Stable isotope analysis has emerged as one of the primary means for examining the structure and dynamics of food webs, and numerous analytical approaches are now commonly used in the field. Techniques range from simple, qualitative inferences based on the isotopic niche, to Bayesian mixing models that can be used to characterize food-web structure at multiple hierarchical levels. We provide a comprehensive review of these techniques, and thus a single reference source to help identify the most useful approaches to apply to a given data set. We structure the review around four general questions: (1) what is the trophic position of an organism in a food web?; (2) which resource pools support consumers?; (3) what additional information does relative position of consumers in isotopic space reveal about food-web structure?; and (4) what is the degree of trophic variability at the intrapopulation level? For each general question, we detail different approaches that have been applied, discussing the strengths and weaknesses of each. We conclude with a set of suggestions that transcend individual analytical approaches, and provide guidance for future applications in the field.

/pdf/applying-stable-isotopes-to-examine-food-web-structure-an-1q6l7odjeq.pdf

Applying stable isotopes to examine food‐web structure: an overview of analytical tools

An overview of the evolutionary causes and consequences of behavioural plasticity

1. Apex predators are often assumed to be dietary generalists and, by feeding on prey from multiple basal nutrient sources, serve to couple discrete food webs. But there is increasing evidence that individual level dietary specialization may be common in many species, and this has not been investigated for many marine apex predators. 2. Because of their position at or near the top of many marine food webs, and the possibility that they can affect populations of their prey and induce trophic cascades, it is important to understand patterns of dietary specialization in shark populations. 3. Stable isotope values from body tissues with different turnover rates were used to quantify patterns of individual specialization in two species of 'generalist' sharks (bull sharks, Carcharhinus leucas, and tiger sharks, Galeocerdo cuvier). 4. Despite wide population-level isotopic niche breadths in both species, isotopic values of individual tiger sharks varied across tissues with different turnover rates. The population niche breadth was explained mostly by variation within individuals suggesting tiger sharks are true generalists. In contrast, isotope values of individual bull sharks were stable through time and their wide population level niche breadth was explained by variation among specialist individuals. 5. Relative resource abundance and spatial variation in food-predation risk tradeoffs may explain the differences in patterns of specialization between shark species. 6. The differences in individual dietary specialization between tiger sharks and bull sharks results in different functional roles in coupling or compartmentalizing distinct food webs. 7. Individual specialization may be an important feature of trophic dynamics of highly mobile marine top predators and should be explicitly considered in studies of marine food webs and the ecological role of top predators.

/pdf/contrasting-patterns-of-individual-specialization-and-21zkjrtyt4.pdf

Contrasting patterns of individual specialization and trophic coupling in two marine apex predators

Colonial breeding is widespread among animals. Some, such as eusocial insects, may use agonistic behavior to partition available foraging habitat into mutually exclusive territories; others, such as breeding seabirds, do not. We found that northern gannets, satellite-tracked from 12 neighboring colonies, nonetheless forage in largely mutually exclusive areas and that these colony-specific home ranges are determined by density-dependent competition. This segregation may be enhanced by individual-level public information transfer, leading to cultural evolution and divergence among colonies.

/pdf/space-partitioning-without-territoriality-in-gannets-4jterr9soo.pdf

Space partitioning without territoriality in gannets.

Summary 1. The large amount of discards produced by commercial fisheries can have major impacts on
marine predator populations: this abundant food may increase populations of some scavengers or
decrease others via accidental bycatch. Yet, despite the conservation implications of discard
practices, the ecology of individual scavengers is poorly understood. 2. Here, we assess the influence of commercial fisheries’ activity on the foraging behaviour of
individual breeding northern gannets Morus bassanus. Using recent developments in stable isotope
mixing models (Stable Isotope Analysis inR or SIAR) we estimate individual discard consumption.
Using GPS tracking and the Vessel Monitoring System (VMS), we investigate behavioural
responses to trawlers. 3. Analysis of conventional diet samples, as well as stable isotope ratios of carbon and nitrogen
in blood (plasma and cells), highlight marked individual differences in the proportion of fishery
discards in the diet. Individual differences in foraging behaviour revealed by stable isotopes show
evidence of both short-termconsistency and behavioural flexibility. 4. At-sea path tortuosity of 25 gannets (tracked using GPS loggers) revealed scale-dependent
adjustments in response to VMS-derived fishing vessel locations, as well as to sea surface temperature,
chlorophyll a concentration and copepod abundance. The results also indicate individual
variability in behavioural response to trawlers. 5. Individual differences in the amount of discards estimated from SIAR were negatively correlated
with differences in foraging trip length and body condition, indicating potential fitness
consequences. 6. Synthesis and applications. The management of commercial fisheries and apex predators is a
daunting task. Ultimately, reducing bycatch and removing dependency on discards remain key
conservation priorities, but managers should also ensure that scavenging species have sufficient
alternative food to meet their energetic needs, to ameliorate potential unforeseen knock-on
consequences. The results of Stable Isotope Analysis (SIAR) reveal intra-population differences in
discard consumption by gannets; differences that have impacts on foraging effort and body condition.
The use of GPS tracking and Vessel Monitoring Systems (VMS) reveal that gannet at-sea
behaviour is influenced by fishing vessels, although this also varies among individuals. A combination
of SIAR, GPS tracking and VMS can be used to study fishery ⁄ scavenger interactions in detail
at the individual level, to answer fundamental questions about scavenging behaviour.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2664.2010.01790.x

Individual responses of seabirds to commercial fisheries revealed using GPS tracking, stable isotopes and vessel monitoring systems

