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Despite the many study systems in which predation has played a major role in phenotypic diversification and speciation, the underlying selective regimes imposed by different predator assemblages have rarely been quantified. We did so for the damselfly genus Enallagma which strongly diverged in antipredator traits when the ancestral species occupying lakes containing fish (hereafter fish lakes) repeatedly invaded fishless lakes with dragonfly larvae as top predators (hereafter dragonfly lakes). In two selection experiments in field enclosures we quantified the selection on two key escape traits of two fish-lake Enallagma species associated with survival selection by fish in the ancestral fish lakes and by dragonfly predators in the invaded fishless, dragonfly lakes. In accordance with the different hunting modes, fish imposed selection for a decreased swimming propensity while dragonfly larvae imposed selection for increased swimming speed in one of the two species. In two complementary quantitative genetic rearing experiments, we found relatively low but significant broad-sense heritabilities for both escape traits. Integrating these estimates for the selection coefficients and the heritabilities suggests that the evolutionary increase in swimming speed associated with the habitat shift may have occurred rapidly. Our study suggests that the phenotypic evolution of ecologically important traits related to habitat shifts may occur at an ecological timescale.

Despite the importance of foraging activity for the growth/predation risk trade-off, studies that demonstrated predator-induced survival selection on foraging activity under semi-natural conditions are relatively rare. Here, we tested for fish-induced selection for reduced foraging activity in two larval Enallagma damselflies using a field enclosure experiment. Fish imposed considerable mortality in both damselfly species and survival selection on foraging activity could be detected in Enallagma geminatum. We did not detect selection in Enallagma hageni, probably because this species already was not eating very much in the absence of fish compared to E. geminatum. Both species responded strongly to the presence of predators by reducing their foraging activity. The documented survival selection on foraging activity was detected despite the already low activity levels in fish lake prey species and despite strong predator-induced plasticity in this trait.

Little is known about the ecological correlates and macro-evolution of life history plasticity to time constraints. Here, we compared age and mass at emergence and their plasticity toward combinations of time constraints (delayed larval development), food level and predation risk (caged dragonfly larvae) by rearing two temporary-pond Lestes damselflies (L. congener and L. forcipatus) and the derived vernal-pond L. dryas from the eggs until adult emergence in outdoor tubs. Life history plasticity under time constraints, low food and predation risk was as predicted by optimality models. Delayed larvae in all three species accelerated development and showed a lower fat content and a substantially elevated mortality rate. At low food, all species emerged later at a smaller mass. Also under predation risk adults of all species emerged later, and (at high food) at a smaller mass. Unexpectedly, delayed larvae did not show a smaller life history response to predation risk imposed by dragonfly larvae. Compared to the two temporary-pond Lestes studied, the derived species that invaded more ephemeral vernal ponds showed a faster development rate and a lower deceleration of development to low food. It also showed a lower acceleration of development to time constraints, possibly reflecting that it reached development rates near to its physiological maximum. Unexpectedly, the vernal-pond Lestes did not slow its development less under predation risk. Our results stress the importance of evaluating ecological and evolutionary correlates of life history plasticity under as realistic conditions as possible.

Despite the survival value of high energy reserves during winter, animals often face energy deficits when entering winter. Compensatory growth in energy reserves during the winter period to buffer such deficits may increase winter survival and alleviate the need for costly compensatory mechanisms before or after winter when predation risk is much higher. However, such compensatory responses in energy reserves during winter have not been demonstrated under field conditions. We explored if Lestes eurinus damselfly larvae can compensate for suboptimal energy reserves during winter at 4°C when their ponds are covered with ice. In a field enclosure experiment, we demonstrated compensatory growth in terms of body mass and energy reserves in larvae whose energy status was previously manipulated in the laboratory. These results were supported by patterns in body mass and energy reserves over winter in two natural unmanipulated populations. Winter survival was high overall and not affected by compensatory growth. We hypothesize that the observed compensatory growth in energy reserves during winter may shape life history decisions in autumn and spring, and may make resource availability during winter as or more important than energy reserves before winter.

