Obligate Predators Research Articles

Unbalanced predatory communities and a lack of microbial degraders characterize the microbiota of a highly sewage-polluted Eastern-Mediterranean stream.

Wastewater pollution of water resources takes a heavy toll on humans and on the environment. In highly polluted water bodies, self-purification is impaired, as the capacity of the riverine microbes to regenerate the ecosystem is overwhelmed. To date, information on the composition, dynamics and functions of the microbial communities in highly sewage-impacted rivers is limited, in particular in arid and semi-arid environments. In this year-long study of the highly sewage-impacted Al-Nar/Kidron stream in the Barr al-Khalil/Judean Desert east of Jerusalem, we show, using 16S and 18S rRNA gene-based community analysis and targeted qPCR, that both the bacterial and micro-eukaryotic communities, while abundant, exhibited low stability and diversity. Hydrolyzers of organics compounds, as well as nitrogen and phosphorus recyclers were lacking, pointing at reduced potential for regeneration. Furthermore, facultative bacterial predators were almost absent, and the obligate predators Bdellovibrio and like organisms were found at very low abundance. Finally, the micro-eukaryotic predatory community differed from those of other freshwater environments. The lack of essential biochemical functions may explain the stream's inability to self-purify, while the very low levels of bacterial predators and the disturbed assemblages of micro-eukaryote predators present in Al-Nar/Kidron may contribute to community instability and disfunction.

Response of Ips bark beetles (Coleoptera: Curculionidae: Scolytinae) to prescribed fire in South Carolina

Ips bark beetles (Ips species; Coleoptera: Curculionidae: Scolytinae) play an important role in forest ecosystems by attacking injured, stressed, or dying pine (Pinus) trees. Management of Ips bark beetles has largely focused on prevention by maintaining tree health through silvicultural treatments including thinning to reduce basal area or prescribed fire. However, there is conflicting evidence of whether prescribed fire leads to increased bark beetle populations and, subsequently, increased tree mortality. Recent outbreaks of Ips bark beetles in the southeastern U.S. have caused renewed interest in the interactions between pine management and bark beetle populations. We monitored Ips populations following prescribed fire in intensively managed pine stands and quantified Ips damage. We captured over 361,000 Ips bark beetles with I. avulsus being the most abundant of the three species collected. While we found high variation among Ips populations across study sites and years, there was a significant increase in bark beetle populations after prescribed fire followed by a significant decline the next year. While Ips populations increased up to three-fold after prescribed burns, damage ratings in burned stands were no different than unburned stands which had significantly lower bark beetle populations. Unburned stands had higher basal area and a higher average tree damage rating, but the lowest number of Ips bark beetles. A total of 14,071 facultative (e.g., Monochamus = 9775) and obligate (e.g., Thanasimus dubius = 4296) predators were collected during the study across all sites and the ratio of predators:Ips also significantly increased following prescribed fire. These data suggest that while Ips are ubiquitous and abundant in pine forests being managed with prescribed fire, natural mechanisms such as predator activity mediate their overall damage and impact to southeastern pine stands.

Carcass provisioning and intra-guild risk avoidance between two sympatric large carnivores

Apex carnivores that rely primarily on predation play a central but complex role within scavenging ecology by potentially suppressing intra-guild competitors, but also facilitating them by providing a reliable supply of carrion. We investigated the competitive relationship between sympatric wolves (Canis lupus) and wolverines (Gulo gulo) in Norway across three seasons. We deployed remote cameras at fresh wolf kills (n = 29) and built Bayesian generalized linear mixed models (GLMMs) to explore the use of fresh wolf-killed prey by sympatric wolves and wolverines. Our results showed that wolves facilitated wolverines by providing scavenging opportunities. Biomass available from wolf kills was influenced by seasonal wolf prey preference and group size. Wolverines visited 100% of wolf kills in fall and winter, whereas only 18% in summer. We found that in winter, wolverines visited wolf kills 3.6 and 6.7 times more often than single wolves and wolf groups revisited their kills, and spent 10 and 25 times as much time at carcasses compared to single wolves and groups of wolves. Thus, wolverines played an important role in the depletion of wolf-killed prey, with potential effects on the scavenging behavior of other guild members. Understanding how globally threatened top predators may function as key species in scavenging processes is important to conservation as this may have community-wide cascading effects and support important ecosystem functions and services.Significance statementLarge carnivores serve a central role within scavenging ecology through the suppression and facilitation of intraguild competitors. The wolf, as an apex obligate predator, can provide a reliable supply of carrion, that can serve as an important resource to facultative scavengers. However, while facultative behavior helps to mitigate the effects of limited prey for scavengers, it can also increase exposure to competition and intraguild predation. Across three seasons, we explored the use of fresh wolf-killed prey by sympatric wolves and wolverines. Our findings reveal that wolves facilitate wolverines by providing scavenging opportunities, where biomass available from kills is influenced by wolves’ prey preference and group size. Wolverines, like wolves, utilized kills heavily during winter, when increased access to food is important to wolverine reproductive rates. Wolverines exhibited caching behavior, possibly reducing exposure to interspecific competition, while serving an important role in the depletion of carrion biomass.

