Small Grazers Research Articles

A grazing crab drives saltmarsh carbon storage and recovery.

Consumers can directly (e.g., consumption) and indirectly (e.g., trophic cascades) influence carbon cycling in blue carbon ecosystems. Previous work found that large grazers have nuanced effects on carbon stocks, yet, small, bioturbating-grazers, which remove plant biomass and alter sediment properties, remain an understudied driver of carbon cycling. We used field-derived and remote sensing data to quantify how the purple marsh crab, Sesarma reticulatum, influenced carbon stocks, flux, and recovery in salt marshes. Sesarma caused a 40%-70% loss in carbon stocks as fronts propagated inland (i.e., ungrazed to recovered transition), with front migration rates accelerating over time. Despite latitudinal differences, front migration rate had no effect on carbon stocks, flux, or time to replacement. When we included Sesarma disturbance in carbon flux calculations, we found it may take 5-100 years for marshes to replace lost carbon, if at all. Combined, we show that small grazers cause a net loss in carbon stocks as they move through the landscape, and irrespective of migration rate, these grazer-driven impacts persist for decades. This work showcases the significant role of consumers in carbon storage and flux, challenging the classic paradigm of plant-sediment feedbacks as the primary ecogeomorphic driver of carbon cycling in blue carbon ecosystems.

Microzooplankton grazers induce chain length plasticity in colonial diatoms

AbstractMany diatoms form long chains and the distribution of chain lengths within a species depends on several environmental factors, among them grazing risk. Larger grazers, such as copepods, efficiently handle and ingest even very long chains but are less efficient with individual cells, whereas most smaller grazers are unable to feed on chains exceeding a few cells in length. Copepod cues make several species shorten their chain length, and theory predicts that cues from small grazers should induce increased, but this remains to be tested, despite the importance of diatoms in marine food webs. Here, we expose three species of chain‐forming diatoms, Skeletonema marinoi, Chaetoceros affinis, and Thalassiosira rotula to cues from various actively feeding micrograzers and record their response. The effect of grazer presence on chain length varies depending on both the type of grazer and the diatom species. For example, S. marinoi increased its chain length when exposed to grazing cues from the heterotrophic dinoflagellate Gyrodinium dominans and the ciliate Euplotes sp. but not to a copepod nauplii (Temora longicornis). C. affinis also responded to cues from grazing G. dominans by increasing chain length, and the response increased with exposure time. Finally, T. rotula did not respond to grazing cues from G. dominans, but rather inhibited the dinoflagellates' ability to feed, presumably through the release of chemical compounds. Our results suggest that some chainforming diatoms can sense and appropriately respond to fast‐growing micrograzers, thus contributing to the success of diatoms in marine environments.

Revisiting the burglar alarm hypothesis: A behavioural cascade mediated by dinoflagellate bioluminescence

Abstract Bioluminescence is widespread among marine organisms and has evolved independently multiple times. While its specific adaptive value is diverse, bioluminescence in most cases mediates fundamental interactions between individuals (predator, prey, mates) and thus impacts ecosystem processes. One hypothesized value of bioluminescence in dinoflagellates is through the ‘burglar alarm’: Grazers of phytoplankton will make the ambient water ‘glow’ as they swim, thereby attracting visual predators of the grazer, thus indirectly protecting the dinoflagellates. However, the most important grazers of dinoflagellates, copepods, are generally too small to elicit dinoflagellates to glow. Only individual cells captured by a copepod will flash, which in turn elicits a powerful escape response in the copepod. Here, we test a variant of this hypothesis that may work for copepods. The behavioural response of the grazer to the flashing of a captured dinoflagellate, rather than the flashing itself, attracts the attention of the grazer's flow‐sensing predators. We demonstrate that bioluminescence in three dinoflagellates reduces the clearance and ingestion rate of nauplii of the copepod Temora longicornis. The presence of bioluminescent cells also elicited an increased frequency of high‐speed jumps of the grazers. The increased jump frequency elevated the detectability of the grazers to a flow‐sensing predator, the copepod Centropages typicus, consequently leading to increased predation mortality of T. longicornis nauplii. The consequent behavioural cascade mediated by bioluminescence works for small grazers that cause only single cells to flash, unlike the traditional description of the burglar alarm. The significance of such behavioural mediated indirect interactions in shaping food webs is becoming increasingly recognized, although rarely demonstrated in pelagic food webs. We argue that the burglar alarm is a secondary adaptation, while the defence of individual cells is the primary adaptive value of bioluminescence in dinoflagellates. Read the free Plain Language Summary for this article on the Journal blog.

