Predator Size Research Articles

Complexity of the prey spectrum of Agaronia propatula (Caenogastropoda: Olividae), a dominant predator in sandy beach ecosystems of Pacific Central America.

Olivid gastropods of the genus Agaronia are dominant predators within invertebrate communities on sandy beaches throughout Pacific Central America. At Playa Grande, on the Pacific Coast of Costa Rica, we observed 327 natural predation events by Agaronia propatula. For each predation event, we documented prey taxa and body size of both predator and prey. The relationship between predator and prey size differed for each of the four main prey taxa: bivalves, crustaceans, heterospecific gastropods, and conspecific gastropods (representing cannibalism). For bivalve prey, there was increased variance in prey size with increasing predator size. Crustaceans were likely subdued only if injured or otherwise incapacitated. Heterospecific gastropods (mostly Olivella semistriata) constituted half of all prey items, but were only captured by small and intermediately sized A. propatula. Large O. semistriata appeared capable of avoiding predation by A. propatula. Cannibalism was more prevalent among large A. propatula than previously estimated. Our findings suggested ontogenetic niche shifts in A. propatula and a significant role of cannibalism in its population dynamics. Also indicated were size-dependent defensive behavior in some prey taxa and a dynamic, fine-scale zonation of the beach. The unexpected complexity of the trophic relations of A. propatula was only revealed though analysis of individual predation events. This highlights the need for detailed investigations into the trophic ecology of marine invertebrates to understand the factors driving ecosystem structuring in sandy beaches.

Multiple factors, including arena size, shape the functional responses of ladybird beetles

Abstract Functional response studies are often used to determine the suitability of predators as biocontrol agents. Ladybird beetles (Coleoptera: Coccinellidae) are often used for the control of crop pests such as aphids. However, most functional response studies on coccinellids compare a limited number of species at different life stages, temperatures or sexes. A large‐scale comparison of ladybird beetle functional responses is needed to evaluate the utility of these species as potential biocontrol predators and to understand the traits that influence the interaction strength between ladybird beetles and their prey. We compiled 158 ladybird beetle functional responses from 30 studies and tested for effects of taxa, traits, temperature and arena size on functional response parameters using linear mixed‐effects models. Our results show that functional response parameters (handling time and space clearance rate) are affected by predator stage, predator mass, prey type, temperature and arena size. Although complicated by interaction terms, space clearance rate generally increased with predator size, temperature and predator stage, while handling time decreased with predator size, temperature and predator stage. Coleopteran prey induced the highest handling times. Our results also show that experimental arena size has a large, consistent effect on space clearance rate. Arena size is more important in determining foraging rates at low prey densities than any other factor considered here, including predator mass and temperature. Efforts to use laboratory‐based functional response experiments to evaluate the efficacy of biocontrol predators are therefore confounded by the choice of arena size. Synthesis and applications. In addition to confirming known body mass and temperature effects, our study reveals previously unclear age‐related effects and the importance of prey types, which can be used to optimize biocontrol programmes. The arena size effect is unexpected and problematic because failure to account for arena size precludes accurate comparison of biocontrol predator effectiveness. We suggest managers and biocontrol practitioners re‐evaluate the efficacy of candidate biocontrol predators, perhaps by statistically controlling for arena size to minimize the influence of this widely unconsidered factor on functional response estimates.

Ontogenetic shifts in predator diet drive tradeoffs between fisheries yield and strength of predator-prey interactions

