Testing the validity of functional response models using molecular gut content analysis for prey choice in soil predators

Analysis of predator–prey interactions is a core concept of animal ecology, explaining structure and dynamics of animal food webs. Measuring the functional response, i.e. the intake rate of a consumer as a function of prey density, is a powerful method to predict the strength of trophic links and assess motives of prey choice, particularly in arthropod communities. However, due to their reductionist set‐up, functional responses, which are based on laboratory feeding experiments, may not display field conditions, possibly leading to skewed results. Here, we tested the validity of functional responses of centipede predators and their prey by comparing them with empirical gut content data from field‐collected predators. Our predator–prey system included lithobiid and geophilomorph centipedes, abundant and widespread predators of forest soils and their soil‐dwelling prey. First, we calculated the body size‐dependent functional responses of centipedes using a published functional response model in which we included natural prey abundances and animal body masses. This allowed us to calculate relative proportions of specific prey taxa in the centipede diet. In a second step, we screened field‐collected centipedes for DNA of eight abundant soil‐living prey taxa and estimated their body size‐dependent proportion of feeding events. We subsequently compared empirical data for each of the eight prey taxa, on proportional feeding events with functional response‐derived data on prey proportions expected in the gut, showing that both approaches significantly correlate in five out of eight predator–prey links for lithobiid centipedes but only in one case for geophilomorph centipedes. Our findings suggest that purely allometric functional response models, which are based on predator–prey body size ratios are too simple to explain predator–prey interactions in a complex system such as soil. We therefore stress that specific prey traits, such as defence mechanisms, must be considered for accurate predictions.     

Trait‐mediated indirect interactions in a marine intertidal system as quantified by functional responses

Studies of trait‐mediated indirect interactions (TMIIs) typically focus on effects higher predators have on per capita consumption by intermediate consumers of a third, basal prey resource. TMIIs are usually evidenced by changes in feeding rates of intermediate consumers and/or differences in densities of this third species. However, understanding and predicting effects of TMIIs on population stability of such basal species requires examination of the type and magnitude of the functional responses exhibited towards them. Here, in a marine intertidal system consisting of a higher‐order fish predator, the shanny Lipophrys pholis, an intermediate predator, the amphipod Echinogammarus marinus, and a basal prey resource, the isopod Jaera nordmanni, we detected TMIIs, demonstrating the importance of habitat complexity in such interactions, by deriving functional responses and exploring consequences for prey population stability. Echinogammarus marinus reacted to fish predator diet cues by reducing activity, a typical anti‐predator response, but did not alter habitat use. Basal prey, Jaera nordmanni, did not respond to fish diet cues with respect to activity, distribution or aggregation behaviour. Echinogammarus marinus exhibited type II functional responses towards J. nordmanni in simple habitat, but type III functional responses in complex habitat. However, while predator cue decreased the magnitude of the type II functional response in simple habitat, it increased the magnitude of the type III functional response in complex habitat. These findings indicate that, in simple habitats, TMIIs may drive down consumption rates within type II responses, however, this interaction may remain de‐stabilising for prey populations. Conversely, in complex habitats, TMIIs may strengthen regulatory influences of intermediate consumers on prey populations, whilst potentially maintaining prey population stability. We thus highlight that TMIIs can have unexpected and complex ramifications throughout communities, but can be unravelled by considering effects on intermediate predator functional response types and magnitudes. Synthesis Higher‐order predators and habitat complexity can influence behaviour of intermediate species, affecting their consumption of prey through trait‐mediated indirect interactions (TMIIs). However, it is not clear how these factors interact to determine prey population stability. Using functional responses (FRs), relating predator consumption to prey density, we detected TMIIs in a marine system. In simple habitats, TMIIs reduced consumption rates, but FRs remained de‐stabilising for prey populations. In complex habitats, TMIIs strengthened prey regulation with population stabilizing FRs. We thus demonstrate that FRs can assess interactions of environmental and biological cues that result in complex and unexpected outcomes for prey populations.     

