Prey Biomass Research Articles

Revisiting the paradigm of shark-driven trophic cascades in coral reef ecosystems.

Global overfishing of higher-level predators has caused cascading effects to lower trophic levels in many marine ecosystems. On coral reefs, which support highly diverse food webs, the degree to which top-down trophic cascades can occur remains equivocal. Using extensive survey data from coral reefs across the relatively unfished northern Great Barrier Reef (nGBR), we quantified the role of reef sharks in structuring coral reef fish assemblages. Using a structural equation modeling (SEM) approach, we explored the interactions between shark abundance and teleost mesopredator and prey functional group density and biomass, while explicitly accounting for the potentially confounding influence of environmental variation across sites. Although a fourfold difference in reef shark density was observed across our survey sites, this had no impact on either the density or biomass of teleost mesopredators or prey, providing evidence for a lack of trophic cascading across nGBR systems. Instead, many functional groups, including sharks, responded positively to environmental drivers. We found reef sharks to be positively associated with habitat complexity. In turn, physical processes such as wave exposure and current velocity were both correlated well with multiple functional groups, reflecting how changes to energetic conditions and food availability, or modification of habitat affect fish distribution. The diversity of species within coral reef food webs and their associations with bottom-up drivers likely buffers against trophic cascading across GBR functional guilds when reef shark assemblages are depleted, as has been demonstrated in other complex ecosystems.

Predator stomach contents can provide accurate indices of prey biomass

Abstract Diet-based annual biomass indices can potentially use predator stomach contents to provide information about prey biomass and may be particularly useful for species that are otherwise poorly sampled, including ecologically important forage fishes. However, diet-based biomass indices may be sensitive to underlying ecological dynamics between predators and prey, such as predator functional responses and changes in overlap in space and time. To evaluate these factors, we fit spatio-temporal models to stomach contents of five Atlantic herring (Clupea harengus) predators and survey catch data for predators and Atlantic herring. We identified drivers of variation in stomach contents, evaluated spatial patterns in stomach content data, and produced predator-specific indices of seasonal Atlantic herring biomass. After controlling for spatio-temporal processes and predator length, diet-based indices of biomass shared similar decadal trends but varied substantially between predators and seasons on shorter time scales. Diet-based indices reflected prey biomass more than prey availability, but weak correlations indicated that not all biological processes were controlled for. Results provide potential guidance for developing diet-based biomass indices and contribute to a body of evidence demonstrating the utility of predator diet data to provide information about relative prey biomass.

Low individual diet variation and high trophic niche overlap between the native polecat and invasive American mink

AbstractIntroductions of invasive species increase competitive interactions in a community proportionally to the similarity between the ecological niches of invasive and native species. Food represents one of the most important resources influencing competition, and differences in the trophic niche of native and invasive species can affect their ability to coexist. Moreover, in sympatric native and invasive species of similar body size and high sexual dimorphism, competition for food between individuals of the same sex may be higher than intraspecific competition between males and females, thus making the overall competitive interactions more complex. The aim of this study was to investigate the intraspecific and intrasexual competition for food resources between two sexually dimorphic mustelids, the native polecat Mustela putorius and invasive American mink Neovison vison, in riparian habitats of Białowieża Forest (Poland). Based on an analysis of 1215 scats, we studied the food habits of eight polecat and 24 mink individuals. The trophic niches of the polecat and mink were narrow and overlapped considerably. The diet of both predators was dominated by amphibians (mainly the common frog Rana temporaria), which comprised up to 94.1% and 89.7% of polecat and mink prey biomass, respectively. Individual diet variation was low within each sex group of both species. Interspecific similarities in body mass of the sexes and intraspecific differences in body mass between the sexes did not affect pairwise niche overlaps. However, we observed some mechanisms of food segregation between the sexes and species that reduced intra‐ and interspecies competition. Polecat males hunted more common toads Bufo bufo than polecat females, mink males and mink females. They also hunted larger frogs than polecat females, and the opposite pattern was recorded in mink. We conclude that two predator species exploiting abundant prey can coexist despite a very high overlap in their food niches.

