I, and effects of Baetis on algal biomass increased with grazer density. There were no fish effects on algal biomass and no effects of grazer density or fish on algal heterogeneity. At the scale of individual rocks Baetis was unselective when food was homogeneously distributed, but chose high-food rocks when it was heterogeneously distributed. Results of these mechanistic experiments showed that Baetis can track resources at the scale of single rocks; and at moderate densities mobile grazers could potentially maintain periphyton distributions observed in natural streams.

Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass ( Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split–split plot design with two levels of dissolved nutrients and four different grazer species combinations ( Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.

Redundancy among three herbivorous insects across an experimental current velocity gradient.

We conducted streamside experiments to determine if the ability of herbivorous insects to remove algal periphyton varies with local current velocity. We used two mayfly species (Baetis bicaudatusand Drunella grandis) and one caddisfly species (Glossosoma verdona), which differ from one another in body morphology and mobility. Periphyton was grown for 30 days on ceramic tiles in constant velocity to create similar initial forage conditions for grazers. Tiles were transferred to three velocity regimes characteristic of the natural streambed: slow (3-5 cm s(-1)), medium (15-20 cm s(-1)) and fast (32-41 cm s(-1)). Four grazer treatments (Baetis, Drunella, and Glossosoma alone, and all species combined) were repeated for each velocity treatment to isolate the effect of local current on grazer ability to crop periphyton. Grazers differed in their abilities to remove periphyton across current treatments. Glossosoma removed significantly (P<0.05) more periphyton at fast versus either slow or medium velocities; Baetis showed a similar (but non-significant) trend; and, Drunella always removed about 75% of periphyton, irrespective of current. At fast current, periphyton removal was equivalent among the species. At medium current, Drunella removed significantly more than both Baetis and Glossosoma, whereas at slow current, Drunella removed more than Baetis, which removed more than Glossosoma. Periphyton removal under the combined three-grazer treatment was similar qualitatively to the combined effects of individual grazers. More periphyton tended to be removed as current increased, with the fast versus slow contrast showing marginal significance (P=0.10). Under all current regimes, the quantity of periphyton removed did not differ from the null model expectation of simple additive effects among individual grazers (i.e., no facilitation or inhibition). These experiments show that for some species, herbivory varies with current, which suggests that the herbivore "function" of cropping periphyton may vary with the environmental context of local current. Under some local velocities, however, different herbivore species "function" similarly and are potentially redundant with respect to periphytic removal. In naturally heterogeneous streams characterized by sharp gradients in local current velocity, we expect current-dependent species interactions to be common and at least partially contribute to intra-guild co-existence of species.

Species-specific impacts of grazing amphipods in an eelgrass-bed community

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 223:201-211 (2001) - doi:10.3354/meps223201 Species-specific impacts of grazing amphipods in an eelgrass-bed community J. Emmett Duffy*, Annie M. Harvilicz School of Marine Science and Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, Virginia 23062-1346, USA *E-mail: jeduffy@vims.edu ABSTRACT: Small, grazing invertebrates often benefit seagrasses by cropping their epiphytic algal competitors. Yet predictive relations between grazer abundance and seagrass performance are elusive, in significant part because of poorly understood diversity in mesograzer feeding biology. We conducted experiments in eelgrass Zostera marina microcosms to explore how differences in feeding between 2 common grazing amphipod taxa affected accumulation and species composition of epiphytes on eelgrass, as well as amphipod population growth, competition and production, over a 4-week period in summer. Gammarus mucronatus and ampithoids (a mixture of Cymadusa compta and Ampithoe longimana) were stocked, singly and in combination, along with a grazer-free control treatment. Amphipod population growth rates indicated that the 2 taxa competed for a common limiting resource, presumably periphyton, which was essentially eliminated in all grazer treatments. Final abundances of both amphipod taxa were 53 to 68% lower in treatments where the other grazer was present than in single-species grazer treatments. A common carrying capacity was also indicated by the nearly identical final biomass of amphipods across treatments, despite 2-fold variation in initial amphipod densities. These results support the hypothesis that the 2 amphipod taxa are roughly equivalent in terms of resource requirements and production rates. Despite this equivalence, subtle differences in diet breadth between amphipod taxa translated into substantial differences in biomass and composition of the fouling assemblage among treatments. Whereas grazer-free eelgrass became heavily fouled with periphyton and tunicates, eelgrass exposed to G. mucronatus alone was overgrown by the red alga Polysiphonia harveyi, which reached a biomass equal to the total fouling mass of grazer-free controls. P. harveyi was nearly absent from all other treatments. In contrast, eelgrass with ampithoids was virtually devoid of all fouling material. Thus, similar mesograzer species can have markedly different impacts on fouling assemblages, and these occur despite strong similarity in grazer energetics and primary food sources. Our results may help to reconcile evidence of diet overlap and diffuse competition among mesograzer species with the different feeding preferences and community impacts shown for several mesograzers in experimental studies. KEY WORDS: Amphipods · Ampithoe longimana · Community structure · Competition · Cymadusa compta · Gammarus mucronatus · Grazing · Mesograzers · Resource limitation · Zostera marina Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 223. Online publication date: November 28, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics.

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m-2 d-1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.