Dealing with time constraints on development: the effect of food availability

Abstract.  1. Life-history theory predicts that organisms should speed up development in response to time constraints. However, acceleration of development carries energetic costs that have to be compensated, e.g. by an increase in foraging rate. For the wing dimorphic water strider Gerris lacustris (L.), the hypothesis was tested that the adjustment of development to time constraints is limited by the availability of food resources.
2. Six cohorts of larvae hatched increasingly late in the season were reared under two feeding regimes. For each cohort and experimental group the physiological time (in degree-days) of larval development was estimated.
3. In both high- and low-food groups there was a significant reduction of physiological time for development towards the end of the season. Furthermore, within cohorts, physiological development time was always lower in the high-food group than in the low-food group. However, there was no significant interaction effect between food treatment and cohort.
4. The results demonstrate that G. lacustris has the flexibility to adjust development to time constraints. In addition, 20% of the 'low-food individuals' developed into the short-winged morph while all of the 'high-food individuals' became long-winged. The limitation of food may thus lead to a reduced allocation of energy into the development of the flight apparatus. This may explain the strong increase in short-wingedness at the end of the season in natural populations, which are highly food limited.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

Effect of starvation time on the prey capture behaviour, functional response and population growth of Asplanchna sieboldi (Rotifera)

1. We examined the effect of different periods of prior starvation(from 30 min to 16 h) on the prey capture behaviour, and functional and numerical responses of the predatory rotifer Asplanchna sieboldi using the rotifer Brachionus calyciflorus as prey.
2. Feeding activity (i.e. encounter, attack, capture and ingestion) by Asplanchna increased significantly with increasing prey densities from 2 to 16 mL⁻⁻¹ and with increasing prior starvation periods from 0.5 to 8 h.
3.  Asplanchna sieboldi showed a type II functional response at all the prior starvation periods tested. The asymptotic prey density was highest after 8 h of starvation.
4. The instantaneous population growth rate of A. sieboldi ranged from 0.089 ± 0.044 (when starved for 8 h in every 24 h and at a prey density of 2 individuals mL⁻⁻¹ for the other 16 h) to 1.015 ± 0.142 in the control (no starvation and at a prey density of 16 individuals mL⁻⁻¹). The effect of starvation time on the numerical response was evident only at the higher prey density.     

Quantifying the costs and benefits of protective egg coating in a Chrysomelid beetle

Abstract.  1. The costs and benefits of behavioural care of offspring can often be easily quantified through observations and experiments. Other forms of parental investment, on the other hand, are usually less amenable to cost–benefit analysis.
2. Here, the costs and benefits are estimated for protective egg coating by a chrysomelid beetle, Cryptocephalus hypochaeridis , where the female spends a considerable amount of time adding extra structural components to each of the eggs after laying them.
3. Adding this protective coating was very costly, both in terms of material and energy used: the mass of the extrachorion is equivalent to half the mass of the egg, and water loss and energy expenditure while coating the egg is equivalent to half what would be lost while laying a further egg.
4. Choice tests with egg predators demonstrated that these high costs are offset by benefits in terms of protection against predation: whereas uncoated eggs are readily eaten by predators, coated eggs are always rejected.     

Effects of prey-size and predator-instar on the predation of Daphnia by Notonecta

Abstract. 1. Attack rates and handling times are measured by a series of separate functional response experiments for each instar of Notonecta glauca attacking four size classes of Daphnia magna as prey. The resulting attack rate and handling time surfaces are complex, with maximum attack rates for small predators attacking small prey, and large predators attacking large prey. Adult Notonecta have lower attack rates than the two previous juvenile instars (4 and 5).
2. The literature on attack rates and handling times in other predator—prey interactions that involve a series of different predator and prey size or age classes is reviewed in the context of the Notonecta-Daphnia results. The data suggest that small predator instars will usually compete with large instars for food, unless there is spatial or temporal separation between them.
3. Complex attack rate and handling time surfaces are to be expected wherever a wide range of prey and predator sizes is involved.
4. Size related changes in attack rates and handling times can introduce very complex dynamics into predator-prey interactions.     

