The effects of human presence, flock size and prey density on shorebird foraging rates

Animals may alter their foraging behaviour in the presence of humans because they perceive humans as potential predators. In this study I determined whether people caused shorebirds to reduce feeding rates at a stopover site in coastal British Columbia, Canada. I controlled for prey density and flock size because these variables may influence both the foraging rates as well as the effect of human disturbance on feeding efficiency. Semipalmated plovers decreased feeding rates when there were more people on the beach (multiple regression: F_1,15=5.86, b=0.59, P=0.029, R²=37.6%). For least sandpipers, the effect of human densities on feeding rates depended on flock size (F_1,21=5.97, P=0.023) and amphipod availability (F_1,21=4.98, P=0.037). This study demonstrated the importance of measuring subtle behavioural changes in foraging rates along with key ecological variables in order to assess the true impact of human disturbance on migratory shorebirds.     

Non-lethal effects of predators on prey growth rates depend on prey density and nutrient additions

A number of studies show that predators can depress prey growth rates by inducing reductions in foraging activity, but the size of this non-lethal effect is quite variable. Here I investigate how prey density and resource productivity may alter the extent to which predators depress the growth rates of their prey. Theory predicts that when resources are overgrazed, an increase in predation risk will have little net effect on individual food intake because the decline in foraging effort will be offset by an increase in resource level. Thus, the non-lethal effects of predators on prey growth rates should depend upon prey density and resource productivity in a predictable manner, with the growth penalty imposed by predators being strongest when resources are undergrazed and weakest when resources are overgrazed. I tested this hypothesis by manipulating predation risk, prey density, and nutrient additions in a mesocosm experiment with the pulmonate snail Helisoma trivolvis . Refuge use by snails was 45% higher in the presence of caged crayfish than in their absence. Snail growth rates were reduced, on average, by 24% in the presence of caged crayfish. However, the magnitude of the growth penalty exacted by crayfish depended on snail density and nutrient additions. When snails were stocked at high density and nutrient additions were low, growth suppression was just 2.6%. At the other extreme, when snails were at low density and nutrient additions were high, growth suppression was 44.6%. Thus, the non-lethal effects of predators on prey growth depend on environmental context, illustrating an important link between individual traits and system-level properties.     

The effects of plant dispersion and prey density on parasitism rates in a naturally patchy habitat

Despite extensive research on parasitoid-prey interactions and especially the effects of heterogeneity in parasitism on stability, sources of heterogeneity other than prey density have been little investigated. This research examines parasitism rates by three parasitoid species in relationship to prey density and habitat spatial pattern. The herbivore Itame andersoni (Geometridae) inhabits a subdivided habitat created by patches of its host plant, Dryas drummondii, in the Wrangell Mountains of Alaska. Dryas colonizes glacial moraines and spreads clonally to form distinct patches. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame is attacked by three parasitoids: an ichneumonid wasp (Campoletis sp.), a braconid wasp (Aleiodes n. sp.), and the tachinid fly (Phyrxe pecosensis). I performed a large survey study at five distinct sites and censused Itame density and parasitism rates in 206 plant patches for 1–3 years. Parasitism rates varied with both plant patch size and isolation and also between sites, and the highest rates of overall parasitism were in the smallest patches. However, the effects of both small- and large-scale heterogeneity on parasitism differed for the three parasitoid species. There was weak evidence that Itame density was positively correlated with parasitism for the braconid and tachinid at the patch scale, but density effects differed for different patch sizes, patch isolations, and sites. At the site scale, there was no evidence of positive, but some indication of negative density-dependent parasitism. These patterns do not appear to be driven by negative interactions between the three parasitoid species, but reflect, rather, individual differences in habitat use and response to prey density. Finally, there was no evidence that parasitism strongly impacted the population dynamics of Itame. These results demonstrate the importance of considering habitat pattern when examining spatial heterogeneity of parasitism and the impacts of parasitoids. Received: 3 June 1999 / Accepted: 4 October 1999     

Effect of light, prey density, and prey type on the feeding rates of Hemimysis anomala

Hemimysis anomala is a near-shore mysid native to the Ponto-Caspian region that was discovered to have invaded Great Lakes ecosystems in 2006. We investigated feeding rates and prey preferences of adult and juvenile Hemimysis in laboratory experiments to gain insight on the potential for Hemimysis to disrupt food webs. For both age groups (AGs), we measured feeding rates as a function of prey abundance (Bosmina longirostris as prey), prey type (B. longirostris, Daphnia pulex, and Mesocyclops sp.), and light levels (no light and dim light). Mean feeding rates on Bosmina increased with prey density and reached 23 ind. (2 h)^?1 for adults and 17 ind. (2 h)^?1 for juveniles. Dim light had little effect on prey selection or feeding rate compared to complete darkness. When feeding rates on alternate prey were compared, both AGs fed at higher rates on Bosmina than Daphnia, but only juveniles fed at significantly higher rates on Bosmina relative to Mesocyclops. No significant differences were observed between feeding rates on Mesocyclops and on Daphnia. Hemimysis feeding rates were on the order of 30–60% of their body weight per day, similar to predatory cladocerans that have been implicated in zooplankton declines in Lakes Huron and Ontario.     