Urchin grazing of kelp gametophytes in warming oceans.

Sea urchins can cause extensive damage to kelp forests, and their overgrazing can create extensive barren areas, leading to a loss of biodiversity. Barrens may persist when the recruitment of kelp, which occurs through the microscopic haploid gametophyte stage, is suppressed. However, the ecology of kelp gametophytes is poorly understood, and here we investigate if grazing by juvenile urchins on kelp gametophytes can suppress kelp recruitment and if this is exacerbated by climate change. We compared grazing of Ecklonia radiata gametophytes by two species of juvenile urchins, the tropical Tripneustes gratilla and the temperate Centrostephanus rodgersii, atwinter (19°C), summer (23°C), and ocean warming (26°C) temperatures for the low-latitude range edge of E. radiata, which is vulnerable to ocean warming. We examined the rate of recovery of gametophytes following grazing and determined whether they survived and formed sporophytes after ingestion by sea urchins. Both T. gratilla and C. rodgersii grazed E. radiata gametophytes, reducing their abundance compared to no grazing controls. Surprisingly, temperature did not influence grazing rates, but gametophytes did not recover from grazing in the ocean warming (26°C) treatment. Gametophytes survived ingestion by both species of sea urchin and formed sporophytes after ingestion by T. gratilla, but not C. rodgersii. These results suggest complex grazer-gametophyte interactions, in which both negative (reduced abundance and poor recovery with warming) and positive (facilitated recruitment) effects are possible. Small grazers may play a more important role in kelp ecosystem function than previously thought and should be considered in our understanding of alternate stable states.

Disruption of ecological networks in lakes by climate change and nutrient fluctuations

Climate change interacts with local processes to threaten biodiversity by disrupting the complex network of ecological interactions. While changes in network interactions drastically affect ecosystems, how ecological networks respond to climate change, in particular warming and nutrient supply fluctuations, is largely unknown. Here, using an equation-free modelling approach on monthly plankton community data in ten Swiss lakes, we show that the number and strength of plankton community interactions fluctuate and respond nonlinearly to water temperature and phosphorus. While lakes show system-specific responses, warming generally reduces network interactions, particularly under high phosphate levels. This network reorganization shifts trophic control of food webs, leading to consumers being controlled by resources. Small grazers and cyanobacteria emerge as sensitive indicators of changes in plankton networks. By exposing the outcomes of a complex interplay between environmental drivers, our results provide tools for studying and advancing our understanding of how climate change impacts entire ecological communities.

Biological responses to change in Antarctic sea ice habitats

Sea ice is a key habitat in the high latitude Southern Ocean and is predicted to change in its extent, thickness and duration in coming decades. The sea-ice cover is instrumental in mediating ocean–atmosphere exchanges and provides an important substrate for organisms from microbes and algae to predators. Antarctic krill,Euphausia superba,is reliant on sea ice during key phases of its life cycle, particularly during the larval stages, for food and refuge from their predators, while other small grazers, including copepods and amphipods, either live in the brine channel system or find food and shelter at the ice-water interface and in gaps between rafted ice blocks. Fish, such as the Antarctic silverfishPleuragramma antarcticum, use platelet ice (loosely-formed frazil crystals) as an essential hatching and nursery ground. In this paper, we apply the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO) to review current knowledge about relationships between sea ice and associated primary production and secondary consumers, their status and the drivers of sea-ice change in this ocean. We then use qualitative network modelling to explore possible responses of lower trophic level sea-ice biota to different perturbations, including warming air and ocean temperatures, increased storminess and reduced annual sea-ice duration. This modelling shows that pelagic algae, copepods, krill and fish are likely to decrease in response to warming temperatures and reduced sea-ice duration, while salp populations will likely increase under conditions of reduced sea-ice duration and increased number of days of >0°C. Differences in responses to these pressures between the five MEASO sectors were also explored. Greater impacts of environmental pressures on ice-related biota occurring presently were found for the West and East Pacific sectors (notably the Ross Sea and western Antarctic Peninsula), with likely flow-on effects to the wider ecosystem. All sectors are expected to be impacted over coming decades. Finally, we highlight priorities for future sea ice biological research to address knowledge gaps in this field.