In some marine ecosystems, overharvesting marine predators has triggered major changes in trophic structure and ecosystem function. However, harvest levels that are deemed sustainable for one species may still lead to unexpected impacts elsewhere in the ecosystem. For example, by imposing an additional source of mortality, even sustainable harvesting can lead to a reduction in the number of large individuals within a population, and this truncation in size structure is typically more severe when the largest, most valuable size classes are targeted. Often small and large individuals within a species differ in important ways, including in what they consume, so a loss in predator-prey interactions could occur even without changes in overall predator biomass. Here we explore whether a truncation in predator size structure alone can reduce or functionally eliminate linkages between predator and prey. For this outcome to occur, a predator’s diet must change as it grows in size. We examined evidence for changes in diets with size among predators in three large marine ecosystems, and used a size-structured population dynamics model to evaluate the extent to which otherwise sustainable fishing results in disproportionate reductions in predation. Modelling suggests that diet shifts occurring late in life history (diet mid-point >45% of the maximum size) led to losses in predation that were more severe than would have been expected from losses in predator biomass. Further, the form of the fishery selectivity was less important than the degree of reduction in biomass within each size class relative to the timing of diet shifts. Empirical diet information demonstrates that piscivores vary widely in their onset to piscivory, and this may buffer the potential impacts of truncation in size structure. However, over half of the piscivores had diet shifts toward specific fish taxa at sizes at or above that which would lead to disproportionate reductions in prey consumption. Information about when and how diets change with predator size could identify ecosystems where harvest may lead to unexpected losses in predator-prey interactions.

An analytical approach to top predator interference on the dynamics of a food chain model

In this paper, a nonlinear mathematical model is proposed and analyzed to study of top predator interference on the dynamics of a food chain model. The mathematical model is formulated using the system of non-linear ordinary differential equations. In this model, there are three state dimensionless variables, viz, size of prey population x, size of intermediate predator y and size of top predator population z. The analytical results are compared with the numerical simulation using MATLAB software and satisfactory results are noticed.

Anthropogenic fragmentation of landscapes: mechanisms for eroding the specificity of plant-herbivore interactions.

Reduced ecological specialization is an emerging, general pattern of ecological networks in fragmented landscapes. In plant-herbivore interactions, reductions in dietary specialization of herbivore communities are consistently associated with fragmented landscapes, but the causes remain poorly understood. We propose several hypothetical bottom-up and top-down mechanisms that may reduce the specificity of plant-herbivore interactions. These include empirically plausible applications and extensions of theory based on reduced habitat patch size and isolation (considered jointly), and habitat edge effects. Bottom-up effects in small, isolated habitat patches may limit availability of suitable hostplants, a constraint that increases with dietary specialization. Poor hostplant quality due to inbreeding in such fragments may especially disadvantage dietary specialist herbivores even when their hostplants are present. Size and isolation of habitat patches may change patterns of predation of herbivores, but whether such putative changes are associated with herbivore dietary specialization should depend on the mobility, size, and diet breadth of predators. Bottom-up edge effects may favor dietary generalist herbivores, yet top-down edge effects may favor dietary specialists owing to reduced predation. An increasingly supported edge effect is trophic ricochets generated by large grazers/browsers, which remove key hostplant species of specialist herbivores. We present empirical evidence that greater deer browsing in small forest fragments disproportionately reduces specialist abundances in lepidopteran assemblages in northeastern USA. Despite indirect evidence for these mechanisms, they have received scant direct testing with experimental approaches at a landscape scale. Identifying their relative contributions to reduced specificity of plant-herbivore interactions in fragmented landscapes is an important research goal.

Tadpoles of the bromeliad-dwelling frog Phyllodytes luteolus are able to prey on mosquito larvae

Mosquito-borne diseases have been spreading rapidly. Although Aedes sp. are believed to prefer man-made habitats for reproduction, its larvae can also be found in bromeliads in anthropic environments. Mosquito populations can be controlled through predation, in which the relative size of predator and prey are important to understand this interaction. Tadpoles of Phyllodytes luteolus live in bromeliad axils and are potential predators of mosquito larvae. Our aim was to assess if these tadpoles are able to prey mosquito larvae, and if the predation rate differs according to the size of predator and prey. We collected P. luteolus tadpoles and Culicidae mosquito larvae from bromeliads of restinga (sandbank) areas. We measured the total length of tadpoles and mosquito larvae, and separated them into three size categories (large, medium and small). As experimental units, we used 50 mL centrifuge tubes with 35 mL of dechlorinated water, without substrate and food. We applied nine treatments composed by a combination of tadpole size categories and mosquito larvae size categories. In each unit, we added one tadpole and five mosquito larvae to evaluate the overall predation rate after 4 days. We found that P. luteolus tadpoles of any size were able to prey on mosquito larvae and verified that large tadpoles preyed a larger number of mosquito larvae than smaller size categories. Our results can contribute to evaluate the importance of tadpoles in the trophic structuring of aquatic environments, especially in phytotelmata.

