Trophic Variability Research Articles

Bat consumption by holostean fishes in the Eocene Lake Messel: insights into the trophic adaptability of extinct gars and bowfins.

Direct evidence of trophic interactions between extinct species is rarely available in the fossil record. Here, we describe fish-mammal associations from the middle Eocene of Messel (Germany), consisting of three specimens of holosteans (one Atractosteus messelensis (Lepisosteidae) and two Cyclurus kehreri (Amiidae)) each preserved with a bat specimen (Palaeochiropteryx tupaiodon) lying in close contact with its jaws. This suggests that these fishes probably died after failed swallowing attempts, with the bat wing membrane entangled in their jaws resulting in a fatal handicap. Based on data from modern gars and bowfins, A. messelensis and C. kehreri may have opportunistically attacked drowning and dying individuals or scavenged on floating/sinking carcasses. This hypothesis is also supported by the unusually high number of bat specimens preserved in the deposits of the Eocene Lake Messel, suggesting that this group of small mammals may have represented a substantial food source for generalist feeders. This is the earliest case of chiropterophagy and the first known evidence of bat consumption by lepisosteid and amiid fishes, emphasizing the high trophic variability and adaptability of these groups throughout their evolutionary histories. The newly described associations provide important information for reconstructing the Eocene Lake Messel palaeoecosystem and its trophic web.

Trophic variability of inter- and intra-copepod species in the South Sea of Korea during summer

Spatial variation and interspecies differences in the trophic position (TP) of copepods were investigated using nitrogen isotope ratios of amino acids. In the summer of 2021, coastal waters and the Changjiang diluted water generated clear seawater temperature and salinity fronts in the South Sea of Korea. Paracalanus parvus s. l. was a dominant species in the copepod community, and the second dominant species differed among inshore, intermediate, and offshore sites. The TP of each copepod species was estimated in two ways, considering only metazoan diets (TPGlu, based on glutamic acid and phenylalanine nitrogen isotope ratios) and both metazoan and protistan diets (TPAla, based on alanine and phenylalanine nitrogen isotope ratios). Both TPGlu and TPAla indicated trophic variability among copepod species and the contribution of protistan diets as a food source in the study area. Calanus sinicus showed a similar herbivorous TP of 2.0 in both TPGlu and TPAla, suggesting little contribution from protistan diets. Two copepod species (P. parvus s. l. and Acartia omorii) exhibited TPGlu values of approximately 2.0 but their TPAla values increased from 0.1 to 0.5, indicating mixed diets of both primary producers and protists. The other three copepods (Pseudocalanus sp., Oithona similis, and O. atlantica) showed a wide range in TPGlu (2.4–3.1) and TPAla (2.7–3.4), suggesting that protistan trophic transfers enhance TPAla (by up to 0.5) in omnivorous copepods. We found a spatial variation in the TPs of copepods among water masses by various controlling factors including surface seawater temperature, salinity, and size-fractionated Chl-a. Our findings support that the TP values could be potential indicative of interspecies variability, providing useful information on the composition of the planktonic food web.

Copepod Assemblages at the Base of Mangrove Food Webs during a Severe Drought

The effect of severe drought on the functional groups that sustain the base of the mangrove food webs in semi-arid areas is largely unknown. We therefore analyzed the intra-annual variation in the assemblages and functional groups of copepods in a shallow, low-inflow estuary of the Brazilian semi-arid coast when the most severe drought ever occurred. The lowest density was found in April (upstream region) and the highest in August (downstream region). Three main functional groups were identified, sorted by spawning strategy, and further subdivided according to feeding strategy, trophic regime, and diel vertical migration behavior. The community was significantly influenced by the extreme drought period, presenting a temporal homogenization in terms of composition, and an expressive and unexpected increase in density in the dry period, possibly due to phytoplankton blooms resistant to hypersalinity and the occurrence of copepod species adapted to stressful conditions. The few stress-tolerant species sustaining the food webs with seasonal variations were observed simplifying the trophic variability. The results indicate that hypersalinity can induce changes in the zooplankton community, increasing copepod mortality risk and, so, promoting alteration in the trophic estuarine dynamic.