Natural dimethyl sulfide gradients would lead marine predators to higher prey biomass

Finding prey is essential to survival, with marine predators hypothesised to track chemicals such as dimethyl sulfide (DMS) while foraging. Many predators are attracted to artificially released DMS, and laboratory experiments have shown that zooplankton grazing on phytoplankton accelerates DMS release. However, whether natural DMS concentrations are useful for predators and correlated to areas of high prey biomass remains a fundamental knowledge gap. Here, we used concurrent hydroacoustic surveys and in situ DMS measurements to present evidence that zooplankton biomass is spatially correlated to natural DMS concentration in air and seawater. Using agent simulations, we also show that following gradients of DMS would lead zooplankton predators to areas of higher prey biomass than swimming randomly. Further understanding of the conditions and scales over which these gradients occur, and how they are used by predators, is essential to predicting the impact of future changes in the ocean on predator foraging success.

Nestling diet and parental food provisioning in a declining mountain passerine reveal high sensitivity to climate change

Mountain ecosystems naturally experience strong seasonal weather variations leading to a brief peak in food availability that constrains bird reproduction. Climate change accentuates both the intra‐ and interannual weather variability, which in turn can reduce the predictability of food resources and hence impact population demography. Yet, relatively little is known about the influence of environmental factors on the breeding ecology of mountain birds. Here, we quantified the nestling diet and provisioning behaviour of the Alpine ring ouzel Turdus torquatus alpestris, an emblematic and declining thrush species typical of central European treeline ecotones, and relate these parameters to local weather conditions. Nests were monitored with camcorders to assess prey provisioning frequency and identify items delivered by parents to nestlings, as well as to estimate prey biomass. Our results indicate the prominence of earthworms (Lumbricidae) in the nestling diet, both in terms of abundance (80%) and biomass (90%). Elevated ambient temperatures negatively impacted both prey provisioning rates and biomass delivered to chicks by parents, while rainfall had a positive effect on the delivered biomass. The mean prey item biomass decreased throughout the breeding season, as did the proportion of earthworms in nestlings' diet. These findings highlight the key role played by local weather in parental provisioning behaviour, probably reflecting the low availability of the staple food source, earthworms, in warm and dry weather contexts. In particular, they underpin how climate alterations, notably increasing ambient temperatures and changing precipitation regimes, could impact mountain birds. Although effects on reproductive performance and population dynamics still ought to be studied, these results further our understanding of the ecological mechanisms potentially at play in the decline of wildlife inhabiting high‐elevation, climate‐sensitive ecosystems.

Comparison of big cat food habits in the Amazon piedmont forest in two Bolivian protected areas

The feeding habits of jaguar (Panthera onca) and puma (Puma concolor) were studied in the lower Tuichi, Hondo (PNANMI Madidi) and Quiquibey (RB Pilón Lajas) river valleys. A total of 122 large felid scats were collected and identified by an experienced local guide, with identifications later confirmed by morphometry. Of this total, 54 were also identified using associated tracks. For jaguar, prey species richness was 25 species considering all scats and 20 species only considering those scats associated with tracks. For puma, 28 and 22 prey species were obtained, respectively. The results indicate that there is low trophic niche overlap between both cats (0.46 and 0.44 respectively). The breadth of the trophic niche indicates that the jaguar is more specialized in its diet (0.28 and 0.42 respectively), mainly due to the high consumption of the white-lipped peccary (Tayassu pecari), which contributes significantly to jaguar prey biomass. The puma has a more generalist tendency (0.56 and 0.58 respectively), with the spotted paca (Cuniculus paca) the most frequently consumed prey and also contributing the most biomass. Despite the fact that both felines share most of their prey, jaguar and puma differ in the frequency of prey consumption.

Ocean acidification boosts reproduction in fish via indirect effects.

