Belostoma Flumineum Research Articles

Prey Preferences for Three Aquatic Hemipterans Provide Insights About Their Coexistence

AbstractWe explored the potential of differences in foraging preferences to contribute to long-term species coexistence in aquatic predatory hemipterans. We hypothesized that predatory hemipterans would have distinct foraging preferences informed by their morphology. We used a prey choice experiment to test whether Belostoma flumineum, Pelocoris biimpressus, and Ranatra australis differed in their relative preferences of amphipod, damselfly naiad, and physid snail prey. We discovered that the predators showed complementarity in their foraging preferences with B. flumineum preferring snails, P. biimpressus damselfly naiads and R. australis amphipods as prey. Our results suggest that the disparate foraging preferences of aquatic hemipterans may facilitate their coexistence in aquatic systems but caution that studies need to explore whether patterns uncovered in the laboratory experiment reflect interactions in nature.

Intrasexual egg laying in Belostoma oxyurum. A note on its ecological determinants (Hemiptera, Belostomatidae)

Intrasexual egg laying was detected in a female giant water bug, Belostoma oxyurum (Dufour) which was capturad carrying six fertilized eggs on its back. The location of the eggs does not belong to the pattern known for incubant males of this species. This finding is attributed to the absolute shortage of a sexual resource constituted by the male back space. Such environmental circuinstance was recorded in the field when this female was captured. The adult population from which it was taken included a hundred percent of both, gravid females and incubant males, and sex ratio was female biased though without statistical significance. The observed phenomenon has also been recorded in the Nearctic species Belostoma flumineum Say. Comparative considerations referred to bothspecies are included.

Parasite-induced vulnerability to predation in larval anurans.

Within communities, pathogens and parasites have the potential to indirectly influence predator-prey interactions. For instance, prey that exhibit pathology or altered traits (e.g. behavioral shifts) following infection could be more prone to predation, which is known as parasite-induced vulnerability to predation (PIVP). PIVP has been frequently documented for pathogens with trophic transmission, because predators are often critical in the pathogen's life cycle. However, for pathogens without trophic transmission, PIVP can lead to a healthy herds effect, thereby reducing transmission in the system. In this study, we explored whether the pathogen ranavirus (family Iridoviridae) enhances vulnerability of 4 species of larval amphibians (spring peepers Pseudacris crucifer, gray treefrogs Hyla versicolor, American toads Anaxyrus americanus, and northern leopard frogs Lithobates pipiens) to 2 common tadpole predators (larval green darners Anax junius [hereinafter Anax] and adult water bugs Belostoma flumineum [hereinafter Belostoma]). For each anuran species, we conducted short-term microcosm experiments to assess predation rates on individuals that were or were not exposed to virus. For 3 of the 4 species, we found that exposure to ranavirus decreased survival rates with Anax between 2- and 9-fold. However, we did not see the same trend with Belostoma, which indicates that predator identity is important in this interaction. More specifically, the higher efficiency of Anax in capturing and consuming prey, relative to Belostoma, may allow Anax to capitalize on trait changes induced by virus exposure and enhance the PIVP effect. Our results indicate that trait-mediated indirect effects could play a role in creating healthy herds in amphibian communities.

Mate selection for the female giant waterbug (Belostoma flumineum Say, 1832); for her, it’s a coin toss or polyandry

The giant waterbug (Belostoma flumineum Say, 1832) female is responsible for mate selection. After copulation, the female deposits her eggs on the back of the male, who is then exclusively responsible for post-copulatory care of the eggs. It is possible for more than one female to oviposition on a single male. The purpose of this study was to determine if a partially-encumbered male was more attractive to a female because he is a ‘proven’ breeder. To test this, a gravid female was paired with a partially-encumbered male and a male free of eggs. In the laboratory, during trials where mating occurred, there was no significant difference between mate choice or the number of eggs deposited between test groups. Additionally, male backspace availability was not a limiting factor, and polyandry was observed within trials. Therefore, a partially-encumbered male has no advantages or disadvantages for being selected as a mate.

Recent changes in reproductive phenology of a K-selected aquatic insect predator, Belostoma flumineum Say (Heteroptera, Belostomatidae).