The influence of trophic status and seasonal environmental variability on morpho-functional traits in tropical man-made shallow lakes

In the tropics, seasons are delimitated by the extent of rainfall resulting in seasonal differences in water parameters shaping phytoplankton community dynamics. Dry periods can intensify eutrophication and often result in seasonal or even perennial cyanobacterial dominance. This study was developed to evaluate phytoplankton response to trophic state and seasonal differences of environmental filters (dry and rainy periods) using the morphology-based functional groups (MBFG) approach. We also aimed at identifying environmental thresholds of each MBFG dominance in six man-made lakes located in the tropical semiarid region of Brazil. Our results showed clear MBFG association with lakes’ trophic states. The dominant groups in mesotrophic conditions were members of MBFGs V (unicellular flagellates) and VI (non-flagellated with a siliceous exoskeleton), and in meso-eutrophic MBFG IV (medium size without specialized traits) dominated. Conversely, MBFG VII (with mucilage and aerotopes) and VIII (nitrogen-fixing cyanobacteria) dominated mostly under eutrophic conditions, though linked to shallower euphotic zones. Light and phosphorous were the most important environmental thresholds associated with MBFG’s dominance. Overall, most of the lakes displayed seasonal differences in environmental filters. In contrast to what was expected, the rainy season was associated with higher nutrients, suspended solids, and reduced euphotic depth compared to the dry season. Our results, overall, show that the effects of seasonality varied across lakes and highlight eutrophication as the main environmental factor for MBFG selection suggesting reduced seasonality effects during dry years in the tropics.

Trophic variability of long tail hake Macruronus magellanicus in the Southwestern Atlantic: movements evidenced by stomach content and stable isotope analysis

Trophic variability of long tail hake Macruronus magellanicus in the Southwestern Atlantic: movements evidenced by stomach content and stable isotope analysis

Application of multiple indices to the evaluation of trophic and ecological status in a tropical macrotidal estuary (Equatorial Margin, Brazil)

ABSTRACT The Paciência river estuary (PRE) is an Amazonian estuary with relevant socio-economic importance; however, it is undergoing a continuous anthropogenic pressure such as the input of untreated domestic effluents. Hence, the phytoplankton community and trophic indices were analysed during three seasons in 2017 to assess the water quality and understand the degree of human influence in a macrotidal estuary. The trophic state of the PRE was determined by applying the trophic state index (TSI), trophic index, and Karydis trophic index, and ecological status by diversity metrics (diversity, evenness, richness, and dominance). Multivariate analysis indicated that TSI and dominance index were the most sensitive indices to changes in environmental conditions. The precipitation regime characterised the estuary in two different trophic scenarios. PRE was hypereutrophic in the dry season and mesotrophic in the rainy/transitional seasons based on the TSI. A high dominance index was recorded in the dry season with blooms of Thalassiosira rotula, reflecting an ecological disturbance. A higher phytoplankton diversity was recorded in the rainy/transitional seasons, indicating a tendency toward a lower stressed ecosystem. Therefore, understanding the trophic state variability in tropical estuaries and its response to changes in phytoplankton ecology is essential to support the management of coastal ecosystems.

VARIAÇÃO INTRAESPECÍFICA EM PEIXES DE RIACHO COM ÊNFASE NA ECOLOGIA TRÓFICA

Assessing intraspecific variability in community structure and functioning are still scarce for stream-dwelling fish. However, measuring ecological traits for individuals is important since species’ response for environmental conditions occurs at the individual level, especially in streams which suffers constantly physical disturbances. Intraspecific variability can be measured for several ecological traits, such as morphology, diet, life-history, and habitat use that varies between populations or within populations. Here, we present some methods and studies that evaluated intraspecific trophic variability in stream-dwelling fish. Results revealing the role of individual variation in structure of biological populations and/or assemblages were selected, and principal issues were discussed. There are a lot of statistical tools that can test individual variation including functional indexes and multivariate analyzes. Results generated from such individual analyzes have high potential to explain the functioning of stream fish populations and communities.

