Under pressure: Short- and long-term response to predation varies in two populations of a live-bearing fish

The non-lethal effects of predation can significantly influence animal behavior and population composition. Research has often centered around prey response to predator exposure in the short term, but fewer studies have highlighted the effects of long-term predator exposures. In addition, studies of responses to predation risk are not always calibrated against the ecological history of predation risk in specific populations. We address these gaps by examining the effects of both long- and short-term predator exposure on the behavior of individuals from populations that have different ecological histories of predation risk. We exposed individuals from high-predation and low-predation populations of the live-bearing freshwater poeciliid, Heterandria formosa, to predators to assess changes in male reproductive behavior toward females. We also assessed longer-term reproductive responses by exposing male and female H. formosa to predators at a random time of day, every day, for 30 days. In the presence of a predator, in the short term, males changed the frequency of their behaviors and females varied in their concentration of cortisol, demonstrating immediate responses to the perceived risk. The magnitude of these changes was larger in the population without a long history of predator exposure. However, we found that males and females did not change their reproductive output when exposed to predators over longer periods of time, suggesting that individuals acclimatize to the level of predation risk they experience. These results also suggest that short-term variation in behavior or stress hormone responses should not be used as proxies for long-term responses or fitness effects. Future work should assess both short-term behavior and long-term responses while simultaneously considering the ecological history of populations.     

Orientation and navigation relative to water flow, prey, conspecifics, and predators by the nudibranch mollusc Tritonia diomedea

Progress in understanding sensory and locomotory systems in Tritonia diomedea has created the potential for the neuroethological study of animal navigation in this species. Our goal is to describe the navigational behaviors to guide further work on how the nervous system integrates information from multiple senses to produce oriented locomotion. Observation of T. diomedea in its habitat has suggested that it uses water flow to navigate relative to prey, predators, and conspecifics. We test these hypotheses in the field by comparing slug orientation in time-lapse videos to flow direction in circumstances with and without prey, predators, or conspecifics upstream. T. diomedea oriented upstream both while crawling and after turning. This trend was strongest before feeding or mating; after feeding or mating, the slugs did not orient significantly to flow. Slugs turned downstream away from an upstream predator but did not react in control situations without an upstream predator. These data support the hypothesis that T. diomedea uses a combination of odors (or some other cue transported downstream) and water flow to navigate relative to prey, predators, and conspecifics. Understanding the context-dependent choice between upstream and downstream crawling in T. diomedea provides an opportunity for further work on the sensory integration underlying navigation behavior.     

Predation and the evolution of prey behavior: an experiment with tree hole mosquitoes

We tested for facultative changes in behavior of an aquaticinsect in response to cues from predation and for evolutionof prey behavior in response to experimental predation regimes.Larvae of the tree hole mosquito Aedes triseriatus reducedfiltering, browsing, and time below the surface in responseto water that had held a feeding larva of the predator Toxorhynchitesrutilus. We subjected experimental A. triseriatus populationsto culling of 50% of the larval population, either by T. rutiluspredation or by random removal. After two generations of laboratoryculling, behavior of the two treatment groups diverged. Aedestriseriatus in control-culled lines retained their facultativeshift from filtering to resting, but tended to lose the response of reduced browsing below the surface in water that had helda feeding predator. Predator-culled lines lost their facultativeresponse of reduced filtering in water that had held a feedingpredator and evolved toward more time resting and less timefiltering in both water that had held a feeding predator andwater that had held only A. triseriatus. Predator-culled linesretained their facultative response of reduced browsing belowthe surface in water that had held a feeding predator. Twofield populations and their reciprocal hybrids responded similarlyto cues from predation and did not differ in their evolutionaryresponse to experimental culling. We conclude that consistentpresence or absence of predation can select rapidly for divergencein prey behavior, including facultative behavioral responsesto predators.     

