Predator-prey Encounter Rates Research Articles

Trait‐based analyses reveal global patterns in diverse diets of albacore tuna (Thunnus alalunga)

AbstractSimplifying complex species interactions can facilitate tracking and predicting functional responses to ecological stressors. This is important for highly migratory pelagic predators, exploiting diverse prey fields as they respond to dynamic environments. We reconstructed the historical resource use of albacore tuna (Thunnus alalunga) globally from the 1900s to 2015 and confirmed highly biodiverse diets with 308 prey species, and an additional 238 taxa at coarser taxonomic resolution. We synthesised prey diversity into seven functional trait guilds using hierarchical divisive clustering algorithms as a function of four traits that describe habitat use and influence predator–prey encounter rates – prey habitat association vertically in the water column, horizontally along the coastal to pelagic gradient, seasonal and diel vertical migratory behaviour. We explored variability in historical composition of albacore diets across geographies based on species identity, individual trait information and functional trait guilds using a multi‐matrix modelling framework. Taxonomic information remains important for trophic ecology, however, species‐based diet composition in albacore tuna was highly variable across geographies and years sampled, making interpretation of these patterns difficult. By simplifying species identity into habitat‐based traits, we highlight changes in prey resources consumed, such as the historical importance of near‐surface epipelagic prey resources from coastal to oceanic habitats, and seasonally migrating continental shelf prey, with less frequent pulses of deeper water and demersal taxa. Trait information and trait guilds serve as useful classification frameworks for identifying functionally redundant food web linkages across biodiverse prey, and will prove useful in tracking predators' foraging responses to changing resource availability.

Fire as a driver and mediator of predator-prey interactions.

ABSTRACTBoth fire and predators have strong influences on the population dynamics and behaviour of animals, and the effects of predators may either be strengthened or weakened by fire. However, knowledge of how fire drives or mediates predator–prey interactions is fragmented and has not been synthesised. Here, we review and synthesise knowledge of how fire influences predator and prey behaviour and interactions. We develop a conceptual model based on predator–prey theory and empirical examples to address four key questions: (i) how and why do predators respond to fire; (ii) how and why does prey vulnerability change post‐fire; (iii) what mechanisms do prey use to reduce predation risk post‐fire; and (iv) what are the outcomes of predator–fire interactions for prey populations? We then discuss these findings in the context of wildlife conservation and ecosystem management before outlining priorities for future research. Fire‐induced changes in vegetation structure, resource availability, and animal behaviour influence predator–prey encounter rates, the amount of time prey are vulnerable during an encounter, and the conditional probability of prey death given an encounter. How a predator responds to fire depends on fire characteristics (e.g. season, severity), their hunting behaviour (ambush or pursuit predator), movement behaviour, territoriality, and intra‐guild dynamics. Prey species that rely on habitat structure for avoiding predation often experience increased predation rates and lower survival in recently burnt areas. By contrast, some prey species benefit from the opening up of habitat after fire because it makes it easier to detect predators and to modify their behaviour appropriately. Reduced prey body condition after fire can increase predation risk either through impaired ability to escape predators, or increased need to forage in risky areas due to being energetically stressed. To reduce risk of predation in the post‐fire environment, prey may change their habitat use, increase sheltering behaviour, change their movement behaviour, or use camouflage through cryptic colouring and background matching. Field experiments and population viability modelling show instances where fire either amplifies or does not amplify the impacts of predators on prey populations, and vice versa. In some instances, intense and sustained post‐fire predation may lead to local extinctions of prey populations. Human disruption of fire regimes is impacting faunal communities, with consequences for predator and prey behaviour and population dynamics. Key areas for future research include: capturing data continuously before, during and after fires; teasing out the relative importance of changes in visibility and shelter availability in different contexts; documenting changes in acoustic and olfactory cues for both predators and prey; addressing taxonomic and geographic biases in the literature; and predicting and testing how changes in fire‐regime characteristics reshape predator–prey interactions. Understanding and managing the consequences for predator–prey communities will be critical for effective ecosystem management and species conservation in this era of global change.