The Enallagma and Lestes damselflies have both diversified and adapted over the past 10-15 million years to the various ecological milieus found along the pond permanence gradient among North American ponds and lakes. Previous articles have explored this diversification process for Enallagma. In this article, we present a phylogenetic hypothesis for the North American Lestes, use this hypothesis to reconstruct Lestes diversification, and compare the diversification processes inferred for Lestes and Enallagma. The results of this study suggest that Lestes began in temporary ponds where large dragonflies are the top predators, while Enallagma began in permanent lakes where fish are the top predators. Starting from these different ancestral habitats, both genera have invaded and adapted to habitats already occupied by the other genus. Moreover, these adaptive habitat shifts involved substantial convergence on the behaviors used to deal with fish and dragonfly predation in both genera and a major life-history shift from diapausing to directly developing eggs in Lestes. However, in Lestes lineages invading fish lakes, swimming speed and morphology did not change to match those of Enallagma species, illustrating that reciprocal shifts between alternative selection regimes are not necessarily evolutionary opposites. Also, the greater sizes and growth rates of Lestes species compared to Enallagma species, which should impart substantial ecological advantages in competition between the genera, were shown to result from phylogenetic inheritance and not from adaptive diversification. This historical analysis of diversification raises new questions about the relationship between the macroevolutionary mechanisms driving lineage diversification and the ecological mechanisms structuring local food webs and regional species assemblages. ispartof: The American naturalist vol:168 issue:6 pages:S50-S72 ispartof: location:United States status: published

The ecological trade-off between foraging gains and predation risk shapes the ecology of many taxa, including larval amphibians and fishes. Is this ecological trade-off reflected at the population level as a negative genetic correlation between traits that contribute to growth and survival? We addressed this question for a single population of Green Frog tadpoles (Rana clamitans) by estimating heritabilities and correlations of traits that mediate the trade-off. Burst speed, growth rate, and size varied among sires and were significantly heritable, but swimming activity in the presence and absence of predator odors was not. Differences among traits in heritability generally accorded with differences in genetic coefficients of variation, with the exception of activity. Growth rate was not correlated with any measure of predator avoidance or escape ability. Likewise, predator escape ability (burst speed) was not correlated with predator avoidance ability (activity and its plasticity). Our results i...

The role of physiology in mediating the growth/predation risk trade-off has been largely ignored. We examined effects of predation risk on relationships between growth and storage molecules in Enallagma aspersum and Ischnura verticalis damselfly larvae that differ in this trade-off. In laboratory and field experiments, both species had similar growth and mortality rates and similar concentrations of storage molecules in the absence of mortality threats. However, in the presence of dragonfly predators Ischnura larvae had higher mortality rates and grew faster than Enallagma larvae. Consistent with the difference in growth rate, Enallagma's total protein concentrations decreased under predation risk while those of Ischnura did not. Glucose and glycogen concentrations were not affected, while triglyceride concentrations were lower under predation risk in Enallagma but not in Ischnura. Species differences at the physiological level to the presence of mortality threats may be crucial to understanding patterns in metamorphic and post-metamorphic traits.

Identifying key traits that shape trade-offs that restrict species to only a subset of environmental gradients is crucial to understanding and predicting species turnover. Previous field experiments have shown that larvae of Lestes damselfly species segregate along the entire gradient of pond permanence and predator presence and that differential predation risk and life history constraints together shape their distribution. Here, we report laboratory experiments that identify key differences in behavior and physiology among species that structure their distributions along this gradient. The absence of adaptive antipredator behavioral responses against large dragonfly larvae and fish of Lestes dryas, the only species to inhabit predator-free vernal ponds that dry each year, is consistent with its high vulnerability to predation and probably the key trait that excludes it from parts of the gradient with predators. The reciprocal dominance of two other Lestes species in permanent waters dominated by either dragonflies or fish can be explained by the lack of effective antipredator behaviors against dragonflies and fish, respectively. Maximal growth rates did not differ among Lestes along the gradient. However, in the natural predator environment of vernal ponds (only conspecific cannibals), the vernal-pond Lestes had higher growth rates than the other Lestes suggesting that this excludes other Lestes from vernal ponds. Similarly, Lestes species that inhabit temporary ponds (i.e., ponds that dry intermittently every few years but not every year) had a higher growth rate than the fishless permanent-pond Lestes in the presence of the syntopic dragonfly predator. These growth differences among Lestes in predator treatments were not due to differences in food intake, but due to differences in physiology. The vernal-pond Lestes converted more assimilated food into body mass compared to the other Lestes in the presence of conspecific larvae, and the temporary-pond Lestes had a higher conversion efficiency than the fishless permanent-pond Lestes in the presence of the syntopic dragonfly predators. In contrast, reductions in growth rate within species in the presence of syntopic predators were both physiologically and behaviorally mediated. The interplay between behavior and physiology may be a common feature of the growth/predation-risk trade-off, and their joint study is therefore critical to mechanistically link phenotype, performance, and community assembly along the freshwater habitat gradient. ispartof: Ecology vol:84 issue:12 pages:3327-3338 status: published

