Presence Of Adult Conspecifics Research Articles

Colonization of novel algal habitats by juveniles of a marine tube-dwelling amphipod.

BackgroundDispersal is an important process affecting population dynamics and connectivity. For marine direct developers, both adults and juveniles may disperse. Although the distribution of juveniles can be initially constrained by their mothers’ choice, they may be able to leave the parental habitat and colonize other habitats. We investigated the effect of habitat quality, patch size and presence of conspecific adults on the colonization of novel habitats by juveniles of the tube-dwelling amphipod Cymadusa filosa associated with the macroalgal host Sargassum filipendula.MethodsWe tested the factors listed above on the colonization of juveniles by manipulating natural and artificial plants in both the field and laboratory.ResultsIn the laboratory, juveniles selected high-quality habitats (i.e., natural alga), where both food and shelter are provided, when low-quality resources (i.e., artificial alga) were also available. In contrast, habitat quality and algal patch size did not affect the colonization by juveniles in the field. Finally, the presence of conspecific adults did not affect the colonization of juveniles under laboratory condition but had a weak effect in the field experiment. Our results suggest that C. filosa juveniles can select and colonize novel habitats, and that such process can be partially affected by habitat quality, but not by patch size. Also, the presence of conspecifics may affect the colonization by juveniles. Successful colonization by this specific developmental stage under different scenarios indicates that juveniles may act as a dispersal agent in this species.

Are we ready for scaling up restoration actions? An insight from Mediterranean macroalgal canopies.

Extensive loss of macroalgal forests advocates for large-scale restoration interventions, to compensate habitat degradation and recover the associated ecological functions and services. Yet, restoration attempts have generally been limited to small spatial extensions, with the principal aim of developing efficient restoration techniques. Here, the success of outplanting Cystoseira amentacea v. stricta germlings cultured in aquaria was experimentally explored at a scale of tens of kms, by means of a multifactorial experimental design. In the intertidal rocky shores of SE Italy, locations with a continuous distribution for hundreds of meters or with few thalli forming patches of few centimeters of C. amentacea canopy were selected. In each location, the effects of adult conspecifics and the exclusion of macrograzers (salema fish and sea urchins) on the survival of germlings were tested. We evaluated the most critical determinants of mortality for germlings, including the overlooked pressure of mesograzers (e.g. amphipods, small mollusks, polychaetes). Despite the high mortality observed during outplanting and early settlement stages, survival of C. amentacea germlings was consistently favored by the exclusion of macrograzers, while the presence of adult conspecifics had no effects. In addition, the cost analysis of the interventions showed the feasibility of the ex-situ method, representing an essential tool for preserving Cystoseira forests. Large scale restoration is possible but requires baseline information with an in-depth knowledge of the species ecology and of the areas to be restored, together with the development of specific cultivation protocols to make consistently efficient restoration interventions.

Implications of feeding frequency, prey size and condition, and intraspecific competition for the commercial aquaculture of the nudibranch Berghia stephanieae

AbstractMarine ornamental aquaculture is a growing industry. Although most species are cultured for aesthetic purposes, there is increasing interest in organisms that provide functional services. Nudibranchs provide an example in which they act as biological controls by consuming aquaria pests. Berghia stephanieae is a commercially relevant nudibranch that feeds on Exaiptasia anemones, which overgrow sessile invertebrates. However, information pertaining to B. stephanieae feeding habits remains unaddressed. Therefore, this study investigated the following: (a) the effect of feeding frequency on fecundity, (b) optimum prey size, (c) the ability to feed on prey fragments, and (d) the influence of intraspecific competition. Distinct feeding frequencies were tested and embryos produced were quantified; growth of nudibranchs offered different anemone sizes was measured; whole and fragmented anemones were offered, with preference recorded; and growth and mortality of juvenile nudibranchs were recorded in the absence and presence of adult conspecifics. Results show that feeding daily produced the highest number of embryos. Juveniles fed on medium‐sized anemones showed the highest growth rate. Smaller juveniles were unable to feed on whole anemones and preferred fragmented anemones. Juvenile growth and survival decreased in the presence of adults. To maximize production, breeders must feed broodstock daily, provide medium‐sized anemones or fragments, and segregate juveniles from adults.