Summary 1. We studied chick diet in a known-age, sexed population of a long-lived seabird, the Brunnich’s guillemot ( Uria lomvia ), over 15 years ( N = 136; 1993‐2007) and attached time‐depth‐temperature recorders to examine foraging behaviour in multiple years ( N = 36; 2004‐07). 2. Adults showed specialization in prey fed to offspring, described by multiple indices calculated over 15 years: 27% of diet diversity was attributable to among-individual variation (withinindividual component of total niche width = 0·73); average similarity of an individual’s diet to the overall diet was 65% (mean proportional similarity between individuals and population = 0·65); diet was significantly more specialized than expected for 70% of individuals (mean likelihood = 0.53). These indices suggest higher specialization than the average for an across-taxa comparison of 49 taxa. 3. Foraging behaviour varied along three axes: flight time, dive depth and dive shape. Individuals showed specialized individual foraging behaviour along each axis. These foraging strategies were reflected in the prey type delivered to their offspring and were maintained over scales of hours to years. 4. Specialization in foraging behaviour and diet was greater over short time spans (hours, days) than over long time spans (years). Regardless of sex or age, the main component of variation in foraging behaviour and chick diet was between individuals. 5. Plasma stable isotope values were similar across years, within a given individual, and variance was low relative to that expected from prey isotope values, suggesting adult diet specialized across years. Stable isotope values were similar among individuals that fed their nestlings similar prey items and there was no difference in trophic level between adults and chicks. We suggest that guillemots specialize on a single foraging strategy regardless of whether chick-provisioning and self-feeding. With little individual difference in body mass and physiology, specialization likely represents learning and memorizing optimal feeding locations and behaviours. 6. There was no difference in survival or reproductive success between specialists and generalists, suggesting these are largely equivalent strategies in terms of evolutionary fitness, presumably because different strategies were advantageous at different levels of prey abundance or predictability. The development of individual specialization may be an important precursor to diversification among seabirds.

Individual specialization in diet by a generalist marine predator reflects specialization in foraging behaviour

. Breeding seabirds are extreme central-place foragers, commuting long distances between colonies and feeding areas. Central-place foraging theory predicts that prey items close to the colony will be preferred over prey items distant from the colony, which can lead to prey depletion near the colony (“Storer-Ashmole's halo”). To investigate the relevance of these ideas to a single-prey loader, we equipped chick-rearing Thick-billed Murres (Uria lomvia) with time—depth recorders (1999–2007) and monitored prey deliveries (1993–2008). Because feeding rates were constant for chicks 3–15 days old, we restricted analyses to that age group. Between-date relationships were examined within individuals to avoid confounding effects of specialization and parental quality. The mass of prey items increased with foraging distance0,5, which suggests that large prey items were depleted by foraging pressure. Foraging distance for pelagic species increased through the season in years without spawning. After accountin...

/pdf/central-place-foraging-in-an-arctic-seabird-provides-1082n2pl5c.pdf

Central-place foraging in an arctic seabird provides evidence for storer-ashmole's halo

. Prey items fed to chicks of eight individually-marked and known specialist Thick-billed Murres (Una lomvia) were watched over a 15–year period at Coats Island, Nunavut, and time-depth recorders attached to eight birds in two separate years. Two males were amphipod specialists, one male a cod specialist and another male a shanny specialist; two females were sculpin specialists and two capelin specialists. Although there was likely some gender-related component to diet, there were clear specializations within each gender. Specialization in diet was mirrored by specialization in foraging behavior: amphipod specialists made V-shaped dives and had short foraging ranges, shanny/cod specialists made many shallow dives, sculpin specialists made deep, U-shaped dives, while capelin specialists made V- or W-shaped dives to moderate depths. There was no clear pattern between specialties and age or reproductive success. The use of longitudinal studies in foraging behavior, as long-term studies of individuals...

/pdf/specialization-in-murres-the-story-of-eight-specialists-7hihuyux54.pdf

Specialization in Murres:The Story of Eight Specialists

Organohalogen contaminants and total mercury in forage fish preyed upon by thick-billed murres in northern Hudson Bay

How energy costs affect foraging decisions is poorly understood for marine animals. To provide data relevant to this topic, we examined the relationship between activity levels and foraging behavior by attaching activity recorders to 29 chick-rearing wing-propelled diving birds (thick-billed murres, Uria lomvia) in 1999–2000. We connected the activity during the final dive bout with the prey item we observed being fed to the chicks. After accounting for changes in activity level with depth, activity was highest during the final dive of a dive bout, reflecting maneuvring during prey capture. Pelagic prey items, especially invertebrates (amphipods), were associated with higher depth-corrected activity, leading to shorter dives for a given depth (presumably due to higher oxygen consumption rates) and, thus, shorter search times (lower bottom time for a given depth). Pelagic prey items were likely captured during active pursuit, with the birds actively seeking and pursuing schooling mid-water prey. In contrast, benthic prey involved low activity and extended search times, suggesting that the birds slowly glided along the bottom in search for prey hidden in the sediments or rocks. We concluded that activity levels are important in determining the foraging tactics of marine predators.

Kerry J. Woo

Papers

Individual specialization in diet by a generalist marine predator reflects specialization in foraging behaviour

Central-place foraging in an arctic seabird provides evidence for storer-ashmole's halo

Specialization in Murres:The Story of Eight Specialists

Organohalogen contaminants and total mercury in forage fish preyed upon by thick-billed murres in northern Hudson Bay

Do activity costs determine foraging tactics for an arctic seabird