Ocean acidification affects species populations and biodiversity through direct negative effects on physiology and behaviour. The indirect effects of elevated CO2 are less well known and can sometimes be counterintuitive. Reproduction lies at the crux of species population replenishment, but we do not know how ocean acidification affects reproduction in the wild. Here, we use natural CO2 vents at a temperate rocky reef and show that even though ocean acidification acts as a direct stressor, it can indirectly increase energy budgets of fish to stimulate reproduction at no cost to physiological homeostasis. Female fish maintained energy levels by compensation: They reduced activity (foraging and aggression) to increase reproduction. In male fish, increased reproductive investment was linked to increased energy intake as mediated by intensified foraging on more abundant prey. Greater biomass of prey at the vents was linked to greater biomass of algae, as mediated by a fertilisation effect of elevated CO2 on primary production. Additionally, the abundance and aggression of paternal carers were elevated at the CO2 vents, which may further boost reproductive success. These positive indirect effects of elevated CO2 were only observed for the species of fish that was generalistic and competitively dominant, but not for 3 species of subordinate and more specialised fishes. Hence, species that capitalise on future resource enrichment can accelerate their reproduction and increase their populations, thereby altering species communities in a future ocean.

Ecological and environmental factors affecting the foraging activity of the White-bellied Heron Ardea insignis (Hume, 1878) in Bhutan

SummaryWhite-bellied Heron Ardea insignis (WBH) is critically endangered, but we lack data on many aspects of its basic ecology and threats to the species are not clearly understood. The goal of this study was to analyse WBH foraging microhabitat selection, foraging behaviour, and prey preferences in two river basins (Punatsangchhu and Mangdechhu) in Bhutan which are likely home to one of the largest remaining populations of WBH. We also explored the relationship between the relative abundance of the WBH and prey biomass catch per unit effort within four foraging river microhabitats (pool, pond, riffle and run). Prey species were sampled in 13 different 100-m thalweg lengths of the rivers using cast nets and electrofishing gear. Riffles and pools were the most commonly used microhabitats; relative abundance was the highest in riffles. The relative abundance of WBH and prey biomass catch per unit effort (CPUE) also showed a weak but significant positive correlation (rs = 0.22). The highest biomass CPUE was observed in riffles while the lowest was found in the ponds. From the 97 prey items caught by the WBH, 95% of the prey were fish. The WBH mainly exploited three genera of fish (Garra, Salmo, and Schizothorax) of which Schizothorax (64%) was the most frequently consumed. This study provides evidence in support of further protection of critical riverine habitat and fish resources for this heron. Regular monitoring of sand and gravel mining, curbing illegal fishing, habitat restoration/mitigation, and developing sustainable alternatives for local people should be urgently implemented by the government and other relevant agencies. Further study is also required for understanding the seasonal variation and abundance of its prey species in their prime habitats along the Punatsangchhu and Mangdechhu basins.

Effects of Hypoxia on Habitat Quality of Reservoir Largemouth Bass, Saugeye, and White Crappie

AbstractHypoxia induced by eutrophication and high summer temperatures is common in most Midwestern reservoirs, with uncertain effects on resident fishes. To evaluate the potential influence of low dissolved oxygen (DO) and high‐temperature conditions on habitat quality for three common sport fishes (Largemouth Bass Micropterus salmoides, saugeye [Walleye Sander vitreus × Sauger S. canadensis], and White Crappie Pomoxis annularis), we used spatially explicit bioenergetics models to measure growth rate potential (GRP) in three Ohio reservoirs of varying productivity and size. Dissolved oxygen, temperature, and prey biomass data were collected throughout each study reservoir during August in 2015 or 2016, when temperature and hypoxia levels were expected to be at or near their peak. Growth rate potential was generally low in hypoxic bottom waters and higher in the epilimnion due to prey concentrating above the oxycline. The amount of high‐quality habitat (HQH; i.e., GRP > 0 g/g/d) always declined owing to hypoxia; however, the extent of the decline depended on the modeled species and reservoir. Among species, hypoxia‐induced losses of HQH were small, differing by less than 8% in each reservoir. Among‐reservoir differences in HQH were greater; reductions in HQH ranged from 31.3% to 42.5% among species in the reservoir with the lowest DO (mean ± SE = 2.7 ± 0.04 mg/L) and highest gape‐appropriate prey biomass (5.1 ± 0.07 to 5.3 ± 0.07 g/m3), whereas they varied by 0.3–0.6% in the reservoir with the highest DO (5.1 ± 0.07 mg/L) and lowest gape‐appropriate prey biomass (0.2 ± 0.004 to 0.6 ± 0.01 g/m3). Even so, the reservoir with the lowest mean DO (i.e., greatest hypoxic extent) had the highest mean GRP and percentage HQH owing to high prey biomass. Collectively, our findings indicate that bottom hypoxia can reduce sport fish habitat quality in temperate reservoirs, but high prey biomass may mitigate the negative effect of hypoxia.