The influence of environmental and social factors on high- and low-cost vigilance in bat-eared foxes

Vigilance is commonly used to assess anti-predator behavior; however, the majority of studies assess only high-cost vigilance, which interrupts any other activities. Low-cost vigilance, by comparison, allows animals to be vigilant while engaged in other activities, thereby reducing the cost of vigilance. Here, we investigate the use of high- and low-cost vigilance in relation to environmental and temporal factors in a wild population of the bat-eared fox (Otocyon megalotis), a small mesopredator, using behavioral observations of eight habituated animals. We hypothesized that vigilance would increase with tall, dense vegetation, high wind speeds, and low ambient light, and that vigilance would decrease in the presence of conspecifics and over the period of a single observation session. Overall, bat-eared foxes spent significantly more time engaged in low-cost vigilance than in high-cost vigilance. High-cost vigilance increased with vegetation height, in the presence of adult conspecifics and in winter. We suggest these effects are due to impeded lines of sight with increases in vegetation height, increased competition in the presence of conspecifics, and increased social interactions in winter. Low-cost vigilance was highest at the start of observational sessions, and we suggest that this is due to observer effects, as foxes demonstrated increases in both high- and low-cost vigilance specifically towards observers during this time. This study demonstrates that foxes are highly dynamic in their vigilance behavior. Due to the low predation pressure in the study area, the use of mostly low-cost vigilance may be sufficient in this environment, and thus foxes can allocate more time to other activities. Our results indicate that, in this environment, vigilance is most likely driven by factors other than predation risk, but that foxes may still respond to certain cues of predation risk. Ultimately, in areas of low predation risk, species susceptible to predation may retain responses to cues of risk, but adapt behaviors to reduce the associated costs. While studies examining perceived risk in herbivores frequently assess vigilance, the examination of vigilance is largely under-investigated in mesopredators. Here, we present findings from one of the first studies to examine high- and low-cost vigilance in a small mesopredator. We demonstrate that in an area with minimal predation pressure low-cost vigilance is more intensively used and that despite the absence of predator sightings, ambush predators may still pose a perceived threat to foxes. Ultimately, when predation pressure is low, social interactions and competition may be some of the costliest activities. Results further indicate that while this small canid habituates quickly to observer presence, the initial presence of observers cannot be considered truly neutral.

Fitness trade‐offs associated with oviposition strategy in the winter cherry bug, Acanthocoris sordidus

AbstractIn phytophagous insects, oviposition strategy is determined by complex trade‐offs among many factors. The winter cherry bug, Acanthocoris sordidus Thunberg (Heteroptera: Coreidae), deposits its eggs not only on host plants, but also on non‐host plants. Oviposition on non‐host plants may cause higher nymph mortality, as nymphs may be unable to find their host plants during the second instar when the first feeding occurs. Therefore, we hypothesised that this uncommon oviposition strategy would be maintained by a trade‐off between egg and nymphal mortality risks. To test this hypothesis, we conducted field surveys to compare egg mortality due to biotic (predation and parasitism) and abiotic factors on host plants vs. non‐host plants. We also conducted a semi‐field experiment to quantify the risk of nymphal mortality caused by oviposition on non‐hosts during the second instar. In terms of mortality risk during the egg stage, a 2‐year field survey revealed that oviposition on non‐host plants resulted in higher survival of eggs than on host plants; this strategy was also particularly effective for avoiding attacks by ground‐living arthropod predators. In terms of mortality risk during the nymphal stage, the semi‐field experiment demonstrated that nymphs placed off host plants do risk being unable to reach host plants. However, the presence of conspecific adults on host plants increased the probability that nymphs reached host plants. Therefore, second instars may have evolved the ability to utilise chemical cues emitted by adults on host plants or by host plants fed by adults to reduce their risk of not reaching host plants. We concluded that oviposition on non‐host plants by A. sordidus females could be maintained by the balance between the benefit of lower egg mortality and the cost of higher nymph mortality compared to oviposition on host plants.

Cannibalism and food availability in the talitrid amphipod Orchestoidea tuberculata

The availability of limiting resources can potentially influence the intensity of intra- and interspecific interactions. Stranded macroalgae exported from adjacent coastal ecosystems supports abundant intertidal consumers on oceanic sandy beaches, including talitrid amphipods, which can be one of the numerically dominant invertebrates of the upper shore. The allochthonous nature of this donor-controlled food subsidy and its unpredictable delivery by waves and currents, results in highly variable and potentially limiting resource availability for these consumers. In Chile, adults of the talitrid amphipod, Orchestoidea tuberculata Nicolet, can influence the survival of juvenile conspecifics through cannibalism, a type of intraspecific interaction we hypothesized could be affected by the availability of macroalgal resources. We experimentally investigated the potential influence of food availability on cannibalism between age classes in O. tuberculata in laboratory mesocosms. Juvenile mortality in presence of conspecific adults was significantly higher when juveniles and adults were maintained without food. However, adult mortality was neither density dependent or food dependent. Further, juveniles did not influence adult mortality, either with or without food. The strong effect of food limitation on juvenile mortality from cannibalism by adults of O. tuberculata found here, supports our hypothesis that food resource availability on beaches can affect this intraspecific interactions. In addition these results provide evidence of the potential importance of biological interaction in the population dynamics of intertidal consumers on oceanic sandy beaches.