Rove beetle communities (Coleoptera: Staphylinidae) in the rock dumps after coal mining

Rock dumps formed as a result of coal mining are unique model for studying the rove beetle communities. In total 60 species, 38 genera, 7 subfamilies of rove beetles were registered using soil traps. The greatest diversity of rove beetles was noted in forb-grass meadows (H` = 2.89) and pine plantations (H` = 2.39). The diversity of rove beetles was low (H` = 1.11) in birch and aspen-fir forests despite the greater number of revealed species. Only two eurytopic and mesophilic species (Drusilla canaliculata and Xantholinus linearis) were dominant in the rock dumps. Most of the rove beetles were eurytopic or forest species, zoophagous or mixophagous, mesophilous or hygrophilous. The number of eurytopic species and obligate predators increased, and the dynamic density of beetles decreased from older to younger dumps. The number of rove beetles depended on the soil temperature and humidity and the vegetation cover. Rove beetle communities are good indicators for monitoring the restoration of areas that have been technically degraded.

Trophic Position of the Species and Site Trophic State Affect Diet Niche and Individual Specialization: From Apex Predator to Herbivore.

Intra-species variability in isotopic niches, specifically isotopic total niche width (ITNW), isotopic individual niche width (IINW), and isotopic individual specialization (IIS), was studied using an innovative approach without sacrificing the vertebrates. Stable isotopes (δ13C, δ15N) in four body tissues differing in isotopic half-life were analyzed from four freshwater fish species representing different trophic positions. ITNW was widest for the apex predator (European catfish) and narrowest for the obligate predator (Northern pike). IINW exhibited a polynomial trend for the European catfish, Northern pike, and Eurasian perch (mesopredator), decreasing with body mass and increasing again after exceeding a certain species-dependent body mass threshold. Thus, for ectotherms, apex predator status is linked rather to its size than to the species. In herbivores (rudd), IINW increased with body mass. The IIS of predators negatively correlated with site trophic state. Therefore, eutrophication can significantly change the foraging behavior of certain species. We assume that the observed trends will occur in other species at similar trophic positions in either aquatic or terrestrial systems. For confirmation, we recommend conducting a similar study on other species in different habitats.

Арктический голец Salvelinus alpinus (Salmonidae) Пясинского залива

The purpose of this work is to analyze some age, size, and demographic parameters of the Arctic char in the Pyasinsky Bay of the Kara Sea. In the control catches, fish aged from 2 to 10 years were noted, females aged 5-7 years and males aged 4-7 years predominated. The commercial length of individuals in catches was 300-820 mm, weight - 320-8100 g. The sex ratio was approximately 60:40 in favor of females. At the age of 5 years, sexually mature individuals were noted, the maturation period was extended to 7-8 years. In Pyasinsky Bay, arctic char is an obligate predator. The diet included polar cod (Boreogadus saida), four-horned slingshot (Myoxocephalus quadricornis), smelt (Osmerus mordax dentex) and whitefish (genus Coregonus), represented mainly by vendace. Occasionally met muksun, and omul. Polar cod formed the basis of feeding – more than 69% of the frequency of occurrence. The introduction of Arctic char into aquaculture is promising for the Krasnoyarsk Territory.