Applying the mean free-path length model to juvenile Chinook salmon migrating in the Sacramento River, California

Population vital rates, such as stage-specific survival, are influenced by individual behavior and movement patterns. Yet few methods exist to incorporate behavior into predator-prey models, omitting a potentially important source of variability in population dynamics. Here were combine results from an acoustic telemetry study of juvenile Chinook salmon (Oncorhynchus tshawytscha) with an existing predator prey model, called the mean free-path length model, originally presented in Anderson et al. (2005). The model describes the probability of predator-prey encounters as a function of the predator density and the movement patterns of predators and prey. Greater predator densities and greater variation in movement vectors should result in higher predator-prey encounter rates, and lower survival for the prey. Fitting this model to data provides insight into mechanisms of mortality for migrating fishes. Here we estimate model parameters for two flow conditions in the Sacramento River, California, examining the importance of natural environmental variation in shaping encounters and prey survival. Survival estimates were similar between the high and low flow conditions, yet travel time was slower at lower flows. The model estimates of mean free-path length were lower when compared to those estimated in the Snake River system, corresponding with lower survival. We discuss the value of model parameters estimated from telemetry data in providing a tool for forecasting population-level responses to structural or hydrodynamic modifications in large river systems, and we explore how the XT model can provide insight into nonlinear and threshold-like responses of migratory fish survival to flow.

Predicting preferred prey of Sumatran tigersPanthera tigris sumatraevia spatio-temporal overlap

AbstractEncounter rates of carnivores with prey are dependent on spatial and temporal overlap, and are often highest with their preferred prey. The Critically Endangered Sumatran tigerPanthera tigris sumatraeis dependent on prey populations, but little is known about its prey preferences. We collected camera-trap data for 7 years (2010–2016) in Bukit Barisan Selatan National Park, Sumatra, to investigate spatial and temporal overlap of tigers with potential prey species. We also developed a novel method to predict predator–prey encounter rates and potential prey preferences from camera-trap data. We documented at least 10 individual tigers, with an overall detection rate of 0.24 detections/100 trap nights. Tigers exhibited a diurnal activity pattern and had highest temporal overlap with wild boarSus scrofaand pig-tailed macaquesMacaca nemestrina, but highest spatial overlap with wild boar and sambar deerRusa unicolor. We created a spatial and temporal composite score and three additional composite scores with adjustments for the spatial overlap and preferred prey mass. Wild boars ranked highest for all composite scores, followed by sambar deer, and both are known as preferred tiger prey in other areas. Spatial and temporal overlaps are often considered as separate indices, but a composite score may facilitate better predictions of encounter rates and potential prey preferences. Our findings suggest that prey management efforts in this area should focus on wild boar and sambar deer, to ensure a robust prey base for this Critically Endangered tiger population.

Intermittency in wind-generated turbulence in Lake Baikal: implications for biophysical couplings in the plankton

We used tri-hourly wind velocity data recorded near Listvyanka in summer and autumn 1998, 2003, 2005, 2007 and 2011 to estimate the related turbulent kinetic energy dissipation rate affecting the surface waters of Laker Baikal. Though no seasonal nor inter-annual significant differences were found in wind-generated turbulence, clear significant differences were found in their stochastic properties. These properties were subsequently used to assess the effects of intermittent turbulence on critical processes such as nutrient fluxes around phytoplankton cells, coagulation of phytoplankton cells and predator-prey encounter rates.

