Role of anaerobic ciliates in planktonic food webs: abundance, feeding, and impact on bacteria in the field

We studied the dynamics of two populations of anaerobic ciliates, Plagiopyla sp. and Metopus sp., and of their potential prey, heterotrophic and phototrophic purple bacteria, in Lake Cisó throughout a 1-year cycle. The abundance of both ciliates was very low (less than 2 individuals per ml). During mixing, Plagiopyla ciliates exhibited high clearance rates (about 100 nl ciliate h), its integrated abundance increased with a net doubling time of 47 days, and its potential doubling times, as calculated from the number of bacteria consumed, ranged between 5 and 8 days. During stratification, the activity of Plagiopyla ciliates was reduced and the population decreased; this was related to the higher amounts of sulfide present. The impact of predation by the Plagiopyla population on bacterioplankton was found to be insignificant, less than 0.1% of bacterial biomass consumed per day. Thus, anaerobic ciliates cannot control the bacterioplankton in Lake Cisó because of both the low abundance over the period studied and the low feeding rates during certain periods. A review of available field studies suggests that this conclusion can be extrapolated to most other anoxic systems.     

Predator-specific enrichment of actinobacteria from a cosmopolitan freshwater clade in mixed continuous culture

We investigated whether individual populations of freshwater bacteria in mixed experimental communities may exhibit specific responses to the presence of different bacterivorous protists. In two successive experiments, a two-stage continuous cultivation system was inoculated with nonaxenic batch cultures of the cryptophyte Cryptomonas sp. Algal exudates provided the sole source of organic carbon for growth of the accompanying microflora. The dynamics of several 16S rRNA-defined bacterial populations were followed in the experimental communities. Although the composition and stability of the two microbial communities differed, numerous members of the first assemblage could again be detected during the second experiment. The introduction of a size-selectively feeding mixotrophic nanoflagellate (Ochromonas sp.) always resulted in an immediate bloom of a single phylotype population of members of the class Actinobacteria (Ac1). These bacteria were phylogenetically affiliated with an uncultured lineage of gram-positive bacteria that have been found in freshwater habitats only. The Ac1 cells were close to the average size of freshwater bacterioplankton and significantly smaller than any of the other experimental community members. In contrast, no increase of the Ac1 population was observed in vessels exposed to the bacterivorous ciliate Cyclidium glaucoma. However, when the Ochromonas sp. was added after the establishment of C. glaucoma, the proportion of population Ac1 within the microbial community rapidly increased. Populations of a beta proteobacterial phylotype related to an Aquabacterium sp. decreased relative to the total bacterial communities following the addition of either predator, albeit to different extents. The community structure of pelagic microbial assemblages can therefore be influenced by the taxonomic composition of the predator community.     

Stepping in Elton's footprints: a general scaling model for body masses and trophic levels across ecosystems

Despite growing awareness of the significance of body-size and predator-prey body-mass ratios for the stability of ecological networks, our understanding of their distribution within ecosystems is incomplete. Here, we study the relationships between predator and prey size, body-mass ratios and predator trophic levels using body-mass estimates of 1313 predators (invertebrates, ectotherm and endotherm vertebrates) from 35 food-webs (marine, stream, lake and terrestrial). Across all ecosystem and predator types, except for streams (which appear to have a different size structure in their predator-prey interactions), we find that (1) geometric mean prey mass increases with predator mass with a power-law exponent greater than unity and (2) predator size increases with trophic level. Consistent with our theoretical derivations, we show that the quantitative nature of these relationships implies systematic decreases in predator-prey body-mass ratios with the trophic level of the predator. Thus, predators are, on an average, more similar in size to their prey at the top of food-webs than that closer to the base. These findings contradict the traditional Eltonian paradigm and have implications for our understanding of body-mass constraints on food-web topology, community dynamics and stability.     

