Cephalopod prey of the wandering albatross Diomedea exulans

Cephalopod beaks from the stomach contents of wandering albatross (Diomedea exulans L.) chicks from Bird Island, South Georgia, were sampled between May and September in 1983 and 1984. Lower beaks were identified and measured, and allometric data were used to calculated mantle length and biomass of the species consumed. A total of 3421 lower beaks were examined, representing 35 species in the 1983 sample and 45 species in the 1984 sample. Eight of the twenty families contributed over 95% of the biomass. In 1984 there were less Onychoteuthidae and more Ommastrephidae than in 1983 and a decrease in the number of species known to occur south of the Antarctic Polar Front. There was a difference in the size-frequency distribution of the cephalopod diet in the two years; in 1984 there was a higher frequency of intermediate-sized specimens, reflecting the greater importance of ommastrephids, especially Illex sp. The energy content of cephalopods in 1984 may have been greater than in 1983. Serial sampling of cephalopod beaks during the austral winter did not reveal evidence of growth. By the age of 200 d, wandering albatross chicks have consumed a total of approximately 100 kg wet weight of cephalopods each.     

Cephalopod hatchling growth: the effects of initial size and seasonal temperatures

Temperature is known to have a strong influence on cephalopod growth during the early exponential growth phase. Most captive growth studies have used constant temperature regimes and assumed that populations are composed of identically sized individuals at hatching, overlooking the effects of seasonal temperature variation and individual hatchling size heterogeneity. This study investigated the relative roles of initial hatchling size and simulated natural seasonal temperature regimes on the growth of 64 captive Octopus pallidus over a 4-month period. Initial weights were recorded, and daily food consumption and fortnightly growth monitored. Two temperature treatments were applied replicating local seasonal water temperatures: spring/summer (14–18°C) and summer/autumn (18–14°C). Overall octopuses in the spring/summer treatment grew at a rate of 1.42% bwd⁻¹ (% body weight per day) compared to 1.72% bwd⁻¹ in the summer/autumn treatment. Initial size influenced growth rate in the summer/autumn treatment with smaller octopuses (<0.25 g) growing faster at 1.82% bwd⁻¹ compared to larger octopuses at 1.68% bwd⁻¹. This was opposite to individuals in the spring/summer treatment where smaller octopuses grew slower at 1.29% bwd⁻¹ compared to larger octopuses at 1.60% bwd⁻¹. Initial size influenced subsequent growth, however, this was dependent on feeding rate and appears to be secondary to the effects of temperature.     

Cephalopod remains from the stomach of a sperm whale stranded off Patagonia

The stomach of a male sperm whale (Physeter macrocephalus) stranded near Comodoro Rivadavia, Argentina, on 21 March 1986, contained 247 cephalopod beaks (128 upper and 119 lower). The lower beaks were identified, their rostral lengths were measured and used to estimate the mantle lengths and wet weight of the representatives of each cephalopod taxon. Fifteen species from nine families were represented. The most numerous species, in order of decreasing abundance, were the large cranchiidMesonychoteuthis hamiltoni (32.8% of total numbers),Mastigoteuthis sp. A (16.0%), andKondakovia longimana andHistioteuthis atlantica (both 13.5%). Almost 85% of the estimated total weight was contributed by two species,Mesonychoteuthis hamiltoni (61.6%) andK. longimana (23.1%). The presence of beaks fromK. longimana, Alluroteuthis antarcticus andM. hamiltoni, which are all thought to be restricted to the Antarctic, suggests that the whale had migrated North prior to stranding.     

Cephalopod remains from sperm whales caught off western Canada

Of 379 cephalopod beaks sampled from stomachs of 20 sperm whales caught to the west of Vancouver Island, Canada, 152 beaks were sorted and measured. Thirteen types of beak were identified as belonging to eleven families. The most numerous species represented are Berryteuthis magister and Moroteuthis robusta comprising 28.9 and 24.3% respectively. These species were estimated to comprise 17.6 and 61.9% respectively of the weight of flesh represented by beaks.     

