Multiple Prey Research Articles

Time‐to‐kill: measuring attack rates in a heterogenous landscape with multiple prey types

Although spatial heterogeneity of prey and landscapes are known to contribute to variation around predator‐prey functional response models, few studies have quantified these effects. We illustrate a new approach using data from winter movement paths of GPS‐collared wolves in the Rocky Mountains of Canada and time‐to‐event models with competing risks for measuring the effect of prey and landscape characteristics on the time‐to‐kill, which is the reciprocal of attack rate (aN) in a Holling's functional response. We evaluated 13 a priori models representing hypothesized mechanisms influencing attack rates in a heterogeneous landscape with two prey types. Models ranged from variants on Holling's disc equation, including search rate and prey density, to a full model including prey density and patchiness, search rates, satiation, and landscape features, which were measured along the wolf's movement path. Movement rates of wolves while searching explained more of the variation in time‐to‐kill than prey densities. Wolves did not compensate for low prey density by increasing movement rates and there was little evidence that spatial aggregation of prey influenced attack rates in this multi‐prey system. The top model for predicting time‐to‐kill included only search rate and landscape features. Wolves killed prey more quickly in flat terrain, likely due to increased vulnerability from accumulated snow, whereas attack rates were lower when wolves hunted near human‐made features presumably due to human disturbance. Understanding the sources of variation in attack rates provides refinements to functional response models that can lead to more effective predator–prey management in human‐dominated landscapes.

The long‐term impacts of predators on prey: inducible defenses, population dynamics, and indirect effects

Despite the amount of research on the inducible defenses of prey against predators, our understanding of the long‐term significance of non‐lethal predators on prey phenotypes, prey population dynamics, and community structure has rarely been explored. Our objectives were to assess the effects of predators on prey defenses, prey population dynamics, and the relative magnitude of density‐ versus trait‐mediated indirect interactions (DMIIs and TMIIs) over multiple prey generations. Using a freshwater snail and three common snail predators, we constructed a series of community treatments with pond mesocosms that manipulated trophic structure, the identity of the top predator, and whether predators were caged or uncaged. We quantified snail phenotypes, snail population size, and resource abundance over multiple snail generations. We found that snails were expressing inducible defenses in our system although the magnitude of the responses varied over time and across predator species. Despite the expression of inducible defenses, caged predators did not reduce snail population size. There also was no evidence of TMIIs throughout the experiment suggesting that TMIIs have a minimal role in the long‐term structure of our communities. The absence of TMIIs was largely driven by the lack of predator‐induced reductions in resource consumption and the lack of consistent reductions in population size with predator cues. In contrast, we detected strong DMIIs associated with lethal predators suggesting that DMIIs are the dominant long‐term mechanism influencing community structure. Our results demonstrate that although predators can have significant effects on prey phenotypes and sometimes cause short‐term TMIIs, there may be few long‐term consequences of these responses on population dynamics and indirect interactions, at least within simple food webs. Research directed towards addressing the long‐term consequences of predator–prey interactions within communities will help to reveal whether the conclusions and predictions generated from short‐term experiments are applicable over ecological and evolutionary timescales.

Conservation in Sustainable-Use Tropical Forest Reserves

trialspecies,therelativemeritsofdifferentapproachesto ensure the long-term persistence of those species remain highly contentious. Most would agree, however, that both establishing protected areas and exercising some form of restraint on extraction of forest resources are among the most effective of all viable conservation measures. Deforestation, wildfires, logging, and hunting are among the leading drivers of species losses in tropical forests, and de facto or de jure protection from these threats can be conferred by either effective enforcement of regulations or physical remoteness. Attempts to assess conservation success of protected areasatlargescaleshaverestedprimarilyonconventional use of remote sensing to quantify spatial or temporal differences in rates of change in land cover due to deforestation and wildfires, rather than on empirical demographic or community-level metrics (Gaston et al. 2008). Q2 However, the former approaches fail to detect most types of subcanopy anthropogenic disturbances that also result, directly or indirectly, in species losses (Peres et al. 2006). Moreover, the effects of habitat loss and degradation on population extirpations and declines are nonlinear, so vegetation cover alone is rarely a robust proxy for the viability of terrestrial biotas. Remotesensing data show vast tracts of apparently intact tropical forests, but in reality levels of hunting and other forms of extraction in these areas are often unsustainable (Peres & Lake 2003). Fundamental questions yet to be answered include whether ostensibly intact protected areas retain full complements of forest species and how the extent of cryptic patterns of disturbance is related to human population density in both protected and unprotected areas. I considered the global to regional emergence of sustainable-use reserves, emphasizing the world’s largest tropical forest region, Amazonia. Sustainable-use reserves often have intermediate levels of disturbance, so I examined the degree of use of natural resources by resident communities and used human population density as a proxy for level of extraction. In both protected and unprotected areas, I also estimated responses of game vertebrate assemblages to hunting on the basis of the relative biomassextractedfromasubsetoftheforestfauna.Iused analysesofcovariance(ANCOVA)toexaminetheassociationbetweenhumandensityandgamebiomassharvested across different reserve categories. Finally, I considered the long-term capacity of sustainable-use forest reserves to maintain populations of all resident species.