The interaction between spatial heterogeneity and genetic feedback in laboratory predator-prey systems

Summary A hybrid experimental design combining laboratory populations and computer simulation was used to study the relative influence of spatial heterogeneity, genetic feedback and predator foraging behavior on the stability of predator-prey systems. Houseflies, Musca domestica, maintained in multicellular or single-cell population cages were used as predator feeding on chemical solutions contained in small glass vials. Feeding, mortality and dispersal of the predators occurred within the cages, but reproduction of the predators and prey as well as dispersal of the prey was controlled by a computer program. Genetic change in the prey was determined partially by the computer model which associated chemical solutions with particular genotypes, and partially by the predators, whose foraging behavior influenced the fitness of each genotype. Three treatments were compared: a genetically polymorphic prey population in a spatially homogeneous environment, a monomorphic prey population in a heterogeneous environment, and a polymorphic prey population in a heterogeneous environment. With the parameters used, the latter treatment, involving an interaction between spatial heterogeneity and genetic feedback, was the most stable. Without genetic feedback in the prey, spatial heterogeneity was insufficient to overcome the destabilizing influence of the predator's foraging behavior. Without spatial heterogeneity, genetic feedback was insufficient to overcome the destabilizing effect of preferential feeding by the predators on palatable prey. The prey population evolved sufficient resistance to cause extinction of the predator population. The results support the hypothesis that population regulation by genetic feedback in predator-prey systems is less likely when predators feed preferentially on susceptible prey and that spatial heterogeneity, by decreasing the relative accessibility of susceptible prey and hence altering the predator's foraging strategy, may increase the likelihood of regulation through genetic feedback.     

Predator kairomones change food web structure and function,regardless of cues from consumed prey

Predation risk in aquatic systems is often assessed by prey through chemical cues, either those released by prey or by the predator itself. Many studies on predation risk focus on simple pairwise interactions, with only a few studies examining community‐level and ecosystem responses to predation risk in species‐rich food webs. Further, of these few community‐level studies, most assume that prey primarily assess predation risk through chemical cues from consumed prey, even heterospecific prey, rather than just those released by the predator. Here, we compared the effects of different predation cues (predator presence with or without consumed prey) on the structure and functioning of a speciose aquatic food web housed in tropical bromeliads. We found that the mere presence of the top predator (a damselfly) had a strong cascading effect on the food web, propagating down to nutrient cycling. This predation risk cue had no effect on the identity of colonizing species, but strongly reduced the abundance and biomass of the macroinvertebrate colonists. As a result, bacterial biomass and nitrogen cycling doubled, with a concomitant decrease in bacterial production, but CO₂ flux was unaffected. These community and ecosystem effects of predator presence cues were not amplified by the addition of chemical cues from consumed prey. Our results show that some of the consequences of predation risk observed in controlled experiments with simplified food webs may be observed in a natural, species‐rich food web.     

Trait‐mediated apparent competition in an intraguild predator–prey system

Apparent competition is of broad interest due to its effects on community structure and potential applications in agriculture, restoration, and medicine. It is well‐established that apparent competition can result from changes in predator abundance or behavior caused by interactions with alternate prey, but no previous empirical study has demonstrated that apparent competition can also result from prey‐induced changes in predator morphology. This trait‐mediated alternative mechanism of apparent competition would expand the range of conditions under which apparent competition might occur in nature and identify new ways to generate or modify apparent competition in applied contexts. Here, as a proof of concept, we show that trait‐mediated apparent competition can arise from inducible offenses and show how it operates using experiments involving three ciliates. When it feeds on Colpidium kleini, the intraguild predator Tetrahymena vorax increases in size to the extent that it can then consume Paramecium aurelia, an even larger prey. When feeding only on bacteria, however, Tetrahymena remains smaller and is unable to consume Paramecium. This trait‐mediated indirect effect leads to the predatory exclusion of Paramecium, while Tetrahymena and Colpidium coexist. Developmental expansions such as those underlying the interactions observed in our study are not limited to ciliates, such as Tetrahymena, but occur among many diverse taxa and may have a surprising degree of influence over the structure and dynamics of food webs.     