Offspring size–number tradeoffs and food quality feedbacks impact population dynamics in a Daphnia–algae system

Population fluctuations can be affected by both extrinsic (e.g. weather patterns, food availability) and intrinsic (e.g. life‐history) factors. A key life‐history tradeoff is the production of offspring size versus number, ranging from many small offspring to few large offspring. Models show that this life‐history tradeoff in offspring size and number, through maturation time, can have significant impacts on population dynamics. However, few manipulative experiments have been conducted that can isolate life‐history effects from impacts of extrinsic factors in consumer–resource systems. We experimentally tested the effect of an offspring size–number tradeoff on population stability and food availability in a consumer–resource system. Using Daphnia pulex, we created a shift from many, small offspring being produced to fewer, larger offspring. Two sets of experiments were performed to examine the interaction of an extrinsic factor (light levels) and intrinsic population structure on dynamics, and we controlled for the ingestion pressure on algal prey at the time of the manipulation. We predicted that the tradeoff would impact internal consumer population characteristics, including biasing the stage structure towards adults, increasing adult size, and increasing average population‐level reproduction. This adult‐dominated stage structure was predicted to then lead to instability and a low quantity–high quality food state. Under all light levels, the manipulated populations became dominated by large adults. Contrary to predictions, the amplitudes of fluctuations in Daphnia biomass were lower in populations shifted to few–large offspring, representing higher stability in these populations. Furthermore, in high light conditions, a stable low Daphnia – high algae biomass (low food quality) state was observed in few–large offspring treatments but not in control (many–small offspring) treatments. Our results show a strong link between light levels as an extrinsic factor and the life‐history tradeoff of consumer offspring size versus number that impacts consumer–resource population dynamics through feedbacks with resource quality.     

Degeneracy-Driven Self-Structuring Dynamics in Selective Repertoires

Numerous biological interactions, such as interactions between T cell receptors or antibodies with antigens, interactions between enzymes and substrates, or interactions between predators and prey are often not strictly specific. In such less specific, or “sloppy,” systems, referred to here as degenerate systems, a given unit of a diverse resource (antigens, enzymatic substrates, prey) is at risk of being recognized and consumed by multiple consumers (lymphocytes, enzymes, predators). In this study, we model generalized degenerate consumer-resource systems of Lotka–Volterra and Verhulst types. In the degenerate systems of Lotka–Volterra, there is a continuum of types of consumer and resource based on variation of a single trait (characteristic, or preference). The consumers experience competition for a continuum of resource types. This non-local interaction system is modeled with partial differential-integral equations and shows spontaneous self-structuring of the consumer population that depends on the degree of interaction degeneracy between resource and consumer, but does not mirror the distribution of resource. We also show that the classical Verhulst (i.e. logistic) single population model can be generalized to a degenerate model, which shows qualitative behavior similar to that in the degenerate Lotka–Volterra model. These results provide better insight into the dynamics of selective systems in biology, suggesting that adaptation of degenerate repertoires is not a simple “mirroring” of the environment by the “fittest” elements of population.     

Biomass encounter rates limit the size scaling of feeding interactions

The rate that consumers encounter resources in space necessarily limits the strength of feeding interactions that shape ecosystems. To explore the link between encounters and feeding, we first compiled the largest available dataset of interactions in the marine benthos by extracting data from published studies and generating new data. These data indicate that the size‐scaling of feeding interactions varies among consumer groups using different strategies (passive or active) to encounter different resource types (mobile or static), with filter feeders exhibiting the weakest feeding interactions. Next, we used these data to develop an agent‐based model of resource biomass encounter rates, underpinned by consumer encounter strategy and resource biomass density. Our model demonstrates that passive strategies for encountering small, dispersed resources limits biomass encounter rates, necessarily limiting the strength of feeding interactions. Our model is based on generalisable assumptions, providing a framework to assess encounter‐based drivers of consumption and coexistence across systems.     

Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects

The risk of predation can drive trophic cascades by causing prey to engage in antipredator behavior (e.g. reduced feeding), but these behaviors can be energetically costly for prey. The effects of predation risk on prey (nonconsumptive effects, NCEs) and emergent indirect effects on basal resources should therefore depend on the ecological context (e.g. resource abundance, prey state) in which prey manage growth/predation risk tradeoffs. Despite an abundance of behavioral research and theory examining state‐dependent responses to risk, there is a lack of empirical data on state‐dependent NCEs and their impact on community‐level processes. We used a rocky intertidal food chain to test model predictions for how resources levels and prey state (age/size) shape the magnitude of NCEs. Risk cues from predatory crabs Carcinus maenas caused juvenile and sub‐adult snails Nucella lapillus to increase their use of refuge habitats and decrease their growth and per capita foraging rates on barnacles Semibalanus balanoides. Increasing resource levels (high barnacle density) and prey state (sub‐adults) enhanced the strength of NCEs. Our results support predictions that NCEs will be stronger in resource‐rich systems that enhance prey state and suggest that the demographic composition of prey populations will influence the role of NCEs in trophic cascades. Contrary to theory, however, we found that resources and prey state had little to no effect on snails in the presence of predation risk. Rather, increases in NCE strength arose because of the strong positive effects of resources and prey state on prey foraging rates in the absence of risk. Hence, a common approach to estimating NCE strength – integrating measurements of prey traits with and without predation risk into a single metric – may mask the underlying mechanisms driving variation in the strength and relative importance of NCEs in ecological communities.     

