Dominance hierarchies in group-living cleaning gobies: causes and foraging consequences

In species living in social groups, aggression among individuals to gain access to limiting resources can lead to the formation of stable social hierarchies. We tested whether dominance rank in social groups of sponge-dwelling cleaning gobies Elacatinus prochilos in Barbados was determined by physical attributes of individuals or by prior experience of dominance, and examined the foraging consequences of dominance rank. Intraspecific aggression within groups resulted in stable dominance hierarchies that were strongly correlated with fish length. Dominant individuals maintained exclusive territories while subordinate fish occupied broader home ranges. Larger, competitively dominant fish were able to monopolize areas inside the sponge lumen with the highest abundance of the polychaete Haplosyllis spp., a favoured prey item, and achieved the highest foraging rates. The removal of a territorial individual from large groups resulted in a domino-like effect in territory relocation of the remaining fish as individuals moved to the territory previously occupied by the individual just above them in the group hierarchy. Individuals added to existing groups generally failed to gain access to territories, despite being formerly dominant in their original groups. When given the opportunity to choose a location in the absence of larger competitors, gobies frequently preferred positions that were previously defended and that had abundant food. These results suggest that intraspecific competition for resources creates the observed dominance structures and provides support for the role of individual physical attributes in the formation and maintenance of dominance hierarchies.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects

The risk of predation can drive trophic cascades by causing prey to engage in antipredator behavior (e.g. reduced feeding), but these behaviors can be energetically costly for prey. The effects of predation risk on prey (nonconsumptive effects, NCEs) and emergent indirect effects on basal resources should therefore depend on the ecological context (e.g. resource abundance, prey state) in which prey manage growth/predation risk tradeoffs. Despite an abundance of behavioral research and theory examining state‐dependent responses to risk, there is a lack of empirical data on state‐dependent NCEs and their impact on community‐level processes. We used a rocky intertidal food chain to test model predictions for how resources levels and prey state (age/size) shape the magnitude of NCEs. Risk cues from predatory crabs Carcinus maenas caused juvenile and sub‐adult snails Nucella lapillus to increase their use of refuge habitats and decrease their growth and per capita foraging rates on barnacles Semibalanus balanoides. Increasing resource levels (high barnacle density) and prey state (sub‐adults) enhanced the strength of NCEs. Our results support predictions that NCEs will be stronger in resource‐rich systems that enhance prey state and suggest that the demographic composition of prey populations will influence the role of NCEs in trophic cascades. Contrary to theory, however, we found that resources and prey state had little to no effect on snails in the presence of predation risk. Rather, increases in NCE strength arose because of the strong positive effects of resources and prey state on prey foraging rates in the absence of risk. Hence, a common approach to estimating NCE strength – integrating measurements of prey traits with and without predation risk into a single metric – may mask the underlying mechanisms driving variation in the strength and relative importance of NCEs in ecological communities.     

Associations between Non‐Lethal Injury,Body Size,and Foraging Ecology in an Amphibian Intraguild Predator

Theoretical treatments of intraguild predation and its effects on behavioral interactions regard the phenomenon as a size‐structured binary response wherein predation among competitors is completely successful or completely unsuccessful. However, intermediate outcomes occur when individuals escape intraguild (IG) interactions with non‐lethal injuries. While the effects of wounds for prey include compromised mobility and increased predation risk, the consequences of similar injuries among top predators are not well understood, despite the implications for species interactions. Using an amphibian IG predator, Ambystoma opacum (Caudata: Ambystomatidae), we examined associations between non‐lethal injuries and predator body size, foraging strategy, microhabitat selection, and intraspecific agonistic interactions. Wounds were common among IG predators, generally increasing in frequency throughout larval ontogeny. Non‐lethal injuries were associated with differences in predator body size and behavior, with injured predators exhibiting smaller body sizes, increased use of benthic microhabitats, reduced agonistic displays, and increased risk of intraspecific aggression. While such effects were not ultimately associated with reduced foraging success, non‐lethal injury could contribute to niche partitioning between injured and healthy predators via habitat selection, but injured predators likely continue to exert predatory pressure on IG and basal prey populations. Our results indicate that studies of top‐down population regulation should incorporate injury‐related modifications to both prey and predator behavior and size structure.     