The enigmatic spherical-horned <i>Tsaidamotherium</i> reveals an unsuccessful grazing adaptation of early giraffoids

<p><i>Tsaidamotherium</i> was previously believed to be an “<i>Ovibos</i>-like” bovid because of its specialized basicranial morphology, which closely parallels that of living <i>Ovibos</i>. However, the plate-like headgear of <i>Tsaidamotherium</i> is so strange that confidently categorizing it as bovid horncores is challenging. In this paper, we conducted a comparative development study on the morphology of <i>Tsaidamotherium</i>. The headgear of a juvenile <i>Tsaidamotherium</i> exhibits a stepwise fusion state, with a distinct seam between the headgear and the calvaria. This morphology contradicts the typical development of bovid horncore anlagen, where fusion occurs in the early stages and is limited to a small area, making the fusion seam unobservable. Developmental conservatism dictates that primary bovid horncores form in pairs and strictly develop on the frontals. However, <i>Tsaidamotherium</i> possesses only one headgear primarily supported by the parietal. Therefore, the <i>Tsaidamotherium</i> headgear cannot be classified as bovid horncores but aligns better with giraffoid ossicones in terms of both development and morphology. <i>Tsaidamotherium</i>, as a miniaturized grazing giraffoid, likely represents a giraffoid lineage that adapted to open-land grazing, competing with various bovids but ultimately facing defeat. This may serve as another example to understand the early evolution and radiation of giraffoids: basal giraffoids exhibited diversity and expanded into open ecosystems. While most failed as small grazers, some survived as high-level browsers, eventually giving rise to <i>Giraffa</i>, which succeeded with an extremely long neck among giraffoids.</p>

Plankton community response to fronts: winners and losers

AbstractFronts are particularly productive regions of the ocean, and biodiversity hotspots for many marine species. Here we use an ocean-ecosystem model to investigate the effect of fronts on plankton ecology. We focus on energetic fronts in Western Boundary Current systems that efficiently inject nutrients into the euphotic layer and which are physical boundaries between productive and oligotrophic provinces. We found that the fronts form an environment distinct from both provinces, favorable to some plankton groups (diatoms, dinoflagellates and large carnivorous zooplankton) and less favorable to others (pico-phytoplankton, coccolithophores and small grazers), and with an overall larger diversity. In agreement with previous understanding, we find that bottom-up abiotic processes (nutrient enrichment) explain the prevalence of groups with fast growing rates (the “winners”). Importantly, our results also show that biotic interactions within the ecosystem may play a larger role than previously thought. We show that the winners can have a negative impact on other plankton species (the “losers”) through two indirect competitive processes: community shading (modification of the light environment by the plankton community leading to light-limitation of some plankton groups) and shared predation (where an increase in one functional group leads to increased grazing by a shared predator on another functional group).

Invertebrate Grazing on Live Turtlegrass (Thalassia testudinum): A Common Interaction That May Facilitate Fungal Growth

In coastal wetlands and tropical reefs, snails can regulate foundation species by feeding on marsh grasses and hard corals. In many cases, their impacts are amplified because they facilitate microbial infection in grazer-induced wounds. Whether snails commonly graze live plants and facilitate microbial growth on plants in tropical seagrass systems is less explored. On a Belizean Caye, we examined patterns in snail-generated grazer scars on the abundant turtlegrass (Thalassia testudinum). Our initial survey showed the occurrence of snail-induced scarring on live turtlegrass blades was common, with 57% of live leaves scarred. Feeding trials demonstrated that two of five common snails (Tegula fasciata–smooth tegula and Smaragdia viridis–emerald nerite) grazed unepiphytized turtlegrass blades and that smooth tegula abundance had a positive relationship with scarring intensity. Subsequent surveys at three Caribbean sites (separated by >150 km) also showed a high occurrence of snail-induced scars on turtlegrass blades. Finally, simulated herbivory experiments and field observations of a turtlegrass bed in Florida, United States suggests that herbivore damage could facilitate fungal growth in live seagrass tissue through mechanical opening of tissue. Combined, these findings reveal that snail grazing on live turtlegrass blades in the Caribbean can be common. Based on these results, we hypothesize that small grazers could be exerting top-down control over turtlegrass growth directly via grazing and/or indirectly by facilitating microbial infection in live seagrass tissue. Further studies are needed to determine the generality and relative importance of direct and indirect effects of gastropod grazing on turtlegrass health.