Dietary shifts across biogeographic scales alter spatial subsidy dynamics

AbstractOver heterogeneous landscapes, organisms and energy move across ecological boundaries and this can have profound effects on overall ecosystem functioning. Both abiotic and biotic factors along habitat boundaries may facilitate or impede key species interactions that drive these energy flows—especially along the land–sea interface. We synthesized the literature detailing estuarine fish diets and habitat characteristics of salt marshes from U.S. East and Gulf coasts to determine patterns and drivers of cross‐boundary trophic transfers at the land–sea interface. Notably, marsh‐platform species (i.e., killifishes, fiddler crabs) appear virtually absent in the diets of transient estuarine fishes in the Gulf of Mexico, while along the South Atlantic and Mid‐Atlantic Bights, marsh‐platform species appear regularly in the diets of many transient estuarine fishes. Tidal amplitude varied across these three biogeographic regions and likely regulates the availability of marsh‐platform species to transient estuarine fishes via both access to the marsh surface for marine predators and emergence of marsh‐resident prey into the adjacent estuary (i.e., higher tidal amplitude increases predator–prey encounter rates). Surprisingly, marsh shoot density was positively correlated with the presence of marsh‐platform species in the diet, but this pattern appears to be mediated by increased tidal amplitude, suggesting the mode and periodicity of abiotic cycles drive diet structure of transient estuarine fishes more so than local habitat structural complexity. Subsequently, these processes likely influence the degree to which “trophic relay” moves energy from the marsh toward the open estuary. Understanding the dynamics that determine energy flows, spatial subsidies, and ultimately, ecosystem‐level productivity, is essential for implementation of holistic ecosystem‐based approaches to conserve and manage complex landscape mosaics.

Planktonic encounter rates with non-spherical encounter zones.

We present general formulae for planktonic predator-prey encounter rates with encounter zones of convex shape and randomly moving point-like prey with ballistic motion. When the predator is not moving, we show that the encounter rate is independent of the shape of the encounter zone around it and proportional to the product of the surface area of the encounter zone and the prey speed. By contrast, the shape of the encounter zone plays a role when both the predator and the prey are moving. Slow predator motion results in only a weak increase of the encounter rate relative to the non-motile predator situation, but it may lead to a significant shift in where prey impact the surface of the encounter zone. By analysing disc-like and rod-like encounter zones with lengthwise and sideways motion, respectively, we show that fast predator motion may significantly influence the encounter rate, depending on the shape and the direction of motion of the encounter zone.

Intra-guild predation among early benthic phase red and blue king crabs: Evidence for a habitat-mediated competitive advantage

The apparent shift in predominance of commercial crab species from blue king crab (Paralithodes platypus) to red king crab (Paralithodes camtschaticus) around the Pribilof Islands, Alaska, USA, may be driven by ecosystem-level processes such as competitive displacement or predation during early life stages. Specifically, intra-guild predation among species during the early benthic phase might be a source of mortality limiting blue king crab population recovery. In a laboratory experiment, we evaluated how varying ratios of prey species (year-0 blue and red king crabs) and habitat type (shell and cobble) affect prey preference of year-1 red king crabs. Year-0 red king crabs were preferentially consumed over blue king crabs in shell habitat, but not cobble, likely because the smaller interstitial spaces in shell reduced predator foraging efficiency and predator–prey encounter rates. Blue king crabs had high levels of crypsis regardless of habitat type or predator presence/absence, while red king crabs were less cryptic overall compared to blue king crabs, but crypsis increased when predators were present. Preference for red king crabs in shell may reflect divergent adaptations between species for minimizing predation. Blue king crab's light, mottled coloration and relatively smooth, flat carapace is well adapted for visual and physical crypsis in shell habitat, while the monochromatic, dark coloration and pronounced spination of red king crabs may make them more conspicuous. Our results show that habitat can mediate intra-guild predation between Paralithodes species and suggest that shell material may provide blue king crabs a competitive advantage over red king crabs. The extent of shell habitat may be an important factor regulating blue king crab distribution or the recovery of depleted populations around the Pribilof Islands.

Variable prey density modifies the effects of seagrass habitat structure on predator−prey interactions

