Lipid extraction has little effect on the delta N-15 of aquatic consumers

Proper application of stable isotopes (e. g., delta N-15 and delta C-13) to food web analysis requires an understanding of all nondietary factors that contribute to isotopic variability. Lipid extraction is often used during stable isotope analysis (SIA), because synthesized lipids have a low delta C-13 and can mask the delta C-13 of a consumer's diet. Recent studies indicate that lipid extraction intended to adjust delta C-13 may also cause shifts in delta N-15, but the magnitude of and reasons for the shift are highly uncertain. We examined a large data set (n = 854) for effects of lipid extraction (using Bligh and dyer's [ 1959] chloroform-methanol solvent mixtures) on the delta N-15 of aquatic consumers. We found no effect of chemically extracting lipids on the delta N-15 of whole zooplankton, unionid mussels, and fish liver samples, and found a small increase in fish muscle delta N-15 of similar to 0.4%. We also detected a negative relationship between the shift in delta N-15 following extraction and the C:N ratio in muscle tissue, suggesting that effects of extraction were greater for tissue with lower lipid content. As long as appropriate techniques such as those from Bligh and dyer (1959) are used, effects of lipid extraction on delta N-15 of aquatic consumers need not be a major consideration in the SIA of food webs.

Estimating trophic link density from quantitative but incomplete diet data

The trophic link density and the stability of food webs are thought to be related, but the nature of this relation is controversial. This article introduces a method for estimating the link density from diet tables which do not cover the complete food web and do not resolve all diet items to species level. A simple formula for the error of this estimate is derived. Link density is determined as a function of a threshold diet fraction below which diet items are ignored (

Preservation methods alter stable isotope values in gelatinous zooplankton: implications for interpreting trophic ecology

Jellyfish are increasingly topical within studies of marine food webs. Stable isotope analysis represents a valuable technique to unravel the complex trophic role of these long-overlooked species. In other taxa, sample preservation has been shown to alter the isotopic values of species under consideration, potentially leading to misinterpretation of trophic ecology. To identify potential preservation effects in jellyfish, we collected Aurelia aurita from Strangford Lough (54(o)22'44.73aEuro(3)N, 5(o)32'53.44aEuro(3)W) during May 2009 and processed them using three different methods prior to isotopic analysis (unpreserved, frozen and preserved in ethanol). A distinct preservation effect was found on delta N-15 values: furthermore, preservation also influenced the positive allometric relationship between individual size and delta N-15 values. Conversely, delta C-13 values remained consistent between the three preservation methods, conflicting with previous findings for other invertebrate, fish and mammalian species. These findings have implications for incorporation of jellyfish into marine food webs and remote sampling regimes where preservation of samples is unavoidable.

Loss of functionally unique species may gradually undermine ecosystems

Functionally unique species contribute to the functional diversity of natural systems, often enhancing ecosystem functioning. An abundance of weakly interacting species increases stability in natural systems, suggesting that loss of weakly linked species may reduce stability. Any link between the functional uniqueness of a species and the strength of its interactions in a food web could therefore have simultaneous effects on ecosystem functioning and stability. Here, we analyse patterns in 213 real food webs and show that highly unique species consistently tend to have the weakest mean interaction strength per unit biomass in the system. This relationship is not a simple consequence of the interdependence of both measures on body size and appears to be driven by the empirical pattern of size structuring in aquatic systems and the trophic position of each species in the web. Food web resolution also has an important effect, with aggregation of species into higher taxonomic groups producing a much weaker relationship. Food webs with fewer unique and less weakly interacting species also show significantly greater variability in their levels of primary production. Thus, the loss of highly unique, weakly interacting species may eventually lead to dramatic state changes and unpredictable levels of ecosystem functioning.