Mixotrophy in diatoms: Molecular mechanism and industrial potential.

Diatoms are microalgae well known for their high variability and high primary productivity, being responsible for about 20% of the annual global carbon fixation. Moreover, they are interesting as potential feedstocks for the production of biofuels and high-value lipids and carotenoids. Diatoms exhibit trophic flexibility and, under certain conditions, they can grow mixotrophically by combing photosynthesis and respiration. So far, only a few species of diatoms have been tested for their mixotrophic metabolism; in some cases, they produced more biomass and with higher lipid content when grown under this condition. Phaeodactylum tricornutum is the most studied diatom species for its mixotrophic metabolism due to available genome sequence and molecular tools. However, studies in additional species are needed to better understand the conservation of this process in diatoms and its potential in industrial applications. Here, we describe the photosynthetic and respiratory pathways involved in mixotrophy and provide an overview of the trophic variability in diatoms. This review also highlights promising areas of industrial applications for diatoms when cultivated under mixotrophy.

Testing the short-term effects of a fish invader on the trophic ecology of a closely related species

The effects of invaders on native species are usually tested using species mean trait values over long time scales. However, considering individual variation over short timescales can help us better understand the interaction between invaders and native species. We compared trophic traits of the non-native guppy (Poecilia reticulata) and the native Brazilian poeciliid Phalloceros harpagos using experiments simulating the early stages of an invasive process. We used short-term mesocosms to simulate an early invasion scenario, where the two species were placed together, and a pre-invasion scenario, where species were kept separated, and analyzed interspecific and intraspecific trophic variability. We also compared the foraging efficiency of species in laboratory experiments. We found no differences on the mean diet of both species between pre and early invasion treatments. At the individual level, in the early invasion scenario, individuals of both species reduced their trophic niche as a probable effect of the presence of the heterospecific. Phalloceros harpagos had higher consumption rates than guppies indicating greater efficiency in feeding on invertebrates. Our results suggest that non-native species were not intrinsically better consumers of high-quality resources. Instead, intraspecific variation might be playing an overlooked role in shaping interactions between species during the early stages of invasion.

Trophic response to ecological conditions of habitats: Evidence from trophic variability of freshwater fish.

To adapt to ecological and environmental conditions, species can change their ecological niche (e.g., interactions among species) and function (e.g., prey‐predation, diet competition, and habitat segregation) at the species and guild levels. Stable isotope analysis of bulk carbon and nitrogen of organisms has conventionally been used to evaluate such adaptabilities in the scenopoetic and bionomic views as the isotopic niche width.Compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids provides trophic information without any disruption of scenopoetic views in the isotope ratios, unlike conventional bulk isotope analysis provides both information and therefore frequently hinders its usefulness for trophic information.We performed CSIA of amino acids to understand the trophic variability of the pike gudgeon Pseudogobio esocinus and largemouth bass Micropterus salmoides as representative specialist and generalist fish species, respectively, from 16 ecologically variable habitats in the four major rivers of Korea.There was little variation (1σ) in the trophic position (TP) among habitats for P. esocinus (± 0.2); however, there was considerably large variation for M. salmoides (± 0.6). The TP of M. salmoides was negatively correlated with the benthic invertebrate indices of the habitats, whereas the TP of P. esocinus showed no significant correlation with any indices. Thus, these two representative fish species have different trophic responses to ecological conditions, which is related to known differences in the trophic niche between specialists (i.e., small niche width) and generalists (i.e., large niche width).Over the past four decades, the conventional bulk isotope analysis has not been capable of deconvoluting “scenopoetic” and “bionomic” information. However, in the present study, we demonstrated that the CSIA of amino acids could isolate trophic niches from the traditional ecological niche composed of trophic and habitat information and evaluated how biological and ecological indices influence the trophic response of specialists and generalists.