The role of predation for seasonal variability patterns among phytoplankton and ciliates

Investigating the mechanisms which underlie the biomass fluctuations of populations and communities is important to better understand the processes which buffer community biomass in a variable environment. Based on long-term data of plankton biomass in Lake Constance (Bodensee), this study aims at explaining the different degree of synchrony among populations observed within two freshwater plankton groups, phytoplankton and ciliates. Established measures of temporal variability such as the variance ratio and cross-correlation coefficients were combined with first-order autoregressive models that allow estimating species interactions from time-series data. We found that predation was an important driver of the observed seasonal variability patterns in phytoplankton and ciliates, and that competitive interactions only played a subordinate role. In Lake Constance copepods and cladocerans, two major invertebrate predator groups, focus their grazing pressure at different times of the season. Model results suggested that compensatory dynamics detected in phytoplankton originate from the differential vulnerability of species to either one of these two predator groups. For ciliates model results advocated that synchrony among species occurs because ciliates tend to be vulnerable to both predator groups. Our findings underline the necessity of extending studies of community variability to multiple trophic levels because accounting for predator-prey interactions may often be more important than accounting for competitive interactions at one trophic level.     

Predation and searching efficiency of a ladybird beetle, Coccinella septempunctata Linnaeus in laboratory environment

The predation and searching efficiency of fourth instar of predatory C. septempunctata at various densities of mustard aphid, Lipaphis erysimi (Kaltenbach) and predator was investigated under laboratory conditions. The feeding rate of predatory stage decreased at increased prey- and predator densities. Highest percent (92.80%) prey consumption was observed at initial prey density and lowest percent (40.86%) prey consumption at highest prey density by the fourth instar, though the total prey consumption increased with increase in either prey- or predator densities. Similarly, the individual prey consumption was also highest at initial predator density and lowest at highest predator density owing to the mutual interference between the predators at higher densities. The area of discovery (searching efficiency) also decreased with increase in prey- and predator densities. Handling time of predator was highest at lower prey densities, which decreased with increased prey densities. The highest percentage of prey consumption at the prey density of 50 revealed that 1:50 predator-prey ratio was the best to reduce the pest population.     

Top–down limits on prey populations may be more severe in larger prey species,despite having fewer predators

Variation in the vulnerability of herbivore prey to predation is linked to body size, yet whether this relationship is size‐nested or size‐partitioned remains debated. If size‐partitioned, predators would be focused on prey within their preferred prey size range. If size‐nested, smaller prey species should become increasingly more vulnerable because increasingly more predators are capable of catching them. Yet, whether either of these strategies manifests in top–down prey population limitation would depend both on the number of potential predator species as well as the total mortality imposed. Here we use a rare ecosystem scale ‘natural experiment’ comparing prey population dynamics between a period of intense predator persecution and hence low predator densities and a period of active predator protection and population recovery. We use three decades of data on herbivore abundance and distribution to test the role of predation as a mechanism of population limitation among prey species that vary widely in body size. Notably, we test this within one of the few remaining systems where a near‐full suite of megaherbivores occur in high density and are thus able to include a thirtyfold range in herbivore body size gradient. We test whether top–down limitation on prey species of particular body size leads to compositional shifts in the mammalian herbivore community. Our results support both size‐nested and size‐partitioning predation but suggest that the relative top–down limiting impact on prey populations may be more severe for intermediate sized species, despite having fewer predators than small species. In addition we show that the gradual recovery of predator populations shifted the herbivore community assemblage towards large‐bodied species and has led to a community that is strongly dominated by large herbivore biomass.     