Using habitat selection theories to predict the spatiotemporal distribution of migratory birds during stopover – a case study of pink‐footed geese Anser brachyrhynchus

Understanding how animals select for habitat and foraging resources therein is a crucial component of basic and applied ecology. The selection process is typically influenced by a variety of environmental conditions including the spatial and temporal variation in the quantity and quality of food resources, predation or disturbance risks, and inter‐ and intraspecific competition. Indeed, some of the most commonly employed ecological theories used to describe how animals choose foraging sites are: nutrient intake maximisation, density‐dependent habitat selection, central‐place foraging, and predation risk effects. Even though these theories are not mutually exclusive, rarely are multiple theoretical models considered concomitantly to assess which theory, or combination thereof, best predicts observed changes in habitat selection over space and time. Here, we tested which of the above theories best‐predicted habitat selection of Svalbard‐breeding pink‐footed geese at their main spring migration stopover site in mid‐Norway by computing a series of resource selection functions (RSFs) and their predictive ability (k‐fold cross validation scores). At this stopover site geese fuel intensively as a preparation for breeding and further migration. We found that the predation risk model and a combination of the density‐dependent and central‐place foraging models best‐predicted habitat selection during stopover as geese selected for larger fields where predation risk is typically lower and selection for foraging sites changed as a function of both distance to the roost site (i.e. central‐place) and changes in local density. In contrast to many other studies, the nutritional value of the available food resources did not appear to be a major limiting factor as geese used different food resources proportional to their availability. Our study shows that in an agricultural landscape where nutritional value of food resources is homogeneously high and resource availability changes rapidly; foraging behaviour of geese is largely a tradeoff between fast refuelling and disturbance/predator avoidance.     

Biotic and abiotic effects on cohort size distributions in fish

Intraspecific variation in body size is common in animals and plants. Body size affects trophic interactions like foraging ability and vulnerability to predation, which in turn affect individual fitness as well as population stability and extinction risk. Experimental and theoretical work has shown that the size distribution of individuals within cohorts is strongly influenced by intraspecific competition for resources, often leading to skewed frequency distributions. However, little is known about the effects of environmental factors such as climate and eutrophication on the cohort size‐structure of natural populations. We use a long‐term time series of scientific monitoring of a freshwater fish (European perch Perca fluviatilis) to investigate the effects of density dependence, predation, nutrient availability, climate and the timing of spawning on the cohort size distributions. We find that the mean length of the fish is best predicted by the extrinsic factors phosphorus concentration and summer temperature, and the densities of the different age‐classes, whereas the skewness of the length distribution is best predicted by phosphorus concentration, summer temperature, abundance of small fish, and the timing of spawning. Higher nutrient levels, temperatures and densities of small fish increase food availability and thus reduce competition, which is reflected in increased mean length and decreased skewness. The timing of spawning affects skewness presumably through changes in the initial size variation of the cohort and the length of the first growth season. Our results indicate that higher temperatures increase the mean length and decrease skewness due to the concurrent eutrophication of the lake. The study thereby highlights the potential impact of human‐induced environmental change on the size structure of fish populations. More studies are needed to understand better the complex mechanisms through which these factors alter the intensity of intraspecific competition in fish communities.     

Consumer versus resource control and the importance of habitat heterogeneity for estuarine bivalves