The timing of critical events like mating, migration, and development has noticeably and recently shifted in many populations of diverse organisms. Here, we report a change in the breeding phenology of giant waterbugs, Belostoma flumineum Say (Heteroptera, Belostomatidae), in the northeastern United States. Waterbugs collected in 2005 and 2006 exhibited previously typical patterns of mating and reproduction: two annual reproductive peaks in which overwintered adults mated in the spring and young adults from a new generation mated in the fall. In 2012 and 2015, despite similar sampling effort, we detected no fall breeding activity in the study area. Reproductive behaviour under controlled laboratory conditions was also different between the earlier (2005 and 2006) and recent (2012 and 2015) years: waterbugs collected in recent years exhibited significant delays in reproduction (>30 days) under similar photoperiod and thermal conditions. We discuss potential causes of this dramatic change in reproductive behaviour, such as climate change, as well as possible negative impacts of the absence of fall reproduction on populations of B. flumineum in the study region.

Range Extension of the Giant Water BugBelostoma flumineumSay 1832 (Hemiptera: Belostomatidae) to Saskatchewan, Canada

Abstract. We collected a single specimen of the giant water bug Belostoma flumineum Say 1832 (Hemiptera: Belostomatidae) during routine macroinvertebrate biomonitoring in the Souris River, Saskatchewan. This is the first record of B. flumineum in the province of Saskatchewan and represents a more northern record than the previously documented locations in Montana, North Dakota, and southeastern Manitoba.

Effects of clothianidin on aquatic communities: Evaluating the impacts of lethal and sublethal exposure to neonicotinoids.

The widespread usage of neonicotinoid insecticides has sparked concern over their effects on non-target organisms. While research has largely focused on terrestrial systems, the low soil binding and high water solubility of neonicotinoids, paired with their extensive use on the landscape, puts aquatic environments at high risk for contamination via runoff events. We assessed the potential threat of these compounds to wetland communities using a combination of field surveys and experimental exposures including concentrations that are representative of what invertebrates experience in the field. In laboratory toxicity experiments, LC50 values ranged from 0.002 ppm to 1.2 ppm for aquatic invertebrates exposed to clothianidin. However, freshwater snails and amphibian larvae showed high tolerance to the chemical with no mortality observed at the highest dissolvable concentration of the insecticide. We also observed behavioral effects of clothianidin. Water bugs, Belostoma flumineum, displayed a dose-dependent reduction in feeding rate following exposure to clothianidin. Similarly, crayfish, Orconectes propinquus, exhibited reduced responsiveness to stimulus with increasing clothianidin concentration. Using a semi-natural mesocosm experiment, we manipulated clothianidin concentration (0.6, 5, and 352 ppb) and the presence of predatory invertebrates to explore community-level effects. We observed high invertebrate predator mortality with increases in clothianidin concentration. With increased predator mortality, prey survival increased by 50% at the highest clothianidin concentration. Thus, clothianidin contamination can result in a top-down trophic cascade in a community dominated by invertebrate predators. In our Indiana field study, we detected clothianidin (max = 176 ppb), imidacloprid (max = 141 ppb), and acetamiprid (max = 7 ppb) in soil samples. In water samples, we detected clothianidin (max = 0.67 ppb), imidacloprid (max = 0.18 ppb), and thiamethoxam (max = 2,568 ppb). Neonicotinoids were detected in >56% of soil samples and >90% of the water samples, which reflects a growing understanding that neonicotinoids are ubiquitous environmental contaminants. Collectively, our results underscore the need for additional research into the effects of neonicotinoids on aquatic communities and ecosystems.

Prey responses to fine-scale variation in predation risk from combined predators

While it is well documented that organisms can express phenotypic plasticity in response to single gradients of environmental variation, our understanding of how organisms integrate information along multiple environmental gradients is limited in many systems. Using the freshwater snail Helisoma trivolvis and two common predators (water bugs Belostoma flumineum and crayfish Orconectes rusticus), we explored how prey integrate information along multiple predation risk gradients (i.e. caged predators fed increasing amounts of prey biomass) that induce opposing phenotypes. When exposed to single predators fed increasing amounts of prey biomass, we detected threshold responses; intermediate amounts of consumed biomass induced phenotypic responses, but higher amounts induced little additional induction. This suggests that additional increases in predator-induced traits with greater predator risk offer minimal increases in fitness or that a limit in the response magnitude was reached. Additionally, the response thresholds were contingent on the predator and focal trait. For shell width, responses were generally detected at a lower amount of consumed biomass by water bugs compared to crayfish. Within the crayfish treatments, we found that the shell thickness response threshold was lower than the shell width response threshold. When we combined gradients of consumed biomass from both predators, we found that the magnitude of response to one predator was often reduced when the other predator was present. Interestingly, these effects were often detected at consumed biomass levels that were lower than the threshold concentration necessary to elicit a response in the single-predator treatments. Moreover, our combined predator treatments revealed that snails shifted from discrete responses to more continuous (i.e. graded) responses. Together, our results reveal that organisms experiencing multiple environmental gradients can integrate this information to make phenotypic decisions and demonstrate the novel result that an exposure to multiple species of predators can lower the response threshold of prey.