Spatial trophic variability of a coastal apex predator, the giant trevally Caranx ignobilis, in the western Indian Ocean

Top predators have substantial downstream effects on the structure, function, and resilience of ecosystems. The influence of top predators on an ecosystem can vary if they occur within multiple habitat types and have a wide niche breadth due to spatiotemporal changes in diet. We examined spatial patterns in trophic position and niche width for an economically important reef-associated fishery species, the giant trevally Caranx ignobilis. We sampled 4 localities in the western Indian Ocean representing different habitats: coral atolls, coastal reefs, and granitic islands. We analyzed isotopic ratios of carbon (13C/12C) and nitrogen (15N/14N), and performed compound-specific amino acid stable isotope analysis (AA-CSIA) to control for baseline nitrogen variation. Our analysis of 12 juveniles and 43 adults revealed wide variation in trophic niche breadth between sampling sites and an offshore to coastal gradient in carbon that drove niche distinctiveness between localities. We observed niche width patterns suggestive of ontogenetic changes in diet and habitat utilization and larger niche sizes at the oceanic island sites than the coastal site. Trophic position estimates ranged from 3.5-5, expanding the trophic range of C. ignobilis relative to previous studies using AA-CSIA and placing it at the equivalent trophic level as many predatory sharks. Our study corroborates prior evidence that C. ignobilis is an important apex predator in reef and island ecosystems. Additionally, we show how evaluating spatiotemporal components of trophic ecology of marine predators is critical for characterizing their functional role and ecosystem influence, allowing for targeted conservation efforts.

Linking intraspecific variability in trophic and functional niches along an environmental gradient

Abstract Intraspecific trophic variability has important ecological and evolutionary implications, and is driven by multiple interacting factors. Functional traits and environmental conditions are important in mediating the trophic niche of individuals because they determine their ability to consume certain prey, their energetic requirements, and resource availability. In this study, we aimed at investigating the interacting effects of functional traits and environmental conditions on several attributes of trophic niche in natural populations. Here, we quantified intraspecific variability in the trophic niche of 12 riverine populations of European minnow (Phoxinus phoxinus) using stable isotope analyses. Functional traits (i.e. morpho‐anatomical traits) and environmental conditions (i.e. upstream–downstream gradient, forest cover) were quantified to identify the determinants of (1) trophic position and resource origin, (2) trophic niche size, and (3) trophic differentiation (β‐diversity) among populations. We demonstrated that trophic position and resource origin covaried with functional traits related to body size and locomotion performance, and that the strength and shape of these relationships varied according to local environmental conditions. The trophic niche size also differed among populations, although no determinant was identified. Finally, trophic β‐diversity was correlated to environmental differentiation among sites. Overall, the determinants of intraspecific variability in trophic niche appeared highly context‐dependent, and related to the interactions between functional traits and environmental conditions. Because populations are currently facing important environmental changes, understanding this context‐dependency is important for predicting food web structure and ecosystem dynamics in a changing world.

Implications of Trophic Variability for Modeling Biomagnification of POPs in Marine Food Webs in the Svalbard Archipelago.

The occurrence of persistent organic pollutants (POPs) in the Arctic has been of constant concern, as these chemicals cause reproductive effects and mortality in organisms. The Arctic acts as a chemical sink, which makes this system an interesting case for bioaccumulation studies. However, as conducting empirical studies for all Arctic species and POPs individually is unfeasible, in silico methods have been developed. Existing bioaccumulation models are predominately validated for temperate food chains, and do not account for a large variation in trophic levels. This study applies Monte Carlo simulations to account for variability in trophic ecology on Svalbard when predicting bioaccumulation of POPs using the optimal modeling for ecotoxicological applications (OMEGA) bioaccumulation model. Trophic magnification factors (TMFs) were calculated accordingly. Comparing our model results with monitored POP residues in biota revealed that, on average, all predictions fell within a factor 6 of the monitored POP residues in biota. Trophic variability did not affect model performance tremendously, with up to a 25% variability in performance metrics. To our knowledge, we were the first to include trophic variability in predicting biomagnification in Arctic ecosystems using a mechanistic biomagnification model. However, considerable amounts of data are required to quantify the implications of trophic variability on biomagnification of POPs in Arctic food webs.

Exploring variability in the diet of depredating sperm whales in the Gulf of Alaska through stable isotope analysis.