A synthesis of subdisciplines: predator–prey interactions, and biodiversity and ecosystem functioning

The last 15 years has seen parallel surges of interest in two research areas that have rarely intersected: biodiversity and ecosystem functioning (BEF), and multispecies predator–prey interactions (PPI). Research addressing role of biodiversity in ecosystem functioning has focused primarily on single trophic‐level systems, emphasizing additive effects of diversity that manifest through resource partitioning and the sampling effect. Conversely, research addressing predator–prey interactions has focused on two trophic‐level systems, emphasizing indirect and non‐additive interactions among species. Here, we use a suite of consumer‐resource models to organize and synthesize the ways in which consumer species diversity affects the densities of both resources and consumer species. Specifically, we consider sampling effects, resource partitioning, indirect effects caused by intraguild interactions and non‐additive effects. We show that the relationship between consumer diversity and the density of resources and consumer species are broadly similar for systems with one vs. two trophic levels, and that indirect and non‐additive interactions generally do little more than modify the impacts of diversity established by the sampling effect and resource partitioning. The broad similarities between systems with one vs. two trophic levels argue for greater communication between researchers studying BEF, and researchers studying multispecies PPI.     

Size-mediated control of perch–midge coupling in Lake Erie transient dead zones

Transient ecosystem-level disturbances such as oxygen depletion (hypoxia) in aquatic systems modulate species distributions and interactions. In highly eutrophic systems, hypoxic areas (“dead zones”) have expanded around the world, temporarily preventing many demersal predators from accessing their food resources. Here, we investigate how yellow perch (Perca flavescens), an exploited, cool-water mesopredator, interact with their dominant invertebrate prey in benthic habitat–non-biting midge (chironomid) larvae–as bottom-water hypoxia develops in central Lake Erie (United States–Canada) during summer. We apply linear mixed-effects models to individual-level data from basin-wide field surveys on size-based interactions between perch and midge larvae under varying habitat conditions and resource attributes. We test if 1) midge populations (larval body size and density) differ among habitat states (unstratified normoxia, stratified normoxia, and stratified hypoxia); and 2) size-based perch–midge interactions (predator–prey mass ratio or PPMR) differ among habitat states with varying temperature and midge density. Midge populations remained highly abundant after bottom-water oxygen depletion. Despite their high densities, midge larvae also maintained their body size in hypoxic water. In contrast, perch on average consumed relatively smaller (by up to ~64%) midges (higher PPMR) in warmer and hypoxic water, while prey size ingested by perch shrunk less in areas with higher midge density. Our analysis shows that hypoxia-tolerant midges largely allow perch to maintain their consumer–resource relationships in contracted habitats through modified size-mediated interactions in dead zones during summer, revealing plasticity of their trophic coupling in the chronically perturbed ecosystem.     

Physiologically structured models – from versatile technique to ecological theory