Effects of sea ice cover, temperature and predation on the stock dynamics of the key Arctic fish species polar cod Boreogadus saida

Climate change has large effects on population dynamics of fish species in high latitude ecosystems. Arctic fish stocks experience multiple pressures with changing abiotic living conditions and increased competition and predation from boreal species. However, there are many unknowns regarding how environmental change influences the dynamics of those populations. Here, we focused on the Barents Sea polar cod, a pan-Arctic zooplanktivorous key fish species physiologically and ecologically adapted to the presence of sea ice. We developed an age-resolved Bayesian state-space model of the dynamics of polar cod based on 30 yr of survey data (1986-2015). Using this model, we quantified how inter-annual changes in abundance were associated with abiotic variables (temperature and sea ice cover) and biotic variables (prey biomasses and a predation index). Using the model output, we used a hindcast scenario approach to investigate to which degree the observed variations in total population size were related to the abiotic or biotic variables. Our results showed that variation in abundance of young polar cod (ages 0 and 1) was best explained by abiotic variables while variation in the older age groups (ages 3 and 4) was best explained by predation. Hindcast scenarios showed that the abiotic variables had a more evident effect than predation on population dynamics, but none of the variables we considered could explain the drastic population decline observed in recent years. Our work shows the advantages of studying age-specific responses as a stepping stone to understand changes at the population level.

Comparison of prey availability for <i>Setophaga petechia gundlachi</i> (Aves: Parulidae) between two mangroves with different vegetation structure

The subspecies Setophaga petechia gundlachi is characterized by its specificity to mangrove habitat during the breeding season. The objective of this work was to compare the prey availability for S. petechia gundlachi during the breeding season in two mangrove sites with different vegetation structure in Havana, Cuba. To describe different levels of vegetation structure between both mangrove sites, we took several variables at 50 plots. Besides, we recorded the foraging behavior of S. petechia gundlachi to establish the design of prey availability sampling. We measured prey availability by using the branch clipping method, about 90 samples were collected at each mangrove site. Bajo de Santa Ana mangrove, with a lower height and diameter at breast height, showed evidence of deforestation. Laguncularia racemosa was the most used mangrove species for foraging and offered the highest biomass of available prey in the Laguna de Cobre-Itabo mangrove site. The greatest biomass of available prey was found at the Laguna de Cobre-Itabo, with a value of 5.1 (CI: 4.0 - 6.2) mg / 100 g of branch clipping. The Bajo de Santa Ana site had a lower value than expected 1.6 (CI: 0.8 - 1.7) mg / 100 g of branch clipping. The results provide evidence of the possible influence of mangrove vegetation structure changes on food availability for S. petechia gundlachi in this urban landscape.