Predatory Bacteria: A Novel Approach to Antibiotic Substitution

Antibiotics are an effective means of treating infectious bacterial diseases. However, the extensive use of antibiotics in the recent past has resulted in antibiotic-resistant bacteria which interfere with disease treatment and recovery. There is an urgent need for novel medicines to treat Gram-negative infections because there are not enough antibiotics in development to address projected and present demands. One novel approach to antibiotic alternatives is the use of living predatory bacteria which can naturally prey upon other Gram-negative bacteria. These predators are ubiquitous in a wide variety of manmade and natural environments and depending upon their feeding habits are classified as obligatory or facultative predators. All the obligate predators are classified under the umbrella terminology Bdellovibrio and like organisms (BALOs). Bdellovibrio bacteriovorus is by far the most studied BALOs and have long been recognized as a potential therapeutic, water clean-up, and biocontrol agent and source for discovering novel biotechnological tools for research. Antibiotics won’t be replaced by pills containing predatory microorganisms any time soon. However, understanding these bacteria could help us prepare for a time when many medications cannot treat infections that are multi-drug resistant. A paradigm change is necessary since it may appear counterintuitive to cure a bacterial infection by giving another bacterium. The burden of an infection on the host must be taken into account while evaluating the prospective implications of treating a Gram-negative bacterial infection with the administration of a live predatory bacterium.

Change in the Guild Structure of a Snake Community in Japan over 30 Years

Long-term field studies have provided evidence of worldwide declines in vertebrate populations, including amphibians and reptiles. However, studies investigating the decline in snake populations are scarce. As obligate predators, snakes play important roles in local ecosystems and are in turn affected by their food resources. In the present study we investigated the seasonal abundance of snakes and their food habits in a rice paddy field and compared the results of the present study with those of a previous study that was conducted at the same site approximately 30 years ago. Our results showed that Elaphe quadrivirgata, a dietary generalist snake that had been one of the dominant species at the site, has drastically declined and that Rhabdophis tigrinus and Gloydius blomhoffii have remained as dominant species. Furthermore, in the present study, most of the stomach contents of R. tigrinus consisted of the Japanese tree frog, Dryophytes japonicus, whereas three frog species, D. japonicus, Rana japonica, and Pelophylax porosus porosus, had been the main prey of R. tigrinus 30 years ago. In addition, fewer individuals of R. tigrinus and G. blomhoffii contained food in their stomachs than 30 years ago. Our findings imply that a change in prey availability may have affected the guild structure of snakes at this study site, although further investigation is necessary to verify the reasons for the decline of the dietary generalist, E. quadrivirgata.

Mosquitoes Eating Mosquitoes: How Toxorhynchites amboinensis, Psorophora ciliata, and Sabethes cyaneus (Diptera: Culicidae) Capture Prey.

High-speed microcinematography was used to elucidate the details of prey capture by the larvae of three predatory mosquito species. The obligate predators Toxorhynchites amboinensis and Psorophora ciliata exhibited a high degree of convergence as both utilize three essential elements: 1) abdominally-generated hemostatic pressure to propel the head towards the prey; 2) lateral palatal brushes (LPB) opening and fanning into anterior-directed basket-like arrangements; 3) simultaneously with the LPB-basket formation, the wide opening of sharp-toothed mandibles. Thus, LPBs and mandibles are used for prey capture by both species. The facultative predator Sabethes cyaneus utilizes a vastly different prey-capture mechanism that involves ventro-lateral body arching and scooping of prey with axially pointed siphons into the grasp of open maxillae bearing elongate apical teeth. Prey consumption, which is typically incomplete in this species, then involves the action of teeth on the mandibles which cut into the held prey. Although prey consumption is incomplete, simple experiments show that Sa. cyaneus do gain nutritionally from consuming mosquito larvae and that they do discriminate heterospecific from conspecific larvae and actively approach heterospecific mosquito prey. These findings indicate that independent evolutionary pathways have produced diverse predatory behaviors and morphologies in aquatic environments where the immature stages of mosquitoes co-occur.

Spider Parental Care and Awe-Inspiring Egg Sac (Cocoon)

Spiders (Arachnida, Araneae) represent one of the largest groups of organisms on Earth with more than 45,000 recorded species found in nearly all terrestrial communities. In these communities, spiders are obligate predators and generalist consumers regulating the density of pests. Spiders have a stupefying array of prey hunting strategies ranging from ambushing to the use of complex silk nares. Spider silk is incredibly tough and can be used for many applications such as wrapping and immobilization, catching prey, as dragline to connect spiders to the web, as ballooning to aid dispersal of juveniles, as shelters in burrows, for mating, and as egg sacs (or cocoons). Typically, spider egg sacs are multilayered, complex structures that physically protect the eggs and hatchlings against parasitoids, predators as well as changing temperatures. Much research has been undertaken to elucidate the ecological role of spiders and the mechanical characteristics of silks. However, few studies have examined the parental care of spiders and the role of egg sacs. This review goes into great detail about spider parental care and the functions of egg sacs.

