Prey Emigration Research Articles

The armored catfish Corydoras aeneus shapes the meiofaunal community due to predation and disturbance-induced water column dispersal

Bottom-feeding fish can exert strong top-down pressure on benthic invertebrates while also causing mechanical disturbance of the sediment. In our laboratory study, we investigated the influence of predation and disturbance-induced water column emigration on meiofauna, triggered by the armored catfish Corydoras aeneus. We hypothesized that both factors would significantly reduce the abundance of meiofauna, depending on their body size. Accordingly, we also expected a change in the nematode community structure. After 24 h, the abundances of nematodes, tardigrades, hydracarina, copepods and oligochaetes were significantly lower than in control aquariums without fish, in the latter two groups primarily due to predation. All other taxa were equally affected by predation and emigration. Within the nematodes, mainly large individuals (≥ 1 mm) were consumed, while smaller worms (< 1 mm) were increasingly dispersed. This, though it did not reduce species numbers, resulted in altered size and species composition. Our results show that the real top-down effects can quickly be overestimated as emigration of prey is overlooked and, on the other hand, that fish may also influence adjacent, undisturbed areas due to subsequent immigration of meiofauna.

Development of a stage-structured process-based predator–prey model to analyse biological control of cotton aphid, Aphis gossypii, by the sevenspot ladybeetle, Coccinella septempunctata, in cotton

Agricultural system diversification is well known to affect the population dynamics of crop pests, but predator–prey dynamics in crop systems are difficult to analyse due to interactions between multiple life stages of predator and prey, the modulating effect of temperature, the actions of additional predators, and the open nature of the system, with immigration and emigration of both predators and prey. To better understand and characterize the predator–prey system dynamics under field conditions, we developed a detailed process-based simulation model for the stage structured population interaction between cotton aphid, Aphis gossypii, and its main natural enemy, the sevenspot ladybeetle Coccinella septempunctata. The model includes interactions between all insects stages as affected by temperature and host growth, and was parameterized based on detailed data collection in the laboratory and the field. The model was tested with independent data. Simulations show that the initial predator–prey ratio, as affected by immigration rates of the aphid and the predators into the crop, is the key factor for biological control. The model is a useful tool for scenario assessments on the effects of crop diversification on pest-natural enemy dynamics.

Space race functional responses.

We derive functional responses under the assumption that predators and prey are engaged in a space race in which prey avoid patches with many predators and predators avoid patches with few or no prey. The resulting functional response models have a simple structure and include functions describing how the emigration of prey and predators depend on interspecific densities. As such, they provide a link between dispersal behaviours and community dynamics. The derived functional response is general but is here modelled in accordance with empirically documented emigration responses. We find that the prey emigration response to predators has stabilizing effects similar to that of the DeAngelis-Beddington functional response, and that the predator emigration response to prey has destabilizing effects similar to that of the Holling type II response. A stability criterion describing the net effect of the two emigration responses on a Lotka-Volterra predator-prey system is presented. The winner of the space race (i.e. whether predators or prey are favoured) is determined by the relationship between the slopes of the species' emigration responses. It is predicted that predators win the space race in poor habitats, where predator and prey densities are low, and that prey are more successful in richer habitats.

Feeding rates of the nemertean Prosorhochmus americanus (Hoplonemertea) on two species of gammaridean amphipods

. The intertidal hoplonemertean Prosorhochmus americanus is a common inhabitant of the fouling community of rock jetties of the southeast coast of the United States. We undertook a laboratory investigation of the feeding rate of this nemertean, which is a suctorial predator of amphipod crustaceans that co-occur in abundance in the fouling community. While submerged in water (simulating high tide), worms fed on the tube-building amphipods Jassa falcata and Corophium cf. insidiosum at rates of 0.19 amphipods nemertean−1 d−1 (n=10) and 0.26 amphipods nemertean−1 d−1 (n=14), respectively. These predation rates were not significantly different (two-tailed t-test, p>0.05), and are similar to those estimated in laboratory studies of other suctorial nemerteans. Many nemerteans are typically more active at night, and indeed, adults of P. americanus consumed more individuals of J. falcata during dark periods than during light periods (χ2 analysis, p<0.05). However, no difference in consumption of individuals of C. cf. insidiosum was observed in dark versus light. We attribute these contrasting results to differences in tube-building behavior exhibited by these two species of amphipod under laboratory conditions. Our results and those of other laboratory investigations suggest that nemerteans that prey on amphipods feed at a rate of ∼0.2 prey items nemertean−1 d−1, but under natural conditions this rate may not be obtained because of limited feeding time, longer foraging distances, and emigration of prey from regions of high nemertean activity.