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 442:59-70 (2011) - DOI: https://doi.org/10.3354/meps09393 Variable prey density modifies the effects of seagrass habitat structure on predator−prey interactions Rachel Lannin, Kevin Hovel* Department of Biology and Coastal & Marine Institute, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA *Email: hovel@sciences.sdsu.edu ABSTRACT: In seagrass habitats, high structural complexity often enhances prey survival by reducing predator–prey encounter rates and predator foraging success. However, faunal density and biomass typically increase with seagrass structural complexity, such that variation in prey or predator density may contribute to or alter relationships between structural complexity and prey survival. We determined whether variability in prey density alters relationships between seagrass structural complexity and (1) prey survival, (2) predator–prey encounter rates, and (3) behavioral components of predator–prey interactions in eelgrass Zostera marina habitat. Using transplanted eelgrass in laboratory mesocosms, we compared trends in mesopredator (juvenile giant kelpfish Heterostichus rostratus) feeding success and behavior, and predator avoidance behavior by prey (grass shrimp Hippolyte californiensis) between experiments in which we (1) held prey density constant while increasing eelgrass shoot density, or (2) increased prey density while increasing eelgrass shoot density. We found that increasing prey density (but not eelgrass density) enhanced proportional prey survival and decreased the probability that mesopredators would attack prey, whereas increasing eelgrass density (but not prey density) decreased the likelihood that prey would attempt to escape from mesopredator attack. Predator–prey encounter rates were influenced both by prey density and eelgrass density, whereas mesopredator attack success was not influenced by either eelgrass density or prey density. Our results suggest that variable prey density modifies relationships between habitat structure and prey survival through a variety of behavioral mechanisms, and that factors that covary with habitat structure, such as prey density, may alter the effects of habitat structure on predation. KEY WORDS: Epifauna · Habitat structure · Predation · Seagrass · Structural complexity · Zostera marina Full text in pdf format PreviousNextCite this article as: Lannin R, Hovel K (2011) Variable prey density modifies the effects of seagrass habitat structure on predator−prey interactions. Mar Ecol Prog Ser 442:59-70. https://doi.org/10.3354/meps09393 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 442. Online publication date: December 05, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Effects of the probability of a predator catching prey on predator–prey system stability

To understand the effect of the probability of a predator catching prey, P catch, on the stability of the predator–prey system, a spatially explicit lattice model consisting of predators, prey, and grass was constructed. The predators and prey randomly move on the lattice space, and the grass grows according to its growth probability. When a predator encounters prey, the predator eats the prey in accordance with the probability P catch. When a prey encounters grass, the prey eats the grass. The predator and prey give birth to offspring according to a birth probability after eating prey or grass, respectively. When a predator or prey is initially introduced or newly born, its health state is set at a high given value. This health state decreases by one with every time step. When the state of an animal decreases to less than zero, the individual dies and is removed from the system. Population densities for predator and prey fluctuated significantly according to P catch. System stability was characterized by the standard deviation ϕ of the fluctuation. The simulation results showed that ϕ for predators increased with an increase of P catch; ϕ for prey reached a maximum at P catch = 0.4; and ϕ for grass fluctuated little regardless of P catch. These results were due to the tradeoff between P catch and the predator–prey encounter rate, which represents the degree of interaction between predator and prey and the average population density, respectively.

Predator–prey encounter rates in turbulent water: Analytical models and numerical tests

In aquatic environments the encounter rates between small predators and their prey are increased by turbulence. We present an expression for the flux of prey into the detective sphere of a small self-propelled predator. We then test this model by direct comparison of theoretical encounter rates with predictions from a numerical experiment where the Navier–Stokes equation is solved explicitly. This allows us to estimate encounter rates numerically under realistic small-scale flow environments, and to explore the accuracy of a simple theoretical formulation of this process. Our analysis includes results for cruising and spiralling motions, as well as pause-travel search behaviour. We find that the analytical model yield surprisingly accurate predictions for models including also the shape of the predator’s perceptive sphere and turbulence conditions. This adds confidence to such simple approximations in applied models of predator–prey encounter rates for a wide range of scale sizes of the predator’s reactive volumes, their motility patterns and turbulence levels.