Sperm whales interact with commercially important groundfish fisheries offshore in the Gulf of Alaska (GOA). This study aims to use stable isotope analysis to better understand the trophic variability of sperm whales and their potential prey, and to use dietary mixing models to estimate the importance of prey species to sperm whale diets. We analysed tissue samples from sperm whales and seven potential prey (five groundfish and two squid species). Samples were analysed for stable carbon and nitrogen isotope ratios, and diet composition was estimated using Bayesian isotopic mixing models. Mixing model results suggest that an isotopically combined sablefish/dogfish group, skates and rockfish make up the largest proportion of sperm whale diets (35%, 28% and 12%) in the GOA. The top prey items of whales that interact more frequently with fishing vessels consisted of skates (49%) and the sablefish/dogfish group (24%). This is the first known study to provide an isotopic baseline of adult male sperm whales and these adult groundfish and offshore squid species, and to assign contributions of prey to whale diets in the GOA. This study provides information to commercial fishermen and fisheries managers to better understand trophic connections of important commercial species.

Within-individual trophic variability drives short-term intraspecific trait variation in natural populations.

Intraspecific trait variability (ITV) maintains functional diversity in populations and communities, and plays a crucial role in ecological and evolutionary processes such as trophic cascades or speciation. Furthermore, functional variation within a species and its populations can help buffer against harmful environmental changes. Trait variability within species can be observed from differences among populations, and between- and within individuals. In animals, ITV can be driven by ontogeny, the environment in which populations live and by within-individual specialization or variation unrelated to growth. However, we still know little about the relative strength of these drivers in determining ITV variation in natural populations. Here, we aimed to (a) measure the relative strength of between- and within-individual effects of body size on ITV over time, and (b) disentangle the trophic changes due to ontogeny from other sources of variability, such as the environment experienced by populations and individual preferences at varying temporal and spatial scales. We used as a model system the endangered marble trout Salmo marmoratus, a freshwater fish living in a restricted geographical area (<900km2 ) that shows marked changes in diet through ontogeny. We investigated two trophic traits, trophic position and resource use, with stable isotopes (δ15 N and δ13 C), and followed over time 238 individually tagged marble trout from six populations to estimate the trophic changes between and within individuals through ontogeny at three different time-scales (short term: 3months, medium term: 1year and long term: 2years). We found that the relative strength of between- and within-individual effects of body size on trophic position and resource use change strongly over time. Both effects played a similar role in ITV over medium- and long-term time-scales, but within-individual effects were significantly driving trophic variability over short-term scales. Apart from ontogenetic shifts, individuals showed variability in trophic traits as big as the variability estimated between populations. Overall, our results show how the relative strengths of ITV drivers change over time. This study evidences the crucial importance of considering effects of time-scales on functional variability at individual, population and species levels.

Resource use by and trophic variability of Armases cinereum (Crustacea, Brachyura) across human-impacted mangrove transition zones.

In Florida, resource use patterns by Armases cinereum (Armases), a highly abundant crab in coastal habitats, may serve as important indicators of habitat condition. Here we investigated feeding patterns of Armases in coastal palm scrub forest to intertidal mangrove forest transition zones (transitions) as well as the relationship between habitat disturbance and Armases’ trophic position across three pairs of geographically separated populations in Tampa FL, USA. Each pair of sites represented an unmodified “natural” location as well as a “disturbed” location lacking upland terrestrial palm scrub forested habitat. Laboratory experiments established a baseline understanding of feeding preference of Armases offered strictly mangrove material as well as sources abundant at the transition. In-situ feeding behavior was examined using MixSIAR mixing models with δ13C and δ15N stable isotope tracers. Armases showed a strong preference for consuming partially-decomposed mangrove material from Avicennia germinans and an equally strong preference for Iva frutescens. Armases also displayed predatory behavior under laboratory conditions, confirming omnivory in the presence of mangrove material. Stable isotopes revealed a pattern of elevated trophic position of Armases in disturbed habitats over paired natural locations. Diet reconstruction provided coarse resolution of in-situ feeding and results show high spatial variation: in natural habitats, Armases appears to rely heavily upon upland plant material compared to disturbed habitats where it may consume more animal prey. Combined, these findings support that Armases trophic position and diet may indicate habitat quality in mangrove transitions in the southeastern United States.