Olfactory Experience Modifies Semiochemical Responses in a Bark Beetle Predator

A typical feature of forest insect pests is their tendency to undergo large fluctuations in abundance, which can jeopardize the persistence of their predaceous natural enemies. One strategy that these predators may adopt to cope with these fluctuations would be to respond to sensory cues for multiple prey species. Another possible adaptation to temporal variation in the prey community could involve the learning of prey cues and switching behavior. We conducted three experiments to investigate the ability of the generalist bark beetle predator Thanasimus dubius (F.) (Coleoptera: Cleridae) to respond to different prey signals and to investigate the effect of olfactory experience. We first conducted a field choice test and a wind tunnel experiment to examine the kairomonal response of individual predators toward prey pheromone components (frontalin, ipsenol, ipsdienol, sulcatol) along with the pine monoterpene α-pinene, which is a volatile compound from the host of the prey. We also presented semiochemically naive predators with two prey pheromone components, frontalin and ipsenol, alone or associated with a reward. Our results showed that T. dubius populations are composed of generalists that can respond to a broad range of kairomonal signals. Naive T. dubius also were more attracted to ipsenol following its association with a reward. This work constitutes the first evidence that the behavior of a predatory insect involved in bark beetle population dynamics is influenced by previous olfactory experience, and provides a potential explanation for the pattern of prey switching observed in field studies.

The allometry of prey preferences.

The distribution of weak and strong non-linear feeding interactions (i.e., functional responses) across the links of complex food webs is critically important for their stability. While empirical advances have unravelled constraints on single-prey functional responses, their validity in the context of complex food webs where most predators have multiple prey remain uncertain. In this study, we present conceptual evidence for the invalidity of strictly density-dependent consumption as the null model in multi-prey experiments. Instead, we employ two-prey functional responses parameterised with allometric scaling relationships of the functional response parameters that were derived from a previous single-prey functional response study as novel null models. Our experiments included predators of different sizes from two taxonomical groups (wolf spiders and ground beetles) simultaneously preying on one small and one large prey species. We define compliance with the null model predictions (based on two independent single-prey functional responses) as passive preferences or passive switching, and deviations from the null model as active preferences or active switching. Our results indicate active and passive preferences for the larger prey by predators that are at least twice the size of the larger prey. Moreover, our approach revealed that active preferences increased significantly with the predator-prey body-mass ratio. Together with prior allometric scaling relationships of functional response parameters, this preference allometry may allow estimating the distribution of functional response parameters across the myriads of interactions in natural ecosystems.

Detecting arthropod intraguild predation in the field

The process of biological control carries a distinct risk that an alien biological control agent (BCA) will become established as an invasive alien species with an associated threat to the local ecosystem biodiversity. It is imperative that a wide-ranging environmental risk assessment (ERA) is performed before the release of any BCA. This should include considering various potential but difficult to observe ecological interactions between the BCA and members of the native community, including disruption of intraguild relationships. Detection of intraguild predation (IGP) events involving predatory arthropods in the field can be done by analyzing their gut contents. Polymerase chain reaction (PCR) is a sensitive and specific tool to identify target prey DNA within a predator’s gut. This paper reviews the efficiency of a DNA based approach for detecting IGP in the field, compared with detection by the use of monoclonal antibodies or gas chromatography. Prey specificity, detection times after prey consumption, capacity for quantification, multiple prey targeting and the time and costs involved in developing and using the different methods are considered.

Merging resource availability with isotope mixing models: the role of neutral interaction assumptions.