Microscale Complexity in the Ocean: Turbulence, Intermittency and Plankton Life

This contribution reviews the nonlinear stochastic properties of turbulent velocity and passive scalar intermittent fluctuations in Eulerian and Lagrangian turbulence. These properties are illustrated with original data sets of (i) velocity fluctuations collected in the field and in the laboratory, and (ii) temperature, salinity and in vivo fluorescence (a proxy of phytoplankton biomass, i.e. unicelled vegetals passively advected by turbulence) sampled from highly turbulent coastal waters. The strength of three of the most popular models describing intermittent fluctuations (the lognormal, log-Lévy and log-Poisson models) to fit the distribution of in vivo fluorescence has subsequently been critically assessed. A theoretical formulation for the stochastic properties of biologically active scalars is also provided and validated. Finally, the potential effect of the intermittent properties of turbulent velocity fluctuations on processes relevant to the life of plankton organisms are theoretically investigated. It is shown that the intermittent nature of microscale turbulence may result in (i) a decrease in the rate of nutrient fluxes towards non-motile phytoplankton cells (6-62 %), (ii) a decrease in the physical coagulation of phytoplankton cells (25-48 %) and in the subsequent phytoplankton aggregate volumes (22-41 %), and (iii) a decrease of the turbulence contribution to predator-prey encounter rates (25-50 %).

Predator–prey encounter rates in freshwater piscivores: effects of prey density and water transparency

One of the most fundamental components of predator-prey models is encounter rate, modelled as the product of prey density and search efficiency. Encounter rates have, however, rarely been measured in empirical studies. In this study, we used a video system approach to estimate how encounter rates between piscivorous fish that use a sit-and-wait foraging strategy and their prey depend on prey density and environmental factors such as turbidity. We first manipulated prey density in a controlled pool and field enclosure experiments where environmental factors were held constant. In a correlative study of 15 freshwater lakes we then estimated encounter rates in natural habitats and related the results to both prey fish density and environmental factors. We found the expected positive dependence of individual encounter rates on prey density in our pool and enclosure experiments, whereas the relation between school encounter rate and prey density was less clear. In the field survey, encounter rates did not correlate with prey density but instead correlated positively with water transparency. Water transparency decreases with increasing prey density along the productivity gradient and will reduce prey detection distance and thus predator search efficiency. Therefore, visual predator-prey encounter rates do not increase, and may even decrease, with increasing productivity despite increasing prey densities.

A plankton population model with biomechanical descriptions of biological processes in an idealised 2D ocean basin

A five component pelagic ecosystem model is coupled to a two dimensional configuration of the Princeton Ocean Model (POM), representing an idealised ocean basin with upwelling and downwelling regions. The formulation of the biological equations is based on biomechanical descriptions of the processes of nutrient uptake, light capture, sinking and predator–prey encounter rates. The biological equations have mathematical similarities to existing process-based models which use empirical descriptions of biological processes. These similarities are exploited to determine the planktonic sizes which best correspond to the microzooplankton parameter set in the Edwards et al. (J. Plankton Res. 22 (2000) 1619) modelling study that uses the Franks et al. NPZ model (Mar. Biol. 91 (1986) 121). Simulations show the biomechanical model produces a deep chlorophyll maximum (DCM) when a stable surface mixed layer is present, and a surface bloom during wind-driven coastal upwelling. The Franks biological model is coupled to the physical configuration used for the biomechanical model, and the output from the two models compared in the coastal upwelling region. The behaviour of the biomechanical model is further investigated by undertaking supplementary simulations with the biological parameter values determined (1) using size-based relationships only, (2) using size-based relationships without sinking of phytoplankton and zooplankton, (3) by doubling the cell radii. These simulations provide a preliminary assessment of the biomechanical, size-based approach, and shed light on physical processes at the scale of individual planktonic cells that are determining the rates of biological processes.

Behavioral mechanisms underlying the refuge value of benthic habitat structure for two flatfishes with differing anti-predator strategies