Effects of prey density and short term food deprivation on the growth and survival of American shad larvae

Growth and survival of 16-and 18-day-old American larvae ( Alosa sapidissima ) increased with prey density (0, 500, and 1000 Artemia l⁻¹), though differences were not always significant. Significant differences were not observed in growth parameters and survival for larvae fed 500 v . 1000 Artemia 1⁻¹ in laboratory experiments. Food deprivation for as little as 2 days had significant effects on survival but growth effects were not detectable until 4 days of starvation. These findings have application to the Susquehanna River where c . 11 million 2.5-week-old shad larvae are released annually as part of a basin wide effort to restore the species. Larvae released at upriver sites where food may be limiting would drift for 2–3 days to reach reservoirs with higher prey densities. Based on these laboratory findings, survival of shad may be reduced significantly if larvae do not feed prior to reaching reservoir environments.     

The effects of food quantity,light, and temperature on clearance rates in freshwater bivalves (Cyrenidae and Unionidae)

Hydrobiologia - Assessing the environmental variables that influence freshwater bivalve filtration activity is key to better understand the functioning of stream ecosystems. In the present study,...     

Predation by Neomysis mercedis: effects of temperature, Daphnia magna size and prey density on ingestion rate and size selectivity

1. Neomysis mercedis predation rates on Daphnia magna were determined under laboratory conditions. There were generally no consistent differences between the number of Daphnia ingested at 10 and 14°C. 2. At each temperature, the number of prey consumed increased with mysid size and decreased with Daphnia size. 3. For small prey the relationship between ingestion rate and prey density represented a Type II functional response. However, for larger prey there was no significant relationship between density of prey and consumption by mysids. 4. The pattern of size-selective predation by Neomysis was studied to test the optimal foraging hypothesis. For prey populations with mixed size classes, the smallest size of prey was consumed most frequently but intermediate size prey provided the greatest biomass. These observations are contrary to our predictions based on calculations of profitability of different sizes of prey.     

The effects of prey density on survival and growth of damselfly larvae

Abstract.

• 1 Instar 10, 11 and 12 Ischnura larvae were maintained throughout the instar on a constant food supply (Daphnia magna of a standard size).
• 2 A very narrow range of prey availability separated cohorts in which all the larvae successfully moulted to the next instar, from those in which all the larvae died (less than one Daphnia per day in the case of instar 10).
• 3 Larvae were able to survive long periods without food at 16°C.
• 4 Prey availability had a marked effect on development rates, which continued to increase at prey densities well in excess of 100% survival by the larvae.
• 5 The data on Ischnura survival and development rates are compared with previously published general models of these two components of the predators' rate of increase.
• 6 We conclude that starvation is very unlikely to be a significant cause of mortality in the field for Ischnura (or indeed most other Odonata larvae), but that prey availability undoubtedly influences development rates. Natural selection should therefore favour larvae that forage optimally to minimize development times.

     

The effects of temperature and prey density on the development rates and growth of damselfly larvae (Odonata: Zygoptera)

Abstract. 1. The effects of prey density and temperature on the feeding and development rates of several late instars of the larvae of three common European damselflies ( Lestes sponsa (Hansemann), Coenagrion puella (L.) and Ischnura elegans van der Linden) were investigated in a laboratory experiment.
2. Functional responses were used to estimate maximum feeding rates. Maximum feeding rates were compared between species, instars and temperatures by expressing prey consumption in terms of prey biomass consumed per unit predator biomass. Lestes was capable of feeding at almost twice the rate of either Coenagrion or Ischnura.
3. Higher feeding rates led to faster development rates and there was an interaction between species and temperature. With the exception of those with very low feeding rates, larvae maintained at higher temperatures, but similar feeding rates, developed faster. Under similar conditions of temperature and feeding rate, Lestes larvae developed faster than larvae of either Coenagrion or Ischnura.
4. Faster development rates at similar rates of food intake were achieved at the cost of reduced size-increases between instars.
5. The differences in the responses of the three species are discussed in the light of their respective life histories, and with reference to a recent model of population regulation in damselflies.     