The relative influence of consumers (top down) and resources (bottom up) on the distribution and abundance of organisms remains a key question in ecology. We examined the relationships between consumer and resource variables along a productivity gradient for a dominant predator–prey interaction in a marine soft‐sediment system. We 1) quantified density and size of the clam Macoma balthica (prey species) in six replicate sites at each of four habitat types (shallow mud, deep mud, muddy sand and detrital mud) in the Rhode River, Chesapeake Bay. We selected one habitat type of high food availability and clam density (shallow mud) and another of low food availability and clam density (muddy sand) for manipulative experiments. Then, we 2) measured M. balthica survival and growth through transplants, 3) measured food availability as sedimentary organic carbon content, 4) quantified predator density, and 5) calculated predator foraging efficiency in the two habitat types. Clam density in the four habitat types differed and was related to sedimentary carbon availability and predator density. One of the habitats, detrital mud, appeared to be a population sink because it only held juvenile Macoma that never survived to reproductive age. Macoma size and growth, and predator (mainly blue crab Callinectes sapidus) densities were positively correlated with productivity and were higher in shallow mud than muddy sand. In contrast, Macoma mortality, local ‘interaction strength’, and predator foraging efficiency were lower in the productive habitat (shallow mud). Thus, predation intensity was inversely correlated with productivity (food availability); consumer and resource effects differed by habitat type; and, at a relatively small spatial scale, consumer and resource forces jointly determined population dynamics in this soft‐sediment marine system.     

The Effect of Feeder Hotspots on the Predictability and Home Range Use of a Small Bird in Winter

Few studies address how resources and predation risk affect movement patterns and the overall spatial use of prey species. Although movement is generally considered to be dangerous, at large scales, movement may be important for predator avoidance and the predictability of such movement may be key. We examine the movement patterns of a small bird (Junco hyemalis) in winter to better understand how these birds might respond to the trade‐off of unpredictable movements for predator avoidance with the foraging benefits of visiting large, predictable food sources. We manipulated resources by adding feeders to junco home ranges and compared the movement patterns of these flocks to those without access to feeders. Juncos with access to feeders were more spatially and temporally predictable, had reduced movement rates and smaller home range sizes. Our results suggest that the influence of resource distribution on junco movements is high. Juncos with highly productive and predictable resource hotspots may place more value on resources than remaining unpredictable. Consequently, they may be employing non‐movement methods of anti‐predator behavior, such as vigilance, at feeders, although this requires further investigation.     

Ecological Risk Aversion and Foraging Behaviors of Juvenile Squirrel Monkeys (Saimiri sciureus)

The ‘ecological risk aversion hypothesis’ [C.H. Janson and C.P. van Schaik, Juvenile Primates, Oxford Univ. Press, New York (1993), pp. 57–74] proposes that the pattern of slow growth characteristic of juvenile primates is a response to ecological risks (predation and starvation) experienced by juveniles. Juveniles are thought to avoid predation risk by positioning themselves near conspecifics, therefore experiencing high levels of feeding competition with older individuals, reduced access to resources and, consequently, high starvation risks during periods of food scarcity. The present study compared the foraging behaviors of juvenile and adult squirrel monkeys, a small neotropical primate characterized by a long juvenile period, to determine how predation and starvation risks affected juvenile behaviors. The study was conducted in Eastern Amazonia, in a seasonal environment. Due to their slow development, small body size and large group sizes, it was expected that juveniles in this species would behave in a manner consistent with the risk aversion hypothesis. However, age differences in foraging efficiency and foraging success were smaller than predicted. There was also no evidence that juveniles sacrificed access to food for predator protection. Adults did not have preferential access to fruit patches and direct competition was rare. Feeding competition for prey, the most common resource in the troop's diet, was negligible. Therefore, the slow growth and long juvenile period of squirrel monkeys do not correspond with evidence of predation or starvation risk, as predicted by the risk aversion hypothesis.     

Group size influences foraging effort independent of predation risk: an experimental study on rainbow trout

Foraging effort, swimming activity, vertical position and flight response were recorded in focal juvenile rainbow trout Oncorhynchus mykiss at three group sizes: without company, or in visual and chemical contact with either one or five companion fish at two levels of predation risk: high (simulated aerial predator attack) or low (no attack). The predator attack induced a pronounced flight reaction as well as a reduction in vertical position, feeding and swimming activity. The foraging effort of the focal fish increased with group size independent of the level of predation risk, which suggests that the group-mediated increase in foraging activity is caused by competition rather than by reduction in perceived risk. The flight response to the predator attack, however, was stronger when the focal fish had company, suggesting that individuals may benefit from copying the anti-predator response of other group members.     

