Trophic levels and trophic tangles: the prevalence of omnivory in real food webs

The concept of trophic levels is one of the oldest in ecology and informs our understanding of energy flow and top-down control within food webs, but it has been criticized for ignoring omnivory. We tested whether trophic levels were apparent in 58 real food webs in four habitat types by examining patterns of trophic position. A large proportion of taxa (64.4%) occupied integer trophic positions, suggesting that discrete trophic levels do exist. Importantly however, the majority of those trophic positions were aggregated around integer values of 0 and 1, representing plants and herbivores. For the majority of the real food webs considered here, secondary consumers were no more likely to occupy an integer trophic position than in randomized food webs. This means that, above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores. Omnivory was most common in marine systems, rarest in streams, and intermediate in lakes and terrestrial food webs. Trophic-level-based concepts such as trophic cascades may apply to systems with short food chains, but they become less valid as food chains lengthen.     

Topological structure and interaction strengths in model food webs

We report the results of carrying out a large number of simulations on a coevolutionary model of multispecies communities. A wide range of parameter values were investigated which allowed a rather complete picture of the change in behaviour of the model as these parameters were varied to be built up. Our main interest was in the nature of the community food webs constructed via the simulations. We identify the range of parameter values which give rise to realistic food webs and give arguments which allow some of the structure which is found to be understood in an intuitive way. Since the webs are evolved according to the rules of the model, the strengths of the predator–prey links are not determined a priori, and emerge from the process of constructing the web. We measure the distribution of these link strengths, and find that there are a large number of weak links, in agreement with recent suggestions. We also review some of the data on food webs available in the literature, and make some tentative comparisons with our results. The difficulties of making such comparisons and the possible future developments of the model are also briefly discussed.     

Population structure of two Antarctic ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, in winter sea ice

The spatial distribution and population structure of two dominant ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, were studied during winter at nine locations in east Antarctic fast ice. These species accounted for at least 90% of the total metazoan abundance at each location. Abundances were high, reaching 175 individuals l^-1 (190,000 m^-2) for D. glacialis and 660 l^-1 (901,000 m^-2) for P. antarctica. These abundances were probably partly supported by the high biomass of ice-algae (Pearson correlation coefficient, r=0.75), as indicated by chlorophyll-a concentrations (1.7-10.1 µg l^-1). The population structures of each species suggested very different life-history strategies. All developmental stages of D. glacialis were isolated from the ice cores, including females with egg sacs, supporting the hypothesis that this species reproduces in the sea ice during winter. This strategy might assist D. glacialis in leading a continually colonising existence, whereby it responds opportunistically to the availability of favourable habitat patches. The populations of P. antarctica were composed primarily of nauplii (>99%), consistent with past observations of a synchronised life cycle for this species. The strong coupling of the developmental cycle of P. antarctica to the growth and decay of sea ice suggests that local extinctions might occur in areas where ice break-out is unpredictable.     

Sources of primary production supporting food webs in an arid coastal embayment

Penaeid shrimp can be useful ecological indicators of linkages between shallow tropical coastal habitats, acting as integrators of carbon and nitrogen sources due to their generalist feeding habits and their mobility between habitats and with tidal cycles. In the current study, the contribution of mangrove, seagrass and microbial mat to the nutrition of two penaeid shrimp species, Penaeus semisulcatus and Metapeneus ensis, in a shallow arid embayment in the Arabian Gulf was assessed through a combination of analysis of stomach contents and dual carbon and nitrogen stable isotope signatures. Shrimp tissue stable isotope signatures identified seagrass as a major source of carbon and nitrogen for both species, contributing 21–38?% (1–99?‰). Microbial mat was also detected as a significant nutritional source for early-stage Metapenaeus affinis postlarvae (1–27?%). However, mangroves were not identified as a significant source, with the range of results including the possibility of a zero contribution. Moreover, the greatest possible contribution of mangroves as source of carbon was less than for the other primary producers. This may be due the high salinity and wide temperature range limiting mangrove productivity as well as the low export of dissolved and particulate organic material out of the mangroves due to low rainfall.     