Stephen A. Forbes, Antecedent Wetland Ecologist? by Arnold van der Valk

Stephen A. Forbes (1844-1930) was an American entomologist/zoologist who was born, raised and largely educated in northern Illinois. He spent most of his professional career as the director of the Illinois Natural History Survey and as a faculty member and administrator at the University of Illinois. Early in his scientific career, he studied fish and bird diets by examining the stomach contents of these animals. In 1887, he published his most famous and influential paper, “The lake as a microcosm,” which contains one of the earliest formulations of what came to be called the ecosystem. In this paper, Forbes describes a hypothetical isolated, small lake as being a microcosm that is in equilibrium. This equilibrium is the result of trophic interactions among the organisms in the microcosm that limit the sizes of both predator and prey populations. Forbes believed that natural selection was responsible for limiting the reproductive capacities of predators and prey. Although energy transfer among trophic levels is not the main focus of his paper, Forbes postulated that food (energy) is one of the main factors structuring ecosystems, but he did not explicitly discuss the energetics of his lake microcosm.

What determines prey selection in owls? Roles of prey traits, prey class, environmental variables, and taxonomic specialization.

Ecological theory suggests that prey size should increase with predator size, but this trend may be masked by other factors affecting prey selection, such as environmental constraints or specific prey preferences of predator species. Owls are an ideal case study for exploring how predator body size affects prey selection in the presence of other factors due to the ease of analyzing their diets from owl pellets and their widespread distributions, allowing interspecific comparisons between variable habitats. Here, we analyze various dimensions of prey resource selection among owls, including prey size, taxonomy (i.e., whether or not particular taxa are favored regardless of their size), and prey traits (movement type, social structure, activity pattern, and diet). We collected pellets of five sympatric owl species (Athene noctua, Tyto alba, Asio otus, Strix aluco, and Bubo bubo) from 78 sites across the Mediterranean Levant. Prey intake was compared between sites, with various environmental variables and owl species as predictors of abundance. Despite significant environmental impacts on prey intake, some key patterns emerge among owl species studied. Owls select prey by predator body size: Larger owls tend to feed on wider ranges of prey sizes, leading to higher means. In addition, guild members show both specialization and generalism in terms of prey taxa, sometimes in contrast with the expectations of the predator–prey body size hypothesis. Our results suggest that while predator body size is an important factor in prey selection, taxon specialization by predator species also has considerable impact.

Do Closely Related Species Influence Each Other's Sexual Size Dimorphism?

Size differences between males and females (sexual size dimorphism) are often seen in a variety of species. In birds of prey in particular, a phenomenon occurs where the female is larger than the male. One of the main hypotheses attempting to explain sexual size dimorphism in birds of prey is that the female and male differ in size to partition resources, like prey. There is also evidence that predator and prey body size are correlated, so predators of similar size may be in direct competition. It has been shown that when two closely related species interact in the same area, they are likely to be in competition for similar resources, like prey. This study is looking at sexual size dimorphism and closely related species interactions, which has never been looked at before in birds of prey. I am using the subfamily Buteoninae (Buteo Hawks) as my focal group. I will be using sexual size dimorphism ratios, estimates of genetic distance between closely related species and proportion of range overlap between different closely related lineages within the subfamily. I am expecting to see that if species are closely related and inhabit the same area they will have a decreased sexual size dimorphism. This is because both species are likely to be competing for the same resources, and to avoid competition the species will diverge in body size from one another. This will cause the male and female of each species to converge in size, reducing their sexual size dimorphism.