Food selection by bacterivorous protists: insight from the analysis of the food vacuole content by means of fluorescence in situ hybridization

A modified fluorescence in situ hybridization (FISH) method was used to analyze bacterial prey composition in protistan food vacuoles in both laboratory and natural populations. Under laboratory conditions, we exposed two bacterial strains (affiliated with beta- and gamma-Proteobacteria -- Aeromonas hydrophila and Pseudomonas fluorescens, respectively) to grazing by three protists: the flagellates Bodo saltans and Goniomonas sp., and the ciliate Cyclidium glaucoma. Both flagellate species preferably ingested A. hydrophila over P. fluorescens, while C. glaucoma showed no clear preferences. Differences were found in the digestion of bacterial prey with B. saltans digesting significantly faster P. fluorescens compared to two other protists. The field study was conducted in a reservoir as part of a larger experiment. We monitored changes in the bacterial prey composition available compared to the bacteria ingested in flagellate food vacuoles. Bacteria detected by probe HGC69a (Actinobacteria) and R-BT065 were negatively selected by flagellates. Bacteria detected by probe CF319a were initially positively selected but along with a temporal shift in bacterial cell size, this trend changed to negative selection during the experiment. Overall, our analysis of protistan food vacuole content indicated marked effects of flagellate prey selectivity on bacterioplankton community composition.     

Individual base model of predator-prey system shows predator dominant dynamics

Individual base model of predator-prey system is constructed. Both predator and prey species have age structure and cohorts of early reproductive age have competitive advantage. The model has linear functional response in predation behavior and includes the effect of interference among predators and delay of population growth from resource intake, not by functional response but by calculation procedure. Each foraging action is calculated successively and surplus or scarce of acquired resources is interpreted into population size through individual birth and death. This model shows that biomass of prey killed by predator is dependent on demand of predator and that heterogeneity in predator population is essential in persistency and stability of predator-prey system. Heterogeneity of predator makes predator individuals of less competing ability die rapidly. Rapid death of weak individuals causes rapid decrease of total demand of predator and that makes enough room for survived predators. Therefore, the biomass of killed prey is dependent on predator's demand. As young or infant population of predator are the more vulnerable to shortage of prey, and when many of them cannot survive to reproductive age, they can stabilize the system by wasting excessive prey with only temporal numerical increase of predator population.     

A Note on the Possible Ecological Significance of Chemotaxis in Certain Ciliated Protozoa1

ABSTRACT. A heat-stable chemoattractant has been isolated from bacterial cultures. This component has a molecular weight in the range of 500–1000 daltons, is produced by both Gram-positive and Gram-negative bacteria, and serves equally well as an attractant for both the bacterial feeding Paramecium and for its natural predator, Didinium. Aspects of the ecological relationship between bacterial feeding ciliates and their ciliate predators are briefly discussed with respect to responses of both predator and prey to such a common chemotactic bacterial factor.     

Experimental evidence for emergent facilitation: promoting the existence of an invertebrate predator by killing its prey

1. Recent theoretical insights have shown that predator species may help each other to persist by size-selective foraging on a shared prey. By feeding on a certain prey stage, a predator may induce a compensatory response in another stage of the same prey species, thereby favouring other predators; a phenomenon referred to as emergent facilitation. 2. To test whether emergent facilitation may occur in a natural system, we performed an enclosure experiment where we mimicked fish predation by selectively removing large zooplankton and subsequently following the response of the invertebrate predator Bythotrephes longimanus. 3. Positive responses to harvest were observed in the biomass of juvenile individuals of the dominant zooplankton Holopedium gibberum and in Bythotrephes densities. Hence, by removing large prey, we increased the biomass of small prey, i.e. stage-specific biomass overcompensation was present in the juvenile stage of Holopedium. This favoured Bythotrephes, which preferentially feed on small Holopedium. 4. We argue that the stage-specific overcompensation occurred as a result of increased per capita fecundity of adult Holopedium and as a result of competitive release following harvest. If shown to be common, emergent facilitation may be a major mechanism behind observed predator extinctions and patterns of predator invasions.     