How grazing management can maximize erosion resistance of salt marshes

Abstract Combining natural saltmarsh habitats with conventional barriers can provide a sustainable and cost‐effective alternative for fully engineered flood protection, provided that a minimal salt marsh width can be guaranteed for a long period. Hence, it is essential to understand both the key factors and management options driving the lateral erodibility/stability of salt marshes. We aimed to determine how salt marsh management (i.e. grazing by large vs. small grazers vs. artificial mowing), marsh elevation and marsh age affect soil stability (i.e. soil collapse) and intrinsic lateral erodibility of salt marshes (i.e. particle‐by‐particle detachment). Soil cores were collected in high and low marshes (above and below 0.5 m MHWL, respectively) of different ages. At these locations, we compared cores from grazed areas to cores inside grazer exclosures, with and without artificial mowing. All cores were exposed to waves in flumes to test their stability and lateral erodibility. All soil cores were characterized by a stable fine‐grained layer deposited on top of readily erodible sand. The thickness of the fine‐grained layer was a key parameter in reducing salt marsh instability (cliff collapse). This layer thickness increased with marsh age and at lower elevations, but decreased with cattle grazing due to compaction. The erosion resistance of the fine‐grained layer increased with (a) large grazers that compacted the soil by trampling, (b) mowing that excluded soil‐bioturbating species, and (c) grazing by small grazers that promoted vegetation types with higher root density. Synthesis and applications. Overall, marshes with thinner cohesive and/or fine‐grained top layers are more sensitive to lateral erosion than marshes with deep cohesive soils, independently of the management. Grazing and artificial mowing can reduce the erodibility of fine‐grained soils, making salt marshes more resilient to lateral erosion. However, compaction by large grazers simultaneously leads to thinner fine‐grained layers and lower elevation, potentially leading to more inundation under sea‐level rise. Hence, to effectively manage salt marshes to enhance their contribution to coastal protection, we recommend (a) moderate/rotational livestock grazing, avoiding high intensity grazing in sediment‐poor systems sensitive to sea‐level rise and (b) investigating measures to preserve small grazers.

Mixed-species grazing management to improve sustainability and biodiversity.

Mixed-grazing systems occur when two or more large herbivores graze together. Body size and anatomical differences between animal species are reflected in differences in their selected diet and foraging behaviour, which can bring about opportunities for complementary pasture use. The extent to which niche separation occurs within farming systems depends on the degree of sward heterogeneity and the scale of the resource available, with cultivated swards generally offering comparatively little occasion for selective grazing, in comparison to native pastures. However, even then there are opportunities for mixed grazing to benefit productivity. A range of studies have shown that if sheep are grazed together with cattle on simple grass/white clover swards, the performance of the sheep is improved in comparison to sheep-only grazing, leading to a higher total output per unit area. Semi-natural vegetation communities offer much more opportunity for selective grazing as they are generally more botanically and structurally diverse. Examples are given of the impact of losing large and small grazers from grazed ecosystems in marginal areas. Loss of cattle grazing from the uplands of Wales has been instrumental in the spread of invasive hill grass species linked to the loss of heathland habitats of international conservation importance. Conversely, the loss of sheep and goats from common lands in the Cantabrian Mountains has led to the progressive expansion of woody vegetation, again at the expense of heathlands. Such examples highlight the role that mixed grazing can play in promoting economic and environmental sustainability, particularly in marginal areas.