Interpopulation Comparisons of Antipredator Defense Behavior of the Thorn‐Tailed Rayadito (Aphrastura spinicauda)

The high impact of predators on breeding success has favored the evolution of diverse behavioral strategies that enhance both individual survivorship and reproductive success. Self‐ and nest‐defense against predators are inter‐related behaviors that vary according to life history traits and the risks involved in defense. In addition, interpopulation differences in the composition of predator guilds, and hence predation pressures, may result in geographic variation in antipredator defense strategies. It is generally accepted that the absence of predators could drive the loss of antipredator behavior. However, it is unclear how species respond if one type of predator (e.g., a specialized nest predator) is absent but others are present. The multipredator hypothesis suggests that the absence of only a few predators is not sufficient to drive a change or loss of antipredator behavior in a species. We examined interpopulation variation in defense behavior in the thorn‐tailed rayadito (Aphrastura spinicauda), a suboscine bird inhabiting a wide latitudinal range in southern South America. In populations where nest predators were either present or absent, we examined the responses of rayaditos to an adult predator (an owl) and to a nest predator (a marsupial). Owl‐defense behavior involved more intense and longer responses than marsupial‐defense. This suggests that adult rayaditos may allocate more energy to defend themselves than their young. However, because the presence of adult predators near the nest also prevents parents from feeding the nestlings, owl‐defense may simultaneously represent a form of nest‐defense. No differences in marsupial‐defense were found among the three sampled populations, despite the absence of native mammalian nest predators in one population. The lack of variation in nest‐defense behavior provides some support for the multipredator hypothesis, although the high interpopulation gene flow in this species could also contribute to this behavior's maintenance.     

Habitat selection and consumption across a landscape of multiple predators

Predator community composition can alter habitat quality for prey by changing the strength and direction of consumptive effects. Whether predator community composition also alters prey density via nonconsumptive effects during habitat selection is not well known, but is important for understanding how changes to predator communities will alter prey populations. We tested the hypothesis that predator community composition (presence of caged trout, caged dragonflies, or caged trout + dragonflies) alters colonization of aquatic mesocosms by ovipositing aquatic insects. In a previous experiment in this system, we found a spatial contagion effect, in which insects avoided pools with predators, but only when predator‐free pools were isolated (~5 m away from predator pools). Here, we removed the isolated predator‐free pools, allowing us to test whether insects would make fine‐scale (~1 m) oviposition decisions in the absence of preferred isolated pools. We also estimated consumptive effects by allowing predators to feed on colonists for 5 days following colonization. All insects collected after 21 days were dipterans, dominated by Chironomidae. Total colonization, measured as the number of developing larvae after 21 days, was not affected by either predator presence or composition. Consumption was significant in the trout only treatment, reducing larval insect density by 46 ± 37% (mean ± SE). No other predator treatment significantly reduced prey density, although the proportion of chironomid larvae in protective cases increased in response to direct predation from dragonflies, indicating an antipredatory behavioral response. Taken together, these results reveal that predator community composition altered larval survival and behavior, but colonizing females either did not or could not assess these risks across small scales during oviposition.     

Chemical defense of toad tadpoles under risk by four predator species

Many organisms use inducible defenses as protection against predators. In animals, inducible defenses may manifest as changes in behavior, morphology, physiology, or life history, and prey species can adjust their defensive responses based on the dangerousness of predators. Analogously, prey may also change the composition and quantity of defensive chemicals when they coexist with different predators, but such predator‐induced plasticity in chemical defenses remains elusive in vertebrates. In this study, we investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and specifically to the presence of different predator species; furthermore, we assessed the adaptive value of the induced defense. We reared tadpoles in the presence or absence of one of four caged predator species in a mesocosm experiment, analyzed the composition and quantity of their bufadienolide toxins, and exposed them to free‐ranging predators. We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis. Fishes and newts consumed only a small percentage of toad tadpoles, suggesting that bufadienolides provided protection against vertebrate predators, irrespective of the rearing environment. Backswimmers consumed toad tadpoles regardless of treatment. Dragonfly larvae were the most voracious predators and consumed more predator‐naïve toad tadpoles than tadpoles raised in the presence of dragonfly cues. These results suggest that tadpoles in our experiment had high enough toxin levels for an effective defense against vertebrate predators even in the absence of predator cues. The lack of predator‐induced phenotypic plasticity in bufadienolide synthesis may be due to local adaptation for constantly high chemical defense against fishes in the study population and/or due to the high density of conspecifics.     

Short-term responses to elevated predator densities: noncompetitive intraguild interactions and behavior

We investigated the short-term response of an arthropod assemblage to elevated generalist predator densities by introducing Chinese mantids (Tenodera sinensis) to field plots in a replicated, controlled experiment. Abundances of carnivorous arthropods were reduced by mantids to a greater extent than herbivores, and cursorial spiders emigrated from treatment plots in greater numbers than from controls. Initially, this emigration consisted only of small spiders that were demonstrated in the laboratory to be prey for mantids. Thus, the initial response of an arthropod assemblage to increased predators, densities was increased interactions among predators, which caused decline in predator population densities in a shorter time than competition for prey would require. Predator avoidance behavior must be considered together with intraguild predation and competition when interpreting the outcome of predator manipulations. Shortterm experiments may be more valuable than longer term studies in detecting this effect.     