Comparative toxicities of organophosphate and pyrethroid insecticides to aquatic macroarthropods

As agricultural expansion and intensification increase to meet the growing global food demand, so too will insecticide use and thus the risk of non-target effects. Insecticide pollution poses a particular threat to aquatic macroarthropods, which play important functional roles in freshwater ecosystems. Thus, understanding the relative toxicities of insecticides to non-target functional groups is critical for predicting effects on ecosystem functions. We exposed two common macroarthropod predators, the crayfish Procambarus alleni and the water bug Belostoma flumineum, to three insecticides in each of two insecticide classes (three organophosphates: chlorpyrifos, malathion, and terbufos; and three pyrethroids: esfenvalerate, λ-cyhalothrin, and permethrin) to assess their toxicities. We generated 150 simulated environmental exposures using the US EPA Surface Water Contamination Calculator to determine the proportion of estimated peak environmental concentrations (EECs) that exceeded the US EPA level of concern (0.5×LC50) for non-endangered aquatic invertebrates. Organophosphate insecticides generated consistently low-risk exposure scenarios (EECs<0.5×LC50) for both P. alleni and B. flumineum. Pyrethroid exposure scenarios presented consistently high risk (EECs>0.5×LC50) to P. alleni, but not to B. flumineum, where only λ-cyhalothrin produced consistently high-risk exposures. Survival analyses demonstrated that insecticide class accounted for 55.7% and 91.1% of explained variance in P. alleni and B. flumineum survival, respectively. Thus, risk to non-target organisms is well predicted by pesticide class. Identifying insecticides that pose low risk to aquatic macroarthropods might help meet increased demands for food while mitigating against potential negative effects on ecosystem functions.

Generalist versus specialist strategies of plasticity: snail responses to predators with different foraging modes

Summary Phenotypic plasticity is a common adaptation to environmental heterogeneity, and theory predicts that the evolution of constitutive versus plastic strategies should depend on the frequency of alternative environments, the magnitude of constraints and the costs of plasticity per se. However, it is unclear how species should evolve when they experience more than two environments that favour divergent phenotypes, particularly when they have absolute constraints on their morphology. We examined the plasticity of three freshwater snail species (Helisoma anceps, H. campanulata and H. trivolvis) in response to three environments: (i) no predator; (ii) shell‐invading water bugs (Belostoma flumineum) and (iii) shell‐crushing crayfish (Orconectes rusticus). We found distinct responses by each snail species to the predator treatments. Helisoma anceps starts with a relatively low, narrow and thick shell that becomes lower and thicker in response to crayfish but is unresponsive to water bugs. In contrast, H. campanulata starts with a relatively high, wide and thin shell that becomes lower and wider in response to water bugs but is unresponsive to crayfish. Helisoma trivolvis starts with a shell of intermediate height and width while the predators induce defences in different directions. These results suggest that H. trivolvis has a generalist plastic strategy while H. anceps and H. campanulata have specialised plastic strategies orientated against a single type of predator at the potential cost of being unable to respond to others. We then performed predation trials to determine predator preferences using a mixture of the three species. After 2 weeks of exposure to crayfish cues, H. anceps had higher survival than both H. trivolvis and H. campanulata with uncaged crayfish. After 2 weeks of exposure to water bug cues, both H. trivolvis and H. campanulata had higher survival than H. anceps with uncaged water bugs. When predation trials were conducted after 5 weeks of exposure to predator cues, H. trivolvis and H. campanulata reached a size refuge from both predators and this shifted predation pressure to H. anceps. Collectively, these results suggest that closely related prey species with different absolute constraints in their morphology had different defences that are either specialised or generalised to alternative environments.