Juvenile flatfish habitat is usually modeled on the basis of sediment grain-size, depth and temperature. Recent evidence indicates that some juvenile flatfishes associate with emergent structures such as sponge, shell and other biogenic and bed-form features of otherwise low-relief shelf habitats. In laboratory experiments we examined the habitat preference and effects of habitat structure upon predation vulnerability of sub-yearling (Age-0) Pacific halibut Hippoglossus stenolepis and northern rock sole Lepidopsetta polyxystra. When given the choice between bare sand or sand with 16% sponge coverage, halibut demonstrated strong preference for sponge, while rock sole showed no preference. Larger Age-2 halibut (used as predators in the subsequent experi- ment) also preferred sponge, but this preference declined with increasing hunger. When allowed to forage for Age-0 flatfishes in either bare sand or sponge, predators consumed more prey in sand and consumed more Age-0 halibut than rock sole. We were able to determine which behavioral processes in the predator-prey interaction were modified by the presence of habitat structure. Predator-prey encounter rates decreased in the sponge habitat as predator search was impeded: predators paused more frequently and swam more slowly to maneuver through the sponges. Sponges also tended to hinder the pursuit of prey. Rock sole utilized stereotypic flatfish defense-mechanisms, relying upon immobility, burial and crypsis, and were less likely to flush at a predator's approach than halibut. Halibut have a less developed ability to mimic sediments, but a deeper/narrower body that confers greater swimming speed, and were more likely to flush as a predator approached. Once they had flushed and were pursued by a predator, halibut were more likely to escape than were rock sole. These experiments support an accumulating body of evidence that emergent structure, in otherwise low-relief benthic habitats, may play an important role in the ecology of some juvenile flatfishes. Removal of emergent structure by towed fishing gear and other anthropogenic and/or natural distur- bance may influence patterns of distribution for juvenile halibut, as fish redistribute to less preferred habitat, and may decrease survival rates through increased losses to predation.

The relative impacts of native and introduced predatory fish on a temporary wetland tadpole assemblage.

Understanding the relative impacts of predators on prey may improve the ability to predict the effects of predator composition changes on prey assemblages. We experimentally examined the relative impact of native and introduced predatory fish on a temporary wetland amphibian assemblage to determine whether these predators exert distinct (unique or non-substitutable) or equivalent (similar) impacts on prey. Predatory fish included the eastern mosquitofish (Gambusia holbrooki), golden topminnow (Fundulus chrysotus), flagfish ( Jordanella floridae), and the introduced walking catfish ( Clarias batrachus). The tadpole assemblage included four common species known to co-occur in temporary wetlands in south-central Florida, USA: the oak toad (Bufo quercicus), pinewoods treefrog (Hyla femoralis), squirrel treefrog (Hyla squirella), and eastern narrowmouth toad (Gastrophryne carolinensis). Tadpoles were exposed to different predators in wading pools under conditions similar to those found in surrounding temporary wetlands (particularly in terms of substrate type, the degree of habitat complexity, and temperature). Native predators were similar with respect to predation rate and prey selectivity, suggesting similar energy requirements and foraging behavior. Conversely, native fish predators, especially G. holbrooki, were distinct from the introduced C. batrachus. In contrast to expectations, C. batrachus were less voracious predators than native fish, particularly G. holbrooki. Moreover, survival of G. carolinensis and H. femoralis were higher in the presence of C. batrachus than G. holbrooki. We suggest that C. batrachus was a less efficient predator than native fish because the foraging behavior of this species resulted in low predator-prey encounter rates, and thus predation rate. In combination with a related field study, our results suggest that native predatory fish play a stronger role than C. batrachus in influencing the spatial distribution and abundance of temporary wetland amphibians in the landscape.

The Norwegian spring-spawning herring - spawning, larval drift and larval retention

The Norwegian spring-spawning herring appear to spawn in areas where the larvae may be temporarily retained (retention areas). This article aims to shed light on the spawning and larval drift of this herring stock with special emphasis on the role of the retention areas. Year class strength seems to be positively correlated with high wind speed over the spawning fields in April and slow northerly larval drift. Most of the Norwegian continental shelf is covered by stratified water all year. In the retention areas, the vertical stratification is reduced during winter/spring compared with the surrounding waters. The windinduced turbulence generated by the high wind speeds in April will, therefore, reach deeper into the retention areas and thereby probably increase the predator-prey encounter rate for the first-feeding herring larvae and favour a high survival rate. The coincidence of slow northerly larval drift and good recruitment supports the conclusion that interannual variations in the degree of retention could be an important recruitment-regulating mechanism for the Norwegian spring-spawning herring.