Temperature, predator-prey interaction strength and population stability

Warming could strongly stabilize or destabilize populations and food webs by changing the interaction strengths between predators and their prey. Predicting the consequences of warming requires understanding how temperature affects ingestion (energy gain) and metabolism (energy loss). Here, we studied the temperature dependence of metabolism and ingestion in laboratory experiments with terrestrial arthropods (beetles and spiders). From this data, we calculated ingestion efficiencies (ingestion/metabolism) and per capita interaction strengths in the short and long term. Additionally, we investigated if and how body mass changes these temperature dependencies. For both predator groups, warming increased metabolic rates substantially, whereas temperature effects on ingestion rates were weak. Accordingly, the ingestion efficiency (the ratio of ingestion to metabolism) decreased in all treatments. This result has two possible consequences: on the one hand, it suggests that warming of natural ecosystems could increase intrinsic population stability, meaning less fluctuations in population density; on the other hand, decreasing ingestion efficiencies may also lead to higher extinction risks because of starvation. Additionally, predicted long-term per capita interaction strengths decreased with warming, which suggests an increase in perturbation stability of populations, i.e., a higher probability of returning to the same equilibrium density after a small perturbation. Together, these results suggest that warming has complex and potentially profound effects on predator-prey interactions and food-web stability.

‘Invasional meltdown’: evidence for unexpected consequences and cumulative impacts of multispecies invasions

Empirical support for ‘invasional meltdown’, where the presence of one invading species facilitates another and compounds negative impacts on indigenous species, is equivocal with few convincing studies. In Ireland, the bank vole was introduced 80 years ago and now occupies a third of the island. The greater white-toothed shrew arrived more recently within the invasive range of the bank vole. We surveyed the abundance of both invasive species and two indigenous species, the wood mouse and pygmy shrew, throughout their respective ranges. The negative effects of invasive on indigenous species were strong and cumulative bringing about species replacement. The greater white-toothed shrew, the second invader, had a positive and synergistic effect on the abundance of the bank vole, the first invader, but a negative and compounding effect on the abundance of the wood mouse and occurrence of the pygmy shrew. The gradual replacement of the wood mouse by the bank vole decreased with distance from the point of the bank vole’s introduction whilst no pygmy shrews were captured where both invasive species were present. Such interactions may not be unique to invasions but characteristic of all multispecies communities. Small mammals are central in terrestrial food webs and compositional changes to this community in Ireland are likely to reverberate throughout the ecosystem. Vegetation composition and structure, invertebrate communities and the productivity of avian and mammalian predators are likely to be affected. Control of these invasive species may only be effected through landscape and habitat management.

Body size in ecological networks

Body size determines a host of species traits that can affect the structure and dynamics of food webs, and other ecological networks, across multiple scales of organization. Measuring body size provides a relatively simple means of encapsulating and condensing a large amount of the biological information embedded within an ecological network. Recently, important advances have been made by incorporating body size into theoretical models that explore food web stability, the patterning of energy fluxes, and responses to perturbations. Because metabolic constraints underpin bodysize scaling relationships, metabolic theory offers a potentially useful new framework within which to develop novel models to describe the structure and functioning of ecological networks and to assess the probable consequences of biodiversity change.

Stable isotopes challenge the perception of ocean sunfish Mola mola as obligate jellyfish predators

Evidence is provided from stable isotope analysis that aggregations of small ocean sunfish Mola mola (total length <1 m) feed broadly within coastal food webs and their classification as obligate predators of gelatinous zooplankton requires revision.