Morphological adaptation of a planktonic diatom to growth in Antarctic sea ice

Chaetoceros dichaeta Ehrenberg is one of the most important planktonic diatom species in the Southern Ocean, making a significant contribution to the total biomass in the region. Our observations on both field and culture material have revealed the existence of a specialized form of C. dichaeta adapted to living in sea ice. This sea ice form differs from the planktonic form by the shape and orientation of the setae and the aperture length between sibling cells. Thus, the diameter of the chain is equivalent to the apical axes of the cells and is accompanied by a two order of magnitude decrease in minimal space requirement. Here, we report for the first time on the extraordinary overwintering strategy of a planktonic diatom in sea ice facilitated by its rapid morphological adaptation to changing environmental conditions. This morphological plasticity enables it to thrive in the confined space of the sea ice brine matrix and retain its numerical dominance in recurrent growing seasons and has likely evolved to optimally exploit the dynamic ecosystem of the seasonally ice-covered seas of the Southern Ocean.     

Origin of compartmentalization in food webs

The response of an ecosystem to perturbations is mediated by both antagonistic and facilitative interactions between species. It is thought that a community's resilience depends crucially on the food web--the network of trophic interactions--and on the food web's degree of compartmentalization. Despite its ecological importance, compartmentalization and the mechanisms that give rise to it remain poorly understood. Here we investigate several definitions of compartments, propose ways to understand the ecological meaning of these definitions, and quantify the degree of compartmentalization of empirical food webs. We find that the compartmentalization observed in empirical food webs can be accounted for solely by the niche organization of species and their diets. By uncovering connections between compartmentalization and species' diet contiguity, our findings help us understand which perturbations can result in fragmentation of the food web and which can lead to catastrophic effects. Additionally, we show that the composition of compartments can be used to address the long-standing question of what determines the ecological niche of a species.     

Responses to extreme seasonality in food supply: diet plasticity in Antarctic brachiopods

The ice gouged, shallow, polar seabed is a challenging place to live, but suspension feeders are particularly rich and abundant there. The extreme seasonality of food supply from phytoplankton at high latitudes, combined with very stable temperatures has not, however, reduced the range of life history adaptations. Some species extend feeding and growing periods by concentrating on smaller, lower biomass but longer duration, fractions of phytoplankton. Here we show that shallow-water Antarctic brachiopods can switch from pelagic to benthic food sources when required. Like most suspension feeders they utilise the extremely abundant summer phytoplankton blooms, but unlike many other groups brachiopods do not appear to become dormant in winter. In shallow sites around the world wind and wave action resuspend benthic material, making it available to filter feeders. Widespread ice disturbance at polar latitudes may also cause resuspension enhancing local food supply. Articulate brachiopods have blind-ended guts and probably feed little once a large bolus of food has been collected, so a resuspension event may represent a significant enhancement of yearly food availability. The key to their ability to utilise unpredictable food sources is the same trait responsible for their polar success; very low maintenance costs and a catholic diet. Utilising resuspended material could be an important factor in global brachiopod distributions. Previously difficult to explain high brachiopod densities in New Zealand, Northwest Europe and the Canadian Pacific Coast may occur because these areas are all in the worlds windiest, and hence strongest resuspension, latitudes.Communicated by J.P. Thorpe, Port Erin     

In situ net primary productivity and photosynthesis of Antarctic sea ice algal, phytoplankton and benthic algal communities

Primary production at Antarctic coastal sites is contributed from sea ice algae, phytoplankton and benthic algae. Oxygen microelectrodes were used to estimate sea ice and benthic primary production at several sites around Casey, a coastal area in eastern Antarctica. Maximum oxygen export from sea ice was 0.95 mmol O₂ m⁻² h⁻¹ (~11.7 mg C m⁻² h⁻¹) while from the sediment it was 6.08 mmol O₂ m⁻² h⁻¹ (~70.8 mg C m⁻² h⁻¹). When the ice was present O₂ export from the benthos was either low or negative. Sea ice algae assimilation rates were up to 3.77 mg C (mg Chl-a)⁻¹ h⁻¹ while those from the benthos were up to 1.53 mg C (mg Chl-a)⁻¹ h⁻¹. The contribution of the major components of primary productivity was assessed using fluorometric techniques. When the ice was present approximately 55–65% of total daily primary production occurred in the sea ice with the remainder unequally partitioned between the sediment and the water column. When the ice was absent, the benthos contributed nearly 90% of the primary production.     