A ubiquitous feature of natural communities is the variation in size that can be observed between organisms, a variation that to a substantial degree is intraspecific. Size variation within species by necessity implies that ecological interactions vary both in intensity and type over the life cycle of an individual. Physiologically structured population models (PSPMs) constitute a modelling approach especially designed to analyse these size‐dependent interactions as they explicitly link individual level processes such as consumption and growth to population dynamics. We discuss two cases where PSPMs have been used to analyse the dynamics of size‐structured populations. In the first case, a model of a size‐structured consumer population feeding on a non‐structured prey was successful in predicting both qualitative (mechanisms) and quantitative (individual growth, survival, cycle amplitude) aspects of the population dynamics of a planktivorous fish population. We conclude that single generation cycles as a result of intercohort competition is a general outcome of size‐structured consumer–resource interactions. In the second case, involving both cohort competition and cannibalism, we show that PSPMs may predict double asymptotic growth trajectories with individuals ending up as giants. These growth trajectories, which have also been observed in field data, could not be predicted from individual level information, but are emergent properties of the population feedback on individual processes. In contrast to the size‐structured consumer–resource model, the dynamics in this case cannot be reduced to simpler lumped stage‐based models, but can only be analysed within the domain of PSPMs. Parameter values used in PSPMs adhere to the individual level and are derived independently from the system at focus, whereas model predictions involve both population level processes and individual level processes under conditions of population feedback. This leads to an increased ability to test model predictions but also to a larger set of variables that is predicted at both the individual and population level. The results turn out to be relatively robust to specific model assumptions and thus render a higher degree of generality than purely individual‐based models. At the same time, PSPMs offer a much higher degree of realism, precision and testing ability than lumped stage‐based or non‐structured models. The results of our analyses so far suggest that also in more complex species configurations only a limited set of mechanisms determines the dynamics of PSPMs. We therefore conclude that there is a high potential for developing an individual‐based, size‐dependent community theory using PSPMs.     

A newly discovered role of evolution in previously published consumer–resource dynamics

Consumer–resource interactions are fundamental components of ecological communities. Classic features of consumer–resource models are that temporal dynamics are often cyclic, with a ¼‐period lag between resource and consumer population peaks. However, there are few published empirical examples of this pattern. Here, we show that many published examples of consumer–resource cycling show instead patterns indicating eco‐evolutionary dynamics. When prey evolve along a trade‐off between defence and competitive ability, two‐species consumer–resource cycles become longer and antiphase (half‐period lag, so consumer maxima coincide with minima of the resource species). Using stringent criteria, we identified 21 two‐species consumer–resource time series, published between 1934 and 1997, suitable to investigate for eco‐evolutionary dynamics. We developed a statistical method to probe for a transition from classic to eco‐evolutionary cycles, and find evidence for eco‐evolutionary type cycles in about half of the studies. We show that rapid prey evolution is the most likely explanation for the observed patterns.     

The negative effects of pathogen‐infected prey on predators: a meta‐analysis

Intra‐guild predation (IGP) – where a top predator (IG_Pred) consumes both a basal resource and a competitor for that resource (IG_Prey) – has become a fundamental part of understanding species interactions and community dynamics. IGP communities composed of intraguild predators and prey have been well studied; however, we know less about IGP communities composed of predators, pathogens, and resources. Resource quality plays an important role in community dynamics and may influence IGP dynamics as well. We conducted a meta‐analysis on predator–pathogen–resource communities to determine whether resource quality mediated by the pathogen affected predator life‐history traits and if these effects met the theoretical constraints of IGP communities. To do this, we summarized results from studies that investigated the use of predators and pathogens to control insect pests. In these systems, the predators are the IG_Pred and pathogens are the IG_Prey. We found that consumer longevity, fecundity, and survival decreased by 26%, 31% and 13% respectively, when predators consumed pathogen‐infected prey, making the infected prey a low quality resource. Predators also significantly preferred healthy prey over infected prey. When we divided consumers by enemy type, strict predators (e.g. wolf spiders) had no preference while parasitoids preferred healthy prey. Our results suggest that communities containing parasitoids and pathogens may rarely exhibit intraguild predation; whereas, communities composed of strict predators and pathogens are more likely dominated by IGP dynamics. In these latter communities, the consumption of low and high quality resources suggests that IGP communities composed of strict predators, pathogens and prey should naturally persist, supporting IGP theory. Synthesis We investigated how consuming pathogen‐infected prey influence important life‐history parameters of insect predators. Pathogens are used in a variety of biocontrol programs, especially to control crop pests. We found that true predators (i.e. wolf spiders) have no preference for healthy or infected prey and have reduced fecundity, survival and longevity consuming infected prey. However, parasitoids avoided infected prey when possible. In biocontrol programs with multiple control agents, parasitoids and pathogens would do a better job controlling pests as predators would reduce the amount of pathogen available and have reduced fitness from consuming infected prey. However, theory suggests that true predators, prey and pathogens may coexist long term.     