Energetic and physiological correlates of prey handling and ingestion in lizards and snakes

In this review, we summarize the energetic and physiological correlates of prey handling and ingestion in lizards and snakes. There were marked differences in the magnitude of aerobic metabolism during prey handling and ingestion between these two groups, although they show a similar pattern of variation as a function of relative prey mass. For lizards, the magnitude of aerobic metabolism during prey handling and ingestion also varied as a function of morphological specializations for a particular habitat, prey type, and behavior. For snakes, interspecific differences in aerobic metabolism during prey handling seem to be correlated with adaptations for prey capture (venom injection vs. constriction). During ingestion by snakes, differences in aerobic metabolism might be due to differences in cranial morphology, although allometric effects might be a potentially confounded effect. Anaerobic metabolism is used for prey handling and ingestion, but its relative contribution to total ATP production seems to be more pronounced in snakes than in lizards. The energetic costs of prey handling and ingestion are trivial for both groups and cannot be used to predict patterns of prey-size selection. For lizards, it seems that morphological and ecological factors set the constraints on prey handling and ingestion. For snakes, besides these two factors, the capacity of the cardio-respiratory system may also be an important factor constraining the capacity for prey handling and ingestion.     

The effects of turbidity and an invasive species on foraging success of rosyside dace (Clinostomus funduloides)

1.  Habitat degradation and biological invasions are important threats to fish diversity worldwide. We experimentally examined the effects of turbidity, velocity and intra- and interspecific competition on prey capture location, reactive distance and prey capture success of native rosyside dace ( Clinostomus funduloides ) and invasive yellowfin shiners ( Notropis lutipinnis ) in Coweeta Creek, North Carolina, U.S.A.
2.  Increased turbidity and velocity produced significant decreases in the number of prey captured forward of the fish's location. It is possible that this represents an increase in the amount of energy expended per prey captured.
3.  We used Akaike's Information Criterion (AIC) to evaluate competing explanatory models for reactive distance (10 generalised linear models, GLM) and prey capture success (9 generalised linear mixed models, GLMM).
4.  Reactive distance decreased by 12% with an increase from 2 to 4 conspecifics, whereas a 10 NTU increase in turbidity reduced reactive distance by 9%. Capture success was affected by velocity, dominance and competition, and varied among species. A 6 cm s⁻¹ increase in velocity produced a 28% decline in capture probability; however, dominant fish were 3.2 times more likely to capture a prey item than non-dominant fish. Yellowfin shiners only were 0.62 times as likely to capture a prey item as rosyside dace. Both intra- and interspecific competition reduced capture probability, and fish in high density intraspecific or interspecific trials were 0.46 times and 0.44 times as likely to capture prey, respectively, as fish in two fish intraspecific trials.
5.  These results suggest behavioural variables are as important as physical factors in determining reactive distance and capture probability by these minnows.     

Rabbits killing birds: modelling the hyperpredation process

1.  Introduced rabbits are known to have catastrophic effects on oceanic islands, either by direct destruction of the vegetative cover, or by the resulting disturbance of indigenous vertebrates.
2.  Another dramatic effect, less well known, but potentially of major importance, is the hyperpredation process. This process, related to apparent competition, predicts that an introduced prey species, well adapted to high predation pressure, could induce the extinction of an indigenous prey, through the sudden increased population size of an introduced predator. In many island ecosystems, the simultaneous presence of introduced feral cats and rabbits is thus potentially a further threat for small vertebrates endemic of these islands.
3.  Through a mathematical model, we tested this hypothesis, using a tri-trophic system comprising an indigenous prey (birds), an introduced prey (rabbits) and an introduced predator (cats), and we demonstrated the theoretical existence of the hyperpredation process.
4.  In addition, the numerical analysis of the model allowed a quantification of this process. It shows that the conditions required for an indigenous species to cope with the hyperpredation process imply very high intrinsic growth rates and/or carrying capacity, as well as behavioural anti-predator response to the introduced predator. Since these conditions are unlikely to be met, this process is a further potential threat to most indigenous vertebrate prey.
5.  Finally, our model shows that, although it can be induced by both types of adaptation together or alone, behavioural adaptations alone are more powerful in generating the hyperpredation process, than are life history traits adaptations.     