Feeding characteristics of two tintinnid ciliate species on phytoplankton including harmful species: effects of prey size on ingestion rates and selectivity

The feeding abilities of two tintinnid ciliates, Favella ehrenbergii and Favella taraikaensis, on 10 species of flagellates including harmful marine algae were examined under single prey conditions, and selective feeding of F. taraikaensis on two species of algae of different sizes was investigated under mixed prey conditions. Ingestion rates calculated from the rate of increases of auto-fluorescent particles inside food vacuoles in individuals ranged from 0.5 to 22.1 cells ind(-1) h(-1) for F. ehrenbergii and from 0.8 to 44.9 cells ind(-1) h(-1) for F. taraikaensis on nine species of prey algae. Significant ingestion rates of both Favella species could not be detected on Heterosigma akashiwo, although some individuals of both species were observed ingesting H. akashiwo in the initial incubation period. The relationship between prey cell volume and ingestion rate could be expressed mathematically for both Favella species, indicating that it is possible to estimate the potential feeding activity of each Favella species on specific algae using an equation, and may be applicable to evaluate the food value of prey alga for both Favella species. When F. taraikensis was fed mixtures of H. circularisquama and Pavlova lutheri, significant ingestion rates of H. circularisquama by F. taraikaensis could not be measured when H. circularisquama accounted for less than 20% of the other prey biomass. However, clear selectivity for H. circularisquama was observed when H. circularisquama reached and exceeded 34% of the other prey biomass. Under mixed prey conditions, it is likely that the selectivity of F. taraikaensis is stronger for larger prey compared to prey algae with a size near the lower limit on which F. taraikaensis can feed.     

The effects of food deprivation on beta-adrenergic responsiveness in male rats

Male rats were deprived of food for varying lengths of time (0-96 h) and their responses to beta-adrenergic stimulation with isoproterenol were tested. Food deprivation for 48 or 96 h attenuated the increase in tail skin temperature normally seen following administration of isoproterenol. The degree of attenuation was dependent on the duration of the deprivation period. Rats deprived of food for 96 h and then refed for 48, 96, or 144 h showed a return of tail skin temperature response to near normal levels. The increase in heart rate observed following administration of isoproterenol was also attenuated following 48 or 96 h of food deprivation. Again, the degree of attenuation was dependent on the duration of the deprivation period. Food deprivation for 96 h led to a decrease in basal plasma levels of T3, T4, and glucose. The increase in plasma glucose following administration of isoproterenol was also attenuated following 96 h of food deprivation. In contrast to the thermal, cardiac, and glucose responses, the dipsogenic response to isoproterenol was increased following food deprivation. The attenuation in beta-adrenergic responses observed in the food-deprived rats might help explain the effects of food deprivation on cold tolerance.     

The effects of food deprivation and re-feeding on bovine adipose-tissue glycogen synthase.

Bovine adipose-tissue glycogen metabolism was studied during food deprivation and re-feeding. Changes in the specific activity of adipose-tissue glycogen synthase paralleled changes in tissue glycogen content: both parameters increased during food deprivation and remained so during the first 10 days of re-feeding. The values for the A0.5 (activation constant) for glucose 6-phosphate of the freshly isolated enzyme from adipose tissue from fed and starved steers were 2.9 +/- 0.1 mM and 0.90 +/- 0.05 mM respectively. Additionally, whereas incubation of adipose-tissue extracts from fed steers did not activate endogenous glycogen synthase (through a presumed phosphoprotein phosphatase mechanism), the enzyme from starved or re-fed (up to 3 days re-feeding) steers was reversibly activated as measured by changes in the value for the A0.5 for glucose 6-phosphate. Thus activation of bovine adipose-tissue glycogen synthase during food deprivation appears to be related to expression of glycogen synthase phosphatase activity. These effects of food deprivation on bovine glycogen metabolism contrast markedly with the effects observed in rat adipose tissue.     

Growth of pike larvae: effects of prey, turbidity and food quality

We studied experimentally the effects of turbidity and prey composition on pike larval growth and hypothesized that pike larval growth varies with turbidity and food quality. We reared the first-feeding pike larvae (Esox lucius) in laboratory tanks with (1) clear or (2) turbid water provided with zooplankton rations from (3) an inner and (4) an outer archipelago site. The sites differ in physical features, salinity, eutrophication status, zooplankton community structure and density. Pike larvae showed the highest weight increase in clear water with zooplankton from the outer site and the poorest weight increase in turbid water with zooplankton as prey from the inner site. Our fatty acid analysis revealed that unsaturated fatty acid levels were highest in the outer site. The relative percentage of copepods was also higher in the outer site. This study supports the hypothesis that turbidity weakens the ability of pike larvae to capture certain prey. Further, zooplankton community composition matters in turbid water, but is not a primary factor in clear water.     