History of co‐occurrence shapes predation effects on functional diversity and structure at low latitudes

Abstract The intensity of biotic interactions is hypothesized to increase towards the equator. Predation can have a stronger effect on prey composition, biomass and taxonomic diversity in tropical regions compared to higher latitudes. Whether predation also shapes patterns of functional diversity and structure of prey communities, however, has rarely been explored across a latitudinal gradient. History of co‐occurrence between predators and prey may also shape the outcome of the interaction, and increased novelty of introduced prey may lead to stronger trait responses. We investigated the influence of predation on trait responses of prey communities across 12 sites in four regions, from the sub‐arctic to the tropics, using a field experiment in invaded coastal habitats. Prey communities experienced ambient or reduced predation pressure during assembly, and the effect of predation on functional diversity and structure of whole communities as well as their native and introduced components was assessed. Overall, predation increased functional diversity in the tropics, but effects were evident for introduced species only. Predation also shaped functional structure of low‐latitude communities, but effects were strongest for native species in subtropical Mexico and introduced species in tropical Panama. Key traits driving shifts in functional structure were related to palatability and reproduction. In the tropics, the strong predation present under ambient conditions reduced the abundance of palatable traits in introduced prey relative to low‐predation treatments, while in the subtropics, ambient predation increased the abundance of palatable traits in native prey. Reproductive traits that require high parental or energetic investment were favoured in low‐predation treatments at both low‐latitude regions. In summary, predation had a stronger influence on trait responses of prey communities at lower latitudes where biotic interactions are expected to be more intense, but changes in functional diversity and structure hinged on co‐occurrence histories of local predators with native or introduced prey. This study provides one of the first examinations of the effects of predation on functional trait responses of prey communities across a latitudinal gradient.

Frequency of Occurrence and Ingested Biomass of Different Prey of the Barn Owl Tyto alba in an Island Ecosystem

The Barn Owl Tyto alba is commonly reported as a non-selective predator of small mammals, and its diet has been thoroughly analyzed also to assess the small mammal assembly composition in many study areas. The aim of this work was to analyze the diet of the Barn Owl in the Elba island through the analysis of 161 pellets collected in September 2020. Undigested fragments were isolated and compared with reference collections. We confirmed that the Barn Owl is a typical predator of field mice (62% of relative frequency), with synanthropic murid rodents as the second category of prey. The frequency of consumption of shrews increased by 9% with respect to the previous work, suggesting that the natural environment of Elba island is still in a good health status. Moreover, fragments of two newborn hares were detected, increasing the knowledge on the local trophic spectrum of the Barn Owl. Finally, the skull of a Geoffroy’s Myotis Myotis emarginatus confirmed the presence of this species in Elba island after over 60 years from the first unconfirmed record. Repeated studies conducted in the same study site may provide useful information on prey population trends and local environmental status.

Foraging habitat of North Atlantic right whales has declined in the Gulf of St. Lawrence, Canada, and may be insufficient for successful reproduction

Climate-induced changes in calanoid copepod (Calanusspp.) availability in traditional feeding areas might explain why a large proportion of the North Atlantic right whaleEubalaena glacialispopulation has fed in the Gulf of St. Lawrence (Canada) in recent years. However, little is known about the distribution of copepods in the gulf, and whether their abundance is sufficient to energetically sustain right whales. We used a mechanistic modelling approach to predict areas within the gulf that have foraging potential for adult female right whales, based on the annual energetic needs of resting, pregnant and lactating females, and their theoretical prey density requirements. We identified suitable foraging areas for right whales by coupling a foraging bioenergetics model with a 12 yr data set (2006-2017) describing the abundance and 3-dimensional distribution of late-stageCalanusspp. in the gulf. Prey densities in the southern gulf (from Shediac Valley to the Magdalen Islands) supported all 3 reproductive states in most (≥6) years. However, foraging habitat became progressively sparse in the southern gulf over time, with noticeably less suitable habitat available after 2014. Few other potentially suitable foraging areas were identified elsewhere in the gulf. Overall, the availability of foraging habitat in the gulf varied considerably between years, and was higher for resting females than for pregnant and lactating females. Our findings are consistent with the recent low calving rates, and indicate that prey biomass in the Gulf of St. Lawrence may be insufficient in most years to support successful reproduction of North Atlantic right whales.