A test of the predator satiation hypothesis, acorn predator size, and acorn preference

Mast seeding is hypothesized to satiate seed predators with heavy production and reduce populations with crop failure, thereby increasing seed survival. Preference for red or white oak acorns could influence recruitment among oak species. We tested the predator satiation hypothesis, acorn preference, and predator size by concurrently measuring acorn production, mouse abundance, and white versus red oak acorn removal rates in exclosures allowing access by mice (HW), squirrels and smaller-sized vertebrates (WW), or all-sized vertebrates (C) for 12 years. Annual removal rate varied, but virtually all acorns were eventually removed from all exclosure types all years except one. Acorns were removed more slowly from HW than from WW or C exclosures, indicating that large vertebrates were not major acorn consumers, locally. Red and white oak acorn removal rates were similar except in two years, when red oak acorns were removed more rapidly. Removal slowed with increasing acorn crops, suggesting that heavy crops can “swamp” predators. Removal rate was negatively correlated with crop size the previous fall. A positive trend between mouse abundance and crop size the previous fall was evident; abundance decreased sharply the year following crop failures but not after moderate or heavy crops, suggesting that poor crops can dampen acorn predation the following year.

Successful escape of bombardier beetles from predator digestive systems.

Some prey animals can escape from the digestive systems of predators after being swallowed. To clarify the ecological factors that determine the success of such an escape, we investigated how the bombardier beetle Pheropsophus jessoensis escapes from two toad species, Bufo japonicus and Btorrenticola, under laboratory conditions. Pheropsophus jessoensis ejects a hot chemical spray from the tip of the abdomen when it is attacked. Although all toads swallowed the bombardier beetles, 43% of the toads vomited the beetles 12-107 min after swallowing them. All the vomited beetles were still alive and active. Our experiment showed that Pjessoensis ejected hot chemicals inside the toads, thereby forcing the toads to vomit. Large beetles escaped more frequently than small beetles, and small toads vomited the beetles more frequently than large toads. Our results demonstrate the importance of the prey-predator size relationship in the successful escape of prey from inside a predator.

Predator size influences intraspecific multiple predator effects in swimming crab-Manila clam system

Interactions between multiple predators, whether conspecific or interspecific, has been the subject of many researches. There is a shortage of studies investigating how different size classes of a single species interact with one another to generate multiple predator effects. Here, we used a comprehensive experimental design, consisting of calculation of per capita consumption rates, to examine multiple predator effects on prey when swimming crabs (Portunus trituberculatus) of multiple sizes foraged on manila clams (Ruditapes philippinarum) using a video recording analysis system in the laboratory. We identified multiple predator effects on prey by comparing observed predation rate to that predicted from the multiple risk model. The observed predation rate of crab was significantly lower than predicted value when a conspecific competitor appeared. Swimming crab significantly decreased foraging time budgets when a conspecific competitor appeared. The proportion of time medium-sized crabs spent searching for and handling clam decreased in the presence of competitors. The encounter rate between crabs and clam decreased while the probability of capture upon encounter and the probability of consumption upon capture increased when a conspecific competitor appeared. The handling time per clam significantly decreased when a conspecific competitor appeared. Most encounters between crabs were fighting (2.31–4.14 times/h), secondly chasing (1.02–3.07 times/h), and threatening (1.24–2.09 times/h), and the fewest encounters were non-aggression (0.13–0.49 times/h). The results indicated that swimming crab foraging behavior was affected by the chasing, threatening and fighting behavior of a conspecific competitor and this affected the predation rate on prey, which led to non-independent multiple predator effects in the swimming crab-Manila clam system.

