Selectivity underlies the dissociation between seasonal prey availability and prey consumption in a generalist predator

Generalist predators are capable of selective foraging, but are predicted to feed in close proportion to prey availability to maximize energetic intake especially when overall prey availability is low. By extension, they are also expected to feed in a more frequency‐dependent manner during winter compared to the more favourable foraging conditions during spring, summer and fall seasons. For 18 months, we observed the foraging patterns of forest‐dwelling wolf spiders from the genus Schizocosa (Araneae: Lycosidae) using PCR‐based gut‐content analysis and simultaneously monitored the activity densities of two common prey: springtails (Collembola) and flies (Diptera). Rates of prey detection within spider guts relative to rates of prey collected in traps were estimated using Roualdes’ c_st model and compared using various linear contrasts to make inferences pertaining to seasonal prey selectivity. Results indicated spiders foraged selectively over the course of the study, contrary to predictions derived from optimal foraging theory. Even during winter, with overall low prey densities, the relative rates of predation compared to available prey differed significantly over time and by prey group. Moreover, these spiders appeared to diversify their diets; the least abundant prey group was consistently overrepresented in the diet within a given season. We suggest that foraging in generalist predators is not necessarily restricted to frequency dependency during winter. In fact, foraging motives other than energy maximization, such as a more nutrient‐focused strategy, may also be optimal for generalist predators during prey‐scarce winters.     

The diet of whiting Merlangius merlangus in the western Baltic Sea

The diet of whiting Merlangius merlangus in the western Baltic Sea was investigated and compared to the diet in the southern North Sea. Clupeids were important prey in both areas, but especially in the western Baltic Sea where they constituted up to 90% of the diet of larger individuals. Gobies, brown shrimps and polychaetes were the main prey of juveniles in the western Baltic Sea, while a wider range of species were consumed in the North Sea. The shift to piscivory occurred at smaller sizes in the western Baltic Sea and the fish prey consumed was proportionately larger than in the southern North Sea. Estimates of prey abundance and food intake of M. merlangus are required to evaluate its predatory significance in the western Baltic Sea, but its diet suggests that it could be just as significant a fish predator here as in the southern North Sea.     

Toxin constraint explains diet choice,survival and population dynamics in a molluscivore shorebird

Recent insights suggest that predators should include (mildly) toxic prey when non-toxic food is scarce. However, the assumption that toxic prey is energetically as profitable as non-toxic prey misses the possibility that non-toxic prey have other ways to avoid being eaten, such as the formation of an indigestible armature. In that case, predators face a trade-off between avoiding toxins and minimizing indigestible ballast intake. Here, we report on the trophic interactions between a shorebird (red knot, Calidris canutus canutus) and its two main bivalve prey, one being mildly toxic but easily digestible, and the other being non-toxic but harder to digest. A novel toxin-based optimal diet model is developed and tested against an existing one that ignores toxin constraints on the basis of data on prey abundance, diet choice, local survival and numbers of red knots at Banc d''Arguin (Mauritania) over 8 years. Observed diet and annual survival rates closely fit the predictions of the toxin-based model, with survival and population size being highest in years when the non-toxic prey is abundant. In the 6 of 8 years when the non-toxic prey is not abundant enough to satisfy the energy requirements, red knots must rely on the toxic alternative.     

Using DNA metabarcoding for simultaneous inference of common vampire bat diet and population structure