Impact of predation on early stages of the armoured catfish Hoplosternum thoracatum (Siluriformes-Callichthyidae) and implications for the syntopic occurrence with other related catfishes in a neotropical multi-predator swamp

This study investigated the role of predators in preventing competitive exclusion among three closely related armoured catfishes (Callichthys callichthys, Hoplosternum littorale and H. thoracatum) that occur synthopically in multi-predator freshwater swamps of Suriname, South America. The potential impact of predation on armoured catfish was determined by combining laboratory measurements of predation rates on five early developmental stages of the armoured catfish H. thoracatum for 24 aquatic predators with field studies of the density of the predators in the swamps. The contribution of a particular predator to the total predation pressure on its prey was determined to a large extent by the density of the predator in the swamp. Seemingly innocuous predators with low or moderate predation rates in the laboratory may be extremely important in the swamps due to their high abundance. Small-sized omnivorous fishes and aquatic invertebrates were major predators of early developmental stages of armoured catfish. Both qualitative and quantitative ontogenetic changes in the predation pressure on armoured catfish were observed. Major predation on eggs, larvae and juveniles of H. thoracatum resulted from a different set of predators in each developmental stage of the prey. In all developmental stages of H. thoracatum the predation pressure involved several predator species and not a single, dominant predator. The potential predation pressure of the 24 predators taken together and the number of predators that were able to prey on H. thoracatum decreased sharply with increasing age (size) of the prey. Even if egg (nest) predation is prevented by the guarding male, the potential impact of the 24 predators on the populations of armoured catfish is large. Predation may account for the high mortality of H. thoracatum observed in the swamps. The high predation pressure on callichthyid catfishes may help to explain the coexistence of three closely related and morphologically quite similar armoured catfishes in Surinamese swamps.     

The Effects of Relaxed and Reversed Selection by Predators on the Antipredator Behavior of the Threespine Stickleback, Gasterosteus aculeatus

Isolation from predators can lead to the reduction or loss of ancestral behavioral defenses in prey, but does not always do so. Predators introduced to populations that have experienced relaxed selection from some ancestral predators can favor the evolution of antipredator behavior that has been lost. We examined these possibilities by eliciting antipredator behavior in three populations of threespine stickleback fish, Gasterosteus aculeatus : an oceanic population thought to resemble the ancestral form, and two populations historically (up to 20 000 yr) devoid of piscine predators (relaxed selection), one of which has been stocked with salmonids for nearly 25 yr (reversed selection). We used three kinds of predator models: a sculpin (ambush predator), a rainbow trout (chasing predator), and an overhead silhouette of an arctic tern. Stickleback reacted differently to the three models, indicating that they distinguished among them. Individuals from all populations responded similarly to the tern model. The ancestral population showed the weakest response to the sculpin model despite being the only population that encounters these predators naturally. Stickleback from the trout-free population displayed slightly reduced responses to the trout model, and recovery times like those in the ancestral population providing only weak evidence for loss of the ancestral antipredator repertoire. Fish from the reverse-selected population exhibited fascinating, elevated responses to both the trout and sculpin models relative to the other two populations. These findings offer initial evidence of (1) a partial alteration of the ancestral behavioral repertoire during a long period of relaxed selection from piscine predators, and (2) rapid acquisition of extreme responses to piscine predators under reverse selection.     

A rare predator exploits prey escape behavior: the role of tail-fanning and plumage contrast in foraging of the painted redstart (Myioborus pictus)