Moult location and diet of auks in the North Sea inferred from coupled light-based and isotopebased geolocation

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 599:239-251 (2018) - DOI: https://doi.org/10.3354/meps12624 Moult location and diet of auks in the North Sea inferred from coupled light-based and isotope-based geolocation Katie St. John Glew1,*, Sarah Wanless2, Michael P. Harris2, Francis Daunt2, Kjell Einar Erikstad3,4, Hallvard Strøm5, Clive N. Trueman1 1Ocean and Earth Science, University of Southampton, Waterfront Campus, Southampton SO143ZH, UK 2Centre for Ecology & Hydrology, Bush Estate, Penicuik EH26 0QB, UK 3Norwegian Institute for Nature Research, Fram Centre, 9296 Tromsø, Norway 4Centre for Biodiversity Dynamics, Department of Biology, Norwegian University of Science and Technology, 7491 Trondheim, Norway 5Norwegian Polar Institute, Fram Centre, Postbox 6606 Langnes, 9296 Tromsø, Norway *Corresponding author: katie.stjohnglew@soton.ac.uk ABSTRACT: Many pelagic seabirds moult their feathers while at sea, which is an energetically costly behaviour. Mortality rates during moult can be high, so spatial and trophic ecology during this critical period is important for understanding demographic patterns. Unfortunately, individual foraging behaviours specifically linked to at-sea moulting are commonly unclear. This paper combines 2 different approaches to geolocation: data from bird-borne geolocation loggers and stable-isotope assignment using carbon and nitrogen isotope maps (isoscapes). Coupling 2 geolocation processes allows some uncertainties associated with isotope-based assignment to be constrained. We applied this approach to quantify species-specific foraging locations and individual trophic variability during feather regrowth in 3 sympatric auk populations breeding on the Isle of May, Scotland (common guillemot Uria aalge, razorbill Alca torda and Atlantic puffin Fratercula arctica). Inferred foraging areas during moult differed between species and feather types. Guillemots likely underwent moult within the southern North Sea, razorbills along the east coast of England and into the southern North Sea and puffins off the east coast of Scotland. Estimates of individual trophic position varied considerably within feather types (up to 1 trophic level difference between individuals), among feather types grown during different time periods and across the 3 species, with guillemots consistently foraging at higher trophic positions than razorbills and puffins. Used in combination, these methods better constrain foraging areas during moulting, and provide a technique to explore individual differences and flexibility in foraging strategy, which is valuable information for both seabird conservation and marine spatial planning. KEY WORDS: Isoscape · Trophic ecology · Foraging · Moult · Atlantic puffin · Common guillemot · Razorbill Full text in pdf format PreviousNextCite this article as: St John Glew K, Wanless S, Harris MP, Daunt F, Erikstad KE, Strøm H, Trueman CN (2018) Moult location and diet of auks in the North Sea inferred from coupled light-based and isotope-based geolocation. Mar Ecol Prog Ser 599:239-251. https://doi.org/10.3354/meps12624 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 599. Online publication date: July 12, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