Metabarcoding diet analysis has become a valuable tool in animal ecology; however, co‐amplified predator sequences are not generally used for anything other than to validate predator identity. Exemplified by the common vampire bat, we demonstrate the use of metabarcoding to infer predator population structure alongside diet assessments. Growing populations of common vampire bats impact human, livestock and wildlife health in Latin America through transmission of pathogens, such as lethal rabies viruses. Techniques to determine large‐scale variation in vampire bat diet and bat population structure would empower locality‐ and species‐specific projections of disease transmission risks. However, previously used methods are not cost‐effective and efficient for large‐scale applications. Using bloodmeal and faecal samples from common vampire bats from coastal, Andean and Amazonian regions of Peru, we showcase metabarcoding as a scalable tool to assess vampire bat population structure and feeding preferences. Dietary metabarcoding was highly effective, detecting vertebrate prey in 93.2% of the samples. Bats predominantly preyed on domestic animals, but fed on tapirs at one Amazonian site. In addition, we identified arthropods in 9.3% of samples, likely reflecting consumption of ectoparasites. Using the same data, we document mitochondrial geographic population structure in the common vampire bat in Peru. Such simultaneous inference of vampire bat diet and population structure can enable new insights into the interplay between vampire bat ecology and disease transmission risks. Importantly, the methodology can be incorporated into metabarcoding diet studies of other animals to couple information on diet and population structure.     

Global species–energy relationship in forest plots: role of abundance,temperature and species climatic tolerances

Aim To evaluate the strength of evidence for hypotheses explaining the relationship between climate and species richness in forest plots. We focused on the effect of energy availability which has been hypothesized to influence species richness: (1) via the effect of productivity on the total number of individuals (the more individuals hypothesis, MIH); (2) through the effect of temperature on metabolic rate (metabolic theory of biodiversity, MTB); or (3) by imposing climatic limits on species distributions. Location Global. Methods We utilized a unique ‘Gentry‐style’ 370 forest plots data set comprising tree counts and individual stem measurements, covering tropical and temperate forests across all six forested continents. We analysed variation in plot species richness and species richness controlled for the number of individuals by using rarefaction. Ordinary least squares (OLS) regression and spatial regressions were used to explore the relative performance of different sets of environmental variables. Results Species richness patterns do not differ whether we use raw number of species or number of species controlled for number of individuals, indicating that number of individuals is not the proximate driver of species richness. Productivity‐related variables (actual evapotranspiration, net primary productivity, normalized difference vegetation index) perform relatively poorly as correlates of tree species richness. The best predictors of species richness consistently include the minimum temperature and precipitation values together with the annual means of these variables. Main conclusion Across the world's forests there is no evidence to support the MIH, and a very limited evidence for a prominent role of productivity as a driver of species richness patterns. The role of temperature is much more important, although this effect is more complex than originally assumed by the MTB. Variation in forest plot diversity appears to be mostly affected by variation in the minimum climatic values. This is consistent with the ‘climatic tolerance hypothesis’ that climatic extremes have acted as a strong constraint on species distribution and diversity.     

Global macroecology of bird assemblages in urbanized and semi‐natural ecosystems

Aim Despite the increasing pace of urbanization, little is known about how this process affects biodiversity globally. We investigate macroecological patterns of bird assemblages in urbanized areas relative to semi‐natural ecosystems. Location World‐wide. Methods We use a database of quantitative bird surveys to compare key assemblage structure parameters for plots in urbanized and semi‐natural ecosystems controlling for spatial autocorrelation and survey methodology. We use the term ‘urbanized’ instead of ‘urban’ ecosystems as many of the plots were not located in the centre of towns but in remnant habitat patches within conurbations. Results Some macroecological relationships were conserved in urbanized landscapes. Species–area, species–abundance and species–biomass relationships did not differ significantly between urbanized and non‐urbanized environments. However, there were differences in the relationships between productivity and assemblage structure. In forests, species richness increased with productivity; in both forests and open habitats, the evenness of species abundances declined as productivity increased. Among urbanized plots, instead, both species richness and the evenness of species abundances were independent of variation in productivity. Main conclusions Remnant habitats within urbanized areas are subject to many ecological alterations, yet key macroecological patterns differ remarkably little in urbanized versus non‐urbanized plots. Our results support the need for increased conservation activities in urbanized landscapes, particularly given the additional benefits of local experiences of biodiversity for the human population. With increasing urbanization world‐wide, broad‐scale efforts are needed to understand and manage the effects of this driver of change on biodiversity.     