Escape response, triggered by an approaching predator, is acommon antipredatory adaptation of arthropods against insectivores.The painted redstart, Myioborus pictus, represents insectivorousbirds that exploit such antipredatory behaviors by flushing,chasing, and preying upon flushed arthropods. In field experimentsI showed that redstarts evoke jump and flight in prey by spreadingwings and tail: this display increased frequency of aerial chasesby redstarts. Artificial models with spread tails also elicitedescape responses more often than models with closed tails and wings.The white patches on black wings and tails additionally help:the frequency of chases decreased when the white patches werecovered with black dye. Black models also tended to elicit escaperesponse less often than black-and-white models did, at leastin some situations. Hence, the prey's ability to detect birdsand to flee could cause the evolution of predators specializedat using conspicuous behavior and contrast in body colorationto elicit and exploit such antipredatory responses. Redstartsconstitute only a small proportion of the predatory guild, andtheir adaptations to exploit the prey's behavior illustratethe theoretically modeled "rare enemy" effect present in multispeciespredator-prey systems. This is the first experimental studyof morphological and behavioral adaptations of a rare predatorthat both elicits and exploits antipredator escape behaviorof its prey against more common predators. Hence, the studydocuments a behavior that could be evolutionarily explainedonly if indirect interactions in predator-prey communities aretaken into account.     

Testing the Threat‐Sensitive Hypothesis with Predator Familiarity and Dietary Specificity

In a system with multiple predators, the threat‐sensitive predator avoidance hypothesis predicts that prey respond differently to predators relative to the risks each poses (e.g., degree of dietary specialization). Aquatic animals often rely heavily on detecting predators via chemical cues (kairomones) and respond with a suite of behaviors including detection and avoidance. However, little is known about how animals respond to kairomones of specialist versus generalist predators. In laboratory experiments, we compared behavioral responses of a poorly studied aquatic salamander, the greater siren (Siren lacertina), to cues from specialist and generalist predator snakes to evaluate threat‐sensitive responses. Sirens exhibited a novel behavior (gill‐flushing) most often in the presence of specialist predator cues. Avoidance behavior (reversing direction following cue detection) was higher in response to specialist predator and novel animal control cues and lowest in response to generalist predator cues. An intermediate response to the animal control, an unfamiliar amphibian predator, indicated that sirens respond cautiously to a novel cue. The gradient of observed responses to different snake cues indicates that sirens may be evaluating predation potential of animals based on their foraging specificity and familiarity.     

Predator-induced delayed maturity in bluegill sunfish (Lepomis macrochirus): variation among populations

Previous studies suggested that differences in age at maturity among populations of bluegill sunfish (Lepomis macrochirus) were not genetically based, but rather were a phenotypic response to the presence of predators. I conducted two experiments to determine if the presence of largemouth bass affected age at maturity in bluegill sunfish. Bluegills from three populations were tested to see if the response to the threat of predation varied among source populations. Juvenile bluegills were maintained in the presence of predators or in controls with no contact with predators. Refuge use and growth were monitored during the experiments and reproductive activity was evaluated when bluegills reached age 1. Bluegills from one population exhibited delayed maturity in the presence of predators. Individuals from the other two populations showed no significant differences between predator and control treatments. The population that responded to the presence of predators had a history of high predation levels over the past 30–40 years. The other populations had a history of low levels of predation. This study suggests that presence of predators can induce phenotypic shifts in age at maturity of bluegills, but that the magnitude of response varies among populations in a manner consistent with historical patterns of coexistence. Received: 7 August 1996 / Accepted: 8 August 1997     

Plastic Responses of a Sessile Prey to Multiple Predators: A Field and Experimental Study

Background

Theory predicts that prey facing a combination of predators with different feeding modes have two options: to express a response against the feeding mode of the most dangerous predator, or to express an intermediate response. Intermediate phenotypes protect equally well against several feeding modes, rather than providing specific protection against a single predator. Anti-predator traits that protect against a common feeding mode displayed by all predators should be expressed regardless of predator combination, as there is no need for trade-offs.

Principal Findings

We studied phenotypic anti-predator responses of zebra mussels to predation threat from a handling-time-limited (crayfish) and a gape-size-limited (roach) predator. Both predators dislodge mussels from the substrate but diverge in their further feeding modes. Mussels increased expression of a non-specific defense trait (attachment strength) against all combinations of predators relative to a control. In response to roach alone, mussels showed a tendency to develop a weaker and more elongated shell. In response to crayfish, mussels developed a harder and rounder shell. When exposed to either a combination of predators or no predator, mussels developed an intermediate phenotype. Mussel growth rate was positively correlated with an elongated weaker shell and negatively correlated with a round strong shell, indicating a trade-off between anti-predator responses. Field observations of prey phenotypes revealed the presence of both anti-predator phenotypes and the trade-off with growth, but intra-specific population density and bottom substrate had a greater influence than predator density.

Conclusions

Our results show that two different predators can exert both functionally equivalent and inverse selection pressures on a single prey. Our field study suggests that abiotic factors and prey population density should be considered when attempting to explain phenotypic diversity in the wild.     