Feeding strategies and diets of breeding grey-headed and wandering albatrosses at South Georgia

The foraging areas and diets of the grey-headed albatross Thalassarche chrysostoma and wandering albatross Diomedea exulans were studied in March/April 2000 at Bird Island, South Georgia, during their respective chick-rearing and brood-guard periods. Oceanographically, March/April 2000 was abnormal, with warm conditions close to South Georgia. These conditions affected albatross foraging behaviour, particularly that of grey-headed albatrosses. Both species tended to forage in different areas of the ocean, with significant differences in trip durations. Grey-headed albatrosses (n=9) foraged mainly in Antarctic waters (predominantly shelf waters of the South Shetland Islands and Antarctic Peninsula, and also in oceanic waters around South Georgia), feeding mainly on krill (Euphausia superba; 77% by mass). Foraging trips lasted 13.3 days (range: 5–26 days), far longer than the 1–3 days found in previous studies. Only one grey-headed albatross was associated with the APF (Antarctic Polar Front), a reported foraging area in recent studies. Wandering albatrosses (n=9) foraged in Antarctic (South Georgia Shelf) and Antarctic Polar Frontal Zone (APFZ) waters, with trips of 1–4 days trip duration (usual for this species), feeding on fish (46% by mass) and cephalopods (32%). One bird was associated with the APF, and two birds foraged on the shelf/shelf break over the Patagonian shelf. These findings suggest that sea surface temperature anomalies, produced by movement of the APF closer to South Georgia or by eddies, may have had an effect on the foraging strategy of grey-headed albatrosses that year (the main prey of grey-headed albatrosses in previous studies, the ommastrephid Martialia hyadesi, is known to be associated with the APF). Also, when both albatross breeding periods overlap, their foraging areas were complementary, which reflected the prey taken.     

The three-dimensional prey field of the northern krill, Meganyctiphanes norvegica, and the escape responses of their copepod prey

In the north Atlantic, Meganyctiphanes norvegica feeds predominantly on copepods, including Calanus spp. To quantify its perceptual field for prey, and the sensory systems underlying prey detection, the responses of tethered krill to free-swimming Calanus spp. were observed in 3D using silhouette video imaging. An attack–which occurred despite the krill’s being tethered—was characterized by a pronounced movement of the krill’s antennae towards the target, followed by a propulsion and opening of the feeding basket. Frequency distributions of prey detection distances were significantly different in the light vs. the dark, with median values of 26.5 mm and 19.5 mm, respectively. There were no significant differences in the angles at which prey were detected by krill (relative to the predator’s longitudinal body axis) in the light vs. the dark. Prey detections were symmetrically distributed on either side of the predator, in both light and dark. However, significant asymmetry was found in the dorsal–ventral direction with 80% of the prey detections located below the midline of the krill’s body axis and, given the placement and orientation of the compound eyes, presumably outside its visual field of view. This indicates that, at least under these conditions, vision was not the main sensory modality involved in the detection of active prey by M. norvegica. However, under some circumstances, vision may provide supplemental information. Avoidance responses of copepod prey were nearly twice the velocity of their nominal background swimming speed (153 ± 48 and 85 ± 75 mm s⁻¹, respectively), on average taking them 43 ± 16 mm away from the predator. This is far beyond the krill’s perceptual range, suggesting that the escape reaction provides an effective deterrent to predation (although perhaps less so for free-swimming krill). This information can be used to parameterize models that assess the role of krill as predators in marine ecosystems.     