Predators Accelerate Nutrient Cycling in a Bromeliad Ecosystem

Conventional ecological theory predicts that predators affect nutrient cycling by decreasing the abundance or activity of prey. By using a predator-detritivore-detritus food chain in bromeliads, we show that predators can increase nutrient cycling by a previously undescribed, but broadly applicable, mechanism: reducing nutrient export by prey emigration. Contrary to expectations, predation on detritivores increases detrital nitrogen uptake by bromeliads. Predation reduces detritivore emergence and hence export of nitrogen from the system. Detritivores therefore benefit their host plant, but only when predators are present. More generally, our results show that predator loss or extinction can dramatically and unexpectedly affect ecosystem functioning.

The role of trout in stream food webs: integrating evidence from field surveys and experiments

1. We evaluated the effects of brown trout on boreal stream food webs using field surveys and enclosure/exclosure experiments. Experimental results were related to prey preference of uncaged trout in the same stream, as well as to a survey of macroinvertebrate densities in streams with vs. without trout. Finally, we assessed the generality of our findings by examining salmonid predation on three groups of macroinvertebrate prey (chironomid midges, epibenthic grazers, invertebrate predators) in a meta-analysis. 2. In a preliminary experiment, invertebrate predators showed a strong negative response to trout, whereas chironomids benefited from trout presence. In the main experiment, trout impact increased with prey size. Trout had the strongest effect on invertebrate predators and cased caddis larvae, whereas Baetis mayfly and chironomid larvae were unaffected. Trout impact on the largest prey seemed mainly consumptive, because prey emigration rates were low and independent of fish presence. Despite strong effects on macroinvertebrates, trout did not induce a trophic cascade on periphyton. Uncaged trout showed a strong preference for the largest prey items (predatory invertebrates and aerial prey), whereas Baetis mayflies and chironomids were avoided by trout. 3. Densities of invertebrate predators were significantly higher in troutless streams. Baetis mayflies also were less abundant in trout streams, whereas densities of chironomids were positively, although non-significantly, related to trout presence. Meta-analysis showed a strong negative impact of trout on invertebrate predators, a negative but variable impact on mobile grazers (mainly mayfly larvae) and a slightly positive impact on chironomid larvae. 4. Being size-selective predators, salmonid fishes have a strong impact on the largest prey types available, and this effect spans several domains of scale. Discrepancies between our experimental findings and those from the field survey and meta-analysis show, however, that for most lotic prey, small-scale experiments do not reflect fish impact reliably at stream-wide scales. 5. Our findings suggest that small-scale experiments will be useful only if the experimental results are evaluated carefully against natural history information about the experimental system and interacting species across a wide array of spatial scales.

Scale dependent effects of predatory fish on stream benthos

In open predation experiments the effects of predators on prey densities can be influenced by predator consumption and by prey movements in to and out of experimental arenas. A published model predicts that the predator effects observed in such experiments are scale dependent over the scale range where there is a transition from movement control (of prey densities) to consumption control. The scale dependence follows from the assumption that per capita rate of emigration out of an experimental arena decreases with increasing arena size.To test this model the effects of a small benthic fish (Cottus gobio) on densities of stream invertebrates was investigated in instream channels of different length (0.5, 2 and 8 m). The effect of fish predation was scale dependent for four prey taxa. For three of these taxa predator effects increased with experimental scale, which is in agreement with model predictions. However, this proved to be a case of “making the right prediction for the wrong reason” as the basic assumption of scale dependent emigration rate was not upheld. By analyzing the behaviour of the model, parameterized with emigration and consumption rates observed in the experimental channels, it was found that observed scale effects occurred because prey emigration in response to the predator treatment was modified by the experimental scale. Further analysis of the parameterized model suggested that the densities of most prey taxa were controlled by prey movements and not by consumption by the sculpins.