BackgroundBayesian mixing models have allowed for the inclusion of uncertainty and prior information in the analysis of trophic interactions using stable isotopes. Formulating prior distributions is relatively straightforward when incorporating dietary data. However, the use of data that are related, but not directly proportional, to diet (such as prey availability data) is often problematic because such information is not necessarily predictive of diet, and the information required to build a reliable prior distribution for all prey species is often unavailable. Omitting prey availability data impacts the estimation of a predator's diet and introduces the strong assumption of consumer ultrageneralism (where all prey are consumed in equal proportions), particularly when multiple prey have similar isotope values.MethodologyWe develop a procedure to incorporate prey availability data into Bayesian mixing models conditional on the similarity of isotope values between two prey. If a pair of prey have similar isotope values (resulting in highly uncertain mixing model results), our model increases the weight of availability data in estimating the contribution of prey to a predator's diet. We test the utility of this method in an intertidal community against independently measured feeding rates.ConclusionsOur results indicate that our weighting procedure increases the accuracy by which consumer diets can be inferred in situations where multiple prey have similar isotope values. This suggests that the exchange of formalism for predictive power is merited, particularly when the relationship between prey availability and a predator's diet cannot be assumed for all species in a system.

Aboriginal responses to Late Quaternary environmental change in a Mediterranean-type region: Zooarchaeological evidence from south-western Australia

South-western Australia is a Mediterranean-type region with characteristic hot dry summers and wet winters. There is little evidence for intensification in resource use by Aboriginal people in this region although elsewhere in Australia intensification is indicated by increasing numbers of seed grinding stones and exploitation of other new plant resources, especially during the mid-late Holocene. These conditions imply departures from models of population growth and intensification of resource use at the Pleistocene–Holocene transition in the Mediterranean Basin. Multiple prey attributes were explored as potential criteria for prey ranking to measure resource depression in zooarchaeological sequences dating from 22,000 to 400 BP at four limestone rock shelters occupied by Aboriginal hunter–gatherers in the far south-west. Assuming the criteria are valid, little evidence was found of switching between higher or lower-ranked resources between Pleistocene and Holocene sequences. In this region changes in stone technology that occur across Australia in the Holocene do not seem to translate to changes in fauna exploitation. However, this may be because of the peculiarities of this Mediterranean-climate region in which the biota were relatively unaffected by post-glacial climate oscillations, meaning that there was little pressure on populations living there.

Scaling-up anti-predator phenotypic responses of prey: impacts over multiple generations in a complex aquatic community

Non-consumptive effects (NCEs) of predators owing to induced changes in prey traits are predicted to influence the structure of ecological communities. However, evidence of the importance of NCEs is limited primarily to simple systems (e.g. two to four species) over relatively short periods (e.g. less than one generation). We examined the NCEs of a fish predator, arising from phenotypic plasticity in zooplankton prey traits, over multiple generations of a diverse zooplankton community. The presence of fish, caged to remove consumptive effects, strongly influenced zooplankton community structure, through both direct and indirect NCE pathways, altering the abundance of many taxa by magnitudes as large as 3 to 10-fold. Presence of fish affected different species of cladocerans and copepods both positively and negatively. A particularly striking result was the reversal of dominance in copepod taxa: presence of fish reduced the ratio of calanoids to cyclopoids from 6.3 to 0.43. Further, the NCE of fish had a strong negative trophic cascade to zooplankton resources (phytoplankton). To our knowledge, this is the first experiment to show that NCEs can influence the abundance of multiple prey species over time spans of multiple prey generations. Our findings demonstrate that adaptive phenotypic plasticity of individuals can scale-up to affect the structure of ecological communities.

Multiple prey cues induce foraging flexibility in a trap-building predator

Predators must be behaviourally flexible to counter the temporal and spatial stochastic fluctuations and response variability of their prey. To ensure behaviours are adequate across environments, animals must regularly assess environmental cues. Spider orb webs are an example of a flexible foraging trait in a predator, as web architectural components vary in response to exposure to different prey types and prey traits. The cues used by orb web spiders to initiate changes in web architecture are not known. Current research predicts that prey nutrients and vibratory stimuli are potential candidates, but how they combine to affect spider foraging decisions is not clear. We performed experiments exposing the giant wood spider, Nephila pilipes, to different prey nutrients and vibratory stimuli. Spiders were fed either large profitable prey with high kinetic energy (crickets) or small prey with low kinetic energy (flies). In two treatments the prey nutrients and vibratory cues came from live prey, but in the other two treatments spiders received dead crickets with webs stimulated by flies and vice versa. The spiders fed on live flies built larger webs with more radii that were less stiff and had greater vibration damping. These web characteristics did not differ between the other three treatment groups. Our results show that in the absence of nutrient and vibratory cues from profitable prey, spiders alter their web architecture to build webs better able to capture the less profitable prey at a cost of more material investment, greater web visibility and reduced vibratory signal clarity.