The influence of intraguild predation on prey suppression and prey release: a meta-analysis

Intraguild predation (IGP) occurs when one predator species consumes another predator species with whom it also competes for shared prey. One question of interest to ecologists is whether multiple predator species suppress prey populations more than a single predator species, and whether this result varies with the presence of IGP. We conducted a meta-analysis to examine this question, and others, regarding the effects of IGP on prey suppression. When predators can potentially consume one another (mutual IGP), prey suppression is greater in the presence of one predator species than in the presence of multiple predator species; however, this result was not found for assemblages with unidirectional or no IGP. With unidirectional IGP, intermediate predators were generally more effective than the top predator at suppressing the shared prey, in agreement with IGP theory. Adding a top predator to an assemblage generally caused prey to be released from predation, while adding an intermediate predator caused prey populations to be suppressed. However, the effects of adding a top or intermediate predator depended on the effectiveness of these predators when they were alone. Effects of IGP varied across different ecosystems (e.g., lentic, lotic, marine, terrestrial invertebrate, and terrestrial vertebrate), with the strongest patterns being driven by terrestrial invertebrates. Finally, although IGP theory is based on equilibrium conditions, data from short-term experiments can inform us about systems that are dominated by transient dynamics. Moreover, short-term experiments may be connected in some way to equilibrium models if the predator and prey densities used in experiments approximate the equilibrium densities in nature.     

Cephalopod remains from the stomach of a sperm whale stranded at Rio Grande do Sul in Brazil

The stomach of a 10.2 m (34 ft) male sperm whale which became stranded at Rio Grande do Sul near Rio Grande, Brazil contained 227 upper cephalopod beaks and 255 lower cephalopod beaks. The lower beaks were identified and measured. Beaks of 14 species belonging to 9 families are present. Lower, rostral lengths are compared with those of the same species collected from stomachs of whales killed off South Africa and in the Antarctic. Estimates of the weight of flesh of the various species represented by beaks are made. Two ommastrephid species are estimated to contribute 33.1% of the weight of flesh represented by beaks while the octopoteuthids contribute 28.3% and the Onychoteuthids 18.5%. Three Antarctic species in the stomach show that the whale had migrated from south of Lat. 40°S prior to being caught and had not eaten much food during the journey which would probably have taken him more than 5 d.     

Transient dynamics and the destabilizing effects of prey heterogeneity

The presence of prey heterogeneity and weakly interacting prey species is frequently viewed as a stabilizer of predator-prey dynamics, countering the destabilizing effects of enrichment and reducing the amplitude of population cycles. However, prior model explorations have largely focused on long-term, dynamic attractors rather than transient dynamics. Recent theoretical work shows that the presence of prey that are defended from predation can have strongly divergent effects on dynamics depending on time scale: prey heterogeneity can counteract the destabilizing effects of enrichment on predator-prey dynamics at long time scales but strongly destabilize systems during transient phases by creating long periods of low predator/prey abundance and increasing extinction probability (an effect that is amplified with increasing enrichment). We tested these general predictions using a planktonic system composed of a zooplankton predator and multiple algal prey. We first parameterized a model of our system to generate predictions and tested these experimentally. Our results qualitatively supported several model predictions. During transient phases, presence of defended algal prey increased predator extinctions at low and high enrichment levels compared to systems with only a single edible prey. This destabilizing effect was moderated at higher dilution rates, as predicted by our model. When examining dynamics beyond initial oscillations, presence of the defended prey increased predator-prey temporal variability at high nutrient enrichment but had no effect at low nutrient levels. Our results highlight the importance of considering transient dynamics when assessing the role of stabilizing factors on the dynamics of food webs.     

Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta, in natural prey assemblages

Predation rates and prey selection of the pelagic mysid shrimp, Mysis mixta, were studied experimentally in the northern Baltic Sea in 1998 during their most intensive growth period, from June to October. Functional responses during 5 months were determined by providing the mysids with a natural zooplankton assemblage, diluted to several different concentrations. The results show that ingestion rate increased, along with mysid growth, from early summer to autumn and that saturation level was reached between 400 and 500 μg C l⁻¹. Ingestion rates increased with increasing prey concentration, and sigmoidal curves explained mostly the variation in ingestion rates (explanatory levels of 86–97%). Prey selection was evident in June, July and August, though weaker during the latter 2 months. Selection differed between the studied months but, generally, copepods were more positively selected than cladocerans. Rotifers were the main prey during June and July, when mysids were small, while larger mysids fed on copepods and cladocerans. Of the copepods, Eurytemora affinis was a truly selected species. This study shows that mysids feed on many zooplankton taxa and that they undergo ontogenetic diet shifts. Received: 19 July 2000 / Accepted: 19 October 2000     