Most organisms must simultaneously find enough food for themselves while trying not to become food for some other organism. Previous field experiments have shown that larvae of Enallagma and Ischnura species are able to coexist in the littoral zones of lakes because they resolve this growth/predation risk trade-off differently: Ischnura species grow more quickly than Enallagma species, but Ischnura species suffer higher mortality rates than Enallagma. We performed a series of laboratory studies to explore the mechanistic basis for the difference in growth between the genera. When held in complete isolation and with unlimited food, larvae of a number of Enallagma species that coexist with fish accumulated mass at much faster rates than Ischnura species. This difference in isolation was due to the fish-lake Enallagma simply ingesting more food. In contrast, when held in the presence of other damselflies or a fish predator, Ischnura had significantly higher growth rates than Enallagma species from fish lakes. All species decreased the amount of food they ingested in the presence of the fish predator as compared to when fish were absent, which resulted in decreased growth in the presence of the predator for all species. However, the interspecific differences in growth rate were due primarily to differences in the abilities of the species to convert ingested food into their own biomass; in the presence of fish, comparably sized larvae ingested nearly identical amounts of food, but Ischnura larvae grew faster because they converted significantly more ingested food into their own biomass than did larvae of Enallagma species from fish lakes. This difference in conversion efficiency between the genera was not apparent when larvae were raised in complete isolation. These results indicate that Enallagma and Ischnura species differ in physiological stress responses to the presence of predators, and this difference may facilitate the coexistence of Enallagma and Ischnura species in the field.

Animals with complex life cycles often show large variation in the size and timing of metamorphosis in response to environmental variability. If fecundity increases with body size and large individuals are more vulnerable to predation, then organisms may not be able to optimize simultaneously size and timing of metamorphosis. The goals of this study were to measure and explain large-scale spatial and temporal patterns of phe- notypic variation in size at metamorphosis of the mayfly, Baetis bicaudatus (Baetidae), from habitats with variable levels of predation risk. Within a single high-elevation watershed in western Colorado, USA, from 1994 to 1996 we measured dry masses of mature larvae of the overwintering and summer generations of Baetis at 28 site-years in streams with and without predatory fish (trout). We also estimated larval growth rates and development times at 16 site-years. Patterns of spatial variation in mayfly size could not be explained by resource (algae) standing stock, competitor densities, or physical-chemical variables. However, size at metamorphosis of males and females of summer generation Baetis was smaller in fish streams than in fishless streams and decreased as densities of predatory stoneflies increased. Furthermore, overwintering individuals matured at larger sizes than summer generation Baetis, and the size of emerging Baetis declined over the summer, but predominantly in trout streams. Theoretical consideration of the effect of predation risk on size and timing of metamorphosis accurately predicted the observed temporal variation in size and timing of mayflies at emergence in fish and fishless streams. Baetis populations had similar growth rates but followed different developmental trajectories in high and low risk environments. In risky environments larval development was accelerated, resulting in metamorphosis of younger and smaller individuals, minimizing exposure of larvae to risk of mortality from trout predation, but at the cost of future reproduction. In safe environ- ments, larvae extended their development, resulting in larger, more fecund adults. Thus, we propose that large-scale patterns of variation in size and timing of metamorphosis represent adaptive phenotypic plasticity, whereby mayflies respond to variation in risk of predation, thereby maximizing their fitness in variable environments.