Relative strengths of trait-mediated and density-mediated indirect effects of a predator vary with resource levels in a freshwater food chain

Predators can affect the density and traits (e.g. morphology, behavior) of their prey, and either change may influence how prey interact with their resources. Thus, predators can interact indirectly with resource species (i.e. two trophic levels below) through two separate mechanisms. The relative strengths of these two kinds of indirect effects have rarely been compared directly, and how their relative importance varies across environmental gradients is virtually unknown. We investigated the relative strength of trait- and density-mediated indirect effects of the predatory insect Belostoma flumineum on algal communities through predation on the pond snail, Physa gyrina , across a gradient of basal resource abundance. Because prey balance the benefits of foraging against the increased risk of predation while foraging, the availability of the prey's resource should influence the strength of anti-predator behavioral responses and hence the strength of trait-mediated indirect interactions. Belostoma presence had positive indirect effects on resources as expected and total predator effects were constant across the basal resource gradient. At low initial resource levels, trait-mediated indirect effects on algal biomass exceeded density-mediated indirect effects, while at high initial resources the reverse was true. Snails showed similar habitat use across the resource gradient suggesting that the anti-predator response was most likely a depression of activity levels.     

Risk‐taking behaviour may explain high predation mortality of GH‐transgenic common carp Cyprinus carpio

The competitive ability and habitat selection of juvenile all‐fish GH‐transgenic common carp Cyprinus carpio and their size‐matched non‐transgenic conspecifics, in the absence and presence of predation risk, under different food distributions, were compared. Unequal‐competitor ideal‐free‐distribution analysis showed that a larger proportion of transgenic C. carpio fed within the system, although they were not overrepresented at a higher‐quantity food source. Moreover, the analysis showed that transgenic C. carpio maintained a faster growth rate, and were more willing to risk exposure to a predator when foraging, thereby supporting the hypothesis that predation selects against maximal growth rates by removing individuals that display increased foraging effort. Without compensatory behaviours that could mitigate the effects of predation risk, the escaped or released transgenic C. carpio with high‐gain and high‐risk performance would grow well but probably suffer high predation mortality in nature.     

Optimizing time and resource allocation trade-offs for investment into morphological and behavioral defense

Prey organisms are confronted with time and resource allocation trade-offs. Time allocation trade-offs partition time, for example, between foraging effort to acquire resources and behavioral defense. Resource allocation trade-offs partition the acquired resources between multiple traits, such as growth or morphological defense. We develop a mathematical model for prey organisms that comprise time and resource allocation trade-offs for multiple defense traits. Fitness is determined by growth and survival during ontogeny. We determine optimal defense strategies for environments that differ in their resource abundance, predation risk, and defense effectiveness. We compare the results with results of simplified models where single defense traits are optimized. Our results indicate that selection acts in favor of integrated traits. The selective advantage of expressing multiple defense traits is most pronounced at intermediate environmental conditions. Optimizing single traits generally leads to a more pronounced response of the defense traits, which implies that studying single traits leads to an overestimation of their response to predation. Behavioral defense and morphological defense compensate for and augment each other depending on predator densities and the effectiveness of the defense mechanisms. In the presence of time constraints, the model shows peak investment into morphological and behavioral defense at intermediate resource levels.     

Food supplementation and predation risk in harsh climate: interactive effects on abundance and body condition of tit species

Food availability and predation risk can have drastic impacts on animal behaviour and populations. The tradeoff between foraging and predator avoidance is crucial for animal survival and will strongly affect individual body mass, since large fat reserves are beneficial to reduce starvation but may increase predation risk. However, two‐factor experiments simultaneously investigating the interactive effects of food and predation risk, are still rare. We studied the effects of food supplementation and natural predation risk imposed by pygmy owls Glaucidium passerinum on the abundance and fat reserves of tit species in boreal forests of north Europe, from January to March in 2012 and in 2013. Food supplementation increased the number of individuals present in a given forest patch, whereas the level of predation risk had no clear impact on the abundance of tit species. The stronger impact of food supply respect to predation risk could be the consequence of the harsh winter conditions in north Europe, with constant below‐zero temperatures and only few (5–7 h) daylight hours available for foraging. Predation risk did not have obvious effects on tit abundance but influenced food consumption and, together with food supplementation, affected the deposition of subcutaneous fat in great tits Parus major. High owl predation risk had detrimental effects on body fat reserves, which may reduce over‐winter survival, but the costs imposed by pygmy owl risk were compensated when food was supplemented. The starvation–predation tradeoff faced by great tits in winter may thus be mediated through variation in body fat reserves. In small species living in harsh environment, this tradeoff appeared thus to be biased towards avoidance of starvation, at the cost of increasing predation risk.     