The effect of bladderwort (Utricularia) predation on microcrustacean prey

SUMMARY 1. The effects of the carnivorous plant Utricularia ( bladderwort) on its microcrustacean and macroinvertebrate prey were studied under seminatural and natural conditions. The results suggest that Utricularia is a strong interactor in littoral communities that influences its prey populations by direct predation and indirect facilitation.
2. In an 8-week enclosure experiment, effects on prey density were compared in three treatments with (1) U. vulgaris with intact trapbladders, (2) U. vulgaris without bladders and (3) no Utricularia present.
3. Utricularia predation caused a decrease in prey density over time, whereas presence of Utricularia without bladders increased prey density. In the controls without Utricularia , prey density was relatively constant over time.
4. Field samples were collected to quantify predation rates of three Utricularia species on two natural prey populations. Daily consumption rates on prey peaked from mid-July to mid-August for all Utricularia species, but were low in June and September. This pattern was explained mainly by a high number of trapbladders at this time, but also by a slight increase in the number of prey caught per bladder. Per capita prey mortality rates caused by Utricularia were substantial and ranged between 0.14 and 0.43 day⁻¹ for copepods, 0.1–0.27 day⁻¹ for ostracods and 0.04–0.2 day⁻¹ for chydorid cladocerans.
5. Predation and facilitation effects were observed for total prey and separately for epiphytic and benthic prey. Planktonic microcrustaceans showed no response to Utricularia presence.     

Energetic constraints, size gradients, and size limits in benthic marine invertebrates

Populations of marine benthic organisms occupy habitats witha range of physical and biological characteristics. In the intertidalzone, energetic costs increase with temperature and aerial exposure,and prey intake increases with immersion time, generating sizegradients with small individuals often found at upper limitsof distribution. Wave action can have similar effects, limitingfeeding time or success, although certain species benefit fromwave dislodgment of their prey; this also results in gradientsof size and morphology. The difference between energy intakeand metabolic (and/or behavioral) costs can be used to determinean energetic optimal size for individuals in such populations.Comparisons of the energetic optimal size to the maximum predictedsize based on mechanical constraints, and the ensuing mortalityschedule, provides a mechanism to study and explain organismsize gradients in intertidal and subtidal habitats. For specieswhere the energetic optimal size is well below the maximum sizethat could persist under a certain set of wave/flow conditions,it is probable that energetic constraints dominate. When theopposite is true, populations of small individuals can dominatehabitats with strong dislodgment or damage probability. Whenthe maximum size of individuals is far below either energeticoptima or mechanical limits, other sources of mortality (e.g.,predation) may favor energy allocation to early reproductionrather than to continued growth. Predictions based on optimalsize models have been tested for a variety of intertidal andsubtidal invertebrates including sea anemones, corals, and octocorals.This paper provides a review of the optimal size concept, andemploys a combination of the optimal energetic size model andlife history modeling approach to explore energy allocationto growth or reproduction as the optimal size is approached.     

Foraging behavior and energetics of Great Egrets and Snowy Egrets at interior rivers and weirs

ABSTRACT.   Wading birds may use different foraging methods and prey capture techniques in particular habitats or under specific conditions. We measured foraging behavior and its energetic costs for Great Egrets ( Ardea alba ) and Snowy Egrets ( Egretta thula ) at two weirs (small overflow dams that raise water levels in a stream or river) and in two naturally flowing rivers in Kansas in May and June 2000 and 2005. We observed 99 randomly selected birds (38 Great Egrets and 61 Snowy Egrets) for 1513 min, and noted strike rate, prey capture rate, capture efficiency, prey size, and social interactions. In addition, 30 of these birds were observed for 504 min to estimate ambulation velocities and foraging energetics. Both species had higher strike rates and prey capture rates in rivers, but caught larger fish at weirs. Capture efficiency was higher for Snowy Egrets at weirs, but did not differ between microhabitats for Great Egrets. Snowy Egrets had higher rates of conspecific aggression at weirs than in rivers, but little aggression was documented for Great Egrets. Established algorithms suggest that, while foraging in rivers, Snowy Egrets had similar costs for changes in velocity. Changes in ambulation velocity for Great Egrets were greater at weirs than rivers. For both species, the percentage of time spent standing was twice as high at weirs as in rivers. Both species also used low-cost foraging strategies at weirs that yielded larger fish, so net energetic gains at weirs were higher than in rivers. Weirs appear to be more important to Snowy Egrets than to Great Egrets. Estimates of energy gains and expenses provide valuable predictive power for understanding egret behavior.     