Previous studies suggest that the evolution of increased caudal lamellae size to increase swimming speed was an adaptation of Enallagma damselflies for coexisting with large, predatory dragonflies in fishless lakes. To test whether dragonfly predation still exerts selection pressures for increased lamellae size, I performed a field experiment in which I manipulated the abilities of dragonfly larvae to inflict mortality on Enallagma boreale larvae and compared differences in lamellae size and shape between treatments. In cages where dragonflies were free to forage on damselflies, surviving E. boreale larvae had lamellae that were larger in lateral surface area, and that were wider relative to their length, as compared with larvae recovered from treatments in which dragonflies were not permitted to forage on damselflies. Selection differentials of about 0.25 phenotypic standard deviation units were measured for both of these characters. These results indicate that dragonfly predation still exerts significant selection pressures on damselfly antipredator adaptations. The results of this study are discussed in the context of studies of adaptation.

Brachionus calyciflorus typically develops long, defensive spines only in response to a kairomone from the predatory rotifer, Asplanchna. However, in the absence of any environmental induction, females of some clones produce daughters with increasingly long spines as they age; late-born individuals can have posterolateral spines as long as those induced by Asplanchna: up to 50% or more of body length. Here, we construct a model using data from life-table and predator-prey experiments to assess how this maternal-age effect can influence the distribution of spine lengths in reproducing populations and provide defense against Asplanchna predation. When Asplanchna is absent, the frequency of individuals with late birth orders rapidly becomes extremely low; thus, any cost associated with the production of long-spined individuals is minimal. When Asplanchna is present at densities too low for spine induction, and preys selectively on individuals with no or short posterolateral spines, the frequency of long-spined individuals rapidly increases until a stable birth-order structure is reached. As a result, mortality from Asplanchna predation is greatly reduced. The pronounced and novel birth-order effect in this rotifer appears to be an effective bet-hedging strategy to limit predation by Asplanchna when its kairomone induces no or less than maximal spine development.

Previous models of diamond-shaped and intraguild predation community modules have represented the essence of the trade-off necessary for a top predator to prevent competitive exclusion among a set of resource-limited consumers. However, at most two consumers can coexist in these models. In this article, I show how intraspecific density dependence in the consumers can permit many more than two consumers to coexist in these community modules. Moreover, responses of the community to removal of the top predator depend on the patterns of the strengths of species interactions relative to the strengths of intraspecific density dependence. If the consumers experience similar strengths of intraspecific density dependence, removing the top predator will in most cases have little effect on consumer species richness. A substantial reduction in consumer species richness with predator removal (i.e., the keystone predation effect) will typically occur only when the consumer that can support a population at the lowest resource abundance also (1) experiences substantially weaker intraspecific density dependence than other consumers and (2) experiences significantly higher levels of mortality from the predator. These results identify how intraspecific density dependence fosters the coexistence of multiple consumers in two important community modules and shapes the responses of these community modules to perturbations such as predator removal.

Species evolve in response to the abiotic and biotic interactions they ex- perience in a community, and so studying the adaptation of species in a community context is a necessary component for understanding the development of community structure. Pre- vious studies have suggested that Enallagma damselflies (Odonata: Coenagrionidae) in- vaded aquatic habitats with large dragonflies as the top predators at least twice, and large changes in morphological structures (e.g., caudal lamellae and abdomen), which should increase swimming speed, are associated with these invasions. This is significant because these species swim away from attacking predators, whereas species in the ancestral habitat (habitats with fish as the top predators) do not swim from attacking predators. We quantified swimming speed over a large ontogenetic size range for larvae of seven Enallagma species, two in dragonfly lakes and five in fish lakes. Results of analysis of covariance and a modification of Felsenstein's method of evolutionary contrasts indicate that a large evo- lutionary increase in the slope of the ontogenetic relationship between swimming speed and loge(body length) is associated with invasions into dragonfly lakes, and this shift results in larvae of dragonfly-lake species being faster swimmers, especially in larger size classes, than larvae of fish-lake species. We also present experimental results demonstrating that damselflies with larger lamellae are faster swimmers and have higher survival when exposed to dragonfly predation. The results of this study indicate that the change in selective en- vironment associated with habitat shifts into dragonfly lakes promoted adaptations for increased swimming speed in Enallagma lineages. The potential ramifications of this ad- aptation for community structure are discussed.