Temperature Mediates Shifts in Individual Aggressiveness, Activity Level, and Social Behavior in a Spider

Although in recent years behavioral syndromes have received a wealth of attention, how traits within syndromes respond to changing environments is not well resolved. Here, we test the effects of temperature on a suite of behavioral traits in the spider Anelosimus studiosus to determine (1) whether there are shifts in individuals’ social tendency, activity level, and foraging behavior in response to temperature, (2) if these traits shift are in the direction predicted by within-population axes of trait covariance, and (3) whether the effects of temperature differ among individuals. In previous work, we documented a behavioral syndrome in A. studiosus where increased tolerance of conspecifics is correlated with decreased activity level and aggressiveness toward prey. Furthermore, there are distinct among-population differences in behavior, where individuals from warm sites tend to be more aggressive and active than individuals from cold sites. Our data here reveal that at warmer temperatures A. studiosus exhibit diminished tolerance of conspecifics, increased activity levels, shorter latencies of attack, and increased tendencies to attack multiple prey items. Furthermore, we found that individual differences in behavior were consistent across temperature regimes for the majority of behavioral traits considered here: social tendency, activity level, and latency of attack. These findings are consistent with the hypothesis that these behaviors are linked together by shared genetic underpinnings (e.g., metabolic differences) and shift non-independently in response to contemporary abiotic environment (i.e., temperature). Furthermore, our data suggest that temperature itself could be responsible for the among-population variation in social structure in A. studiosus.

Field evidence for pervasive indirect effects of fishing on prey foraging behavior

The indirect, ecosystem-level consequences of ocean fishing, and particularly the mechanisms driving them, are poorly understood. Most studies focus on density-mediated trophic cascades, where removal of predators alternately causes increases and decreases in abundances of lower trophic levels. However, cascades could also be driven by where and when prey forage rather than solely by prey abundance. Over a large gradient of fishing intensity in the central Pacific's remote northern Line Islands, including a nearly pristine, baseline coral reef system, we found that changes in predation risk elicit strong behavioral responses in foraging patterns across multiple prey fish species. These responses were observed as a function of both short-term ("acute") risk and longer-term ("chronic") risk, as well as when prey were exposed to model predators to isolate the effect of perceived predation risk from other potentially confounding factors. Compared to numerical prey responses, antipredator behavioral responses such as these can potentially have far greater net impacts (by occurring over entire assemblages) and operate over shorter temporal scales (with potentially instantaneous response times) in transmitting top-down effects. A rich body of literature exists on both the direct effects of human removal of predators from ecosystems and predators' effects on prey behavior. Our results draw together these lines of research and provide the first empirical evidence that large-scale human removal of predators from a natural ecosystem indirectly alters prey behavior. These behavioral changes may, in turn, drive previously unsuspected alterations in reef food webs.

Is Meat Flavor a Factor in Hunters’ Prey Choice Decisions?

By focusing on the caloric composition of hunted prey species, optimal foraging research has shown that hunters usually make economically rational prey choice decisions. However, research by meat scientists suggests that the gustatory appeal of wildlife meats may vary dramatically. In this study, behavioral research indicates that Mayangna and Miskito hunters in Nicaragua inconsistently pursue multiple prey types in the optimal diet set. We use cognitive methods, including unconstrained pile sorts and cultural consensus analysis, to investigate the hypothesis that these partial preferences are influenced by considerations of meat flavor. Native informants exhibit high agreement on the relative appeal of different meats. Given the absence of other noteworthy differences between spider monkeys (Ateles geoffroyi) and howler monkeys (Alouatta palliata), the unappealing flavor of howler monkeys seems to be a factor in the partial preference for this species.

Multiplex reactions for the molecular detection of predation on pest and nonpest invertebrates in agroecosystems

Species- and group-specific PCR primers were developed to study predation on pest and nonpest invertebrate species by generalist carabid predators in agroecosystems. To ensure the amplification of degraded DNA in predator gut samples, amplicons were designed to be less than 300 bp. Specificity of primers was assessed by cross-amplification against a panel of target and nontarget invertebrate species. The new primers were combined with previously published primers for slugs and collembolla in multiplex reactions to simultaneously screen each predator for the presence of multiple prey. All prey species were detected in a screen of the gut contents of field-caught predators.