The probabilistic niche model reveals substantial variation in the niche structure of empirical food webs

The structure of food webs, complex networks of interspecies feeding interactions, plays a crucial role in ecosystem resilience and function, and understanding food web structure remains a central problem in ecology. Previous studies have shown that key features of empirical food webs can be reproduced by low-dimensional "niche" models. Here we examine the form and variability of food web niche structure by fitting a probabilistic niche model to 37 empirical food webs, a much larger number of food webs than used in previous studies. The model relaxes previous assumptions about parameter distributions and hierarchy and returns parameter estimates for each species in each web. The model significantly outperforms previous niche model variants and also performs well for several webs where a body-size-based niche model performs poorly, implying that traits other than body size are important in structuring these webs' niche space. Parameter estimates frequently violate previous models' assumptions: in 19 of 37 webs, parameter values are not significantly hierarchical, 32 of 37 webs have nonuniform niche value distributions, and 15 of 37 webs lack a correlation between niche width and niche position. Extending the model to a two-dimensional niche space yields networks with a mixture of one- and two-dimensional niches and provides a significantly better fit for webs with a large number of species and links. These results confirm that food webs are strongly niche-structured but reveal substantial variation in the form of the niche structuring, a result with fundamental implications for ecosystem resilience and function.     

Oceanic variability and coastal topography shape genetic structure in a long-dispersing sea urchin

Understanding the scale of marine population connectivity is critical for the conservation and sustainable management of marine resources. For many marine species adults are benthic and relatively immobile, so patterns of larval dispersal and recruitment provide the key to understanding marine population connectivity. Contrary to previous expectations, recent studies have often detected unexpectedly low dispersal and fine-scale population structure in the sea, leading to a paradigm shift in how marine systems are viewed. Nonetheless, the link between fine-scale marine population structure and the underlying physical and biological processes has not been made. Here we show that patterns of genetic structure and population connectivity in the broadcast-spawning and long-distance dispersing sea urchin Centrostephanus rodgersii are influenced by physical oceanographic and geographic variables. Despite weak genetic differentiation and no isolation-by-distance over thousands of kilometers among samples from eastern Australia and northern New Zealand, fine-scale genetic structure was associated with sea surface temperature (SST) variability and geography along the southeastern Australian coast. The zone of high SST variability is characterized by periodic shedding of eddies from the East Australian Current, and we suggest that ocean current circulation may, through its influence on larval transport and recruitment, interact with the genetic consequences of large variance in individual reproductive success to generate patterns of fine-scale patchy genetic structure. If proven consistent across species, our findings suggest that the optimal scale for fisheries management and reserve design should vary among localities in relation to regional oceanographic variability and coastal geography.     

A Bayesian hierarchical model of Antarctic fur seal foraging and pup growth related to sea ice and prey abundance

We created a Bayesian hierarchical model (BHM) to investigate ecosystem relationships between the physical ecosystem (sea ice extent), a prey measure (krill density), predator behaviors (diving and foraging effort of female Antarctic fur seals, Arctocephalus gazella, with pups) and predator characteristics (mass of maternal fur seals and pups). We collected data on Antarctic fur seals from 1987/1988 to 1994/1995 at Seal Island, Antarctica. The BHM allowed us to link together predators and prey into a model that uses all the data efficiently and accounts for major sources of uncertainty. Based on the literature, we made hypotheses about the relationships in the model, which we compared with the model outcome after fitting the BHM. For each BHM parameter, we calculated the mean of the posterior density and the 95% credible interval. Our model confirmed others' findings that increased sea ice was related to increased krill density. Higher krill density led to reduced dive intensity of maternal fur seals, as measured by dive depth and duration, and to less time spent foraging by maternal fur seals. Heavier maternal fur seals and lower maternal foraging effort resulted in heavier pups at 22 d. No relationship was found between krill density and maternal mass, or between maternal mass and foraging effort on pup growth rates between 22 and 85 days of age. Maternal mass may have reflected environmental conditions prior to the pup provisioning season, rather than summer prey densities. Maternal mass and foraging effort were not related to pup growth rates between 22 and 85 d, possibly indicating that food was not limiting, food sources other than krill were being used, or differences occurred before pups reached age 22 d.     