Sublethal Effects of Pesticides on Predator–Prey Interactions in Amphibians

Increasing evidence suggests that contaminants in the environment can have important consequences on organismal interactions. While we have a good understanding of the lethal effects of contaminants on organisms, we have a weak understanding of how contaminants can affect organisms by altering the interactions that they have with other species in the community. Using tadpoles of two anuran species (Bullfrogs, Lithobates [Rana] catesbeianus; Green Frogs, L. clamitans), we investigated the effects of low nominal concentrations (1 and 10 ppb) of two pesticides (malathion and endosulfan) on tadpole activity and survival when exposed to four predator treatments (no predators; water bugs, Belostoma flumineum; newts, Notophthalmus viridescens; and dragonfly larvae, Anax junius). In both anuran species, adding predators reduced tadpole activity and survival, with increasing rates of mortality occurring with water bugs, newts, and dragonflies, respectively. Additionally, the highest concentration of endosulfan cau...

Termination of parental care in male giant waterbugs, Belostoma flumineum Say (Heteroptera: Belostomatidae) is associated with breeding season, egg pad size, and presence of females

Reproductive effort and/or success sometimes increase with the age of a parent. We found that reproductive effort was greatest in older egg-brooding male giant waterbugs, Belostoma flumineum Say. Consistent with previous studies, males with large egg pads were unlikely to discard them unhatched under any condition. However, males with small egg pads were significantly more likely to discard the eggs unhatched in the autumn (when breeding adults are young) than in the spring. By brooding smaller pads, older males invested more heavily in each offspring. Males bearing small egg pads were also significantly less likely to discard them in the presence of females. Three kinds of hypotheses have previously been proposed to explain why reproductive investment and/or success might increase with age: selection hypotheses predict differential survival of better parents; experience hypotheses predict that more experienced parents achieve greater success without additional cost per offspring; and residual reproduction hypotheses predict that older parents invest more per offspring. It is difficult to experimentally untangle these hypotheses, but the results of this study provide cautious support for a residual reproduction hypothesis that older males with few remaining reproductive opportunities invest more heavily per offspring by retaining small egg pads.

What Doesn't Kill You Makes You Sluggish: How Sublethal Pesticides Alter Predator–Prey Interactions

Pesticides commonly occur in ecological communities at relatively low concentrations, leading to growing interest in determining the sublethal effects of pesticides. Such effects should affect individuals and, in turn, alter interspecific interactions. We sought to determine how sublethal concentrations (0.1 and 1.0 mg/L) of two common pesticides (carbaryl and malathion) affected predator and prey behavior as well as subsequent predation rates. We conducted a series of experiments using three species of larval amphibians (Gray Treefrogs, Hyla versicolor; Green Frogs, Rana clamitans; and American Bullfrogs, R. catesbeiana) and three species of their predators (larval dragonflies, Anax junius; adult water bugs, Belostoma flumineum; and adult Red-spotted Newts, Notophthalmus viridescens). We found that the pesticides frequently reduced the activity of all three tadpole species. For the two invertebrate predators (Anax and Belostoma), the pesticides were lethal, precluding us from examining sublethal effects on predator–prey interactions. However, newt survival was high and the addition of the pesticides reduced the predation rates of newts in one of the three tadpole species. There were no effects of the pesticides on the striking frequency of the newts or on their prey capture efficiency. Thus, the mechanism underlying the pesticide-induced reduction in predation rates remains unclear. What is clear is that sublethal concentrations of pesticides have the potential to alter prey behavior and species interactions and thereby alter the composition of ecological communities.

Foraging and refuge use by a pond snail: Effects of physiological state, predators, and resources

The costs and benefits of anti-predator behavioral responses should be functions of the actual risk of predation, the availability of the prey's resources, and the physiological state of the prey. For example, a food-stressed individual risks starvation when hiding from predators, while a well-fed organism can better afford to hide (and pay the cost of not foraging). Similarly, the benefits of resource acquisition are probably highest for the prey in the poorest state, while there may be diminishing returns for prey nearing satiation. Empirical studies of state-dependent behavior are only beginning, however, and few studies have investigated interactions between all three potentially important factors. Here I present the results of a laboratory experiment where I manipulated the physiological state of pond snails ( Physa gyrina), the abundance of algal resources, and predation cues ( Belostoma flumineum waterbugs consuming snails) in a full factorial design to assess their direct effects on snail behavior and indirect effects on algal biomass. On average, snails foraged more when resources were abundant, and when predators were absent. Snails also foraged more when previously exposed to physiological stress. Snails spent more time at the water's surface (a refuging behavior) in the presence of predation cues on average, but predation, resource levels, and prey state had interactive effects on refuge use. There was a consistent positive trait-mediated indirect effect of predators on algal biomass, across all resource levels and prey states.