The significance of some methodological effects on filtration and ingestion rates of the rotiferBrachionus plicatilis

Filtration rate(F) and ingestion rate(I) were measured in the rotiferBrachionus plicatilis feeding on the flagellateDunaliella spec. and on yeast cells(Saccharomyces cerevisiae). 60-min experiments in rotating bottles served as a standard for testing methodological effects on levels ofF andI. A lack of rotation reducedF values by 40 %, and a rise in temperature from 18° to 23.5 °C increased them by 42 %. Ingestion rates increased significantly up to a particle (yeast) concentration of ca. 600–800 cells · l^–1; then they remained constant, whereas filtration rates decreased beyond this threshold. Rotifer density (up to 1000 ind · ml^–1) and previous starvation (up to 40 h) did not significantly influence food uptake rates. The duration of the experiment proved to have the most significant effect onF andI values: in 240-min experiments, these values were on the average more than 90 % lower than in 15-min experiments. From this finding it is concluded that ingestion rates obtained from short-term experiments (60 min or less) cannot be used in energy budgets, because they severely overestimate the actual long-term feeding capacity of the rotifers. At the lower end of the particle size spectrum (2 to 3 µm) there are not only food cells, but apparently also contaminating faecal particles. Their number increased with increasing duration of experiments and lead to an underestimation ofF andI. Elemental analyses of rotifers and their food suggest thatB. plicatilis can ingest up to 0.6 mJ or ca. 14 % of its own body carbon within 15 min. The long term average was estimated as 3.4 mJ · ind^–1 · d^–1 or ca. 75 % of body carbon · d^–1.Contribution to research project "Experimentelle Marine Ökosystemanalyse" sponsored by the Bundesministerium für Forschung und Technologie, Bonn (Grant No. MFU - 0328/1)     

The effect of food deprivation on self-control

Two experiments examined the effect of food deprivation on choice in a discrete-trials self-control paradigm, choice between a larger, more-delayed reinforcer and a smaller, less-delayed reinforcer. In Experiment 1, four pigeons were each deprived to 65%, 80%, and 90% of their free-feeding weights, and the delay to the smaller reinforcer was varied. Deprivation level did not affect choice, but the rate of ineffective key pecks made during the reinforcer delays increased as deprivation increased. In Experiment 2, four pigeons were exposed to conditions in which they were fed up to their 80% free-feeding weights following experimental sessions, and in which they were given no postsession feedings. Both the pigeons' weights and their latencies to insert their heads into the food hopper when food was available were lower when the pigeons were not fed following experimental sessions. Choice showed no change. Deprivation level affects response rate and eating behavior in these procedures, but not choice.     

Feeding rates in the chaetognath Sagitta elegans: effects of prey size, prey swimming behaviour and small-scale turbulence

The gut contents of Sagitta elegans were sampled twice daily(noon and midnight) during 9 days in October at an anchor stationin the northern North Sea. Observations of the ambient preyfield and of turbulent dissipation rates were collected simultaneously.The average number of prey per chaetognath was among the highestever recorded, 0.57 ± 0.10. Total gut content was independentof ambient prey concentration, suggesting that feeding ratewas saturated. Clearance rates were estimated from gut contentsand ambient prey concentrations and a literature-based estimateof digestion time. The clearance rate to prey size showed adome-shaped relationship. The maximum clearance rates, about100–300 ml h^-1, were observed for prey sizes correspondingto 6–10% of Sagitta length. Clearance rates varied notonly with prey size, but also with prey type. For example, copepodmales were cleared at rates up to an order of magnitude higherthan similarly sized females, probably owing to differencesin swimming behaviour. Sagitta elegans is an ambush predatorthat perceives its prey by hydromechanical signals. Faster swimmingprey generates stronger signals and is, hence, perceived atlonger distances. We develop a simple prey encounter rate modelby describing the swimming prey as a ‘force dipole’and assuming that a critical signal strength is required toelicit an attack. By fitting the model to the observations,a critical signal strength of 10^-2 cm s^-1 is estimated; thisis very similar to estimates for copepods that also perceiveprey by mechanoreceptory setae. Gut contents were independentof turbulent dissipation rate. Because feeding rates were saturated,we did not expect to see positive effects of turbulence. However,the strong wind-generated turbulent dissipation rates observedduring the study (10^-3–10^-1 cm² s^-3 in the upper mixedlayer) could lead to negative effects by interfering with preyperception. At a dissipation rate of 10^-2 cm² s^-3 a 10-mm longS. elegans would experience fluid signals of order 0.3 cm s^-1due to turbulence, 30 times stronger than the signal strengthfrom the prey. It is, therefore, suggested that S. elegans isable to separate prey signals from turbulence signals due totheir different spatial characteristics.     