Ecosystem engineering by large grazers enhances carbon stocks in a tidal salt marsh

Grazers can have a large impact on ecosystem processes and are known to change vegetation composition. However, knowledge of how the long-term presence of grazers affects soil carbon sequestration is limited. In this study, we estimated total accumulated organic carbon in soils of a back-barrier salt marsh and determined how this is affected by long-term grazing by both small and large grazers in relation to age of the ecosystem. In young marshes, where small grazers predominate, hare and geese have a limited effect on total accumulated organic carbon. In older, mature marshes, where large grazers predominate, cattle substantially enhanced carbon content in the marsh soil. We ascribe this to a shift in biomass distribution in the local vegetation towards the roots in combination with trampling effects on the soil chemistry. These large grazers thus act as ecosystem engineers: their known effect on soil compaction (based on a previous study) enhances anoxic conditions in the marsh soil, thereby reducing the oxygen available for organic carbon decomposition by the local microbial community. This study showed that the indirect effects of grazing can significantly enhance soil carbon storage through changing soil abiotic conditions. This process should be taken into account when estimating the role of ecosystems in reducing carbon dioxide concentration in the atmosphere. Ultimately, we propose a testable conceptual framework that includes 3 pathways by which grazers can alter carbon storage: (1) through above-ground biomass removal, (2) through alteration of biomass distribution towards the roots and/or (3) by changing soil abiotic conditions that affect decomposition.

Individual- and population-level effects of Odocoileus virginianus herbivory on the rare forest herb Scutellaria montana

Odocoileus virginianus (white-tailed deer) grazing can impact rare plant species dramatically given their risk for local extirpation and extinction. To determine if O. virginianus management could aid conservation of federally threatened Scutellaria montana (large-flowered skullcap), we conducted an exclosure experiment across a large occurrence of this rare species in Catoosa County, Georgia, USA. We aimed to: (1) quantify the effects of O. virginianus grazing on S. montana individuals, and (2) evaluate the potential of O. virginianus to influence S. montana populations. A lesser percentage of S. montana individuals protected from O. virginianus were grazed than plants accessible to grazers and additional protection from smaller grazers did not reduce grazing, suggesting that O. virginianus primarily do graze S. montana. But grazing did not significantly influence S. montana individuals as evidenced by changes in stem height or the number of leaves per plant assessed during two single growing seasons or across those growing seasons. At the population-level, grazing impacts were buffered by a lack of grazer preferences for specific plant life stages. Although mostly not significant, our findings are biologically interesting given the numerous ecological concerns associated with O. virginianus abundance, including their demonstrated and proposed impact on rare plants.

Major consequences of minor damage: impacts of small grazers on fast-growing kelps.

Damage by small herbivores can have disproportionately large effects on the fitness of individual plants if damage is concentrated on valuable tissues or on select individuals within a population. In marine systems, the impact of tissue loss on the growth rates of habitat-forming algae is poorly understood. We quantified the grazing damage by an isopod Amphoroidea typa on two species of large kelps, Lessonia spicata and Macrocystis pyrifera, in temperate Chile to test whether non-lethal grazing damage could reduce kelp growth rates and photosynthetic efficiency. For L. spicata, grazing damage was widespread in the field, unevenly distributed on several spatial scales (among individuals and among tissue types) and negatively correlated with blade growth rates. In field experiments, feeding by A. typa reduced the concentration of photosynthetic pigments and led to large reductions (~80%) in blade growth rates despite limited loss of kelp biomass (0.5% per day). For M. pyrifera, rates of damage in the field were lower and high densities of grazers were unable to reduce growth rates in field experiments. These results demonstrate that even low per capita grazing rates can result in large reductions in the growth of a kelp, due the spatial clustering of herbivores in the field and the selective removal of photosynthetically active tissues. The impacts of small herbivores on plant performance are thus not easily predicted from consumption rates or abundance in the field, and vary with plant species due to variation in their ability to compensate for damage.