The Effect of Adult Aggression on Habitat Selection by Settlers of Two Coral-Dwelling Damselfishes

Coral-reef fishes experience a major challenge when facing settlement in a multi-threat environment, within which, using settlement cues, they need to select a suitable site. Studies in laboratories and artificial setups have shown that the presence of conspecific adults often serves as a positive settlement cue, whose value is explained by the increased survival of juveniles in an already proven fit environment. However, settlement in already inhabited corals may expose the recruits to adult aggression. Daily observations and manipulation experiments were used in the present study, which was conducted in the natural reef. We revealed differential strategies of settlers, which do not necessarily join conspecific adults. Dascyllus aruanus prefer to settle near (not with) their aggressive adults, and to join them only after gaining in size; whereas Dascyllus marginatus settlers in densely populated reefs settle independently of their adult distribution. Our results present different solutions to the challenges faced by fish recruits while selecting their microhabitat, and emphasize the complexity of habitat selection by the naïve settlers. Although laboratory experiments are important to the understanding of fish habitat selection, further studies in natural habitats are essential in order to elucidate the actual patterns of settlement and habitat selection, which are crucial for the survival of coral-reef fish populations.

The effect of exposure to conspecifics on restlessness in the parasitoid waspNasonia vitripennis(Hymenoptera: Pteromalidae)

AbstractWhen habitat quality is variable, there should be strong selection for the ability to detect and respond to the variation. Adult females of the parasitoid waspNasonia vitripennis(Walker) are known to increase their restlessness (the proportion of time in locomotion) both during and after exposure to a poor quality host. Doing so provides a mechanism for leaving a poor host and potentially finding a better host. This study examined whether restlessness also changes in response to competition as indicated by the presence of adult conspecifics. Both restlessness and the probability of dispersing across an inhospitable environment were greater when a female was with another female than when she was alone. However, restlessness did not remain elevated after the other female was removed. In contrast with females, restlessness of males did not increase either during or after exposure to other males, and the probability of dispersing across an inhospitable environment was unaffected by the presence of another male. The difference between females and males may be related to differences in dispersal ability and in the abundance and distribution of hosts versus mates.

Risk-sensitive foraging by juvenile signal crayfish (Pacifastacus leniusculus)

Feeding behavior of carnivorous and herbivorous organisms is commonly explained in terms of nutritional requirements. However, feeding behavior of omnivorous organisms is difficult to predict and may not be based on nutritional requirements alone. To assess the plasticity of feeding behaviors exhibited by an omnivore, we conducted a feeding experiment using juvenile signal crayfish ( Pacifastacus leniusculus (Dana, 1852)), both alone and in the presence of conspecifics and predators. Crayfish were presented with three types of food (chironomids, mayflies, and woody debris) in experimental enclosures, and the food type consumed and time to consumption were recorded. Although the same overall food choice (chironomid larvae) was made in all trials, the time it took for the juvenile crayfish to make a food selection was longer in the presence of adult conspecifics (both male and female) and adult cutthroat trout ( Oncorhynchus clarkii (Richardson, 1836)). Food selection also took longer, although not significantly, in the presence of conspecific juvenile crayfish, and no changes in behavior were noted in the presence of young-of-the-year (YOY) cutthroat trout. The altered behavior of the juvenile crayfish observed in the presence of adult fish and crayfish can lead to context-dependent effects of this omnivore on its community. Ecologists must be increasingly aware that the feeding of omnivores may not be solely based on nutritional criteria.

Massive recruitment of the barnacle Semibalanus balanoides in the Clyde Sea (Scotland, UK) in the spring of 2000

In April 2000 a massive recruitment of the barnacle Semibalanus balanoides was observed in the Clyde Sea. At one location 700 cyprids 1−1 of this species were recorded. This is ∼3500 times more abundant than previously recorded, and resulted in metamorphosis of some cyprids whilst still in the plankton, as well as massive settlement, with recruits found on adults and in the splash zone. The maximum density recorded was 109 settlers cm−2. Over the next 60 d mortality averaged 85%, resulting in a mean density of 8.4 recruits cm−2 in June 2000. Recruitment varied spatially at the km and m scale (site, shore height) but there was no evidence that it was affected by the presence of adult conspecifics.