Taste-rejection by predators and the evolution of unpalatability in prey

Aposematic species advertise their unpalatability to potential predators using conspicuous warning colouration. The initial evolution of aposematism is thought to occur by warningly coloured mutants emerging in an already unpalatable cryptic species. However, possessing defence chemicals is often costly, and it is difficult to understand what the selective benefits might be for a mutation causing its bearer to be defended in a population of otherwise palatable cryptic prey. One solution to this problem is that chemically defended individuals are tasted and rejected by predators, and are, therefore, more likely to survive predatory attacks than undefended individuals. Using naïve domestic chicks Gallus gallus domesticus as predators and cryptic green chick crumbs as prey, we asked whether the accuracy with which birds discriminated between palatable and unpalatable prey was affected by the palatability of the unpalatable prey (moderately or highly defended), or their frequency in the population (10 or 25%). Birds could discriminate between green prey on the basis of their defences, and showed better discrimination between palatable and unpalatable prey when defended crumbs were highly unpalatable, compared to when they were moderately unpalatable. Although there was no detectable effect of the frequency of unpalatable prey in the population on predator taste-rejection behaviour in our main analysis, frequency did appear to affect the strategies that birds used in their foraging decisions when prey were only moderately unpalatable. How birds used taste to reject prey also suggests that birds may be able to monitor and regulate their chemical intake according to the frequency and defence levels of the unpalatable prey. Taken together, these results show that avian predators can generate selection for unpalatability in cryptic prey by sampling and taste-rejecting prey, but that a relatively large chemical difference between palatable and unpalatable prey may be necessary before unpalatable prey can enjoy a selective advantage. The exact nature of this evolutionary dynamic will depend on other environmental factors, such as defence costs and prey availability, but it provides a mechanism by which defences can evolve in a cryptic population.     

Invasive prey impacts the abundance and distribution of native predators

While an extensive literature exists on the negative effects of invasive species, little is known about their facilitative effects on native species, particularly the role of invasives as trophic subsidies to native predators. The invasive gypsy moth (Lymantria dispar) undergoes periodic outbreaks during which it represents a super-abundant food source for predators capable of consuming it, particularly native cuckoos (Coccyzus erythropthalmus and C. americanus). We examined how gypsy moth outbreaks affect the abundance and distribution of cuckoos using the North American Breeding Bird Survey and 29 years of U.S. Forest Service gypsy moth defoliation records. Abundances of both Black-billed and Yellow-billed Cuckoos were significantly above average during outbreaks, but populations were average or below average in preceding and subsequent years, suggesting that cuckoos are immigrating to defoliations during outbreak years. Spatial analyses showed that cuckoo abundances approximately 40-150 km outside of defoliation areas were significantly below average, and these under-occupied breeding areas extended in all four compass directions around outbreaks. This result supports the idea that cuckoos locate gypsy moth outbreaks during a post-migratory nomadic phase. By shifting the annual distribution of cuckoos, gypsy moths may be shifting the trophic impact of cuckoos across large distances, which could affect native insect herbivores and plants.     