Does differential predation permit invasive and native mosquito larvae to coexist in Florida?

Abstract.  1. The hypothesis that selective predation on larvae of the invasive Aedes albopictus (Skuse) could account for its stable coexistence with the native mosquito species and inferior competitor Ochlerotatus triseriatus (Say) in Florida treeholes and container systems was tested experimentally.
2. Functional responses of the two dipteran predators Toxorhynchites rutilus (Coquillett) and Corethrella appendiculata (Grabham) were evaluated separately for A. albopictus and O. triseriatus prey. Both predators exhibited type II functional responses and consistently consumed more of the invasive species. Handling time of T. rutilus feeding upon O. triseriatus was significantly longer than when preying upon the invasive species.
3. When either predator species was offered varying ratios of the two prey species, A. albopictus was consumed preferentially. The absence of a prey ratio effect on preference indicated that switching probably does not occur.
4. The higher maximum feeding rate upon, and preference for, A. albopictus suggests that differential predation may foster coexistence of the invasive and native mosquito prey species in Florida.     

Effects of light and microcrustacean prey on growth and investment in carnivory in Utricularia vulgaris

SUMMARY 1. In a 5-week enclosure experiment, we studied the effects of light (ambient light, low light) and prey availability (no prey, prey added) on growth and investment in carnivory in Utricularia vulgaris .
2. Investment in carnivory, measured as the proportion of biomass allocated to bladders, was strongly affected by our manipulations of light intensity and prey density. In the treatment with high prey density the light reduction decreased the investment in bladders from 25% to zero. The effect of prey density on investment in bladders was negative. Because prey addition increased the concentration of nutrients, especially phosphorus, we propose that the effect of the prey treatment on investment reflected altered nutrient concentrations.
3. Availability of prey increased growth and apical biomass of Utricularia . As Utricularia had very few bladders in some treatments we suggest that the effect was due to a combination of live prey trapped and increased nutrient availability from dead prey.
4. Abundance of periphyton on Utricularia and on the enclosure walls was highest in the treatments with high prey density where nutrient concentrations were highest. Thus we interpret the response of periphyton as primarily reflecting nutrient availability.     

Photoperiod and food restriction differentially affect reproductive and immune responses in Siberian hamsters Phodopus sungorus

1.  Organisms must contend with seasonal fluctuations in energy availability. To maintain a positive energy balance year-round, a number of adaptations have evolved including seasonal changes in reproduction, energetics and immunity. Photoperiod is the primary environmental signal most animals use to predict seasonal events. Despite the established link between energetics and immune function, little is known regarding how changes in energy availability affect immunity.
2.  The goal of the present study was to determine the effects of food restriction on photoperiodic changes in reproduction and immune function in the Siberian hamster ( Phodopus sungorus ). Adult hamsters were housed in long or short days and were food restricted or fed ad libitum . Immune responses were quantified by measuring specific antibody production and bacterial killing capacity.
3.  Food restriction decreased body and relative reproductive masses in long-day animals. Antibody responses, but not bacterial killing ability, were enhanced in food restricted short-day animals as compared with ad libitum fed controls. We also found differential effects of body fat on immune responses depending on the immune measure.
4.  The effects of food restriction on immune function appear to vary based on the restriction regimen, the response measured, and the physiological state of the organism including energy balance, metabolic rate and reproductive status.
5.  In conclusion, these results suggest that a wide range of factors can differentially affect immune function. In addition, these effects may vary based on the specific response examined. Future studies should include a variety of measurements to provide a more integrative and accurate picture of reproductive, energetic, and photoperiodic effects on immune function.     