Some Consequences of Size Variability in Juvenile Prickly Sculpin, Cottus asper

Variation in size among fishes can have important ecological consequences. The origin and persistence of variation in size among a cohort of juvenile prickly sculpin, Cottus asper, was explored experimentally by manipulating the size variability in juvenile fish and the presence of conspecific adult fishes in a factorial design. I found that adult fish significantly suppressed the growth and survivorship of juveniles, while the variance treatment alone had no effect on growth or survivorship. The presence of adults was also associated with a decrease in size variability in the high variance treatment. An analysis of juvenile growth patterns revealed no evidence for size dependent growth or temporal correlations in growth (`growth autocorrelation'), and are consistent with a lack of amplification of size variance through time. Juvenile C. asper appear to have been feeding on a relatively homogeneous resource and utilizing those resources similarly. Thus, patterns of individual growth and the presence of adult conspecifics may both serve to dampen size differences in this coastal fish species.

Behavioral Analysis of Social Effects on the Problem‐Solving Ability in the House Mouse

AbstractThe effects of the social context on the problem‐solving ability in the house mouse was evaluated by means of a puzzle box containing food, which could be opened by rotating a revolving door. The aim of the study was to clarify which mechanisms of social learning promote the acquisition of this complex motor skill. Young mice were exposed to the puzzle box in the presence of (i) demonstrators opening of the puzzle box; (ii) adults unable to open the puzzle box; and (iii) adults confined in one part of the cage and not manipulating the puzzle box. Results of the detailed analysis of the sequence of behaviors showed that (i) young mice successful in opening the puzzle box did not copy the sequence of actions performed by demonstrators; and (ii) the presence of adult conspecifics in proximity of the problem apparatus increased the rate of success of young mice. Results suggest that trial‐and‐error learning, social exposure and stimulus enhancement all play a role in the acquisition of the opening ability.

Growth of hatchling earthworms in the presence of adults: interactions in laboratory culture

Hatchling Lumbricus terrestris, Aporrectodea longa, Allolobophora chlorotica and L. rubellus were cultured in 1-l pots at 15°C in darkness. Each species was grown in isolation and also in the presence of selected adult species representing a range of earthworm ecological groupings. The experiments were continued for 24–36 weeks (until animals matured). The effects of adult presence were determined by measuring survival, growth rate, maturation and cocoon production of original hatchlings. In all treatments greater than 90% of animals survived. Both inter- and intra-specific interactions negatively influenced growth, maturation and fecundity of experimental earthworms. The intensity of interactions was directly related to the degree of niche overlap between pairings. Growth of both anecic and endogeic species was significantly (P <0.05) reduced by the presence of adult L. rubellus. The behaviour of anecic hatchlings was seen to change from a sub-surface to a deep burrowing mode after 12 weeks (at approx. 1 g). Growth and cocoon production of A. chlorotica was enhanced by the presence of A. longa. Early growth of L. terrestris was significantly (P <0.05) greater in the presence of conspecific adults but such an advantage (possibly mediated by midden formation/use) decreased with age. Results indicated that the outcome of all inter-specific interactions could not be pre-determined by simple reference to ecological grouping. Furthermore the stage of individual earthworm development was seen to influence the intensity of inter- and intra-specific interactions. The results may help to inform field-based studies on earthworm interactions.

Parental care in the whitefly Aleyrodes singularis

Summary1. Patches of Aleyrodes singularis nymphs are characterised by a distinctive phenotype composed of the nymphs' exuviae, which are piled on the nymph, and by a covering layer of wax secreted by the adults; these characteristics have been found to confer defensive properties against natural enemies.2. In contrast to the behaviour typical for ovipositing females of other aleyrodids, A. singularis females tend to remain near the patch of their progeny throughout their development. These mothers were therefore tested to show whether they exhibited active defensive behaviour towards natural enemies, beyond their contribution to passive defence achieved through the secretion of wax on the immatures.3. The behaviour of whitefly adults differed significantly when performed in the presence of conspecific adults from their behaviour in the presence of natural enemies (either a parasitoid or a predator). The differences were expressed in the mean time devoted to some behavioural events, the frequency at which events were performed, and the number of transitions between pairs of events.4. Most of the recorded behavioural differences were associated with departure of the natural enemies, facilitating immature survival.5. This is the first report of active behavioural changes that convey defence of immature offspring for the family Aleyrodidae. Conditions characterising these findings and their relationship with those in which parental care is expected are discussed.