Predators mediate the effects of a fungal pathogen on prey: an experiment with grasshoppers,wolf spiders,and fungal pathogens

1. Prey interact with multiple kinds of enemies such as predators, parasites, and pathogens. Interactions among enemies can alter prey dynamics but they are often studied separately. 2. During the summers of 2005–2006, we conducted a field experiment to examine interactions among grasshoppers, spider predators, and a lethal fungal pathogen of grasshoppers. Grasshopper nymphs were stocked into field enclosures. Predation was manipulated by adding spiders to enclosures on day 1, day 5, or day 10 of the experiment, or no spiders were added. We monitored grasshopper survival and grasshopper mortality from fungal pathogens for 4 weeks. 3. Fungal pathogens were abundant in 2005 but not in 2006, probably because of favourable weather conditions in 2005. When fungal pathogens were abundant, spider presence reduced grasshopper mortality from fungal pathogens, but only when spiders were present early in the experiment (added on day 1 or day 5). 4. The outcome of predator–prey interactions varied between years, probably as a result of differences in pathogen prevalence. In 2005, spider presence reduced the number of deaths from the pathogen, leading to a slight trend of increased grasshopper density. However, in 2006, when pathogens were not an important source of mortality, spider predation was compensatory.     

The Biology of Tiger Sharks, Galeocerdo Cuvier, in Shark Bay, Western Australia: Sex Ratio, Size Distribution, Diet, and Seasonal Changes in Catch Rates

Tiger sharks, Galeocerdo cuvier, are apex predators in a variety of nearshore ecosystems throughout the world. This study investigates the biology of tiger sharks in the shallow seagrass ecosystem of Shark Bay, Western Australia. Tiger sharks (n = 252) were the most commonly caught species (94%) compared to other large sharks. Tiger sharks ranged from 148–407cm TL. The overall sex ratio was biased towards females (1.8:1), but the sex ratio of mature animals (> 300cm TL) did not differ from 1:1. Contrary to previous accounts, tiger sharks were caught more often in all habitats during daylight hours than at night. Tiger shark catch rates were highly correlated with water temperature and were highest when water temperatures were above 19°C. The seasonal abundance of tiger sharks is correlated to both water temperature and the occurrence of their main prey: sea snakes and dugongs, Dugong dugon. Stomach contents analysis indicated that sea turtles and smaller elasmobranchs were also common prey. The importance of major seagrass grazers (dugongs and green sea turtles, Chelonia mydas) in the diet of tiger sharks suggests the possibility that these sharks are keystone predators in this ecosystem.     

The effect of colour variation in predators on the behaviour of pollinators: Australian crab spiders and native bees

1. Australian crab spiders exploit the plant–pollinator mutualism by reflecting UV light that attracts pollinators to the flowers where they sit. However, spider UV reflection seems to vary broadly within and between individuals and species, and we are still lacking any comparative studies of prey and/or predator behaviour towards spider colour variation. 2. Here we looked at the natural variation in the coloration of two species of Australian crab spiders, Thomisus spectabilis and Diaea evanida, collected from the field. Furthermore, we examined how two species of native bees responded to variation in colour contrast generated by spiders sitting in flowers compared with vacant flowers. We used data from a bee choice experiment with D. evanida spiders and Trigona carbonaria bees and also published data on T. spectabilis spiders and Austroplebeia australis bees. 3. In the field both spider species were always achromatically (from a distance) undetectable but chromatically (at closer range) detectable for bees. Experimentally, we showed species‐specific differences in bee behaviour towards particular spider colour variation: T. carbonaria bees did not show any preference for any colour contrasts generated by D. evanida spiders but A. australis bees were more likely to reject flowers with more contrasting T. spectabilis spiders. 4. Our study suggests that some of the spider colour variation that we encounter in the field may be partly explained by the spider's ability to adjust the reflectance properties of its colour relative to the behaviour of the species of prey available.     