Few But Relatively Large Prey: Trophic Ecology of Chauliodus sloani (Pisces: Stomiidae) in Deep Waters of the Central Mediterranean Sea

The trophic ecology of Chauliodus sloani (Stomiidae) was thoroughly investigated by stomach content analysis for the first time in the Mediterranean. Overall 206 individuals (64.0–260.0 mm SL) were collected from 2013 to 2015. C. sloani can be defined a specialist predator which feeds exclusively on mesopelagic fish belonging to Gonostomatidae, Myctophidae, Paralepididae, Phosichthyidae, Sternoptychidae and Stomiidae. Gonostomatidae (%IRI = 26.02), Myctophidae (%IRI = 24.77) and Sternoptychidae (%IRI = 24.35) were important food items for C. sloani. Maurolicus muelleri (%IRI = 36.43), Cyclothone braueri (%IRI = 28.26) and Vinciguerria attenuata (%IRI = 12.97) were the most important prey. Cases of cannibalism were also observed. The examination of food size spectrum, in relation to predator length, demonstrated that C. sloani developed a feeding strategy aimed to maximizing energy input and based on the capture of few and relatively large prey: more than 50% of prey items exceeded the value of 20% for ratio between prey and predator size (SL) and the 5.4% of prey measured more than 50% of predator size. Differences in food composition across seasons were found; C. braueri and M. muelleri were more abundant in autumn, V. attenuata during spring. Prey items mainly belongs to weakly vertical migrating fauna, usually concentrated at 400 m Deep Scattering Layer.

Heterogeneity in body size and habitat complexity influence community structure

ABSTRACTFood web studies commonly ignore individual variation within a population and variation in the environment and assume these factors have insignificant effects on community dynamics relative to interspecific interactions. However, variation in body sizes within a population (size structure) and the physical structure of habitats (habitat complexity) can both affect interspecific interactions independently, with possible interactive effects. Using experimental mesocosms, we examined effects of predator size structure and habitat complexity on predation by two common predators in fishless ponds: larval aquatic beetles (Cybister fimbriolatus) and larval dragonflies (Anax junius). Cannibalism, intraguild predation, and predation on shared prey were measured at two levels of habitat complexity crossed with six size-structured pairs of predators. We found that highly complex habitats sustain a higher prey density because prey can take refuge from predators. Additionally, size structure had direct (size better predicted predation rate than did species identity) and indirect (IGP and cannibalism lowered predation rates) effects on consumptive interactions, which changed the composition and density of the predator guild. The identified size- and habitat-mediated mechanisms can change the frequencies of intraguild vs. interguild predation (a balance important in determining top–down control of predators), and therefore we argue that these sources of heterogeneity should be included in community ecology studies where possible.

Seasonal dynamics of Zooplankton functional groups in relation to environmental variables in Xiquanyan Reservoir, Northeast China

In this study, the concept of functional feeding groups was used to classify and model the seasonal variation of zooplankton functional groups in relation to environmental variables. A total of 48 zooplankton species were observed in the reservoir and grouped into 8 functional groups. Both environmental variable and the biomass of zooplankton functional groups vary spatially and seasonally. Water temperature, water transparency, total nitrogen and nitrates were significantly higher in summer, while chlorophyll-aand ammonium were higher in autumn and spring, respectively. Biomass of zooplankton was significantly higher in summer (245.81 μg/L), followed by autumn (196.54 μg/L) and spring (54.25 μg/L). Group RF (rotifer filter feeders) dominated in spring, accounting for 80% of the total biomass. In summer and autumn, group RC (rotifer carnivore) and SCF (small copepods and cladocerans filter feeders) were the dominant, respectively. Total nitrogen, total phosphorus, ammonium, chlorophyll-aand water transparency were the major factor influencing zooplankton community. Group RF was positively influenced by ammonium and total phosphorus, while RC, SCF and MCF (middle copepods and cladocerans filter feeders) were positively correlated with chlorophyll-a. Top-down control of phytoplankton by groups RC, SCF and MCF in Xiquanyan reservoir is not strong enough to produce negative effect. Increase in predator size biomass did not strengthen top-down control on prey. It is quite clear that the zooplankton function groups of Xiquanyan reservoir followed a predictable seasonal pattern. This therefore highlights the significance of environmental variables in structuring plankton composition in the reservoir.