Trophic coupling within the microbial food web: a study with fine temporal resolution in a eutrophic freshwater ecosystem

• 1 The major components of the microbial food web (dissolved organic carbon, bacteria, protozoa, rotifers and algae) of Priest Pot, a small freshwater pond, were investigated over a period of 5 months. Water samples were collected from the epilimnion every 1–3 days.
• 2 Time series analysis helped identify the trophic relationships within the planktonic community. There were strong predator—prey relationships between both ciliates and large rotifers and the total nanoplankton, between rotifers and small ciliates and between the total microzooplankton community and phytoplankton. Small rotifers and small ciliates probably share the same food resources. The major bacterivores in the system could not be identified with our methods. However, our previous results point to a dominating role of nanoplanktonic (2–20 μm) heterotrophic protists as the main grazers of bacteria.
• 3 Rotifers are the major type of metazoan zooplankton in Priest Pot; crustacean zooplankton are absent from the community. Bacterial production probably reaches rotifers via a variety of pathways: there may be a three-step link from bacteria to bacterivorous nanoplankton, to ciliates and then to rotifers. Furthermore, a strong correlation between the nanoplankton and rotifers suggests a direct link between these components, implying a much shorter pathway. Some of the rotifers in the pond can graze directly on bacteria, and many of the larger planktonic organisms (large ciliates and rotifers) are algivores. The latter two predator—prey relationships suggest an efficient transfer of bacterial and primary production to higher trophic levels.

     

Preference and consumption of Macrolophus pygmaeus preying on mixed instar assemblages of Myzus persicae

This study examines the effects of changes in the prey frequency and abundance on prey selection among the four instars of Myzus persicae by the predator Macrolophus pygmaeus under laboratory conditions. The central hypothesis was that M. pygmaeus will become more selective as prey density increases. It was also observed that M. pygmaeus can occasionally abandon a prey item that had already been killed (non-consumptive prey mortality). It was assumed that the frequency of this behavior would increase with the prey size and prey density. For these purposes prey selection was evaluated by simultaneously presenting all instars of M. persicae to the predator in equal proportions and at increasing densities. M. pygmaeus showed a higher predation rate and a higher preference for smaller prey instars at all prey densities. However, if the predation rate by the predator is expressed in terms of biomass consumed, then biomass gain was higher when feeding on the larger instars of M. persicae. The prey selectivity was indicated by the total prey mortality (consumptive plus non-consumptive prey mortality) as well as by the non-consumptive prey mortality, was associated with relatively high prey densities, depending on the prey instar. Therefore, we argued that the predatory impact of M. pygmaeus on the various instars of the aphid depends not only on prey traits but also on their relative abundance in a patch. Observed decreases in biomass gain from larger prey were likely the result of high prey availability at densities before saturation, which might have caused confusion in the predator’s prey selection.     

Macroecological patterns and niche structure in a new marine food web

The integration of detailed information on feeding interactions with measures of abundance and body mass of individuals provides a powerful platform for understanding ecosystem organisation. Metabolism and, by proxy, body mass constrain the flux, turnover and storage of energy and biomass in food webs. Here, we present the first food web data for Lough Hyne, a species rich Irish Sea Lough. Through the application of individual-and size-based analysis of the abundance-body mass relationship, we tested predictions derived from the metabolic theory of ecology. We found that individual body mass constrained the flux of biomass and determined its distribution within the food web. Body mass was also an important determinant of diet width and niche overlap, and predator diets were nested hierarchically, such that diet width increased with body mass. We applied a novel measure of predator-prey biomass flux which revealed that most interactions in Lough Hyne were weak, whereas only a few were strong. Further, the patterning of interaction strength between prey sharing a common predator revealed that strong interactions were nearly always coupled with weak interactions. Our findings illustrate that important insights into the organisation, structure and stability of ecosystems can be achieved through the theoretical exploration of detailed empirical data.     

An emerging infectious disease triggering large-scale hyperpredation

Hyperpredation refers to an enhanced predation pressure on a secondary prey due to either an increase in the abundance of a predator population or a sudden drop in the abundance of the main prey. This scarcely documented mechanism has been previously studied in scenarios in which the introduction of a feral prey caused overexploitation of native prey. Here we provide evidence of a previously unreported link between Emergent Infectious Diseases (EIDs) and hyperpredation on a predator-prey community. We show how a viral outbreak caused the population collapse of a host prey at a large spatial scale, which subsequently promoted higher-than-normal predation intensity on a second prey from shared predators. Thus, the disease left a population dynamic fingerprint both in the primary host prey, through direct mortality from the disease, and indirectly in the secondary prey, through hyperpredation. This resulted in synchronized prey population dynamics at a large spatio-temporal scale. We therefore provide evidence for a novel mechanism by which EIDs can disrupt a predator-prey interaction from the individual behavior to the population dynamics. This mechanism can pose a further threat to biodiversity through the human-aided disruption of ecological interactions at large spatial and temporal scales.     