Diurnal changes in photosynthesis of Antarctic fast ice algal communities determined by pulse amplitude modulation fluorometry

The aim of this project was to determine both the diurnal changes in photosynthetic activity of Antarctic sea ice algae and also the protective mechanisms they use to mitigate the effects of in situ UV radiation. Changes in the diurnal photosynthetic parameters of fast ice algal communities at McMurdo Sound were measured in situ, using a custom designed monitoring pulse amplitude modulation fluorometer. The sea ice microalgae were able to adapt rapidly to either increasing or decreasing ambient irradiances. ΔF/F_m' values were between 0.2 and 0.51, while E_k varied between 2.1 and 18 μmol photons m^-2 s^-1. ΔF/F_m', E_k, and relative electron transfer rate (rETR) all varied sequentially over the course of a day. rETR and E_k were highest at midday at the highest irradiances, when there was apparent midday down regulation of photosynthesis, while ΔF/F_m' was highest at midnight. The effects of natural UV radiation on sea ice were examined, but it was not possible to detect the effect of either UVB or UVA and UVB on photosynthesis. This was considered to be largely because of the large spatial and temporal heterogeneity of the under ice community, changing irradiances throughout the day and the relatively small change caused by UV.     

Multiple anthropogenic stressors and the structural properties of food webs

Coastal environments are among the most productive on the planet, providing a wide range of ecosystem services. Development and exploitation mean that they are faced with stresses from a number of anthropogenic sources. Such stresses are typically studied in isolation, but multiple stressors can combine in unexpected ways to alter the structure of ecological systems. Here, we experimentally explore the impacts of inorganic nutrients and organic matter on a range of food web properties. We find that these two stressors combine additively to produce significant increases in connectance and mean food chain length. Such increases are typically associated with enhanced robustness to secondary extinctions and productivity, respectively. Despite these apparent beneficial effects, we find a simplification of web structure in terms of taxon richness and diversity, and altered proportions of basal and top species. These effects are driven by a reduction in community assembly and lower consistency in a range of system properties as a result of the multiple stressors. Consequently, impacted food webs are likely to be more vulnerable to human- or climate-induced perturbations in the long-term.     

Potential trophic biomasses and trace-substance concentrations in unstructured marine food webs

It is proposed that unstructured food webs may more closely resemble real marine food webs than does the conventional, structured model. An unstructured food-web model leads to a set of very simple expressions for the potential partition of matter in the food web in steady state, including the potential fluxes of material and biomasses of trophic types and the concentration of trace substances in the members and materials of such a food web. The approach may explain some anomalies of relative predator-prey biomasses and of trace-element distribution, and may be of further use for analyzing and predicting (a) the tropho-dynamic parameters of marine systems, (b) the trophic positions, and the steadystate fluxes and biomasses of marine organisms, (c) the distribution of trace materials in marine biota; and for relating findings among these areas. Other matters, such as limitations of food conversion, indicated by concentration factors of trace substances, the possibilities of non-causal association of anomalously high levels of trace substances (including pollutants) with diseased or otherwise abnormal marine creatures, and an inverse relationship of early concentrations of newly introduced trace substances and their eventual concentrations in various organisms, are also developed in this approach.     

Grazing of phytoplankton by copepods in eastern Antarctic coastal waters

Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke‐out. Phytoplankton biomass increased again from mid‐ to late‐February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break‐out, whereas larval polychaetes and ctenophores dominated after ice break‐out. Oncaeacurvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C␣m⁻³␣d⁻¹) was recorded on 21 December when O.␣curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m⁻³ d⁻¹); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February (⋍5 mg C␣m⁻³␣d⁻¹). The main species responsible for grazing during this period were O. curvata, Oithonasimilis, Calanoidesacutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity. Received: 11 October 1996 / Accepted: 22 October 1996     

Skeletal changes in the test and jaws of the sea urchinDiadema antillarum in response to food limitation

When food-limited,Diadema antillarum increases the relative size of Aristotle's lantern compared to the size of the test. This is accomplished primarily by decreasing the size of the test: the demipyramid grows at a reduced rate as the test shrinks. These results suggest that an altered relationship between demipyramid and test size can provide evidence for food-limitation in field populations. The sizes of the lantern and test of museum and fossil specimens can be investigated for the presence of food limitation and biotic interactions in recent and past times.     