Experimental predatory behavior of the stone crab Eriphia verrucosa (Forskål, 1775) (Decapoda, Brachyura, Eriphiidae)

Abstract Predatory behavior of the crab Eriphia verrucosa (Forskal, 1775) on the oyster Magallana angulata (Lamarck, 1819), the mussel Mytilus galloprovincialis Lamarck, 1819, the limpet Patella depressa Pennant, 1777, the hermit crab Clibanarius erythropus (Latreille, 1818), the gastropod Phorcus lineatus (da Costa, 1778) and the sea urchin Paracentrotus lividus (Lamarck, 1816) was experimentally studied. Taken into account that the sampled population of E. verrucosa was infected with the parasitic barnacle Sacculina eriphiae Smith, 1906, four predator categories were established regarding crab sex and parasite presence: uninfected and infected males and females, respectively. These four crab categories were fed on the six preys offered, but prey remains suggest that the crab obtains the flesh of each prey using different methods of attack. Irrespective of the prey species, uninfected females consistently presented lower percentages of crab feeding and total consumption. On average, it was found that a significantly higher percentage of crabs feed on M. galloprovincialis, P. depressa and Ph. lineatus than on the other three prey species (chi-squared test, p < 0.05). In terms of mean eaten biomass, the preys most eaten were P. lividus and P. depressa and the least eaten was C. erythropus (ANOVA test, p < 0.05). For crabs feeding on M. galloprovincialis and Ph. lineatus there was a significant positive correlation (p < 0.05) between the predator size and prey size, as well as between the predator size and total prey consumption. According to these results, potential effects of this top predator on intertidal communities are also discussed.

Giving predators a wide berth: quantifying behavioral predator shadows in colonizing aquatic beetles.

The perceived quality of habitat patches in complex landscapes is highly context dependent. Characteristics of neighboring patches in such complex landscapes can influence perceived habitat quality, altering colonization dynamics and community structure. Spatial contagion of predation risk across patches has been observed over smaller spatial scales in aquatic systems. Naturally colonizing aquatic beetles were used to examine the spatial dynamics of risk contagion by quantifying the size of predator shadows around fish patches across spatial scales potentially involving numerous patches in natural landscapes. These consisted of fish free, replicate experimental mesocosm arrays radiating from larger central mesocosms containing fish, and allowed examination of the effect of distance to fish on beetle abundance, rarified species richness, and variation in species responses. Overall, beetles avoided pools closer to fish, but species varied in colonization pattern, resulting in species-specific predator shadows and potential behavioral species sorting. The spatial and phylogenetic extent of contagion and other context-dependent effects has implications for the role of complex behavior in the dynamics of communities and metacommunities.

Ontogenetic prey size selection in snakes: predator size and functional limitations to handling minimum prey sizes

As body size increases, some predators eliminate small prey from their diet exhibiting an ontogenetic shift toward larger prey. In contrast, some predators show a telescoping pattern of prey size in which both large and small prey are consumed with increasing predator size. To explore a functional explanation for the two feeding patterns, I examined feeding effort as both handling time and number of upper jaw movements during ingestion of fish of consistent size. I used a range of body sizes from two snake species that exhibit ontogenetic shifts in prey size (Nerodia fasciata and N. rhombifer) and a species that exhibits telescoping prey size with increased body size (Thamnophis proximus). For the two Nerodia species, individuals with small or large heads exhibited greater difficulty in feeding effort compared to snakes of intermediate size. However, for T. proximus measures of feeding effort were negatively correlated with head length and snout–vent length (SVL). These data indicate that ontogenetic shifters of prey size develop trophic morphology large enough that feeding effort increases for disproportionately small prey. I also compared changes in body size among the two diet strategies for active foraging snake species using data gleaned from the literature to determine if increased change in body size and thereby feeding morphology is observable in snakes regardless of prey type or foraging habitat. Of the 30 species sampled from literature, snakes that exhibit ontogenetic shifts in prey size have a greater magnitude of change in SVL than species that have telescoping prey size patterns. Based upon the results of the two data sets above, I conclude that ontogenetic shifts away from small prey occur in snakes due, in part, to growth of body size and feeding structures beyond what is efficient for handling small prey.