Adaptive prey behavior and the dynamics of intraguild predation systems

Intraguild predation constitutes a widespread interaction occurring across different taxa, trophic positions and ecosystems, and its endogenous dynamical properties have been shown to affect the abundance and persistence of the involved populations as well as those connected with them within food webs. Although optimal foraging decisions displayed by predators are known to exert a stabilizing influence on the dynamics of intraguild predation systems, few is known about the corresponding influence of adaptive prey decisions in spite of its commonness in nature. In this study, we analyze the effect that adaptive antipredator behavior exerts on the stability and persistence of the populations involved in intraguild predation systems. Our results indicate that adaptive prey behavior in the form of inducible defenses act as a stabilizing mechanism and show that, in the same direction that adaptive foraging, enhances the parameter space in which species can coexist through promoting persistence of the IG-prey. At high levels of enrichment, the intraguild predation system exhibits unstable dynamics and zones of multiples attractors. In addition, we show that the equilibrium density of the IG-predator could be increased at intermediate values of defense effectiveness. Finally we conclude that adaptive prey behavior is an important mechanism leading to species coexistence in intraguild predation systems and consequently enhancing stability of food webs.     

Mechanics of prey selection by ephyrae of the scyphomedusa Aurelia aurita

In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (Linnaeus) revealed the importance of relatively large (>1 mm) prey in the diet of these scyphozoan predators. These studies were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 through 1996. Rotifers were the only small prey ingested in quantity, and then only when they were unusually abundant in the plankton. Copepod nauplii, similar in size to rotifers and equally abundant, were rarely consumed. Since copepods evince rapid escape responses, this observation suggested a role for prey escape in determining prey vulnerability, while the predominance of large prey in the diet suggested a role for prey size. Using two dimensional video observations of free-swimming ephyrae and their prey in the laboratory we tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanisms for capture of large versus small prey and for prey of equal size but differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey were 4 to 12 times greater than for small prey taxa. Capture efficiencies for prey of equal size also differed significantly, indicating that other factors influence the outcome of predator–prey interactions. Most prey captures occurred while the ephyrae were swimming and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently drawn into the subumbrella of swimming ephyrae despite average potential escape velocities (25.7 mm s⁻¹) that exceeded mean maximum velocity of fluid flows around the ephyrae (13.1 mm s⁻¹). Large prey were more likely than small prey to contact nematocyst-bearing surfaces both before and after entrainment in flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued swimming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal swimming (copepod nauplii and barnacle nauplii). Copepod nauplii were the prey least likely to be captured because they either “played dead” and were expelled from the subumbrella of the ephyrae before contacting a surface, or they eventually escaped at high velocity. These observations indicate that size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey. Received: 9 April 1997 / Accepted: 5 September 1997     

Scale-dependent response diversity of seabirds to prey in the North Sea

Functional response diversity is defined as the diversity of responses to environmental change among species that contribute to the same ecosystem function. Because different ecological processes dominate on different spatial and temporal scales, response diversity is likely to be scale dependent. Using three extensive data sets on seabirds, pelagic fish, and zooplankton, we investigate the strength and diversity in the response of seabirds to prey in the North Sea over three scales of ecological organization. Two-stage analyses were used to partition the variance in the abundance of predators and prey among the different scales of investigation: variation from year to year, variation among habitats, and variation on the local patch scale. On the year-to-year scale, we found a strong and synchronous response of seabirds to the abundance of prey, resulting in low response diversity. Conversely, as different seabird species were found in habitats dominated by different prey species, we found a high diversity in the response of seabirds to prey on the habitat scale. Finally, on the local patch scale, seabirds were organized in multispecies patches. These patches were weakly associated with patches of prey, resulting in a weak response strength and a low response diversity. We suggest that ecological similarities among seabird species resulted in low response diversity on the year-to-year scale. On the habitat scale, we suggest that high response diversity was due to interspecific competition and niche segregation among seabird species. On the local patch scale, we suggest that facilitation with respect to the detection and accessibility of prey patches resulted in overlapping distribution of seabirds but weak associations with prey. The observed scale dependencies in response strength and diversity have implications for how the seabird community will respond to different environmental disturbances.     