The effects of spatial scale and isoscape on consumer isotopic niche width

Abstract The mean and variance of ecological variables are dependent on sampling attributes such as the coverage of environmental heterogeneity (sampling extent) and spatial scale. Trophic niche width is often approximated by bulk tissue stable isotopes of C and N, that is, the population isotopic niche. However, recent studies suggest that environmental heterogeneity (experienced by individuals) may be more important in defining the isotopic niche width than trophic variability. We hypothesised that isotopic niche width will increase monotonically with spatial scale, largely produced by environmental variation, for example, nutrient source. To refine this hypothesis, by describing the shapes of isotope scaling curves, we explored a previously published dataset describing three Chilean intertidal species representing different feeding guilds (grazing snails, suspension‐feeding mussel). We tested these hypotheses on a new, larger dataset describing three functionally analogous intertidal species from Northern Ireland. We generated isotopic variance–area curves from a spatially explicit bootstrap and investigated the scale dependency of environment–isotope relationships, including wave exposure and sub‐habitat heterogeneity. Spatial scale explained 50% of the variance in population isotopic niche widths (bivariate C‐N ellipse area) by simple, nonlinear relationships. Finer scales (&lt;1–10 km lag) accounted for most variance. Scale dependence was strong for δ15N variance, of which &gt;40% was explained by modelling linear coefficients. A δ15N baseline gradient, or isoscape, dominated δ15N variance scaling patterns, from sheltered, terrestrially influenced embayments to exposed, pelagic‐dominated coastline. Consumer δ13C variance had a weaker scale dependence, plateauing at mesoscales (&gt;20 km lag). We show that isotopic niche width is strongly dependent on sampling spatial extent, which controls the environmental heterogeneity experienced by individual consumers. Environmental heterogeneity must be accounted for before isotopic niche width can be considered to accurately represent trophic niche width. Studies conducted at different spatial scales are likely to identify different environment–isotope relationships. We recommend that spatial scale should be incorporated into sampling designs explicitly, easiest by maintaining a consistent lag distance or area within which populations are sampled. Identified isoscapes can be de‐trended, where necessary. A plain language summary is available for this article.

Long-Term Monitoring of Dolphin Biosonar Activity in Deep Pelagic Waters of the Mediterranean Sea

Dolphins emit short ultrasonic pulses (clicks) to acquire information about the surrounding environment, prey and habitat features. We investigated Delphinidae activity over multiple temporal scales through the detection of their echolocation clicks, using long-term Passive Acoustic Monitoring (PAM). The Istituto Nazionale di Fisica Nucleare operates multidisciplinary seafloor observatories in a deep area of the Central Mediterranean Sea. The Ocean noise Detection Experiment collected data offshore the Gulf of Catania from January 2005 to November 2006, allowing the study of temporal patterns of dolphin activity in this deep pelagic zone for the first time. Nearly 5,500 five-minute recordings acquired over two years were examined using spectrogram analysis and through development and testing of an automatic detection algorithm. Echolocation activity of dolphins was mostly confined to nighttime and crepuscular hours, in contrast with communicative signals (whistles). Seasonal variation, with a peak number of clicks in August, was also evident, but no effect of lunar cycle was observed. Temporal trends in echolocation corresponded to environmental and trophic variability known in the deep pelagic waters of the Ionian Sea. Long-term PAM and the continued development of automatic analysis techniques are essential to advancing the study of pelagic marine mammal distribution and behaviour patterns.

Resource composition mediates the effects of intraspecific variability in nutrient recycling on ecosystem processes

Despite the growing evidence for individual variation in trophic niche within populations, its potential indirect effects on ecosystem processes remains poorly understood. In particular, few studies have investigated how intraspecific trophic variability can modulate the effects of consumers on ecosystems through potential changes in nutrient excretion rates. Here, we first quantified the level of intraspecific trophic variability in 11 wild populations of the omnivorous fish Lepomis gibbosus. Outputs from stomach content and stable isotope analyses revealed that the degree of trophic specialization and trophic positions were highly variable between and within these wild populations. There was intrapopulation variation in trophic position of more than one trophic level, suggesting that individuals consumed a range of plant and animal resources. We then experimentally manipulated intraspecific trophic variability to assess how it can modulate consumer‐mediated nutrient effects on relevant processes of ecosystem functioning. Specifically, three food sources varying in nutrient quality (e.g. plant material, macro‐invertebrate and fish meat) were used individually or in combination to simulate seven diet treatments. Results indicated that intraspecific variability in growth and nitrogen excretion rates were more related to the composition of the diet rather than the degree of specialization, and increased with the trophic position of the diet consumed. We subsequently used microcosms and showed that critical ecosystem functions, such as primary production and community respiration, were affected by the variability in excretory products, and this effect was biomass‐dependent. These results highlight the importance of considering variation within species to better assess the effects of individuals on ecosystems and, more specifically, the effects of consumer‐mediated nutrient recycling because the body size and the trophic ecology of individuals are affected by a large spectrum of natural and human‐induced environmental changes.