Geometric factors influencing the diet of vertebrate predators in marine and terrestrial environments

Predator–prey relationships are vital to ecosystem function and there is a need for greater predictive understanding of these interactions. We develop a geometric foraging model predicting minimum prey size scaling in marine and terrestrial vertebrate predators taking into account habitat dimensionality and biological traits. Our model predicts positive predator–prey size relationships on land but negative relationships in the sea. To test the model, we compiled data on diets of 794 predators (mammals, snakes, sharks and rays). Consistent with predictions, both terrestrial endotherm and ectotherm predators have significantly positive predator–prey size relationships. Marine predators, however, exhibit greater variation. Some of the largest predators specialise on small invertebrates while others are large vertebrate specialists. Prey–predator mass ratios were generally higher for ectothermic than endothermic predators, although dietary patterns were similar. Model‐based simulations of predator–prey relationships were consistent with observed relationships, suggesting that our approach provides insights into both trends and diversity in predator–prey interactions.     

Metabarcoding for the parallel identification of several hundred predators and their prey: Application to bat species diet analysis

Assessing diet variability is of main importance to better understand the biology of bats and design conservation strategies. Although the advent of metabarcoding has facilitated such analyses, this approach does not come without challenges. Biases may occur throughout the whole experiment, from fieldwork to biostatistics, resulting in the detection of false negatives, false positives or low taxonomic resolution. We detail a rigorous metabarcoding approach based on a short COI minibarcode and two‐step PCR protocol enabling the “all at once” taxonomic identification of bats and their arthropod prey for several hundreds of samples. Our study includes faecal pellets collected in France from 357 bats representing 16 species, as well as insect mock communities that mimic bat meals of known composition, negative and positive controls. All samples were analysed using three replicates. We compare the efficiency of DNA extraction methods, and we evaluate the effectiveness of our protocol using identification success, taxonomic resolution, sensitivity and amplification biases. Our parallel identification strategy of predators and prey reduces the risk of mis‐assigning prey to wrong predators and decreases the number of molecular steps. Controls and replicates enable to filter the data and limit the risk of false positives, hence guaranteeing high confidence results for both prey occurrence and bat species identification. We validate 551 COI variants from arthropod including 18 orders, 117 family, 282 genus and 290 species. Our method therefore provides a rapid, resolutive and cost‐effective screening tool for addressing evolutionary ecological issues or developing “chirosurveillance” and conservation strategies.     

Causes and consequences of ontogenetic dietary shifts: a global synthesis using fish models

Ontogenetic dietary shifts (ODSs), the changes in diet utilisation occurring over the life span of an individual consumer, are widespread in the animal kingdom. Understanding ODSs provides fundamental insights into the biological and ecological processes that function at the individual, population and community levels, and is critical for the development and testing of hypotheses around key concepts in trophic theory on model organisms. Here, we synthesise historic and contemporary research on ODSs in fishes, and identify where further research is required. Numerous biotic and abiotic factors can directly or indirectly influence ODSs, but the most influential of these may vary spatially, temporally and interspecifically. Within the constraints imposed by prey availability, we identified competition and predation risk as the major drivers of ODSs in fishes. These drivers do not directly affect the trophic ontogeny of fishes, but may have an indirect effect on diet trajectories through ontogenetic changes in habitat use and concomitant changes in prey availability. The synthesis provides compelling evidence that ODSs can have profound ecological consequences for fish by, for example, enhancing individual growth and lifetime reproductive output or reducing the risk of mortality. ODSs may also influence food‐web dynamics and facilitate the coexistence of sympatric species through resource partitioning, but we currently lack a holistic understanding of the consequences of ODSs for population, community and ecosystem processes and functioning. Studies attempting to address these knowledge gaps have largely focused on theoretical approaches, but empirical research under natural conditions, including phylogenetic and evolutionary considerations, is required to test the concepts. Research focusing on inter‐individual variation in ontogenetic trajectories has also been limited, with the complex relationships between individual behaviour and environmental heterogeneity representing a particularly promising area for future research.     