Nanoplankton mixotrophy in the eastern equatorial Pacific

Heterotrophic bacteria, cyanobacteria, and picoeukaryotic algae dominate the plankton community of high nutrient-low chlorophyll (HNLC) areas of the eastern equatorial Pacific (EEP). While grazing on these picoplankton is often attributed to aplastidic zooflagellates, mixotrophic nanoflagellates (phagotrophic phototrophs) may also exert a large grazing pressure. We assessed the relative contributions of mixotrophic nanoplankton and obligate heterotrophs to picoplankton phagotrophy in mixed-layer water of the EEP using 0.8-μm Fluorescently-Labeled Bacteria (FLB). Obligate heterotrophs and phototrophs were distinguished from their ratios of microscopically measured red (chlorophyll a) to green (proflavin-stained protein) fluorescence. Sampling sites were located along a nutrient gradient formed by a tropical instability wave at 0.5°N between 123.5°W and 128°W and at 1.75°N, 125°W. The majority of ingested particles were found within 3-5 μm flagellates, with 54% of the demonstrated phagotrophs belonging to the high-pigment putatively phototrophic population and obligate heterotrophs responsible for 51% of the demonstrated phagotrophy due to their greater propensity to ingest multiple prey. The importance of mixotrophy as a means of alleviating nutrient stress is indicated by a strong inverse relationship between the proportion of community FLB uptake by mixotrophs and ambient nutrient concentration. Low ambient Fe concentration and a demonstrated community response to Fe-addition in shipboard grow-out experiments suggest that mixotrophs were primarily engaging in phagotrophy to offset Fe-deficiencies.

Prey size selection and bottom type influence multiple predator effects in a crab–bivalve system

Consumption by multiple conspecific or interspecific predators on shared prey is not always predicted when data from isolated predators are combined. Predator interactions can result in non-independent predator effects on prey, and may be influenced by prey size selection and bottom type. We examined stone crab Menippe mercenaria and blue crab Callinectes sapidus predation of hard clams Mercenaria mercenaria. Interactions between these predators may become common in mid-Atlantic estuaries as stone crab abundance increases with a poleward range shift driven by climate change. Crabs in isolation, in conspecific pairs and in the interspecific pair were offered clams in 5 size classes simultaneously (10–60 mm shell length [SL]) or 1 size class (10–20 mm SL). Trials were on sand and hard bottom. On sand, all predator pairs had independent effects on prey, regardless of the presence or absence of multiple prey sizes. On hard bottom, when multiple prey sizes were present, blue crab conspecific and the interspecific pairs consumed less than predicted from isolated crabs. Strong selection of small clams by blue crabs led to conspecific interactions that reduced foraging compared to isolated crabs. Because stone crabs consumed all clam sizes, behaviours other than prey size selection caused the non-independent effect on prey by the interspecific pair. When multiple prey sizes were absent on hard bottom, most predator pairs had independent effects on prey. Thus, an expected poleward range shift of stone crabs may not increase interactions with blue crabs when foraging on sand. However, on hard bottom, increased predator interactions could reduce overall predation risk for prey, especially when consuming multiple prey sizes. In a climate-changed midAtlantic estuary, stone crabs will consume a wide range of clam sizes, and the size refuge large clams usually have from blue crab predation will be lost.

The influence of diet on foraging habitat models: a case study using nursing Antarctic fur seals

Predictable sources of food underpin lifetime reproductive output in long lived animals. The most important foraging areas of top marine predators are therefore likely to be related to environmental features that enhance productivity in predictable spatial and temporal patterns. Even so, although productive areas within the marine environment are distributed patchily in space and time, most studies assess the relationships between feeding activity and proximate, not long term, environmental characteristics. In addition, individuals within a population may exploit different prey types, and these are often associated with different hydrographic features. Until now, models attempting to associate core foraging areas (CFAs) of marine predators with the environmental characteristics of those areas have not considered the diet of individual animals, despite the influence this could have on these relationships. We used bathymetry and multi‐year (n=24) mean sea surface temperature and variability as predictors of CFAs of lactating Antarctic fur seals Arctocephalus gazella at Heard Island. The effect of prey types on the predictability of these models was explored by matching diet and foraging trip data of individual seals (n=40 seals, n=1 trip each). Differences in diet between seals were mirrored by their spatial behaviour. Foraging strategies differed both between and within groups of seals consuming different diets. Long‐term environmental parameters were useful for predicting the foraging activity of seals that consumed a single prey type with relatively specific habitat preferences, but not for those that consumed single or multiple prey types associated with more varied habitats. Ignoring individual variation in predator diet probably contributes to the poor performance of foraging habitat models. These findings highlight the importance of incorporating individual specialization in foraging behaviour into ecological models and management of predator populations.