Habitat-dependent foraging in a classic predator–prey system: a fable from snowshoe hares

Current research contrasting prey habitat use has documented, with virtual unanimity, habitat differences in predation risk. Relatively few studies have considered, either in theory or in practice, simultaneous patterns in prey density. Linear predator–prey models predict that prey habitat preferences should switch toward the safer habitat with increasing prey and predator densities. The density‐dependent preference can be revealed by regression of prey density in safe habitat versus that in the riskier one (the isodar). But at this scale, the predation risk can be revealed only with simultaneous estimates of the number of predators, or with their experimental removal. Theories of optimal foraging demonstrate that we can measure predation risk by giving‐up densities of resource in foraging patches. The foraging theory cannot yet predict the expected pattern as predator and prey populations covary. Both problems are solved by measuring isodars and giving‐up densities in the same predator–prey system. I applied the two approaches to the classic predator–prey dynamics of snowshoe hares in northwestern Ontario, Canada. Hares occupied regenerating cutovers and adjacent mature‐forest habitat equally, and in a manner consistent with density‐dependent habitat selection. Independent measures of predation risk based on experimental, as well as natural, giving‐up densities agreed generally with the equal preference between habitats revealed by the isodar. There was no apparent difference in predation risk between habitats despite obvious differences in physical structure. Complementary studies contrasting a pair of habitats with more extreme differences confirmed that hares do alter their giving‐up densities when one habitat is clearly superior to another. The results are thereby consistent with theories of adaptive behaviour. But the results also demonstrate, when evaluating differences in habitat, that it is crucial to let the organisms we study define their own habitat preference.     

The effect of predation pressure and predator adaptive foraging on the relative importance of consumptive and non‐consumptive predator net effects in a freshwater model system

An important challenge in community ecology is identifying the functional characteristics capable of predicting the nature and strength of predator effects on food webs. We developed an individual‐based model, based on a shallow lake model system, to evaluate the total, consumptive, and non‐consumptive indirect effect that predators have on basal resources when the predators differ in their foraging types (active adaptive foraging or sedentary foraging). Overall, both predator types caused similar total indirect effects on lower trophic levels. However, the nature net effects of predators diverged between predator foraging types. Active predators caused larger non‐consumptive effects, relative to the total indirect effect, irrespective of predation pressure levels. On the other hand, sedentary predators caused larger non‐consumptive effects for lower predation pressure levels, but consumptive effects became more important as predation pressure increased. Our simulations showed that the reliance on a particular mechanism driving consumer–resource interactions is altered by predator foraging behavior and highlight the importance of both prey and predator foraging behaviors to predict the causes and consequences of cascading effects observed in food webs.     

Modeling forage mediated aggregation in a gregarious ruminant

Two explanations exist for the evolutionary origin of grouping in primary consumers: reduction of individual predation risk and resource‐mediated aggregation. While several studies have assessed relationships between aggregation and predation risk, few studies have examined the circumstances under which resource‐mediated aggregation can lead to stable group formation. Using a model, we examined if forage preference alone can generate stable aggregation, and what were the circumstances of its emergence and stability. The model was a spatially explicit grazing model using empirically derived parameters to simulate large ruminant foraging in a meadow. Simulation results indicated that aggregation can spontaneously arise if grazers exhibit preference for forage of higher nutritional quality, usually associated with intermediate stages of forage growth. In this case, foragers could establish and maintain ‘islands’ of high quality forage as a result of revisiting continuous paths of previously grazed patches. However, aggregation was an intermittent phenomenon and occurred only within a narrow range of parameters. If grazer density was low compared to the amount of forage, the grazers’ foraging paths intersected too rarely to form contiguous islands of high forage quality; if their density was too high, the entire available area was uniformly utilized and foraging movements resembled unbounded random walks. We conclude that it is difficult to conceive of the evolution of grouping without the involvement of predators, since the relationship between grazer and forage abundance is ultimately co‐regulated by predator abundance, and because in modern grazers, predator avoidance and foraging behavior seem to be functionally inseparable. Future research should consider the reinforcing effects of predator avoidance as well as foraging behavior on consumer aggregation.     