Thysanoptera as predators: their diversity and significance as biological control agents.

Some species of thrips have evolved predatory behaviours, with their reported prey range including thrips, mites, scale-insects, whitefly, psyllid nymphs, and eggs of moths and beetles. However, our current understanding of the biology and potential of these thrips as biological control agents is insufficient, limiting our understanding of their role in pest management. This paper assesses published information on the diversity of predatory thrips and the available biological data on their diet breadth. About 100 species within three different families - Aeolothripidae, Phlaeothripidae and Thripidae - seem likely to be predatory, although some conclusions are speculative. Both facultative and obligate predators can be found among the species with prey records. Obligate predators feed primarily on arthropod tissues, whereas facultative predators consume plant tissues in addition to prey. Quantitative information regarding developmental biology, longevity and fecundity, was obtained for only 11 species. Obligate predators, particularly members of Scolothrips and Franklinothrips, are efficient biological control agents. To better augment the use of predatory thrips in pest management, further increased investment in mass rearing, life history and ecological studies, including interactions with other organisms and their habitat, are needed on these natural enemies. © 2022 Society of Chemical Industry.

Bdellovibrio lyse multiple pathogenic bacteria and protect crucian carp Carassius auratus gibelio from Aeromonas veronii infections

Bdellovibrio are obligate predators of Gram-negative bacteria that could parasitize other bacteria and cause their lysis. It has been considered as a potential probiotic and may replace antibiotics. In order to better explore the inhibitory activity of Bdellovibrio against common aquatic pathogens and its potential use in aquaculture, the Aeromonas hydrophilia was used as the host bacteria for culturing Bdellovibrio. An absolute qPCR detection system with specific primers were also established for the amount determination of Bdellovibrio sp.. The morphological characteristics of the cultured Bdellovibrio sp. were detected by scanning electron microscope. By detecting absorbance at OD600nm and double layer plates plaque experiments, the Bdellovibrio sp. exhibited a certain lysis effect on the selected aquatic pathogens including Aeromonas veronii, Aeromonas hydrophil ia, Vibrio fluvialis, Escherichia coli, Vibrio anguillarum, Vibrio cholerae and Citrobacter freundii. Adding the cultured Bdellovibrio sp. to water could significantly reduce the mortality rate of Carassius auratus caused by the infections with A. vironii. Meanwhile, the amount of Bdellovibrio sp. on intestines and gills of C. auratus could increase over time after treatment, indicating Bdellovibrio may colonized in fish. The present study revealed that Bdellovibrio may play important roles as an alternative to antibiotics in protecting aquatic animals from pathogenic bacteria. These results will provide a certain theoretical basis for the application of Bdellovibrio in aquaculture.

Generalization of threats attributed to large carnivores in areas of high human-wildlife conflict.

Fear-induced generalization of threats to noninimical stimuli is a behavioral tendency of humans to minimize exposure to potential threats. In human-carnivore conflict zones, people often generalize their fear of predation by obligate carnivores to nonobligate carnivores despite differences in species' predation rates. We investigated the effect of a perceived threat of large obligate carnivores to livestock on tolerance and perception of striped hyena (Hyaena hyaena) in an area of high human-carnivore conflict. We surveyed 197 households through asemistructured questionnaire to determine people's perception and tolerance of striped hyenas in Sathyamangalam and Mudumalai Tiger Reserves after identification of the current distribution range of hyena determined through camera trap and sign surveys. Through the random forest algorithm, we modeled the level of tolerance of striped hyena as a function of loss of livestock to predation and from disease, the perceived threat of predation by hyena, and other socioeconomic attributes. Animal husbandry was a major source of income but was severely affected by livestock loss due to predation and disease. Sixty-nine percent of people were uncertain about predatory behavior of hyena; out of that, 23% reported a negative conservation attitude. Only 6 instances of hyena depredation on livestock and 2 on dogs were reported. Our model confirmed that economic instability associated with increased loss to predation and disease, livestock dependency, and a decrease in family annual income negatively affected people's tolerance of hyena. Perceptual uncertainty related to predatory behavior of hyena also negatively affected people's tolerance. In our study area, economic instability and perceptual uncertainty led to generalization of fear of large carnivores to a nonobligate predator. Such generalization may affect the attitude of people toward many other species. Understanding the role of economic instability and perceptual uncertainty should facilitate conservation of species, such as the hyena, that are vulnerable to false generalization.

Mutualistic interplay between bacteriophages and bacteria in the human gut.