Prey size selection and cannibalistic behaviour of juvenile barramundi Lates calcarifer

This study assessed the cannibalistic behaviour of juvenile barramundi Lates calcarifer and examined the relationship between prey size selection and energy gain of cannibals. Prey handling time and capture success by cannibals were used to estimate the ratio of energy gain to energy cost in prey selection. Cannibals selected smaller prey despite its capability of ingesting larger prey individuals. In behavioural analysis, prey handling time significantly increased with prey size, but it was not significantly affected by cannibal size. Conversely, capture success significantly decreased with the increase of both prey and cannibal sizes. The profitability indices showed that the smaller prey provides the most energy return for cannibals of all size classes. These results indicate that L. calcarifer cannibals select smaller prey for more profitable return. The behavioural analysis, however, indicates that L. calcarifer cannibals attack prey of all size at a similar rate but ingest smaller prey more often, suggesting that prey size selection is passively orientated rather than at the predator's choice. The increase of prey escape ability and morphological constraint contribute to the reduction of intracohort cannibalism as fish grow larger. This study contributes to the understanding of intracohort cannibalism and development of strategies to reduce fish cannibalistic mortalities.     

Effects of prey dispersal on predator–prey interactions in streams

1. We studied the effect of mesh size (6 and 3 mm) on interactions between brown trout ( Salmo trutta ) and benthic invertebrates in enclosures placed in a stream in southern Sweden. We also compared how different prey exchange rates affected interactions between trout and invertebrates.
2. Trout had strong impacts on some benthic taxa, and different mesh sizes produced different patterns. Trout affected the abundance of 10 of the 21 taxa examined, six in enclosures with 3 mm mesh and six in enclosures with 6 mm mesh. The abundance of nine of the prey taxa was lower in the presence of trout, only leptocerids were more numerous in the presence of trout.
3. Our measurements of prey immigration/emigration, together with trout diet data, suggest that direct consumption by trout, rather than avoidance behaviour by prey, explains most decreases in prey abundance. There was avoidance behaviour by only two of the twenty-one prey taxa, with trout inducing emigration of the mayflies Baetis rhodani and Paraleptophlebia sp.
4. Trout indirectly increased periphyton biomass in both 3 and 6 mm enclosures. The effect of trout on periphyton was probably due to strong effects of trout on the grazer, Baetis rhodani , Heptagenia sp. and Paralepthoplebia sp.
5. Our results suggest that mesh size, through its effects on exchange rates of prey, may affect interactions between predators and prey in running waters, but that the effects of dispersal and predation on invertebrates are taxon specific.     

Context-dependent running speed in funnel-web spiders from divergent populations

1.  Locomotor performance can influence individual fitness through several ecological contexts, such as prey capture and predator escape. One means of determining which contexts act as significant selective forces on running speed is to quantify individual speed in each context. The underlying hypothesis is that animals will exhibit their highest speeds in contexts most crucial to fitness.
2.  We measured running speeds in three ecological contexts (prey capture, fleeing predators and territory defence) in lab-reared offspring of the funnel-web spider Agelenopsis aperta collected from two arid grassland and two riparian populations. Arid populations experience little predation pressure, are prey limited, and are highly territorial; riparian populations experience high predation, have high prey availability, and are less territorial in nature.
3.  The offspring of arid individuals exhibited their highest burst speeds in territory defence, and ran more slowly in response to predator threats. The offspring of riparian populations, however, ran fastest when responding to predatory threats and displayed lower velocities in prey capture and territory defence. Thus, our findings support the hypothesis that A. aperta are selected to exhibit their highest speeds in contexts most important to their fitness.
4.  Contextual use of running speed can differ among conspecific populations experiencing differing selective forces on locomotion.     