The impact of a sit‐and‐wait predator: separating consumption and prey emigration

Reviews of the impact of invertebrate predators in enclosure/exclosure experiments suggest that much of the apparent depletion of prey is due to prey emigration induced by the predators. However, these generalisations derive mainly from studies of invertebrate predators that are predominantly active searchers (usually stoneflies) and of prey with strong avoidance responses (mainly mayflies).We examined the impact of a large sit‐and‐wait predator, the nymph of the dragonfly Cordulegaster boltonii, which has recently invaded Broadstone Stream as a new top predator. Field enclosure/exclosure experiments were conducted to assess the impact of the invader on the benthos. Depletion of prey varied seasonally and among taxa, and was highest when prey density and encounter rates were high. Mobile prey, although least likely to show a statistically significant response because of high exchange rates, were those most strongly depleted.Experimental channels were used to separate the relative contribution of consumption and emigration to total impact for the two most depleted prey species. Depletion of prey was due solely to consumption and predators did not induce emigration. We therefore urge caution in making generalisations about the impacts of invertebrate predators, since sit‐and‐wait and searching predators potentially have very different impacts.

SPATIAL SCALES OF CARBON FLOW IN A RIVER FOOD WEB

Spatial extents of food webs that support stream and river consumers are largely unknown, but such information is essential for basic understanding and management of lotic ecosystems. We used predictable variation in algal δ13C with water velocity, and measurements of consumer δ13C and δ15N to examine carbon flow and trophic structure in food webs of the South Fork Eel River in Northern California. Analyses of δ13C showed that the most abundant macroinvertebrate groups (collector-gatherers and scrapers) relied on algae from local sources within their riffle or shallow pool habitats. In contrast, filter-feeding invertebrates in riffles relied in part on algal production derived from upstream shallow pools. Riffle invertebrate predators also relied in part on consumers of pool-derived algal carbon. One abundant taxon drifting from shallow pools and riffles (baetid mayflies) relied on algal production derived from the habitats from which they dispersed. The trophic linkage from pool algae to riffle invertebrate predators was thus mediated through either predation on pool herbivores dispersing into riffles, or on filter feeders. Algal production in shallow pool habitats dominated the resource base of vertebrate predators in all habitats at the end of the summer. We could not distinguish between the trophic roles of riffle algae and terrestrial detritus, but both carbon sources appeared to play minor roles for vertebrate consumers. In shallow pools, small vertebrates, including three-spined stickleback (Gasterosteus aculeatus), roach (Hesperoleucas symmetricus), and rough-skinned newts (Taricha granulosa), relied on invertebrate prey derived from local pool habitats. During the most productive summer period, growth of all size classes of steelhead and resident rainbow trout (Oncorhynchus mykiss) in all habitats (shallow pools, riffles, and deep unproductive pools) was largely derived from algal production in shallow pools. Preliminary data suggest that the strong role of shallow pool algae in riffle steelhead growth during summer periods was due to drift of pool invertebrates to riffles, rather than movement of riffle trout. Data for δ15N showed that resident rainbow trout (25–33 cm standard length) in deep pools preyed upon small size classes of juvenile steelhead that were most often found in riffles or shallow pools. While many invertebrate consumers relied primarily on algal production derived from local habitats, our study shows that growth of top predators in the river is strongly linked to food webs in adjacent habitats. These results suggest a key role for emigration of aquatic prey in determining carbon flow to top predators.

THE EFFECT OF HYDROLOGICAL DISTURBANCE ON THE IMPACT OF A BENTHIC INVERTEBRATE PREDATOR

The harsh–benign model of community dynamics predicts that the impact of predation will decline as abiotic conditions become more stressful to biota. Experiments were conducted to determine whether hydrological disturbance altered the impact of an invertebrate predator in stream benthic communities. The impact of a predatory stonefly, Cosmioperla kuna, on its mayfly prey was measured in experimental stream channels receiving constant or variable flow (flooding) regimes over a one-week period. Contrary to predictions of the harsh–benign hypothesis, the impact of Cosmioperla on its two major prey taxa was either unchanged or increased by artificial floods, despite increased predator emigration from variable-flow channels. Predator impacts in variable-flow treatments were apparently strongly influenced by predator-induced prey emigration during floods. The results of this study show that nonlethal predator effects may be important during abiotic disturbance, and that it may not be reasonable to predict the impact of predation solely on the basis of the relative tolerances of predators and prey to prevailing abiotic conditions.