The importance of genetic variability and population differentiation in the Eurasian lynx Lynx lynx for conservation,in the context of habitat and climate change

• 1 The Eurasian lynx Lynx lynx occupies a variety of environmental and climatic conditions, and the majority of present‐day European populations have either recovered from severe demographic bottlenecks, or are living in fragmented habitat. These factors may have affected the genetic variability of lynx populations. We summarize available data on genetics, population status and ecology of these felids to shed light on the pattern and mechanisms behind their genetic variability and population differentiation in Europe.
• 2 Genetic studies conducted so far, based on mtDNA and microsatellites, have shown that the Eurasian lynx has low to moderate genetic variability. Variability is lowest in the north (Scandinavian bottlenecked population), but is also low in the Carpathian region. A trend towards loss of genetic variation has been noted in fragmented and reintroduced populations. Genetically, the populations are highly differentiated from each other.
• 3 There are clear relationships between the pattern of lynx genetic variability, differentiation between the populations, and such factors as population history (demographic bottlenecks), social interactions and habitat fragmentation. The genetic divergence between lynx populations is also strongly correlated with the depth and duration of snow cover.
• 4 Our review provides evidence that the lynx is undergoing significant genetic differentiation, due to several factors. To enable better planning of conservation programmes for the Eurasian lynx, researchers should identify the Evolutionarily Significant Units among its populations, using different classes of molecular markers.

     

The behavioural response of adult Petromyzon marinus to damage‐released alarm and predator cues

Using semi‐natural enclosures, this study investigated (1) whether adult sea lamprey Petromyzon marinus show avoidance of damage‐released conspecific cues, damage‐released heterospecific cues and predator cues and (2) whether this is a general response to injured heterospecific fishes or a specific response to injured P. marinus. Ten replicate groups of 10 adult P. marinus, separated by sex, were exposed to one of the following nine stimuli: deionized water (control), extracts prepared from adult P. marinus, decayed adult P. marinus (conspecific stimuli), sympatric white sucker Catostomus commersonii, Amazon sailfin catfish Pterygoplichthys pardalis (heterospecific stimuli), 2‐phenylethylamine (PEA HCl) solution, northern water snake Nerodia sipedon washing, human saliva (predator cues) and an adult P. marinus extract and human saliva combination (a damage‐released conspecific cue and a predator cue). Adult P. marinus showed a significant avoidance response to the adult P. marinus extract as well as to C. commersonii, human saliva, PEA and the adult P. marinus extract and human saliva combination. For mobile P. marinus, the N. sipedon washing induced behaviour consistent with predator inspection. Exposure to the P. pardalis extract did not induce a significant avoidance response during the stimulus release period. Mobile adult female P. marinus showed a stronger avoidance behaviour than mobile adult male P. marinus in response to the adult P. marinus extract and the adult P. marinus extract and human saliva combination. The findings support the continued investigation of natural damage‐released alarm cue and predator‐based repellents for the behavioural manipulation of P. marinus populations in the Laurentian Great Lakes.     

Use of Highly Specific Molecular Markers Reveals Positive Correlation between Abundances of Mesodinium cf. major and Its Preferred Prey,Teleaulax amphioxeia,During Red Water Blooms in the Columbia River Estuary

In a previous study, Teleaulax amphioxeia—the preferred prey of Mesodinium in the Columbia River estuary—were undetectable within intense annual blooms, suggesting blooms are prey‐limited or prey are acquired outside of bloom patches. We used a novel molecular approach specifically targeting the prey (i.e., Unique Sequence Element [USE] within the ribosomal RNA 28S D2 regions of T. amphioxeia nucleus and nucleomorph) in estuarine water samples acquired autonomously with an Environmental Sample Processor integrated within a monitoring network (ESP‐SATURN). This new approach allowed for both more specific detection of the prey and better constraint of sample variability. A positive correlation was observed between abundances of M. cf. major and T. amphioxeia during bloom periods. The correlation was stronger at depth (> 8.2 m) and weak or nonexistent in the surface, suggesting that predator–prey dynamics become uncoupled when stratification is strong. We confirmed exclusive selectivity for T. amphioxeia by M. cf. major and observed the incorporation of the prey nucleus into a 4‐nuclei complex, where it remained functionally active. The specific biomarker for T. amphioxeia was also recovered in M. cf. major samples from a Namibian coastal bloom, suggesting that a specific predator–prey relationship might be widespread between M. cf. major and T. amphioxeia.     