Coexistence and community structure in a consumer resource model with implicit stoichiometry

We combine stoichiometry theory and optimal foraging theory into the MacArthur consumer-resource model. This generates predictions for diet choice, coexistence, and community structure of heterotroph communities. Tradeoffs in consumer resource-garnering traits influence community outcomes. With scarce resources, consumers forage opportunistically for complementary resources and may coexist via tradeoffs in resource encounter rates. In contrast to single currency models, stoichiometry permits multiple equilibria. These alternative stable states occur when tradeoffs in resource encounter rates are stronger than tradeoffs in elemental conversion efficiencies. With abundant resources consumers exhibit partially selective diets for essential resources and may coexist via tradeoffs in elemental conversion efficiencies. These results differ from single currency models, where adaptive diet selection is either opportunistic or selective. Interestingly, communities composed of efficient consumers share many of the same properties as communities based on substitutable resources. However, communities composed of relatively inefficient consumers behave similarly to plant communities as characterized by Tilman’s consumer resource theory. The results of our model indicate that the effects of stoichiometry theory on community ecology are dependent upon both consumer foraging behavior and the nature of resource garnering tradeoffs.     

Predatory fish sounds can alter crab foraging behaviour and influence bivalve abundance

The risk of predation can have large effects on ecological communities via changes in prey behaviour, morphology and reproduction. Although prey can use a variety of sensory signals to detect predation risk, relatively little is known regarding the effects of predator acoustic cues on prey foraging behaviour. Here we show that an ecologically important marine crab species can detect sound across a range of frequencies, probably in response to particle acceleration. Further, crabs suppress their resource consumption in the presence of experimental acoustic stimuli from multiple predatory fish species, and the sign and strength of this response is similar to that elicited by water-borne chemical cues. When acoustic and chemical cues were combined, consumption differed from expectations based on independent cue effects, suggesting redundancies among cue types. These results highlight that predator acoustic cues may influence prey behaviour across a range of vertebrate and invertebrate taxa, with the potential for cascading effects on resource abundance.     

Benefits and Costs of Dominance in the Angelfish Centropyge bicolor

Social groups are often structured by dominance hierarchies in which subordinates consistently defer to dominants. High‐ranking individuals benefit by gaining inequitable access to resources, and often achieve higher reproductive success; but may also suffer costs associated with maintaining dominance. We used a large‐scale field study to investigate the benefits and costs of dominance in the angelfish Centropyge bicolor, a sequential hermaphrodite. Each haremic group contains a single linear body size‐based hierarchy with the male being most dominant, followed by several females in descending size order. Compared to their subordinate females, dominant males clearly benefited from disproportionately high spawning frequencies, but bore costs in lower foraging rates and greater aggressive defence of their large territories. Within the female hierarchy, more dominant individuals benefited from higher spawning frequencies and larger home ranges, but displayed neither higher foraging rates nor spawn order priority. However, dominance in females was also linked to aggressiveness, particularly towards immediate subordinates, suggesting that females were using energetically costly aggression to maintain their high rank. We further showed by experimentally removing dominant females that the linear hierarchy was also a social queue, with subordinates growing to inherit higher rank with its attendant benefits and costs when dominants disappeared. We suggest that in C. bicolor, the primary benefit of high rank is increased reproductive success in terms of current spawning frequency and the prospect of inheriting the male position in the near future, which may be traded off against the cost of aggressively defending rank and territory.