The effect of prey species and environmental complexity on the functional response of Franklinothrips orizabensis: a test of the fractal foraging model

Abstract. 1. The hypothesis proposed by the fractal foraging model, that prey procurement by predators declines as habitat complexity increases, was tested. To evaluate this hypothesis, the effect of two prey species, second-instar larvae of Scirtothrips perseae Nakahara and Heliothrips haemorrhoidalis (Bouché) (both Thysanoptera: Thripidae), and environmental complexity on the functional response of the predatory thrips Franklinothrips orizabensis Johansen (Thysanoptera: Aeolothripidae) was examined.
2. The Koch curve, a well-studied fractal, was used to generate the shapes of experimental arenas to test the effect of environmental complexity on the functional response of F. orizabensis. Complexity was defined in terms of the number of acute vertices in which prey thrips could seek refuge and the length of the perimeter that had to be searched by the predator. Four shapes were tested: a circle (zero acute vertices, circumference = 186.61 mm), equilateral triangle (three acute vertices, perimeter = 240 mm), star (six acute vertices, perimeter = 277.13 mm), and snowflake (18 acute vertices, perimeter = 333.65 mm). All shapes were scaled so that the area of each arena was 2771 mm².
3.  Franklinothrips orizabensis exhibited a Type II functional response for both prey species and all four experimental arenas tested. Significantly fewer S. perseae larvae were killed in the most complex arena (i.e. the snowflake) when prey densities exceeded 16. For H. haemorrhoidalis , significant differences in mortality were observed only when prey densities equalled four. These results demonstrated that the fractal foraging model was supported when certain prey densities coincided with particular levels of environmental complexity.
4. Subtle changes in environmental complexity and different prey species of varying density can affect prey–predator interactions significantly.     

Colony productivity and foundress behaviour of a native wasp versus an invasive social wasp

Abstract.  1. Colony productivity, prey utilisation, and foundress behaviour of a North American native wasp ( Polistes fuscatus ) versus an European invasive wasp ( Polistes dominulus ) were investigated in a controlled field experiment with optimal versus natural foraging conditions. Colonies with the optimal prey foraging conditions were provided with prey ad libitum within an enclosed area. The other colonies foraged in the adjacent field–woodland but had the same nest conditions as the other treatment.
2. When given prey ad libitum , both wasp species captured similar amounts of prey and the conversion to total offspring biomass was similar. But P. dominulus colonies produced 2.5 times the number of workers as P. fuscatus colonies, reflecting the smaller size of P. dominulus wasps.
3. Foundresses of P. dominulus were observed more often building or repairing the nest, thereby contributing to the production of colonies with twice as many cells as colonies of P. fuscatus . Foundresses of P. dominulus showed more acts of aggression toward workers than did P. fuscatus foundresses, which was not a function of adult density on the nest.
4. At the end of the experiment, P. dominulus colonies with optimal prey foraging conditions still had a high level of egg-laying and peaked in the number of pupae then, whereas egg-laying and the number of pupae per colony of the other treatments began to decline 2–3 weeks earlier. These results indicate that P. dominulus is more opportunistic than P. fuscatus , which may account in part for P. dominulus 's success as an introduced species in North America.     

The energetics of feeding strikes in larval carp, Cyprinus carpio

The prey intake of larval carp is described from high-speed (200–1250 frames s⁻¹) films with synchronous lateral and ventral views. Even in first-feeding carp larvae, the operculars are functional in sealing effectively the opercular slit until the moment of prey intake, and the maxillaries close off the corners of the mouth, preventing leak flow. In reducing the distance between larva and prey during attack, the relative importance of sucking the prey towards the mouth and swimming forward is variable; overall they are about equally important. The volume and the velocity of the water sucked into the mouth cavity during prey uptake are calculated. The energy costs of suction, i.e., accelerating the water sucked into the mouth cavity, during prey intake are estimated from these values. The energy costs of suction and swimming are in the same order of magnitude. Together they form only a fraction of 1% of the energetic content of the prey, so considerations about energy expenditure seem unimportant in a strategy to optimize the prey attack. During searching, however, they will be important. Power requirements during attack may also be important.