Evidence of population-level lateralized behaviour in giant water bugs, Belostoma flumineum Say (Heteroptera: Belostomatidae): T-maze turning is left biased

Lateralized behaviour occurs in diverse animals, but relatively few studies examine the phenomenon in invertebrates. Here we report a population-level left turn bias in the giant water bug Belostoma flumineum Say (Heteroptera: Belostomatidae) in an underwater T-maze. Individuals made significantly more left turns than right turns, including when they were naïve and first introduced to the maze. Water bugs also showed significantly longer runs of consecutive left turns than right turns (i.e. LLLLL). The length of these runs, however, did not increase with experience in the maze, suggesting that the effect is not the result of learning. There were also no differences in turning bias between male and female water bugs. The proximate mechanism(s) underlying the left turn bias is unknown, but directional cues in the environment were eliminated by rotating the maze 180 degrees between experiments, suggesting the mechanism(s) is endogenous. To our knowledge this is the first study of lateralized behaviour in the Heteroptera or in a swimming invertebrate animal.

Dragonfly predators influence biomass and density of pond snails

Studies in lakes show that fish and crayfish predators play an important role in determining the abundance of freshwater snails. In contrast, there are few studies of snails and their predators in shallow ponds and marshes. Ponds often lack fish and crayfish but have abundant insect populations. Here we present the results of field surveys, laboratory foraging trials, and an outdoor mesocosm experiment, testing the hypothesis that insects are important predators of pulmonate snails. In laboratory foraging trials, conducted with ten species of insects, most insect taxa consumed snails, and larval dragonflies were especially effective predators. The field surveys showed that dragonflies constitute the majority of the insect biomass in fishless ponds. More focused foraging trials evaluated the ability of the dragonflies Anax junius and Pantala hymenaea to prey upon different sizes and species of pulmonate snails (Helisoma trivolvis, Physa acuta, and Stagnicola elodes). Anax junius consumed all three species up to the maximum size tested. Pantala hymenaea consumed snails with a shell height of 3 mm and smaller, but did not kill larger snails. P. acuta were more vulnerable to predators than were H. trivolvis or S. elodes. In the mesocosm experiment, conducted with predator treatments of A. junius, P. hymenaea, and the hemipteran Belostoma flumineum, insect predators had a pronounced negative effect on snail biomass and density. A. junius and B. flumineum reduced biomass and density to a similar degree, and both reduced biomass more than did P. hymenaea. Predators did not have a strong effect on species composition. A model suggested that A. junius and P. hymenaea have the largest effects on snail biomass in the field. Given that both pulmonate snails and dragonfly nymphs are widespread and abundant in marshes and ponds, snail assemblages in these water bodies are likely regulated in large part by odonate predation.

HOW FLEXIBLE IS PHENOTYPIC PLASTICITY? DEVELOPMENTAL WINDOWS FOR TRAIT INDUCTION AND REVERSAL

Inducible defenses allow prey to modulate their phenotypic responses to the level of predation risk in the environment and reduce the cost of constitutive defenses. Inherent in this statement is that prey must alter their phenotypes during development in order to form these defenses. This has lead many ecologists and evolutionary biologists to call for studies that examine developmental plasticity to provide insights into the importance of development in controlling the trajectories of trait formation, the integration of phenotypes over ontogeny, and the establishment of developmental windows for trait formation and reversal. By moving away from studies that focus on a single point in development, we can obtain a more complete understanding of the phenotypic decisions and limitations of prey. We exposed freshwater snails (Helisoma trivolvis) to environments in which predatory water bugs (Belostoma flumineum) were always absent, always present, or added and removed at different points in development. We discovered that snails formed morphological defenses against water bugs. Importantly, after the initial induction of defenses, snails showed similar developmental trajectories as snails reared without predators. Further, the snails possessed wide developmental windows for inducible defenses that extended past sexual maturity. However, being induced later in development appeared to have an associated cost (i.e., decreased shell thickness) that was not found when water bugs were always present. This epiphenotype (i.e., new shell formation as an extension of the current shell) suggests that resource limitation plays an important role in responses to temporal variation in predation risk and may have critical ecological costs that limit the benefits of the inducible defense. Lastly, the ability of snails to completely reverse their defenses was limited to early in ontogeny due to the constraints associated with modular growth of shell material. In sum, we demonstrate that taking a developmental perspective is extremely valuable for understanding the ecology of inducible defenses.