The benefits of being big: effects of body size on energy budgets of three wintering goose species grazing Carex beds in the Yangtze River floodplain, China

Herbivores of different body size vary in food selection because of their different metabolic requirements and abilities to harvest and digest food. Compared with smaller grazers, larger ones require higher food quantity but can tolerate poorer quality. This divergence may also explain habitat partitioning in the distribution of closely related species. By estimating daily energy expenditure (based on observed activity budgets) and energy intake (using the indigestible marker method in food and faeces), we compared the field energy budgets of three wintering herbivorous goose species differing in body size feeding on the same Carex meadows. Throughout the winter, the larger Bean Geese Anser fabalis serrirostris and Greater White-fronted Geese Anser albifrons maintained positive energy budgets grazing lower quality Carex, in contrast to the smaller Lesser White-fronted Geese Anser erythropus which failed to do so and could only maintain positive energy budgets by grazing high-quality Alopecurus, Cynodon and Eleocharis. However, all three species failed to maintain positive energy balance and lost mass in mid-winter. These results have important implications for explaining the divergent distribution patterns of these species on their wintering grounds in China.

Are Small Grazers and/or Viruses a Structuring Factor of the Free-Living Bacterial Community in Lake Geneva?

Two experiments were conducted to test whether viruses, small auto- and heterotrophic nanoflagellates were key factors regulating at relatively short-term (4 days) and nearby periods (April vs. May) the bacterial community in surface waters of Lake Geneva. 2.5 L containing polycarbonate bottles were incubated in situ with either μm filtered water with additions of either virus-free water or a viral concentrate. Abundances of viruses, prokaryotes and small autotrophs were obtained each day using flow cytometry, while bacterial richness was assessed using 16S rDNA PCR-DGGE and auto- and heterotrophic flagellates counted with epifluorescence microscopy at t0 and t96. Transmission electron microscopy was also used to assess virus-induced bacterial mortality at the start and the end of the experiments. Cloning-sequencing was applied on PCR products obtained after excision of selected DGGE bands to highlight more specifically the identity of bacteria of interest in the context of the experiment. The autotrophs and grazer presence and/or the virus enrichment resulted in different effects on the structure of the bacterial community and the impact was also different with the period. In May, bacterial structure changes seemed to be related to the impact or influence of the eukaryotes (including nanoflagellate grazers), while viruses might have a higher impact on the bacterial community structure the month before. This study provides new persuasive evidence that the presence of viruses and small eukaryotes are likely to drive bacterial community composition and shifts on the short-term in lacustrine ecosystems. More interestingly, such effects seem to be different between viruses and grazers, the ones sustaining, the others reducing bacterial community composition.

The role of subtropical zooplankton as grazers of phytoplankton under different predation levels

Summary1. Large zooplankton such asDaphniaplay a fundamental role as consumers of phytoplankton in temperate lakes. These organisms are scarce in subtropical lakes where smaller cladocerans or copepods take this niche. However, such smaller grazers appear to be less able to exert an effective top–down control on the phytoplankton community.2. We experimentally analysed the ability of zooplankton typical of subtropical, nutrient‐rich lakes to graze effectively on the phytoplankton community. We conducted two outdoor mesocosm experiments in a hypertrophic lake, with combinations of three different zooplankton densities and three different omnivorous fish densities. In the first experiment, the zooplankton community was dominated by a small‐sized cladoceran (Moina micrura) and in the second by a calanoid copepod (Notodiaptomus incompositus). The phytoplankton community also differed between experiments, with dominance of large size classes and less palatable species in the first experiment and edible sizes in the second.3. In both experiments, the effect of fish on the largest zooplankton was strong and negative, and low fish densities were sufficient to eliminate the larger zooplankton. Fish presence had positive effects on the biovolume of the largest phytoplankton size fraction (30–100 μm) in the first experiment. This effect was more pronounced in combination with high zooplankton biomass, suggesting that nutrient recycling by both fish and zooplankton may have been an important mechanism promoting phytoplankton growth.4. None of the zooplankton communities tested had significant top–down effects on the phytoplankton community. In view of the phytoplankton species that dominated the communities at the end of both experiments, inedibility, toxicity and antigrazer defences may explain the absence of significant effects of zooplankton grazing.5. Our results support the idea that in subtropical nutrient‐rich lakes, drastic removal of small omnivorous fish may be needed to allow an increase in zooplankton biomass. In addition, our results imply that for such a change to result in effective top–down control of phytoplankton, a shift in zooplankton community composition is essential too, as the experimental increase in small‐sized grazers had little effect on the phytoplankton communities.