Mortality of post‐settlement abalone Haliotis iris caused by conspecific adults and wave exposure

Experimental studies of post‐settlement abalone (Haliotis iris Martyn) indicate that wave exposure and conspecific adults may be important sources of mortality. In aquarium studies, exposure of post‐settlement abalone to grazing by sea urchins (Evechinus chloroticus) and starfish (Patiriella regularis), animals common in the subtidal habitat of H. iris, resulted in similar survival to control treatments (no grazers). However, in the presence of conspecific adults, the survival of post‐settlement juveniles was approximately half that of control treatments. The major cause of mortality was evidently pedal smothering by adults. Similar results were obtained in field studies in which survival of recently settled abalone was greater in the absence of conspecific adults. Field studies also revealed that the differences between two bays exposed to contrasting wave action were a significant source of variation in post‐settlement mortality. Survival of recruits was greater in the sheltered bay compared with the bay exposed to ocean swells.

Settlement of Pomatoceros lamarkii (Serpulidae) larvae on biofilmed surfaces and the effect of aerial drying.

The settlement responses of Pomatoceros lamarkii (Polychaeta: Serpulidae) larvae to biofilms of varying age on slate surfaces and to dried biofilms on slate surfaces were investigated in the laboratory. Settlement experiments were performed as multi-treatment, still water assays. Larvae did not settle on clean, non-biofilmed slates but settled on biofilms up to 28 days old. Settlement intensity was closely related to the bacterial density of a biofilm. Drying a biofilmed surface for 1-2 h at 20 degrees C to simulate a single tidal emersion completely negated the former inductive effect of the biofilm. Drying also negated the larval settlement-inducing effect that normally results from the presence of conspecific adults. The settlement inhibition lasted for approximately 5 days following a single drying event. Larvae settled readily on biofilms exposed to formalin and antibiotics. Treating biofilms with formalin or antibiotics before or after drying had no effect upon larval avoidance of dried biofilms. Freeze-drying a biofilm had the same effect as aerial drying. The biofilm drying effect could not be mimicked by exposing biofilms to hyper-saline seawater. The finding that P. lamarkii larvae do not settle on dried biofilms could have significance in explaining the natural distribution of this species in the intertidal.

Ontogenetic differences in the circadian locomotor activity rhythm of the talitrid amphipod crustacean Orchestoidea tuberculata

Adult talitrid amphipods often display a nocturnal activity pattern, which has been shown in a number of cases to be partly controlled by an endogenous rhythm of circadian period. Juvenile talitrids have been little studied in the past, but evidence from field studies indicates that they may display a different diel pattern of locomotor activity from that of adults. This study presents evidence from the field in south–central Chile that juvenile Orchestoidea tuberculata Nicolet show crepuscular activity peaks, contrasting with the nocturnal peaks shown by adult conspecifics. Under constant conditions in the laboratory, both juveniles and adults exhibit endogenous rhythms of circadian periodicity. However, the phasing of activity differs in juveniles and adults, corresponding to the differences observed in the field. It is suggested that differing phasing of peak locomotor activity may be a mechanism to reduce contact between juvenile and adult talitrids. Experimental evidence indicates a significant negative influence on survival of juveniles in the presence of adult conspecifics by intraspecific predation. Other factors influencing the timing of surface activity are also discussed.

THE ROLE OF DENSITY DEPENDENCE IN THE POPULATION DYNAMICS OF A TROPICAL PALM

The role of density dependence in the population dynamics of tropical trees has been a subject of considerable debate. Here, we present data on the demography of the edible palm Euterpe edulis, classified into seven size categories and monitored over three years. On average, each adult palm contributed 98 seedling recruits per year into the population. The pattern of mortality was similar to that of other palms, with mortality being highest among the smallest plants. Those plants with a diameter at soil level >20 mm had an annual mortality <7%. Density dependence was found to act only on the seedling stage of the life cycle. The probability of survival and transition of seedlings to the next size class were affected both by the density of seedlings and the presence of conspecific adults. Matrix modeling indicated that the true finite rate of population increase (λ) was 1.28 and that the observed reverse “J”-shaped size distribution of plants was a consequence of the density dependence operating in the population. Elasticity analysis showed that the survival elements in the matrix contributed most to the value of λ, and that the position of the transition matrix in growth–survival–fecundity (G–L–F) space was influenced by density. The matrix model incorporating density dependence predicted size distributions and densities approximating the maximum observed in the field. Spatial simulations indicated that the predictions from the matrix model relating to the size structure of plants are robust, but that the predictions of densities are sensitive to the precise spatial dynamics of the population.