Bacteriophages (phages) are often described as obligate predators of their bacterial hosts, and phage predation is one of the leading forces controlling the density and distribution of bacterial populations. Every 48 h half of all bacteria on Earth are killed by phages. Efficient killing also forms the basis of phage therapy in humans and animals and the use of phages as food preservatives. In turn, bacteria have a plethora of resistance systems against phage attack, but very few bacterial species, if any, have entirely escaped phage predation. However, in complex communities and environments such as the human gut, this antagonistic model of attack and counter-defence does not fully describe the scope of phage-bacterium interactions. In this Review, we explore some of the more mutualistic aspects of phage-bacterium interactions in the human gut, and we suggest that the relationship between phages and their bacterial hosts in the gut is best characterized not as a fight to the death between enemies but rather as a mutualistic relationship between partners.

Cross-kingdom interactions and functional patterns of active microbiota matter in governing deadwood decay.

Microbial community members are the primary microbial colonizers and active decomposers of deadwood. This study placed sterilized standardized beech and spruce sapwood specimens on the forest ground of 8 beech- and 8 spruce-dominated forest sites. After 370 days, specimens were assessed for mass loss, nitrogen (N) content and 15N isotopic signature, hydrolytic and lignin-modifying enzyme activities. Each specimen was incubated with bromodeoxyuridine (BrdU) to label metabolically active fungal and bacterial community members, which were assessed using amplicon sequencing. Fungal saprotrophs colonized the deadwood accompanied by a distinct bacterial community that was capable of cellulose degradation, aromatic depolymerization, and N2 fixation. The latter were governed by the genus Sphingomonas, which was co-present with the majority of saprotrophic fungi regardless of whether beech or spruce specimens were decayed. Moreover, the richness of the diazotrophic Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium group was significantly correlated with mass loss, N content and 15N isotopic signature. By contrast, presence of obligate predator Bdellovibrio spp. shifted bacterial community composition and were linked to decreased beech deadwood decay rates. Our study provides the first account of the composition and function of metabolically active wood-colonizing bacterial and fungal communities, highlighting cross-kingdom interactions during the early and intermediate stages of wood decay.

Сукцессии зоопланктона малых мелководных озер после проведения мероприятий по экореабилитации

The need for eco-rehabilitation of water bodies affected by strong anthropogenic influence increases greatly with an increase in the area of urbanized territories. The effectiveness of such measures may be evaluated by studying successions of hydrobiont communities in a rehabilitated water body. In 2017, actions were taken to rehabilitate the Lebyazhye Lakes system (Kazan), which included deepening the basins of Bolshoe Lebyazhye Lake and Svetloe Lebyazhye Lake to 4 m, isolating the bed of future basins with bentonite mats, supplying water from Izumrudnoye Lake through a pressure conduit, filling the lakes with water, and improvement of the shores. Then, in 2018–2021, zooplankton indicators of these lakes were studied and models of its food webs were proposed. After the work was completed, the lakes were successfully colonized by zooplankton: in 2018, the identified assemblage included 106 species, with 51 species of rotifers (46%), 36 species of cladocerans (34%) and 19 species of copepods (18%). However, in the absence of higher aquatic plants, a strong “bloom” of water was observed with the predominance of blue-green algae in the composition of phytoplankton and low water transparency. In 2020–2021, the zooplankton composition showed a decrease in the total number of species and the number of species in the sample, a decrease in the production in communities, a change in the composition of dominant species towards representatives of the genus Brachionus, and an increase in the variability of biomass dynamics. All these suggested instability and the gradual simplification of the community. In 2021, obligate predators disappeared from one of the lakes, which led to the shortening of food chains.

Predatory bacteria in the haemolymph of the cultured spiny lobster Panulirus ornatus.

Bdellovibrio and like organisms (BALOs) are Gram-negative obligate predators of other bacteria in a range of environments. The recent discovery of BALOs in the circulatory system of cultured spiny lobster P. ornatus warrants more investigation. We used a combination of co-culture agar and broth assays and transmission electron microscopy to show a Halobacteriovorax sp. strain Hbv preyed upon the model prey bacterium Vibrio sp. strain Vib. The haemolymph microbiome of juvenile P. ornatus was characterised following injection of phosphate buffered saline (control) or prey and/or predator bacteria for 3 d. The predator Hbv had no effect on survival compared to the control after 3 d. However, when compared to the prey only treatment group, lobsters injected with both prey and predator showed significantly lower abundance of genus Vibrio in the haemolymph bacterial community composition. This study indicates that predatory bacteria are not pathogenic and may assist in controlling microbial population growth in the haemolymph of lobsters.