Clicking throughout the year: sperm whale clicks in relation to environmental conditions off the west coast of South Africa

Knowledge of cetacean occurrence and behaviour in southern African waters is limited, and passive acoustic monitoring has the potential to address this gap efficiently. Seasonal acoustic occurrence and diel-vocalizing patterns of sperm whales in relation to environmental conditions are described here using passive acoustic monitoring data collected off the west coast of South Africa. Four autonomous acoustic recorders (AARs) were deployed on 3 oceanographic moorings from July 2014 to January 2017. Sperm whale clicks were detected year round in most recording sites, with peaks in acoustic occurrence in summer and late winter through spring. Diel-vocalizing patterns were detected in winter, spring and summer. Higher percentages of sperm whale clicks were recorded by AARs deployed at 1100 m water depth compared to those concurrently deployed at 850 and 4500 m, likely inferring that the whales exhibited some preference to water depths around 1100 m. Acoustic propagation modelling suggested a maximum detection range of 83 km in winter for sperm whale clicks produced at 1100 m. Random forest models classified daylight regime, sea surface height anomaly and month of the year as the most important predictors of sperm whale acoustic occurrence. The continuous acoustic occurrence of sperm whales suggests that the study area supports large biomasses of prey to sustain this species’ food requirements year round. This is the first study to describe the seasonal acoustic occurrence and diel-vocalizing patterns of sperm whales off the west coast of South Africa, extending knowledge of the species previously available only through whaling records.

Stock dynamics and predator–prey effects of Atlantic bonito and bluefish as top predators in the Black Sea

AbstractThis study uses surplus production model-based methods to assess data-poor stocks and estimate key reference points for Atlantic bonito (Sarda sarda) and bluefish (Pomatomus saltatrix) in the Black Sea. Our results demonstrate that the catch maximum sustainable yield (CMSY) method, using catch data only, yields similar results to the more accurate Bayesian Schaefer model (BSM) method, fitted with commercial catch-per-unit-of-effort data, and therefore is suitable in assessing data-poor stocks. We explore the ecological impacts of the two stocks on other commercial species and compare impacts of predation and fishing. Prior to 1995, the consumption of bonito and bluefish on anchovy, horse mackerel, and sprat exceeded the removal of those prey species by the fisheries. Later on, the trends reversed, with catches of prey species becoming more than three times higher than their predation by bonito and bluefish. Horse mackerel, the main prey of bluefish, has declined to critical levels since 1995, which is likely contributing to the general decline in bluefish, along with overfishing. Heavy fishing of bonito and bluefish has caused their current depleted states and combined with their significant impact on prey fish contributed to the ecosystem regime shift in the Black Sea. Due to the present steady positioning of low stock regimes, the recovery of the two stocks need decisive and possibly prolonged rebuilding measures, including a reduction in fishing pressure, efficient control of under-sized catch, and ensuring sufficient prey biomass availability.

Metabarcoding Fecal DNA Reveals Extent of Halichoerus grypus (Gray Seal) Foraging on Invertebrates and Incidence of Parasite Exposure

Halichoerus grypus (Gray Seal) is a top predator in New England waters, yet little data exists on seals foraging on invertebrate prey. We combined analysis of hard parts of scat samples (n = 98) from 2 breeding sites in Massachusetts with metabarcoding of fecal DNA. Invertebrates were detected in 4 samples via analysis of hard parts (4.1%) but in 35 samples (35.7%) using a metabarcoding approach. Metabarcoding also detected parasite DNA in 82.7% of scat samples. Nematode DNA (78.6%) was more prevalent than trematode (28.6%) or cestode (22.5%) DNA. Prey species, biomass, and diversity within each sample were not affected by seal sex, parasite exposure, or type of parasitic exposure. Previous diet studies may have underestimated the diversity and abundance of invertebrates in the diet of Gray Seals.