A cross‐system meta‐analysis reveals coupled predation effects on prey biomass and diversity

Predator diversity and abundance are under strong human pressure in all types of ecosystems. Whereas predator potentially control standing biomass and species interactions in food webs, their effects on prey biomass and especially prey biodiversity have not yet been systematically quantified. Here, we test the effects of predation in a cross‐system meta‐analysis of prey diversity and biomass responses to local manipulation of predator presence. We found 291 predator removal experiments from 87 studies assessing both diversity and biomass responses. Across ecosystem types, predator presence significantly decreased both biomass and diversity of prey across ecosystems. Predation effects were highly similar between ecosystem types, whereas previous studies had shown that herbivory or decomposition effects differed fundamentally between terrestrial and aquatic systems based on different stoichiometry of plant material. Such stoichiometric differences between systems are unlikely for carnivorous predators, where effect sizes on species richness strongly correlated to effect sizes on biomass. However, the negative predation effect on prey biomass was ameliorated significantly with increasing prey richness and increasing species richness of the manipulated predator assemblage. Moreover, with increasing richness of the predator assemblage present, the overall negative effects of predation on prey richness switched to positive effects. Our meta‐analysis revealed strong general relationships between predator diversity, prey diversity and the interaction strength between trophic levels in terms of biomass. This study indicates that anthropogenic changes in predator abundance and diversity will potentially have strong effects on trophic interactions across ecosystems. Synthesis The past centuries we have experienced a dramatic loss of top–predator abundance and diversity in most types of ecosystems. To understand the direct consequences of predator loss on a global scale, we quantitatively summarized experiments testing predation effects on prey communities in a cross‐system meta‐analysis. Across ecosystem types, predator presence significantly decreased both biomass and diversity of prey, and predation effects were highly similar. However, with increasing predator richness, the overall negative effects of predation on prey richness switched to positive ones. Anthropogenic changes in predator communities will potentially have strong effects on prey diversity, biomass, and trophic interactions across ecosystems.     

Systematic deviations from linear size spectra of lake fish communities are correlated with predator–prey interactions and lake‐use intensity

Size structure of organisms at logarithmic scale (i.e. size spectrum) can often be described by a linear function with a negative slope; however, substantial deviations from linearity have often been found in natural systems. Theoretical studies suggest that greater nonlinearity in community size spectrum is associated with high predator–prey size ratios but low predator–prey abundance ratios; however, empirical evaluation of the effects of predator–prey interactions on nonlinear structures remains scarce. Here, we aim to empirically explore the pattern of the size‐specific residuals (i.e. deviations from the linear regression between the logarithmic fish abundance and the logarithmic mean fish size) by using size spectra of fish communities in 74 German lakes. We found that nonlinearity was strong in lakes with high predator–prey abundance ratios but at low predator–prey size ratios. More specifically, our results suggest that only large predators, even if occurring in low abundances, can control the density of prey fishes in a broad range of size classes in a community and thus promote linearity in the size spectrum. In turn, the lack of large predator fishes may cause high abundances of fish in intermediate size classes, resulting in nonlinear size spectra in these lakes. Moreover, these lakes were characterized by a more intense human use including high fishing pressure and high total phosphorus concentrations, which have negative impacts on the abundance of large, predatory fish. Our findings indicate that nonlinear size spectra may reflect dynamical processes potentially caused by predator–prey interactions. This opens a new perspective in the research on size spectrum, and can be relevant to further quantify the efficiency of energy transfer in aquatic food webs.