Selection often operates not directly on phenotypic traits but on performance which is important as several traits may contribute to a single performance measure (many-to-one mapping). Although largely ignored in the context of selection, this asks for studies that link all relevant phenotypes with performance and fitness. In an enclosure experiment, we studied links between phenotypic traits, swimming performance and survival in two Enallagma damselflies. Predatory dragonflies imposed survival selection for increased swimming propensity and speed only in E. annexum; probably E. aspersum was buffered by the former species’ presence. Accordingly, more circular caudal lamellae, structures involved in generating thrust while swimming, were selected for only in E. annexum. Other phenotypic traits that contributed to swimming speed were apparently not under selection, probably because of many-to-one mapping (functional redundancy). Our results indicate that not only the phenotypic distributions of syntopic prey organisms but also many-to-one mapping should be considered when documenting phenotype–performance–fitness relationships.

Survey data from New England showed that assemblages of Lestes damselflies are organized along the entire gradient of pond permanence and predator presence. One assemblage occupies vernal ponds lacking large dragonfly predators and fish; four are largely confined to temporary ponds that typically contain dragonfly predators; one dominates fishless permanent ponds and lakes where dragonflies are the top predators; and one dominates permanent ponds and lakes where fish are the top predators. We determined the role of life history and predation in maintaining this striking pattern by conducting a series of transplant experiments in the field and a laboratory experiment manipulating presence and absence of local predators. Life history (1) shaped the ability of species to cope with drying regime, thereby excluding temporary-pond Lestes from vernal ponds and permanent-water Lestes from temporary ponds, and (2) generated size differences among species due to differences in the timing of hatching. This mediated the exclusion of temporary-pond Lestes from permanent water bodies through asymmetric intraguild predation by permanent-water Lestes. Dragonfly predation on permanent-water Lestes had an indirect positive effect on the survival of temporary-pond Lestes; however, this effect apparently is too small to allow coexistence of both Lestes groups. Predation by large dragonfly larvae excluded the Lestes species of vernal ponds from temporary ponds, and differential vulnerability to large dragonfly larvae and fish shaped the reciprocal dominance of L. eurinus and L. vigilax in fishless and fish-containing permanent water bodies, respectively. Taken together, these results show that life history constraints and predation both shape the distributions of Lestes species along the pond permanence gradient in New England. We discuss the importance of this freshwater habitat gradient in shaping local and regional species diversity. ispartof: Ecology vol:84 issue:6 pages:1576-1587 status: published

In a large behavioral experiment we reconstructed the evolution of behavioral responses to predators to explore how interactions with predators have shaped the evolution of their prey's behavior. All Enallagma damselfly species reduced both movement and feeding in the presence of coexisting predators. Some Enallagma species inhabit water bodies with both fish and dragonflies, and these species responded to the presence of both predators, whereas other Enallagma species inhabit water bodies that have only large dragonflies as predators, and these species only responded to the presence of dragonflies. Lineages that shifted to live with large dragonflies showed no evolution in behaviors expressed in the presence of dragonflies, but they evolved greater movement in the absence of predators and greater movement and feeding in the presence of fish. These results suggest that Enallagma species have evolutionarily lost the ability to recognize fish as a predator. Because species coexisting with only dragonfly predators have also evolved the ability to escape attacking dragonfly predators by swimming, the decreased predation risk associated with foraging appears to have shifted the balance of the foraging/predation risk trade-off to allow increased activity in the absence of mortality threats to evolve in these lineages. Our results suggest that evolution in response to changes in predation regime may have greater consequences for characters expressed in the absence of mortality threats because of how the balance between the conflicting demands of growth and predation risk are altered.

Previous studies have shown that two or three lineages of Enallagma damselflies, which historically co- existed with fish, recently invaded and adapted to living with large dragonfly predators in fishless waters. In adapting to live with these new predators, lineages shifted behaviorally to using swimming as an evasive tactic against attacking predators, evolved morphological features that made them faster swimmers, and evolved biochemical features to increase refueling strenuous activities like swimming. However, these habitat shifts have occurred in only one of the two primary clades within the genus in North America. Here, I show that clade-level differences exist among species in the ancestral, fish-lake habitat that should make habitat shifts easier to accomplish in the clade in which they have occurred. Specifically, fish-lake species in the clade in which habitat shifts occurred have much higher propensities to swim in the laboratory, swim faster when they do swim, and have higher mass-specific activities for arginine kinase than do species in the other primary clade, in which no extant species are found in fishless waters. These results are discussed in the context of the dynamics of founder events and the potential implications for community structure.