Timing of spring sea-ice retreat and summer seabird-prey associations in the northern Bering Sea

To understand the effect of an unusually early sea-ice retreat in the northern Bering Sea in the spring of 2018 on the marine ecosystem of the northern Bering Sea, we compared at-sea observations of seabird density and acoustic observations of prey (fish and zooplankton) biomass during shipboard surveys around St. Lawrence Island in the summers of 2017 and 2018. Densities of foraging seabirds in 2018 (piscivorous divers: 4.7 birds km−2, planktivorous divers: 5.1, shearwaters: 0.7, surface feeders: 6.6) were lower than those in 2017 (piscivorous divers: 14.7 birds km−2, planktivorous divers: 10.3, shearwaters: 11.9, surface feeders:11.9). Acoustically-determined prey biomass in 2018 (fish: 6.4 m2nmi−2, zooplankton: 2.3) was also lower than in 2017 (fish: 18.4 m2nmi−2, zooplankton: 5.5). Similarly, biomass of macrozooplankton (amphipods, euphausiids, Neocalanus spp. and Calanus marshallae) sampled using bongo nets was smaller in 2018. At scales of 5–30 km, correlations between the seabird density and prey biomass were weaker (−0.2–0.3 of Pearson's r) in 2018 than those in 2017 (0.4–0.9) for all seabirds except planktivorous divers. We suggest that the lack of sea ice, and the resulting lack of ice-edge phytoplankton blooms, may weaken trophic linkages by causing a low biomass of secondary consumers and hence a reduced density of seabirds.

The resilience of weed seedbank regulation by carabid beetles, at continental scales, to alternative prey

Carabids are generalist predators that contribute to the agricultural ecosystem service of seedbank regulation via weed seed predation. To facilitate adoption of this ecosystem services by farmers, knowledge of weed seed predation and the resilience of seedbank regulation with co-varying availability of alternative prey is crucial. Using assessments of the seedbank and predation on seed cards in 57 cereal fields across Europe, we demonstrate a regulatory effect on the soil seedbank, at a continental scale, by groups formed of omnivore, seed-eating (granivore + omnivore) and all species of carabids just prior to the crop-harvest. Regulation was associated with a positive relationship between the activity-density of carabids and seed predation, as measured on seed cards. We found that per capita seed consumption on the cards co-varied negatively with the biomass of alternative prey, i.e. Aphididae, Collembola and total alternative prey biomass. Our results underline the importance of weed seedbank regulation by carabids, across geographically significant scales, and indicate that the effectiveness of this biocontrol may depend on the availability of alternative prey that disrupt the weed seed predation.

Top‐down regulation of filamentous cyanobacteria varies among a raptorial versus current feeding copepod across multiple prey generations

Abstract Although considered a key functional trait, little is known about how zooplankton feeding mode affects top‐down regulation of phytoplankton communities. Indeed, copepods are expected to promote the dominance of toxic phytoplankton by selective removal of their edible competitors; however, empirical evidence comparing the effect among calanoid and cyclopoid copepods is lacking. We compared the top‐down effects of two copepods with contrasting feeding modes—the calanoid Notodiaptomus iheringi (current feeder) and the cyclopoid Thermocyclops decipiens (ambush feeder) — on the relative and absolute biomass of the filamentous cyanobacterium Raphidiopsis raciborskii co‐cultured with the nutritious eukaryotic phytoplankton Cryptomonas obovata in a week‐long laboratory assay. The current feeder had a stronger top‐down effect on the biomass of both prey throughout the experiment, with mass‐specific clearance rates 3–5× higher than ambush feeder. By the end of the experiment, the current feeder significantly reduced cyanobacteria biomass compared to controls while the ambush feeder did not. During the week‐long experiment, the current feeder switched from grazing on edible prey to cyanobacteria as the former became less abundant. Contrary to expectation, neither of the copepod species promoted cyanobacterial dominance by the end of the experiment. This is because both grazers, but especially the current feeder, initially increased but subsequently decreased the relative contribution of cyanobacteria to total phytoplankton biomass. Moreover, both copepods decreased the length of cyanobacteria filaments by c. 70% Current feeders can switch from edible prey to cyanobacteria when the abundance of shortened filaments surpasses the abundance of edible prey. While top‐down regulation of phytoplankton can be stronger for current feeding copepods, ambush feeding copepods can have a significant role during blooms by shortening cyanobacterial filaments. Hence, the broader role of contrasting copepod feeding traits on phytoplankton communities merits further study.