The dynamics of top-down and bottom-up effects in food webs of varying prey diversity, composition, and productivity

Prey diversity is thought to mediate the strength of top-down and bottom-up effects, but few experiments directly test this hypothesis. I assembled food webs of bacteria and bacterivorous protist prey in laboratory microcosms with all combinations of five productivity levels, two top predator treatments (present or absent), and three prey compositions. Depauperate food chains contained one of two edible prey species, while more diverse food webs contained both edible prey species plus two additional less-edible/inedible prey. Equilibrium theory predicts that prey diversity should weaken the top-down and bottom-up effects on trophic level biomasses, due to density compensation among prey species. Top-down effects should increase with productivity in food chains, but decrease with productivity in food webs. Results revealed highly dynamic top-down effects, the strength of which varied more over time than among treatments. Further, top-down effects did not merely vary in absolute strength over time, but also in relative strength across different prey compositions and productivity levels. It might be expected that equilibrium models would qualitatively reproduce time-averaged results. However, time-averaged data largely failed to support equilibrium predictions. This failure may reflect strong temporal variability in treatment effects combined with nonlinear density dependence of species' per-capita growth rates. Strong temporal variability in the strength of top-down effects has not previously been demonstrated, but likely is common in nature as well.     

Effects of prey density,prey size and predator size on rates of feeding by an intertidal predatory gastropod Morula marginalba Blainville (Muricidae), on several species of prey

As a prerequisite for models of foraging behaviour of the whelk, Morula marginalba Blainville (Muricidae), the effects of variation in density of prey on the rate of feeding of the predator were examined in field conditions for three coexisting species of prey. Densities of prey used were those at which the prey, two limpets and a barnacle, occurred naturally in the rocky intertidal habitat.Large limpets, Cellana tramoserica (Sowerby) can resist attacks by predatory gastropods by raising the mantle over the outside of the shell. These experiments showed that no C. tramoserica were killed by Morula marginalba even at very great densities and with no alternative prey present. For the small limpet Patelloida latistrigata (Angas), one of the whelk's most highly preferred prey, juveniles were eaten 1.4 times as fast as adults. Fitting the random predator equation gave greater attack coefficients and shorter handling times for juvenile than adult limpets.Sizes of both predator and prey affected rates of eating barnacles, Tesseropora rosea (Krauss), but not in a simple way. Whelks of 15-mm aperture length ate adult barnacles 4.2 times faster than did 12-mm whelks, but there was no significant difference in the rates at which the two sizes of snail ate juvenile barnacles.Rates of feeding on T. rosea and Patelloida latistrigata increased significantly with prey density. These results form a basis for including the density of prey in models of spatial dispersion of the predatory gastropod Morula marginalba.     

The effect of prey density on food intake by adultCheilomenes sulphurea [Col.: Coccinellidae]

The feeding rate of males of the coccinellidCheilomenes sulphurea (Ol.) was checked in 3 types of exposure arenas of 210, 476 and 2973 cm², at 3 levels of prey offered daily (40, 80, 120Aphis fabae Scopoli aphids per arena). Prey numbers affected the feeding rate much more than the size of the exposure arena.C. sulphurea males thus showed a rather high searching efficiency. The data are apparently near to the satiation threshold whereHolling's functional response curves of type II and III resemble each other.

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Résumé La consommation de proie par les males deCheilomenes sulphurea (Ol.) est étudiée dans 3 types d'enceintes dont l'espace expérimental est de 210, 476 et 2973 cm², avec 3 densités de proie offerte chaque jour: 40, 80 et 120 individus du puceronAphis fabae, Scopoli. La quantité de proies a modifié la consommation de manière plus importante que la taille de l'espace expérimental utilisé. Les males deC. sulphurea ont démontré ainsi une capacité à trouver la proie très élevée. Les niveaux de densité de proie sont évidemment près du seuil de saturation définis dans les courbes II et III de la réaction fonctionnelle deHolling.

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The experiments were performed when the 2nd author worked as a participant in a UNESCO course at the Czechoslovak Academy of Sciences, Prague.