A generalized functional response for predators that switch between multiple prey species

We develop a theory for the food intake of a predator that can switch between multiple prey species. The theory addresses empirical observations of prey switching and is based on the behavioural assumption that a predator tends to continue feeding on prey that are similar to the prey it has consumed last, in terms of, e.g., their morphology, defences, location, habitat choice, or behaviour. From a predator's dietary history and the assumed similarity relationship among prey species, we derive a general closed-form multi-species functional response for describing predators switching between multiple prey species. Our theory includes the Holling type II functional response as a special case and makes consistent predictions when populations of equivalent prey are aggregated or split. An analysis of the derived functional response enables us to highlight the following five main findings. (1) Prey switching leads to an approximate power-law relationship between ratios of prey abundance and prey intake, consistent with experimental data. (2) In agreement with empirical observations, the theory predicts an upper limit of 2 for the exponent of such power laws. (3) Our theory predicts deviations from power-law switching at very low and very high prey-abundance ratios. (4) The theory can predict the diet composition of a predator feeding on multiple prey species from diet observations for predators feeding only on pairs of prey species. (5) Predators foraging on more prey species will show less pronounced prey switching than predators foraging on fewer prey species, thus providing a natural explanation for the known difficulties of observing prey switching in the field. (C) 2013 Elsevier Ltd. All rights reserved.

Predation in the marine intertidal amphipod Echinogammarus marinus Leach: implications of inter- and intra-individual variation

Studies of competition, predator–prey dynamics and food webs typically consider conspecifics as equal, however, individuals from the same population that are seemingly identical can show considerable variation with regards to a number of processes. Such phenomena may be demonstrated in terms of diet, and the quantities and types of resources that are consumed are commonly considered. The marine amphipod Echinogammarus marinus, a recently demonstrated predator on intertidal rocky shores, has been shown to consume a wide range of food types but it is unknown how this may vary between individuals. Here, we investigated the variation that occurs both among and within individuals of a population of E. marinus with respect to the mean numbers consumed of a common prey item, the isopod Jaera nordmanni. First, by comparing the length of starvation times, used as a proxy for hunger level, individuals maintained without food for up to 24 h consumed significantly less prey during feeding trials than those starved for 48 h and longer. The degree of inter-individual variation within each starvation period was also found to differ, with greater variation among individuals starved for shorter periods of time than those starved for longer time periods. Secondly, we tested whether individual amphipods tracked over time consumed consistently similar numbers of prey or whether they showed intra-individual variation, and if so, to what degree. We found that the numbers of prey consumed per individual could be predicted in the short-term between consecutive feeding trials, however over the long-term this relationship broke down. These results are discussed with respect to potential physiological and behavioural mechanisms, as well as the implications that such variation may have for stability of prey populations in the field.

One size fits all: stability of metabolic scaling under warming and ocean acidification in echinoderms

Responses by marine species to ocean acidification (OA) have recently been shown to be modulated by external factors including temperature, food supply and salinity. However the role of a fundamental biological parameter relevant to all organisms, that of body size, in governing responses to multiple stressors has been almost entirely overlooked. Recent consensus suggests allometric scaling of metabolism with body size differs between species, the commonly cited 'universal' mass scaling exponent (b) of A3/4 representing an average of exponents that naturally vary. One model, the Metabolic-Level Boundaries hypothesis, provides a testable prediction: that b will decrease within species under increasing temperature. However, no previous studies have examined how metabolic scaling may be directly affected by OA. We acclimated a wide body-mass range of three common NE Atlantic echinoderms (the sea star Asterias rubens, the brittlestars Ophiothrix fragilis and Amphiura filiformis) to two levels of pCO(2) and three temperatures, and metabolic rates were determined using closed-chamber respirometry. The results show that contrary to some models these echinoderm species possess a notable degree of stability in metabolic scaling under different abiotic conditions; the mass scaling exponent (b) varied in value between species, but not within species under different conditions. Additionally, we found no effect of OA on metabolic rates in any species. These data suggest responses to abiotic stressors are not modulated by body size in these species, as reflected in the stability of the metabolic scaling relationship. Such equivalence in response across ontogenetic size ranges has important implications for the stability of ecological food webs.