The importance of live coral for small-sized herbivorous reef fishes in physically challenging environments

Tropical coral reefs are facing increasing levels of disturbance, with predictions of ongoing reductions in live coral cover. The response of herbivorous fishes to loss of coral cover was investigated by analysing the relationship between the abundance of different sized herbivorous fishes and live coral cover, on a high flow reef slope and a sheltered back reef on Ningaloo Reef, Western Australia. We found positive relationships between coral cover and the abundance of small (10–20 cm) excavating and scraping parrotfishes on the slope, but a negative relationship between corals and small (10–15 cm) grazers. These relationships were not evident on back reefs. Generally, the abundance of large individuals were not correlated with live coral in either habitat. We suggest that in physically demanding environments small parrotfishes may utilise the structural complexity of corals to resist strong water flow and/or to reduce predation. Small acanthurid grazers may school to reduce energy costs and minimise predation risk. The loss of coral cover appears to be particularly important for small fishes in high energy habitats, especially non-schooling species.

Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers

1. Theory predicts that small grazers are regulated by the digestive quality of grass, while large grazers extract sufficient nutrients from low-quality forage and are regulated by its abundance instead. In addition, predation potentially affects populations of small grazers more than large grazers, because predators have difficulty capturing and handling large prey. 2. We analyse the spatial distribution of five grazer species of different body size in relation to gradients of food availability and predation risk. Specifically, we investigate how the quality of grass, the abundance of grass biomass and the associated risks of predation affect the habitat use of small, intermediate and large savanna grazers at a landscape level. 3. Resource selection functions of five mammalian grazer species surveyed over a 21-year period in Serengeti are calculated using logistic regressions. Variables included in the analyses are grass nitrogen, rainfall, topographic wetness index, woody cover, drainage lines, landscape curvature, water and human habitation. Structural equation modelling (SEM) is used to aggregate predictor variables into 'composites' representing food quality, food abundance and predation risk. Subsequently, SEM is used to investigate species' habitat use, defined as their recurrence in 5 × 5 km cells across repeated censuses. 4. The distribution of small grazers is constrained by predation and food quality, whereas the distribution of large grazers is relatively unconstrained. The distribution of the largest grazer (African buffalo) is primarily associated with forage abundance but not predation risk, while the distributions of the smallest grazers (Thomson's gazelle and Grant's gazelle) are associated with high grass quality and negatively with the risk of predation. The distributions of intermediate sized grazers (Coke's hartebeest and topi) suggest they optimize access to grass biomass of sufficient quality in relatively predator-safe areas. 5. The results illustrate how top-down (vegetation-mediated predation risk) and bottom-up factors (biomass and nutrient content of vegetation) predictably contribute to the division of niche space for herbivores that vary in body size. Furthermore, diverse grazing assemblages are composed of herbivores of many body sizes (rather than similar body sizes), because these herbivores best exploit the resources of different habitat types.

Annual cycle of distribution of three ice-associated copepods along the coast near Dumont d'Urville, Terre Adélie (Antarctica)

In polar regions sea ice is a site of enhanced primary production during winter and provides important habitat for small grazers, such as copepods. We sampled zooplankton from the sea ice and water column throughout 2005, near Dumont d'Urville station (Terre Adélie, Antarctica). Three species of ice-associated copepods were found: two calanoid copepods Paralabidocera antarctica and Stephos longipes and the harpacticoid copepod Drescheriella glacialis. P. antarctica was the most abundant of the three and was closely associated with the sea ice during most of the year. This species had a one year life cycle with a probable over-wintering period in the sea ice as nauplii and a short copepodite phase in spring. Reproduction and spawning occurred in early summer. A comparison with two other populations (near Syowa and Davis stations) along the east coast of Antarctica showed that there was a temporal shift in the life cycles of the three populations, which was linked to variability in sea ice conditions. D. glacialis was the second most abundant copepod and was more common during the winter than during summer, indicating its preference for the sea ice habitat. In autumn, the presence of D. glacialis in the deeper part of the water column suggested that this species colonised the sea ice from the benthos. S. longipes was found only in the water column near Dumont d'Urville and was not very abundant. In Terre Adélie particular environmental conditions, such as the absence of a permanent sea ice zone throughout the year, a longer time of open water, strong katabatic winds and the presence of polynyas, have influenced both the abundance and distribution of the three common ice-associated copepods.