Trophic role of small cyclopoid copepod nauplii in the microbial food web: a case study in the coastal upwelling system off central Chile

Copepod grazing impact on planktonic communities has commonly been underestimated due to the lack of information on naupliar feeding behaviour and ingestion rates. That is particularly true for small cyclopoid copepods, whose nauplii are mainly in the microzooplankton size range (<200 μm). The trophic role of Oithona spp. nauplii was investigated off Concepción (central Chile, ~36°S) during the highly productive upwelling season, when maximum abundances of these nauplii were expected. Diet composition, ingestion rates, and food-type preferences were assessed through grazing experiments with different size fractions of natural planktonic assemblages (<3, <20, <100, and <125 μm) and cultures of the nanoflagellate Isochrysis galbana. When the Oithona spp. nauplii were offered a wide range of size fractions as food (pico- to microplankton), they mostly ingested small (2–5 μm) nanoflagellates (5–63 × 10³ cells nauplius⁻¹ day⁻¹). No ingestion on microplankton was detected, and picoplankton was mainly ingested when it was the only food available. Daily carbon (C) uptake by the nauplii ranged between 28 and 775 ng C nauplius⁻¹, representing an overall mean of 378% of their body C. Our relatively high ingestion rate estimates can be explained by methodological constraints in previous studies on naupliar feeding, including those dealing with “over-crowding” and “edge” effects. Overall, the grazing impact of the Oithona spp. nauplii on the prey C standing stocks amounts up to 21% (average = 13%) for picoplankton and 54% (average = 28%) for nanoplankton. These estimates imply that the nauplii of the most dominant cyclopoid copepods exert a significant control on the abundances of nanoplankton assemblages and, thereby, represent an important trophic link between the classical and microbial food webs in this coastal upwelling system.     

Polonium-210 and lead-210 in Antarctic marine biota and sea water

Concentrations of the naturally-ocurring radionuclides ²¹⁰Po and ²¹⁰Pb were measured in krill (Euphausia superba), mesozooplankton, phytoplankton and sea water collected during the South African SIBEX cruise to the Antarctic in autumn 1984. The data reported constitute the first substantial measurements on ²¹⁰Po and ²¹⁰Pb in such samples in the Antarctic Ocean. The concentrations of ²¹⁰Po in mesozooplankton and phytoplankton are unexceptional in comparison with those from other oceans. The SIBEX E. superba, however, have higher levels of ²¹⁰Po than usually found in euphausiids. The ²¹⁰Po data, combined with reasonable estimates of biological quantitites such as the fractional assimilation, are used to obtain information about the diet of E. superba. It is suggested that the higher ²¹⁰Po in the SIBEX E. superba reflects a change from an almost entirely phytoplanktonic diet in summer to a more omnivorous diet as winter approaches. The data show that there are allometric relationships between the ²¹⁰Po content of euphausiids and animal size; these are discussed briefly. The limited sea-water data presented are characterized by unusually high ²¹⁰Po:²¹⁰Pb activity ratios and need further investigation.     

POM in macro-/meiofaunal food webs associated with three flow regimes at deep-sea hydrothermal vents on Axial Volcano,Juan de Fuca Ridge

Deep-sea hydrothermal vent ecosystems host both symbiotic and non-symbiotic invertebrates. The non-symbiotic vent fauna is generally assumed to rely on free-living chemoautotrophic bacteria as their main food source but other sources such as detritus have recently been suggested to be a part of the invertebrate diets. Little is known about how food availability influences the distribution of vent organisms on a small scale. In addition, the feeding ecology and role of small, often numerically dominant invertebrates, the meiofauna is poorly understood at vents. In this study, we used stable carbon and nitrogen isotopic analysis to investigate the role of particulate detritus in the diets of macro- and meiobenthic invertebrates within three vent assemblages at Axial Volcano, Juan de Fuca Ridge, and Northeast Pacific. Particulate organic matter of a detrital origin became more important in the diet of invertebrates in assemblages typically associated with low-hydrothermal flow intensities. Meiobenthic species occupied several different feeding guilds and trophic levels in the assemblages investigated. We conclude that small-scale spatial variability in food sources is an important feature of vent food webs and that spatial patterns observed here and elsewhere are shaped by variations in hydrothermal discharge.