Semantic Differences in Sifaka (Propithecus verreauxi) Alarm Calls: A Reflection of Genetic or Cultural Variants?

In this study, we compared the usage of alarm calls and anti‐predator strategies between a captive and a wild lemur population. The wild lemur population was studied earlier in Western Madagascar ( Fichtel & Kappeler 2002 ). The captive population was studied in outdoor enclosures of the Duke University Primate Center. Alarm calls and anti‐predator behavior were elicited by conducting experiments with both vocal and visual dummies. We scored the subjects’ immediate behavioral responses, including alarm calls, from video recordings made during the experiments. In principle, both populations have a mixed alarm call system with functionally referential alarm calls for aerial predators and general alarm calls for terrestrial and aerial predators and for situations associated with high arousal, such as group encounters. Although wild and captive sifakas exhibit the same alarm call system and use the same alarm call types, we discovered striking differences in the usage and perception of some of the alarm calls. We argue that these differences indicate either an evolutionary drift in the meaning of these calls or reflect cultural variation. The latter possibility is consistent with our understanding of the ontogeny of call usage and comprehension.     

Lethal and non-lethal effects of multiple indigenous predators on the invasive golden apple snail (Pomacea canaliculata)

1. We investigated the individual and combined effects of two predators (the climbing perch, Anabas testudineus, and the wetland crab, Esanthelphusa nimoafi) indigenous to wetlands in Laos, on the behaviour and survival of the invasive South American golden apple snail (Pomacea canaliculata). The snail is considered a pest, consuming large amounts of rice and other aquatic vegetation in the region. 2. Snail avoidance reactions to released predator chemical cues were investigated in aquaria while the effects of predators on a mixed snail population were studied in field enclosures that contained native aquatic plants (Salvinia cucullata, Ludwigia adscendens and Ipomoea aquatica). 3. In the aquaria experiment, neonate (2–3 mm) and medium‐sized snails (8–10 mm) responded to fish chemical cues by going to the surface, whereas adult snails (35–40 mm) went to the bottom. In contrast, no size class of snails reacted to chemical cues released by crabs. 4. In the field experiment, fish reduced the abundance of neonate snails, and crabs reduced the abundance of all size classes. The effect of the combined predators could not be predicted from the mortality rate observed in single predator treatments. The survival of neonate and medium‐sized snails was greater and of adults less than expected. The presence of predators did not affect egg production. Snails consumed significant amounts of plants despite the presence of predators. 5. Our findings suggest that some indigenous Asian predators have lethal and sublethal effects on P. canaliculata that depend on snail size and predator type. When in the presence of several predators the response of snails to one predator may either increase or decrease the vulnerability of snails to the others.     

Match and mismatch: Integrating consumptive effects of predators,prey traits,and habitat selection in colonizing aquatic insects

Predators are a particularly critical component of habitat quality, as they affect survival, morphology, behavior, population size, and community structure through both consumptive and non‐consumptive effects. Non‐consumptive effects can often exceed consumptive effects, but their relative importance is undetermined in many systems. Our objective was to determine the consumptive and non‐consumptive effects of a predaceous aquatic insect, Notonecta irrorata, on colonizing aquatic beetles. We tested how N. irrorata affected survival and habitat selection of colonizing aquatic beetles, how beetle traits contributed to their vulnerability to predation by N. irrorata, and how combined consumptive and non‐consumptive effects affected populations and community structure. Predation vulnerabilities ranged from 0% to 95% mortality, with size, swimming, and exoskeleton traits generating species‐specific vulnerabilities. Habitat selection ranged from predator avoidance to preferentially colonizing predator patches. Attraction of Dytiscidae to N. irrorata may be a natural ecological trap given similar cues produced by these taxa. Hence, species‐specific habitat selection by prey can be either predator‐avoidance responses that reduce consumptive effects, or responses that magnify predator effects. Notonecta irrorata had both strong consumptive and non‐consumptive effects on populations and communities, while combined effects predicted even more distinct communities and populations across patches with or without predators. Our results illustrate that an aquatic invertebrate predator can have functionally unique consumptive effects on prey, attracting and repelling prey, while prey have functionally unique responses to predators. Determining species‐specific consumptive and non‐consumptive effects is important to understand patterns of species diversity across landscapes.     