Previous studies have shown that at least two lineages of Enallagma damselflies (Odonata: Coenagrionidae) shifted from inhabiting lakes with fish as top predators to inhabiting ponds and lakes with large dragonflies as the top predators. In adapting to living with the new predator type, these lineages evolved much greater swimming speeds to avoid attacking dragonflies. In this paper, I test whether biochemical adaptations to fuel swimming arose in concert with previously identified morphological changes that increase swimming speed. I assayed the mass-specific enzyme activities of three enzymes involved in fueling strenuous activity: pyruvate kinase and lactate dehydrogenase (enzymes involved in glycolysis) and arginine kinase (the enzyme that recharges the ATP pool). Enzyme activities were determined for 14 Enallagma species from across the genus. Species that coexist with dragonfly predators had significantly higher mass-specific arginine kinase activities than species that coexist with fish, and the results of evolutionary contrasts analyses indicate that this difference between the two groups is the result of evolutionary change associated with the habitat shifts of lineages from fish lakes to dragonfly lakes. Although significant evolution was documented for lactate dehydrogenase and pyruvate kinase across the genus, evolutionary change in the activities of these enzymes was not consistent with adaptation to coexisting with dragonfly predators. Swimming bouts to avoid dragonfly predators last for only a few seconds, and the action of arginine kinase to phosphorylate ADP to make ATP will extend the duration of maximal exertion for swimming for a few seconds. However, much longer time periods (over 45 sec) are required to generate ATP via glycolysis. Therefore, selection may have favored adaptation only at the arginine kinase locus.

We examined diet composition of postmetamorphic bullfrogs (Rana catesbeiana) and green frogs (R. clamitans) co-occurring at two study sites in southwest Michigan to gain insight into the nature of potential interactions between the species. Observations during sample collection indicated that bull- frogs tended to be found in the water and green frogs tended to be on land within a few meters of the water's edge. This habitat difference was reflected in diet composition. The percentage of the diet composed of aquatic prey items was significantly higher for bullfrogs on three of four collection dates. Comparisons of adult and juvenile classes of the two species indicated interspecific diet similarity was negatively related to the body size difference between classes. Juvenile frogs were common in the diet of adult bullfrogs, but were almost never consumed by green frogs. The small size of frogs consumed by adult bullfrogs indicated that juvenile green frogs constituted the great majority of frogs eaten. Our results suggest that, because of differences in habitat and body size, the opportunity for substantial competition between these species is probably small, and is restricted to individuals of similar body size. The potential for predatory interactions, however, may be substantial, and is highly asymmetrical, with the interaction largely re- stricted to adult bullfrogs preying on juvenile green frogs.

Invertebrates that graze periphyton growing on stones in coldwater streams in Michigan are at greatest risk of predation from a benthic-feeding fish (the mottled sculpin, Cottus bairdi) while on the top surface of substrates, where periphyton is most abundant, and during the day, when sculpin feed most actively. We conducted laboratory experiments with larvae of two strong exploitative competitors for periphyton (the mayfly Baetis tri- caudatus and the caddisfly Glossosoma nigrior) to test if and how their foraging behavior was affected by presence of sculpin when sculpin were not allowed to attack prey. We incorporated four experimental factors in a factorial design: sculpin presence/absence, the abundance (low, high) of food that was patchily distributed on the top surface of substrates, grazer hunger level (fed, starved), and time of day (day, night). In feeding trials, Baetis was much more vulnerable than Glossosoma to sculpin pre- dation. Similarly, only the foraging behavior of Baetis was strongly affected by the presence of sculpin. Baetis larvae responded to the presence of sculpin by: (1) significantly reducing the time spent on the top surface of substrates, and their movement rate while moving within and between food patches, and (2) significantly increasing the proportion of time spent in food patches, and the proportion of individuals that left the substrate by drifting downstream. Baetis larvae accepted greater risk of predation (by spending more time on top) when food abundance on top was high and when larvae had been starved for 27 h. Also, Baetis behavior was strongly affected by sculpin presence during the day but not at night. Collectively, these results suggest that Baetis larvae made adaptive compromises between feeding and avoiding sculpin. In contrast, Glossosoma behavior was not affected by the presence of sculpin. Glossosoma responded most strongly to alterations in food density and their own hunger level. Our results suggest that the presence of sculpin, even though the fish consumed no larvae, might indirectly affect the outcome of competitive interactions between Baetis and Glossosoma through the grazers' differential behavioral responses to the predator.