Relative strengths of trait‐mediated and density‐mediated indirect effects of a predator vary with resource levels in a freshwater food chain

Predators can affect the density and traits (e.g. morphology, behavior) of their prey, and either change may influence how prey interact with their resources. Thus, predators can interact indirectly with resource species (i.e. two trophic levels below) through two separate mechanisms. The relative strengths of these two kinds of indirect effects have rarely been compared directly, and how their relative importance varies across environmental gradients is virtually unknown. We investigated the relative strength of trait‐ and density‐mediated indirect effects of the predatory insect Belostoma flumineum on algal communities through predation on the pond snail, Physa gyrina, across a gradient of basal resource abundance. Because prey balance the benefits of foraging against the increased risk of predation while foraging, the availability of the prey's resource should influence the strength of anti‐predator behavioral responses and hence the strength of trait‐mediated indirect interactions. Belostoma presence had positive indirect effects on resources as expected and total predator effects were constant across the basal resource gradient. At low initial resource levels, trait‐mediated indirect effects on algal biomass exceeded density‐mediated indirect effects, while at high initial resources the reverse was true. Snails showed similar habitat use across the resource gradient suggesting that the anti‐predator response was most likely a depression of activity levels.

RELATIVE STRENGTH OF TOP‐DOWN, BOTTOM‐UP, AND CONSUMER SPECIES RICHNESS EFFECTS ON POND ECOSYSTEMS

Theory and experiments demonstrate that the number of species in a local area can determine rates of ecosystem processes, but we know little about how the strength of that control compares with other influences or how it might vary across ecological gradients. Here I report results of a pond mesocosm experiment where consumer (snail) species richness, predation intensity, and nutrient availability were crossed in a full‐factorial design. This design allowed a direct comparison of the strength of the different factors on food web properties and ecosystem functioning (i.e., system‐level fluxes of energy or matter), and an evaluation of how the factors may interact.Systems with higher snail species richness had greater secondary production, consumer biomass, ecosystem respiration, and macrophyte stem growth, and lower epiphyton and periphyton biomass. However, snail species richness effects on periphyton and epiphyton were context dependent; predators reduced the effect of increasing snail richness on the biomass of attached algae. Species richness effects were most often the result of a biological mechanism (e.g., differential resource use), rather than being solely artifacts of the experimental design (e.g., sampling effects). The effects of nutrient enrichment and predation were mostly predictable from simple food chain models; increases in nutrient availability led to increased algal biomass, primary production, and snail biomass, while predators decreased snail biomass and indirectly augmented algae.Snail species richness effects on the biomass of many functional groups were as strong or stronger than those of a substantial nutrient enrichment or of the addition of a voracious predator (Belostoma flumineum). Nutrient enrichment had the most pronounced effects on several system‐level processes (e.g., primary production, sedimentation). Species richness had weaker effects on ecosystem properties than on particular functional groups, probably because of compensatory responses among different producer functional groups. In summary, this experiment suggests that the number of consumer species in a system can have large and meaningful effects on the distribution of biomass in a food web, and that these effects can depend on ecological context.

Experimental evidence for alternative stable equilibria in a benthic pond food web

AbstractBoth local and initial conditions have been implicated in causing variation in the strengths of predator effects among natural communities. In this study, I performed a controlled mesocosm experiment using two common snails, Physella gyrina and Helisoma trivolvis, and their insect predator, Belostoma flumineum, to explicitly quantify and understand the causes of previously observed variation in food web interactions in natural ecosystems. The results from a controlled mesocosm experiment showed that: (1) at low productivity, snails were dominated by small‐vulnerable species (P. gyrina), predator effects were strong and predator effects caused a trophic cascade extending to producers; (2) at high productivity, snails were dominated by large‐invulnerable species (H. trivolvis), predator effects were weak and there was no trophic cascade to producers; (3) at intermediate productivity, alternative stable equilibria were implicated, such that depending on initial conditions, snails were dominated by either small‐vulnerable or large‐invulnerable species and predator effects were either strong or weak. Thus, this study represents one of the first to quantify how variation in food web structure can result from the local environment, initial conditions and their interaction.