Community and single cell analyses reveal complex predatory interactions between bacteria in high diversity systems

A fundamental question in community ecology is the role of predator–prey interactions in food-web stability and species coexistence. Although microbial microcosms offer powerful systems to investigate it, interrogating the environment is much more arduous. Here, we show in a 1-year survey that the obligate predators Bdellovibrio and like organisms (BALOs) can regulate prey populations, possibly in a density-dependent manner, in the naturally complex, species-rich environments of wastewater treatment plants. Abundant as well as rarer prey populations are affected, leading to an oscillating predatory landscape shifting at various temporal scales in which the total population remains stable. Shifts, along with differential prey range, explain co-existence of the numerous predators through niche partitioning. We validate these sequence-based findings using single-cell sorting combined with fluorescent hybridization and community sequencing. Our approach should be applicable for deciphering community interactions in other systems.

In vivo predation and modification of the Mediterranean fruit fly Ceratitis capitata (Wiedemann) gut microbiome by the bacterial predator Bdellovibrio bacteriovorus.

The Mediterranean fruit fly (the medfly) causes major losses of agricultural fruits. Its microbiome is mainly composed of various Enterobacteriaceae that contribute to nutrient acquisition and are associated with the fly's development. Moreover, the performance of males produced by the sterile insect technique is improved by providing mass-reared insects with specific gut bacteria. Bdellovibrio and like organisms (BALOs) are obligate predators of Gram-negative bacteria that efficiently preys upon diverse Enterobacteriaceae, making it a potential disruptor of the fly's microbiome. We hypothesized that the fly's microbiome can be targeted to control the insect. Inoculation of B. bacteriovorus as free-swimming or encapsulated cells into gut extracts significantly reduced gut bacterial abundance, sustaining predator survival. Similar treatments applied to adult flies showed that the predators also survived in the gut environment. While addition of the predators did not affect total gut bacterial abundance and end-point fly mortality, a shift in the gut community structure, measured by high-throughput community sequencing was observed. The bacterial predator of bacteria B. bacteriovorus can prey and survive invivo in the medfly gut. This study establishes the potential of BALOs to affect the microbiome of insect hosts.

The Functional Significance of Bacterial Predators.

Predation structures food webs, influences energy flow, and alters rates and pathways of nutrient cycling through ecosystems, effects that are well documented for macroscopic predators. In the microbial world, predatory bacteria are common, yet little is known about their rates of growth and roles in energy flows through microbial food webs, in part because these are difficult to quantify. Here, we show that growth and carbon uptake were higher in predatory bacteria compared to nonpredatory bacteria, a finding across 15 sites, synthesizing 82 experiments and over 100,000 taxon-specific measurements of element flow into newly synthesized bacterial DNA. Obligate predatory bacteria grew 36% faster and assimilated carbon at rates 211% higher than nonpredatory bacteria. These differences were less pronounced for facultative predators (6% higher growth rates, 17% higher carbon assimilation rates), though high growth and carbon assimilation rates were observed for some facultative predators, such as members of the genera Lysobacter and Cytophaga, both capable of gliding motility and wolf-pack hunting behavior. Added carbon substrates disproportionately stimulated growth of obligate predators, with responses 63% higher than those of nonpredators for the Bdellovibrionales and 81% higher for the Vampirovibrionales, whereas responses of facultative predators to substrate addition were no different from those of nonpredators. This finding supports the ecological theory that higher productivity increases predator control of lower trophic levels. These findings also indicate that the functional significance of bacterial predators increases with energy flow and that predatory bacteria influence element flow through microbial food webs.IMPORTANCE The word "predator" may conjure images of leopards killing and eating impala on the African savannah or of great white sharks attacking elephant seals off the coast of California. But microorganisms are also predators, including bacteria that kill and eat other bacteria. While predatory bacteria have been found in many environments, it has been challenging to document their importance in nature. This study quantified the growth of predatory and nonpredatory bacteria in soils (and one stream) by tracking isotopically labeled substrates into newly synthesized DNA. Predatory bacteria were more active than nonpredators, and obligate predators, such as Bdellovibrionales and Vampirovibrionales, increased in growth rate in response to added substrates at the base of the food chain, strong evidence of trophic control. This work provides quantitative measures of predator activity and suggests that predatory bacteria-along with protists, nematodes, and phages-are active and important in microbial food webs.