Effects of substratum and conspecific adults on the metamorphosis of Chasmagnathus granulata (Dana) (Decapoda: Grapsidae) megalopae

The ability of marine invertebrate larvae to delay their metamorphosis in the absence of adequate environmental cues has been reported for numerous sedentary and sessile species. In the present study, the effect of various substrata and the presence of conspecific adults on the metamorphosis of a mobile species, the crab Chasmagnathus granulata, was evaluated. The duration of the megalopa stage in experiments with six different substrata and in the presence or absence of conspecific adults was compared in a laboratory study. In addition, the influence of natural substrata was compared with that of artificial substrata of similar grain size or texture. In a further experiment, the two most effective cues (natural mud and conspecific adults) were tested as single vs. combined factors. Natural mud and unidentified chemical cues from conspecific adults had the strongest accelerating effects on development duration to metamorphosis. With the exception of nylon threads (artificial filamentous substratum), none of the artificial substrata had a significant effect on the duration of the megalopa stage. Simultaneous exposure to natural mud and water containing chemical cues from conspecific adults accelerated metamorphosis more than each of these factors separately. Megalopae that were reared without a substratum (control) delayed their metamorphosis by 29% (about 3 days) compared with those in simultaneous contact with natural mud and rearing water of adult conspecifics. The results indicate that the metamorphosis of the megalopa of C. granulata is influenced by the presence or absence of environmental stimuli that are associated with the preferred adult habitat.

Cannibalism as the cause of an ontogenetic shift in habitat use by fry of the threespine stickleback.

In Crystal Lake, British Columbia, small fry (≤15 mm SL) of the threespine stickleback (Gasterosteus aculeatus) are concentrated in vegetation while larger fry are not. Because fry in all size classes feed primarily on zooplankton, even when in vegetation, we hypothesized that size-limited predation was responsible for the observed shift in habitat use with size. The major predators on fry in Crystal Lake are adult threespine stickleback, the water scorpion, Ranatra sp. (Hemiptera: Nepidae), backswimmers, Notonecta spp. (Hemiptera: Notonectidae), and dragonfly naiads of the genus Aeshna (Odonata: Aeschnidae). On the basis of distribution and hunting behavior we excluded the insects Ranatra sp., and Notonecta sp. as causal agents for this shift in resource by fry in water >0.25 m deep. Ranatra was found almost exclusively near the shoreline in water <0.25 m deep, and both insects hunted primarily as ambush predators within vegetation. Such predators seemed more likely to drive vulnerable fry from vegetation than to restrict them to it. In contrast, Aeshna naiads and adult stickleback frequently hunted outside of vegetation. In prey preference experiments the naiads did not show the decline in predation efficiency on fry >15 mm SL that would be expected if size-limited predation by this insect was responsible for the observed shift in resource use by fry. Adult stickleback only fed on fry <15 mm SL, and in an experimental situation, consumed fry at a rate 10 times greater than that exhibited by any of the insects. Predation experiments demonstrated that small fry (11-15 mm) spent more time in vegetation in the presence of adult conspecifics than they did in control pools, as would be expected if size-limited cannibalism caused small, vulnerable fry to be restricted to vegetation. Fry >15 mm SL were found outside of vegetation more often than in control treatments. The probable cause of this result is that adults become aggressive toward fry at this size, and often could be seen chasing large fry from vegetation during the experiments. Dragonfly naiads (Aeshna spp.) spent most of their time in vegetation in the experimental pools. Both size classes of fry spent less time in vegetation in the presence of dragonfly naiads than they did in control treatments, an apparent reflection of their similar vulnerabilities to these naiads. The presence of vegetation in pools reduced predation rates by adult stickleback on small fry. Because the experiments presented here indicate that fry are capable of rapidly assessing predation risk and of altering their behavior adaptively, we conclude that small fry occupy vegetation as a refuge from cannibalism. Once fry have reached the size-threshold at which they are no longer vulnerable to adult conspecifics they are able to forage farther from vegetation thereby reducing risk of predation by insects in vegetation and possibly acquiring more abundant food resources.