Towards a mechanistic understanding of temperature and enrichment effects on species interaction strength,omnivory and food‐web structure

Revealing the links between species functional traits, interaction strength and food‐web structure is of paramount importance for understanding and predicting the relationships between food‐web diversity and stability in a rapidly changing world. However, little is known about the interactive effects of environmental perturbations on individual species, trophic interactions and ecosystem functioning. Here, we combined modelling and laboratory experiments to investigate the effects of warming and enrichment on a terrestrial tritrophic system. We found that the food‐web structure is highly variable and switches between exploitative competition and omnivory depending on the effects of temperature and enrichment on foraging behaviour and species interaction strength. Our model contributes to identifying the mechanisms that explain how environmental effects cascade through the food web and influence its topology. We conclude that considering environmental factors and flexible food‐web structure is crucial to improve our ability to predict the impacts of global changes on ecosystem diversity and stability.     

Dying from the lesser of three evils: facilitation and non‐consumptive effects emerge in a model with multiple predators

Prey modify their behaviour to avoid predation, but dilemmas arise when predators vary in hunting style. Behaviours that successfully evade one predator sometimes facilitate exposure to another predator, forcing the prey to choose the lesser of two evils. In such cases, we need to quantify behavioural strategies in a mix of predators. We model optimal behaviour of Atlantic cod Gadus morhua larvae in a water column, and find the minimal vulnerability from three common predator groups with different hunting modes; 1) ambush predators that sit‐and‐wait for approaching fish larvae; 2) cruising invertebrates that eat larvae in their path; and 3) fish which are visually hunting predators. We use a state‐dependent model to find optimal behaviours (vertical position and swimming speed over a diel light cycle) under any given exposure to the three distinct modes of predation. We then vary abundance of each predator and quantify direct and indirect effects of predation. The nature and strength of direct and indirect effects varied with predator type and abundance. Larvae escaped about half the mortality from fish by swimming deeper to avoid light, but their activity level and cumulative predation from ambush predators increased. When ambush invertebrates dominated, it was optimal to be less active but in more lit habitats, and predation from fish increased. Against cruising predators, there was no remedy. In all cases, the shift in behaviour allowed growth to remain almost the same, while total predation were cut by one third. In early life stages with high and size‐dependent mortality rates, growth rate can be a poor measure of the importance of behavioural strategies.     

Elevational gradients in species abundance,assemblage structure and energy use of rainforest birds in the Australian Wet Tropics bioregion

Elevational patterns of species richness, local abundance and assemblage structure of rainforest birds of north‐eastern Australia were explored using data from extensive standardized surveys throughout the region. Eighty‐two species of birds were recorded with strong turnover in assemblage structure across the elevational gradient and high levels of regional endemism in the uplands. Both species richness and bird abundance exhibited a humped‐shaped pattern with elevation with the highest values being between 600 and 800 m elevation. While much of the variability in species richness could be explained by the species–area relationship, analyses of net primary productivity (NPP) and total daily energy consumption of the bird community (energy use) suggest that ecosystem energy flow or constraints may be a significant determinant of species richness. Species richness is positively correlated with local bird abundance which itself is closely related to total energy use of the bird community. We suggest the hypothesis that lower NPP limits bird abundance and energy use in the uplands (>500 m) and that low bird energy use and species richness in the lowlands is limited by a seasonal bottleneck in available primary productivity and/or a species pool previously truncated by an extinction filter imposed by the almost complete disappearance of rainforest in the lowlands during the glacial maxima. We suggest that some of the previously predicted impacts of global warming on biodiversity in the uplands may be partially ameliorated by increases in NPP because of increasing temperatures. However, these relationships are complex and require further data specifically in regard to direct estimates of primary productivity and detailed estimates of energy flow within the assemblage.