Application of a model of scale dependence to quantify scale domains in open predation experiments

We use a model of open predation experiments to define scale domains that differ in terms of the controlling processes and scale dependence of predator impacts. For experimental arenas that are small compared to the movements of the prey (small scale domain) the model predicts that predator impacts are scale independent and controlled by prey movements. For arenas of intermediate scale we predict that predator impacts are scale dependent and controlled by both prey movements and direct predation, and for the largest scale domain we predict weak scale dependence and predation control. We propose that the scale‐domain concept is useful when designing and interpreting field experiments. As an illustration we apply the concept to experiments examining predator effects on the stream benthos. First, we test two key assumptions of the underlying model: that area‐specific prey migration rates decrease with increasing size of experimental arenas and that predation rates are independent of arena size. For this purpose we used published estimates of prey emigration and predator consumption rates for nine studies examining the effects of stream predators on benthic prey. We found that prey per capita emigration rates but not predation rates decreased with increasing arena length. Second, we demonstrate a method for identifying the scale domain of real experiments. The model of predation experiments was parameterized using experimental data and the expected spatial and temporal scale dependence of predator impacts on prey in these experiments was simulated. The simulations suggest that the studies conducted in the largest arenas (length 15–35 m) should be classified as large‐scale, consumption‐controlled experiments, whereas the experiments conducted in smaller arenas (length 1.5–6 m) should be classified as small or intermediate‐scale. We also attempted to determine the scale domain of the experiments in a large data set, including results from most published stream predation experiments. The majority of arenas used in these experiments (73%) were smaller than 1 m in length. Our data on the scale dependence of predation and prey migration rate suggest that experiments in this scale range (&lt;1 m) should be classified as small‐scale, movement‐controlled experiments for most prey taxa.

Do herbivore-induced plant volatiles influence predator migration and local dynamics of herbivorous and predatory mites?

If predators lack information on the prey's position, prey have more chance to escape predation and will therefore reach higher population densities. One of the many possible cues that predators may use to find their prey are herbivore-induced plant volatiles. Although their effects on the behaviour of foraging predators have been well studied, little is known about how these prey-related odours affect predator-prey dynamics on a plant. We hypothesise that herbivore-induced plant volatiles provide the major cue eliciting predator arrestment on prey-infested leaves and that the response to these volatiles ultimately leads to lower prey densities. To test this hypothesis experimentally, we created two types of odour-saturated environments: one with herbivore-induced plant volatiles (treatment), and one with green-leaf volatiles (control). An odour-free environment could not be tested because herbivores require plants for population growth. We measured the rate at which predatory mites (Phytoseiulus persimilis) immigrate, emigrate and exploit a single leaf infested by two-spotted spider mites (Tetranychus urticae). The experiments did not show a significant difference between treatment and control. At best, there was a somewhat higher rate of predator (and possibly also prey) emigration in the treatment. The lack of a pronounced difference between treatment and control indicates that at the spatial scale of the experiments random searching for prey was as effective as directional searching. Alternatively, predators were arrested in the prey patch by responding not merely to herbivore-induced plant volatiles, but also to other prey-related cues, such as web and faeces. Based on our current experience we advocate to increase the spatial scale of the experiment (> 1 m2) and we provide other suggestions for improving the set-up.

The impact of vertebrate and invertebrate predators on a stream benthic community.

I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m2 (20 leeches/enclosure) and 1.3 trout/m2 (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus , Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators.

Infaunal predation regulates benthic recruitment: an experimental study of the influence of the predator Nephtys hombergii (Savigny) on recruits of Nereis diversicolor (O.F. Müller)