Cooperatively breeding Arabian babblers call differently when mobbing in different predator-induced situations

Cooperatively breeding Arabian babblers (Turdoides squamiceps) havea repertoire of different calls that they use in predator-induced contexts.We investigated their vocal mobbing behavior in two different predator-inducedsituations. We presented territorial groups of babblers a perched,stuffed owl representing an avian predator and a cat representinga ground predator. Babblers approached in both situations andmobbed the predators with the same two call types. In both predator-inducedsituations their first call was a short, metallic-sounding "tzwick."In response to the cat, babblers continued to primarily usetzwicks. However, in continued response to the owl, the babblersprimarily used long trills. The experiments indicate that differencesin use of two common call types during mobbing provide informationon differences in predator-induced situations, although no calltype per se identified a specific situation. The experiments suggestthat the short tzwicks indicate a higher risk or urgency thantrills and that combinations of both call types may providegraded information about differences in predator-induced situations.The relatively stable groups of these cooperative breeders mighthave favored evolution for using different calls in differentmobbing situations.     

Biotic resistance on the increase: native predators structure invasive zebra mussel populations

1. Abundant native predators, parasites and pathogens that switch to consuming a hyper‐successful exotic species may be able to control the invasive population. Native predators may, however, need time to adapt to feed effectively on an exotic resource. In this case, mortality on an exotic population from native predators could increase over time even without a numerical increase in the predator population. 2. We measured mortality of zebra mussels (Dreissena polymorpha) in the Hudson River both in controls open to predation and in exclosures that excluded large predators to estimate mortality of zebra mussels from large predators and other causes. 3. We found that predation by the blue crab (Callinectes sapidus), and perhaps other predators, causes high mortality on zebra mussels in the Hudson River estuary. This predation apparently led to increased mortality and altered population structure in the invader over time. 4. Long‐term data from the Hudson River suggest that components of the invaded ecosystem, like rotifers, are recovering through predator‐caused release from zebra mussel grazing. Increased mortality on hyper‐successful exotic populations over time may be a common phenomenon with both ecological and management implications.     

Isolation from mammalian predators differentially affects two congeners

Evolutionary isolation from predators can profoundly influencethe morphology, physiology, and behavior of prey, but littleis known about how species respond to the loss of only someof their predators. We studied antipredator behavior of tammarwallabies (Macropus eugenii) and western gray kangaroos (Macropusfuliginosus) on Kangaroo Island (KI), South Australia, andat Tutanning Nature Reserve on the mainland of western Australia.Both species on KI have been isolated from native mammalian predators for several thousand years. On KI, wallabies (becauseof their size) are vulnerable to diurnal aerial predators.In contrast, on the mainland both species have been exposedcontinuously to native and introduced mammalian and avian predators.At both locations, wallabies modified the amount of time they allocated to vigilance and foraging in response to group size,whereas kangaroos did so only at the higher risk Tutanningsite. Both species modified overall time budgets (they werewarier at the higher risk site), and both species modifiedspace-use patterns as a function of risk. At the higher risk site, tammars were closer to cover, whereas kangaroos were,on average, farther from cover. We hypothesize that the presenceof a single predator, even if it is active at a different timeof day, may profoundly affect the way a species responds tothe loss of other predators by maintaining certain antipredatorbehaviors. Such an effect of ancestral predators may be expected as long as species encounter some predators.     

Coexistence in streams: do source–sink dynamics allow salamanders to persist with fish predators?

Theory suggests that source–sink dynamics can allow coexistence of intraguild predators and prey, but empirical evidence for this coexistence mechanism is limited. We used capture–mark–recapture, genetic methods, and stable isotopes to test whether source–sink dynamics promote coexistence between stream fishes, the intraguild predator, and stream salamanders (Dicamptodon aterrimus), the intraguild prey. Salamander populations from upstream reaches without fish were predicted to maintain or supplement sink populations in downstream reaches with fish. We found instead that downstream reaches with fish were not sinks even though fish consumed salamander larvae—apparent survival, recruitment, and population growth rate did not differ between upstream and downstream reaches. There was also no difference between upstream and downstream reaches in net emigration. We did find that D. aterrimus moved frequently along streams, but believe that this is a response to seasonal habitat changes rather than intraguild predation. Our study provides empirical evidence that local-scale mechanisms are more important than dispersal dynamics to coexistence of streams salamanders and fish. More broadly, it shows the value of empirical data on dispersal and gene flow for distinguishing between local and spatial mechanisms of coexistence.