14C as a tool to trace terrestrial carbon in a complex lake system: implications for foodweb structure and carbon cycling

Carbon and nitrogen stable isotope analysis (SIA) has identified the terrestrial subsidy of freshwater food-webs but relies on different 13C fractionation in aquatic and terrestrial primary producers. However dissolved inorganic carbon (DIC) is partly comprised of 13C depleted respiration of terrestrial C and ‘old’ C derived from weathering of catchment geology. SIA thus fails to differentiate between the contribution of old and recently fixed terrestrial C. DIC in alkaline lakes is partially derived from weathering of 14C-free carbonaceous bedrock This
yields an artificial age offset leading samples to appear significantly older than their actual age. As such, 14C can be used as a biomarker to identify the proportion of autochthonous C in the food-web. With terrestrial C inputs likely to increase, the origin and utilisation of ‘old’ or ‘recent’ allochthonous C in the food-web can also be determined. Stable isotopes and 14C were measured for biota, particulate organic matter (POM), DIC and dissolved organic carbon (DOC) from Lough Erne, Northern Ireland, a humic but alkaline lake. High winter δ15N values in calanoid zooplankton (δ15N =24‰) relative to phytoplankton and POM (δ15N =6‰ and 12‰ respectively) may reflect several microbial trophic levels between terrestrial C and calanoids. Furthermore winter calanoid 14C ages are consistent with DOC from inflowing rivers (87 and 75 years BP respectively) but not phytoplankton (355 years BP). Summer calanoid δ13N, δ15N and 14C (312 years BP) indicate greater reliance on phytoplankton. There is also temporal and spatial variation in DIC, DOC and POM C isotopes.

The Fate of Terrestrial Carbon in a Complex Lake System: Lough Erne

Lake food webs were in the past viewed as being fuelled solely by primary production – i.e. by photosynthetic plants and algae. However this has changed as the exports of terrestrial areas into lakes have been taken into account. Previously, terrestrial carbon in lakes was thought to have been buried in sediments or exported to the atmosphere, however recent studies have indicated that terrestrial carbon can supplement primary production in some lakes, or in others be the dominant source of production for the lake food web. In this study radiocarbon has been used in conjunction with stable carbon and nitrogen isotopes to show the utilisation of terrestrial carbon in the food web. The fate of terrestrial carbon in the lake will be discussed as well as the possible mechanisms for the transfer of terrestrial carbon for utilisation in the lake.

Not all jellyfish are equal: isotopic evidence for inter- and intraspecific variation in jellyfish trophic ecology

Jellyfish are highly topical within studies of pelagic food-webs and there is a growing realisation that their role is more complex than once thought. Efforts being made to include jellyfish within fisheries and ecosystem models are an important step forward, but our present understanding of their underlying trophic ecology can lead to their oversimplification in these models. Gelatinous zooplankton represent a polyphyletic assemblage spanning >2,000 species that inhabit coastal seas to the deep-ocean and employ a wide variety of foraging strategies. Despite this diversity, many contemporary modelling approaches include jellyfish as a single functional group feeding at one or two trophic levels at most. Recent reviews have drawn attention to this issue and highlighted the need for improved communication between biologists and theoreticians if this problem is to be overcome. We used stable isotopes to investigate the trophic ecology of three co-occurring scyphozoan jellyfish species (Aurelia aurita, Cyanea lamarckii and C. capillata) within a temperate, coastal food-web in the NE Atlantic. Using information on individual size, time of year and ;delta C-13 and delta N-15 stable isotope values, we examined: (1) whether all jellyfish could be considered as a single functional group, or showed distinct inter-specific differences in trophic ecology; (2) Were size-based shifts in trophic position, found previously in A. aurita, a common trait across species?; (3) When considered collectively, did the trophic position of three sympatric species remain constant over time? Differences in delta N-15 (trophic position) were evident between all three species, with size-based and temporal shifts in delta N-15 apparent in A. aurita and C. capillata. The isotopic niche width for all species combined increased throughout the season, reflecting temporal shifts in trophic position and seasonal succession in these gelatinous species. Taken together, these findings support previous assertions that jellyfish require more robust inclusion in marine fisheries or ecosystem models.