Field and laboratory experiments were carried out to examine predatory activity effects of the infaunal polychaete Nephtys hombergii on the recruitment of Nereis diversicolor. Using N. diversicolor juveniles as prey, we tested the effects of (1) the temperature variations on predatory activity and (2) the predator density ( d=114 and 342 ind. m −2) on prey mortality, on their growth and their escape behaviour. In the field, N. diversicolor alone ( d=13750 ind. m −2), and N. hombergii ( d=114 and 342 ind. m −2) and N. diversicolor juveniles together were enclosed in 15-cm dia. PVC cylinders during 12 weeks, from 7 March to 30 May 1995, in a mudflat of the Rance Estuary. In the presence of the predator, the biomass of N. diversicolor juveniles declined 8–12× more than in control cylinders, whereas their individual weight was increased. Although it had little effect on the biomass of prey, predator density regulated the consumption by N. hombergii. Predation activity was minimal, but effective, at a temperature of 7°C, increased between 9 and 11°C, and became constant above 11°C. In the laboratory, gradual or sudden temperature variations had little effect on predatory activity of N. hombergii, presumably because they were already physiologically active. Reduction in the abundance of infauna may be due to prey emigration as well as predation: beyond their mortality, N. hombergii induced, proportional to its density, the emigration of juveniles. The experimental results suggest that infaunal predation, by regulating recruit densities, should be considered in addition to the interactions between adults as a strong force structuring soft-bottom communities.

Amphipod (Gammarus minus) responses to predators and predator impact on amphipod density.

Recent theoretical work suggests that predator impact on local prey density will be the result of interactions between prey emigration responses to predators and predator consumption of prey. Whether prey increase or decrease their movement rates in response to predators will greatly influence the impact that predators have on prey density. In stream systems the type of predator, benthic versus water-column, is expected to influence whether prey increase or decrease their movement rates. Experiments were conducted to examine the response of amphipods (Gammarus minus) to benthic and water-column predators and to examine the interplay between amphipod response to predators and predator consumption of prey in determining prey density. Amphipods did not respond to nor were they consumed by the benthic predator. Thus, this predator had no impact on amphipod density. In contrast, amphipods did respond to two species of water-column predators (the predatory fish bluegills, Lepomis macrochirus, and striped shiners, Luxilus chrysocephalus) by decreasing their activity rates. This response led to similar positive effects on amphipod density at night by both species of predatory fish. However, striped shiners did not consume many amphipods, suggesting their impact on the whole amphipod "population" was zero. In contrast, bluegills consumed a significant number of amphipods, and thus had a negative impact on the amphipod "population". These results lend support to theoretical work which suggests that prey behavioral responses to predators can mask the true impact that predators have on prey populations when experiments are conducted at small scales.

A Review of the Drift and Activity Responses of Stream Prey to Predator Presence

In streams the impact that predators have on prey density is unclear. A recent review suggests that predators have on average a small to moderate negative effect on prey density and that predatory invertebrates have a stronger impact than predatory vertebrates. We suggest that a reason for this difference between predatory invertebrates and vertebrates is differences in prey emigration responses to the two types of predators. If prey increase their emigration rates in the presence of predators, predator impacts on local prey density will be strong; conversely, if prey decrease their emigration rates in the presence of predators, prey density might actually build up in patches containing predators and predator impact on prey density will be weak or even positive. We reviewed 22 studies that examined the impact that predatory vertebrates and invertebrates have on prey drift and activity (crawling and rates of emergence from refuge). Our review revealed that predatory invertebrates cause prey to increase drift rates more often than expected by chance. Predatory vertebrates had variable effects on prey drift rates (sometimes increasing and sometimes decreasing prey drift rates), but they caused prey to decrease activity more often than expected by chance. These results provide a potential mechanism for the relatively large impact that predatory invertebrates have on prey density as compared to the impact that predatory vertebrates have. We suggest directions for future research that include examining the impact that both types of predators have on prey emigration behavior and benthic density in the same system

Predator impacts on stream benthic prey.

The impact that predators have on benthic, macroinvertebrate prey density in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factors appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thus might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration responses to the two types of predators. In addition, predatory vertebrates have more complex prey communities than predatory invertebrates; this complexity might reduce the impact that predatory vertebrates have on prey because of indirect effects. I conducted a meta-analysis on the results of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic prey density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have different impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a significant negative effect on prey density, predatory invertebrates have a significantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivorous invertebrate prey. Three methodological variables (mesh size of enclosures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differences in impact between predatory invertebrates and vertebrates. No correlation exists between mesh size and predator impact. Over all predators, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exist between these variables. Also, a correlation was found between enclosure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within the same system, and (2) examine their impacts on the same spatial scale.