Alternative prey and the dynamics of intraguild predation: theoretical perspectives

A rich body of theoretical literature now exists focused on the three-species module of intraguild predation (IGP), in which a top predator both attacks and competes with an intermediate predator. Simple models of intraguild predation are often unstable, either because one consumer is excluded, or because sustained oscillations emerge from long feedback loops. Yet, many natural IGP systems robustly persist. Standard models of intraguild predation simplify natural systems in crucial ways that could influence persistence; in particular, many empirical IGP systems are embedded in communities with alternative prey species. We briefly review the key conclusions of standard three-species IGP theory, and then present results of theoretical explorations of how alternative prey can influence the persistence and stability of a focal intraguild predation interaction.     

Clam predation by whelks (Busycon spp.): Experimental tests of the importance of prey size,prey density,and seagrass cover

In 57 l-m² samples within a meadow of Halodule wrightii in Bogue Sound, North Carolina, USA, densities of the clams Mercenaria mercenaria and Chione cancellata were positively associated with seagrass cover. Where seagrass was experimentally removed, marked individuals of both clam species exhibited high rates of mortality in fine sand sediments during two successive experiments spanning 13 months. In the unaltered (control) seagrass meadow, M. mercenaria density remained constant over 13 months and C. cancellata density declined at a slower rate than in the unvegetated plots. Seagrass provides these clams with a refuge from whelk (Busycon carica, B. contrarium, and B. canaliculatum) predation, the major cause of mortality and population decline in experimentally unvegetated plots. In 2 factorial field experiments in unvegetated substratum in which densities of M. mercenaria and C. cancellata were varied independently, first over 5 levels (0 X, 1/2X, 1 X, 2 X, 4 X) and subsequently over 4 levels (0 X, 1/4 X, 1 X, 4 X), there was no repeatable intra- or interspecific effect of density on percent survival, or on the rate of any mortality type. Whelk predation fell preferentially on larger size classes of both species, whereas factors which contribute to clam disappearance usually acted more intensely on smaller sizes. Experimental exclusion of large predators by caging demonstrated that even in unvegetated substratum survivorship of both clam species was high in the absence of whelks and other predators. Individuals of C. cancellata live closer to the sediment surface than those of M. mercenaria, which may explain why seagrass does not serve as effectively to protect them from whelk predation. The mechanism of whelk inhibition may depend upon sediment binding by the H. wrightii root mat, which produces a demonstrable decrease in the physical penetrability of surface sediments.     

The choice of two prey types that minimises the probability of starvation

Summary In this paper we investigate the optimal diet of a forager faced with two prey types. Classical optimal foraging theory, based on the maximization of the mean net rate of energetic gain , predicts that the optimal policy is either to take only the more profitable prey type or to take both prey types. The decision between these policies does not depend on the forager's energy reserves or the time available for foraging. We develop two alternative models, based on the minimization of the probability of starvation. In the first model, foraging occurs continuously, and it is optimal to take a prey type if and only if it increases the forager's energy reserves. In the second model foraging stops at dusk, and the forager dies during the night if its reserves at dusk are too low. The optimal policy, which has to be found numerically by dynamic programming, depends on the forager's reserves and the time left till dusk. In general the optimal policy is either to take both types or to take only the more profitable type. Taking both types is optimal when reserves are low, and there is some evidence that this occurs. The models show that factors that have been ignored in classical models may be of importance.     

Environmental chemical cues associated with prey and subsequent prey preference in the wolf spider Hogna carolinensis Hentz (Araneae,Lycosidae)

The purpose of this study was to determine if environmental chemical cues associated with prey can affect subsequent prey choice in wolf spiderlings (Hogna carolinensis). After emergence from the egg sac, three groups of 10 spiderlings were each fed for one-week on one of three naturally-occurring prey species: group 1 fed on nymphs of the field cricket Gryllus pennsylvanicus; group 2 (house cricket, Acheta domesticus); group 3 (mole cricket, Gryllotalpa hexadactyla). They were then tested for subsequent prey preference in choice tests conducted in a plastic arena. Each spiderlings was presented simultaneously with one individual of each prey species in a randomized design. Spiderlings exhibited a significant first preference for the original diet. Thus, experience with certain foods (environmental chemical cues) encountered by newly hatched spiderlings can affect subsequent prey preference in this species.