Fishing-induced changes in predation pressure by perch (Perca fluviatilis) regulate littoral benthic macroinvertebrate biomass, density, and community structure

We aimed to study whether the varying changes in predation pressure by perch (Perca fluviatilis) reflect the biomass, density, and community structure of the benthic macroinvertebrates. Prey preference is size-dependent, and overall predation pressure is density dependent, and thus the size structure of the P. fluviatilis population should affect the structure of the macroinvertebrate community, and the population density of P. fluviatilis should reflect the overall density of benthic macroinvertebrates. We sampled the littoral benthic community in a boreal lake that had been divided into two parts that were subjected to two different fishing procedures during 2007–2012 period and analyzed the macroinvertebrate diet of fish. The benthic macroinvertebrate community reflected the predation pressure. Total macroinvertebrate biomass increased during the study period in the lake division with a non-size-selective fishing procedure (NSF), i.e., all invertivorous perch size-classes targeted, but decreased in the section with negatively size-selective fishing procedure (SSF), i.e., large invertivorous individuals ≥ 16 cm were not targeted. This difference was a result of the increase in large-sized species, such as Odonata, for the NSF procedure and decrease in the SSF procedure. In contrast to total biomass, total macroinvertebrate density did not show a response to predator size structure but rather total macroinvertebrate density decreased with increasing fish density. The study demonstrates the effect of predation pressure of P. fluviatilis on benthic communities, thus highlighting the keystone predator role of the species in boreal lakes and gives more insight on the multiple effects of fish predation on littoral benthic communities.

Size‐based interactions and trophic transfer efficiency are modified by fish predation and cyanobacteria blooms in Lake Mývatn, Iceland

Abstract Trophic cascade studies have so far mostly focused on changes in the abundance, biomass, or average size of prey and predators. In contrast, individual size‐based interactions, playing a key role in the trophic structure and functioning of aquatic ecosystems, have been less explored. We conducted a 3‐month in situ experiment in Lake Mývatn, Iceland, with two fish treatments (with and without fish, Gasterosteus aculeatus). After the first month of the experiment, Anabaena blooms appeared in the lake. We studied the effects of fish predation and occurrence of cyanobacteria blooms on the individual size structure (i.e. the distribution of the number of organisms over a size range) of zooplankton and phytoplankton. We also assessed the potential consequences for trophic transfer efficiency (TTE) (measured as the predator to prey biomass ratio) in the planktonic food web. Our results showed that fish predation and cyanobacteria bloom had a negative relationship with size diversity of zooplankton, which became dominated by small‐sized individuals in both cases. The phytoplankton size diversity changed over time particularly due to the blooming of large‐sized Anabaena, and its increase was apparently mainly driven by changes in resources. Low zooplankton size diversity related to fish predation reduced TTE, particularly in the enclosures with fish. This may be because low zooplankton size diversity represents a lower partition of resources among consumers, thereby decreasing the trophic energy transfer. With the occurrence of Anabaena bloom, high phytoplankton size diversity coincided with a lower energy transfer in all enclosures likely due to reduced zooplankton grazing when large‐sized colony‐forming Anabaena dominated. In conclusion, our results indicate that both top‐down and bottom‐up forces significantly influence the size structure of planktonic communities. The changes in size structure were related to shifts in the energy transfer efficiency of the Lake Mývatn food web. Thus, our study underpins the importance of taking into account size‐based interactions in the study of trophic cascades, particularly in a warming climate where strong planktivorous fish predation and frequent cyanobacteria blooms may occur.