The Young, the Weak and the Sick: Evidence of Natural Selection by Predation

It is assumed that predators mainly prey on substandard individuals, but even though some studies partially support this idea, evidence with large sample sizes, exhaustive analysis of prey and robust analysis is lacking. We gathered data from a culling program of yellow-legged gulls killed by two methods: by the use of raptors or by shooting at random. We compared both data sets to assess whether birds of prey killed randomly or by relying on specific individual features of the prey. We carried out a meticulous post-mortem examination of individuals, and analysing multiple prey characteristics simultaneously we show that raptors did not hunt randomly, but rather preferentially predate on juveniles, sick gulls, and individuals with poor muscle condition. Strikingly, gulls with an unusually good muscle condition were also predated more than expected, supporting the mass-dependent predation risk theory. This article provides a reliable example of how natural selection may operate in the wild and proves that predators mainly prey on substandard individuals.

Influence of wing loading on the trade-off between pursuit-diving and flight in common guillemots and razorbills

Species of bird that use their wings for underwater propulsion are thought to face evolutionary trade-offs between flight and diving, leading to the prediction that species with different wing areas relative to body mass (i.e. different wing loadings) also differ in the relative importance of flight and diving activity during foraging trips. We tested this hypothesis for two similarly sized species of Alcidae (common guillemots and razorbills) by using bird-borne devices to examine three-dimensional foraging behaviour at a single colony. Guillemots have 30% higher wing loading than razorbills and, in keeping with this difference, razorbills spent twice as long in flight as a proportion of trip duration whereas guillemots spent twice as long in diving activity. Razorbills made a large number of short, relatively shallow dives and spent little time in the bottom phase of the dive whereas guillemots made fewer dives but frequently attained depths suggesting that they were near the seabed (ca. 35-70 m). The bottom phase of dives by guillemots was relatively long, indicating that they spent considerable time searching for and pursuing prey. Guillemots also spent a greater proportion of each dive bout underwater and had faster rates of descent, indicating that they were more adept at maximising time for pursuit and capture of prey. These differences in foraging behaviour may partly reflect guillemots feeding their chicks single large prey obtained near the bottom and razorbills feeding their chicks multiple prey from the water column. Nonetheless, our data support the notion that interspecific differences in wing loadings of auks reflect an evolutionary trade-off between aerial and underwater locomotion.

Multiple prey effects: Agonistic behaviors between prey species enhances consumption by their shared predator

Multiple prey species may affect consumption by their shared predator in a non-additive fashion which cannot be predicted by summing pairwise predator–prey interactions within food webs. By manipulating prey species richness and identity in experimental mesocosms, we sought to explore the effects of interspecific prey interactions between pinfish Lagodon rhomboides and pigfish Orthopristis chrysoptera, and between blue crabs Callinectes sapidus and common mud crabs Panopeus herbstii on the consumption rates of a shared predator species, gulf toadfish Opsanus beta. We also determined the effects of these interactions on the survivorship of the prey species as mediated by the predator. In both fish and crab prey trials, interspecific interactions between multiple prey increased the overall consumption rate of O. beta, and in each case, one prey species was harmed (decreased survivorship) while the other prey species was not significantly affected by these interactions (all relative to single-prey treatments). In trials run with fish prey, behavioral observations revealed that L. rhomboides aggressively chased O. chrysoptera out of a seagrass refuge and into a surrounding sand matrix where O. beta foraged, thus increasing O. chrysoptera vulnerability to predation. In trials run with crab prey, C. sapidus failed to reduce their activity in the presence of O. beta. In addition, P. herbstii displaced C. sapidus from the seagrass refuge in mixed prey treatments, and so these conspicuous behaviors primarily occurred in the unstructured sand habitat where C. sapidus were more vulnerable to predation. We conclude that multiple prey effects, including those involving risk-enhancing, agonistic interactions between prey, can be important determinants of predator–prey outcomes. This may be especially true when refuge space is limited.