Prey Behavior, Prey Dispersal, and Predator Impacts on Stream Prey

A recent model, designed with stream systems in mind, suggested that prey exchange (movement of prey among patches) tends to reduce predator impacts on prey density; that is, rapid prey immigration into a patch with predators can swamp local effects of predators on prey density. The previous model, however, included two assumptions that influence the model's qualitative predictions. First, it assumed that the system's focal patches are surrounded by an environment that has no predators and a constant prey density. More importantly, it assumed that prey do not alter their per capita emigration rates in response to the presence of predators. We extended the earlier model by relaxing these assumptions. Specifically, we: (1) addressed predator impacts in patches surrounded by background environments that do not have predators, (2) allowed the background environment to have a constant or decreasing prey density (i.e., we examined situations in which predators deplete prey), and (3) accounted for the fact that prey per capita emigration rates are often altered by the presence of predators. Our most interesting results concerned the profound effects of prey emigration behavior on the relationship between prey exchange and predator impact. If prey do not alter their emigration rates in response to predators, then, as predicted by the earlier model, high prey exchange should result in very low predator impact. If, however, prey disperse out of patches in response to the presence of predators (i.e., if prey per capita emigration rates are higher out of patches with predators than out of predator—free patches), then even very high prey exchange rates cannot swamp predator impact; instead, prey emigration adds to predator impact. Thus, depending on prey emigration behavior, increased overall prey exchange can either decrease or increase predator impact. Predators can also suppress prey emigration (e.g., if prey hide in refuges so effectively that they disperse at low rates from predator patches). In that case, high prey exchange rates tend to result in “negative predator impacts” (i.e., higher prey density in patches with predators). The details of the above relationships are influenced by whether or not the background environment has predators. If the background has predators, but predators do not deplete prey (e.g., if predation is offset by recruitment), then predator impact depends only on the ratio of per capita emigration rates out of predator and predator—free patches; that is, attack rates do not influence impact. In contrast, if predators can deplete prey, then attack rates influence predator impact. In that situation, if attack rates are highly relative to prey emigration rates out of predator—free patches, then predator impact steadily increases over time. A literature review suggested that prey alterations in per capita emigration rates in response to the presence of predators can potentially explain some surprising natural phenonomena, including the existence of “negative predator impacts,” and the apparent tendency for invertebrate predators (primarily, stoneflies) to have stronger impacts, and the apparent tendency for vertebrate predators (primarily, fish). Finally, we discussed possible adaptive links between prey escape success, refuge use, dispersal behavior, and predator impacts. This discussion raised a “paradox of danger” that due to their effects on prey exchange, more dangerous predators might often have unexpectedly weak effects on local prey density. In this context, we suggest a framework for studying relationships between prey behavior, prey exchange, and predator impacts.

Increased emigration of the amphipod Rhepoxyniusabronius (Barnard) and the polychaete Nephtys caeca (Fabricius) in the presence of invertebrate predators

Laboratory experiments tested whether the emigration of two infaunal organisms, the amphipod Rhepoxynius abronius (Barnard) and the polychaete Nephtys caeca (Fabricius) increased in the presence of potential predators. Significantly more Rhepoxynius abronius migrated into the water column in the presence of an epibenthic predator, the dungeness crab Cancer magister Dana, and two infaunal predatory polychaetes Glycera convoluta Keferstein and Nephtys caeca, than in their absence. Emigration of small N. caeca was greater in the presence of Glycera convoluta than in the presence of large Nephtys caeca or in the control. These results demonstrate that avoidance of predators may sometimes explain the occurrence of benthic organisms in the water column and indicate that reductions of infauna by predators in predator inclusion experiments may be due to prey emigration as well as direct predation.

Prey and predator emigration responses in the acarine system Tetranychus urticae-Phytoseiulus persimilis.

Some of the processes that influence the emigration of prey and predatory mites from bean plants were investigated experimentally. The emigration of the prey depends on the damage they cause to the plants and on predator density. The predator's emigration rate is a decreasing function of prey density, and does not change (or it slightly decreases) when prey and predator numbers are increased maintaining the same prey/predator ratio. The probability of emigration of the predators is independent of their own density when prey are absent and density dependent when prey density is kep constant. Forty three per cent of the variability in the predator's instantaneous rate of emigration in the different experiments is accounted for by a two parameter negative exponential function of capture rate (number of prey eaten per predator and per unit of time).