Prey Patchiness, Predator Survival and Fish Recruitment

A simple mathematical model is presented for the population dynamics of fish larvae when the main food supply, in this case copepods, is spatially patchy in its distribution. Encounters by an individual predator (larva) with prey patches, and with individual prey within patches, are represented by Poisson processes. It is demonstrated analytically, and confirmed by numerical experiments, that prey patchiness fails to alter mean predator–prey encounter rates from their values for homogeneous prey distributions. Individual variance in encounter rate is, however, much affected. This has significant consequences for the (small) numbers of larvae surviving to metamorphosis and recruitment to the adult fish population.

Turbulence intermittency, small-scale phytoplankton patchiness and encounter rates in plankton: where do we go from here?

Turbulence is widely recognized to enhance contact rates between planktonic predators and their prey. However, previous estimates of contact rates are implicitly based on homogeneous distributions of both turbulent kinetic energy dissipation rates and phytoplanktonic prey, while turbulent processes and phytoplankton cell distributions have now been demonstrated to be highly intermittent even on small scales. Turbulent kinetic energy dissipation rates and intermittent (i.e. patchy) phytoplankton distributions can be wholly parameterized in the frame of universal multifractals. Using this framework and assuming statistical independence between turbulent kinetic energy dissipation rate and phytoplankton distributions, we evaluated the effect of intermittent turbulence and the potential effects of zooplankton behavioral responses to small-scale phytoplankton patchiness on predator–prey encounter rates. Our results indicated that the effects of turbulence on predator–prey encounter rates is about 35% less important when intermittently fluctuating turbulent dissipation rates are considered instead of a mean dissipation value. Taking into account zooplankton behavioral adaptations to phytoplankton patchiness increased encounter rates up to a factor of 60.

Predation rates of nemertean predators: the case of a rocky shore hoplonemertean feeding on amphipods

Estimates of the predation rates of benthic nemerteans are often based on observations of single individuals, and consequently they may not be representative for all members of a population of these predators. Herein we conducted controlled and repeatable laboratory experiments on the predation rate of the hoplonemertean Amphiporus nelsoni Sanchez 1973, which is common at exposed rocky shores along the central Chilean coast. During the austral fall (April, May 2000), nemerteans were observed in relatively high numbers crawling in the intertidal zone during early morning or late-afternoon low tides. When these nemerteans were offered living amphipods held by a forceps, they immediately attacked the amphipods and fed on them. In the laboratory experiments, nemerteans preferred the amphipod Hyale maroubrae Stebbing, 1899, which is also very common in the natural habitat of A. nelsoni. The nemerteans preyed to a higher extent on small males and non-ovigerous females than would have been expected from their abundance. We suggest that these (non-reproductive) stages of H. maroubrae are very mobile and therefore have a high likelihood of encounters with nemerteans. Predation rates reached maxima when nemerteans were provided prey densities of four or more of their preferred prey species, H. maroubrae, furthermore indicating that encounter rates with prey may affect predation rates. In long-term laboratory experiments, A. nelsoni consumed more amphipods during low tide conditions than during high tide conditions. Many nemerteans in the field prefer particular environmental conditions (e.g. nocturnal low tides), which restricts the time available for successful feeding. In the long-term experiment, predation rates of A. nelsoni never exceeded 0.5 amphipods nemertean−1 d−1. Maximum feeding events were 3 or 4 amphipods nemertean−1 d−1, but this only occurred during 10 out of a possible 2634 occasions. Nemerteans that had consumed 3 or 4 amphipods during 1 day, consumed substantially less prey during the following days. Towards the end of the long-term experiment, average predation rates decreased to ∼0.2 amphipods nemertean−1 d−1, corresponding to predation rates reported for other nemertean species (0.1–0.3 prey items nemertean−1 d−1). We suggest that predation rates from laboratory experiments represent maximum estimates that may not be directly transferable to field populations. Additionally, low predator–prey encounter rates with preferred prey in the field may further limit the predation impact of nemertean predators in natural habitats.