Multichannel feeding by spider functional groups is driven by feeding strategies and resource availability

Multichannel feeding, whereby consumers feed across resource channels such as upon herbivore and detritivore resources, acts to link discrete compartments of a food web with implications for ecosystem functioning and stability. Currently however, we have little understanding which feeding strategies of consumers underlie multichannel feeding. We therefore link spider functional group and resource density‐dependent or density‐independent feeding strategies to multichannel feeding by quantifying not only consumer diet, but also the relative availability of resources. Here we analysed herbivore (green) and detritivore (brown) prey use by spider communities in grasslands, and tested if available prey biomass proportions were linked to observed spider dietary proportions. Different spider functional groups each linked green and brown resource channels, but while green prey were always consumed in proportion to their relative biomass, brown prey were consumed independently of proportion by some functional groups. Additionally, we found greater intraguild predation by cursorial spiders when green resources were relatively scarcer, suggesting green prey was preferred, and needed to be compensated for when rare. Overall, we observed a stronger consumer connection to the green than brown resource channel, yet this green connection was more variable due to greater range in green resource availability across grasslands and density‐dependent consumption on green prey. Consequently, multichannel feeding by spiders was determined by density‐dependent and density‐independent feeding strategies that varied by spider functional group and across resources channels. Our results demonstrate that the role of multichannel feeding by spiders in linking separate food web compartments is a dynamic component of food web structure in these wild grasslands.     

The impact of an introduced predator on a threatened galaxiid fish is reduced by the availability of complex habitats

1. The availability of complex habitats such as macrophytes may be vital in determining the outcomes of interactions between introduced predators and native prey. Introduced brown trout (Salmo trutta) have impacted numerous small native freshwater fishes in the southern hemisphere, but the potential role of complex habitats in determining the direct outcomes of brown trout – native fish interactions has not been experimentally evaluated. 2. An in‐lake enclosure experiment was used to evaluate the importance of structurally complex habitats in affecting the direct impacts of brown trout on a threatened galaxiid fish. Five Galaxias auratus and a single brown trout were added to enclosures containing one of three different habitat types (artificial macrophytes, rocks and bare silt substrate). The experiment also had control enclosures without brown trout. Habitat‐dependence of predation risk was assessed by analysis of G. auratus losses to predation, and stomach contents of remaining fish were analysed to determine if brown trout directly affect the feeding of G. auratus and whether this is also habitat‐dependent. 3. Predation risk of G. auratus differed significantly between habitat types, with the highest mortality in enclosures with only bare silt substrate and the lowest in enclosures containing artificial macrophytes. This result highlights the importance of availability of complex habitats for trout – native fish interactions and suggests that increasing habitat degradation and loss in fresh waters may exacerbate the direct impacts of introduced predators. 4. Stomach contents analyses were restricted to fish in enclosures with artificial macrophytes and rocks, as most fish were consumed in enclosures with brown trout and only bare silt substrate. These analyses suggest that brown trout do not directly affect the feeding of G. auratus in complex habitats, but it is still unknown whether its feeding is reduced if complex habitats are unavailable.     

Human–Cougar interactions in the wildland–urban interface of Colorado's front range

As human populations continue to expand across the world, the need to understand and manage wildlife populations within the wildland – urban interface is becoming commonplace. This is especially true for large carnivores as these species are not always tolerated by the public and can pose a risk to human safety. Unfortunately, information on wildlife species within the wildland – urban interface is sparse, and knowledge from wildland ecosystems does not always translate well to human‐dominated systems. Across western North America, cougars (Puma concolor) are routinely utilizing wildland – urban habitats while human use of these areas for homes and recreation is increasing. From 2007 to 2015, we studied cougar resource selection, human–cougar interaction, and cougar conflict management within the wildland – urban landscape of the northern Front Range in Colorado, USA. Resource selection of cougars within this landscape was typical of cougars in more remote settings but cougar interactions with humans tended to occur in locations cougars typically selected against, especially those in proximity to human structures. Within higher housing density areas, 83% of cougar use occurred at night, suggesting cougars generally avoided human activity by partitioning time. Only 24% of monitored cougars were reported for some type of conflict behavior but 39% of cougars sampled during feeding site investigations of GPS collar data were found to consume domestic prey items. Aversive conditioning was difficult to implement and generally ineffective for altering cougar behaviors but was thought to potentially have long‐term benefits of reinforcing fear of humans in cougars within human‐dominated areas experiencing little cougar hunting pressure. Cougars are able to exploit wildland – urban landscapes effectively, and conflict is relatively uncommon compared with the proportion of cougar use. Individual characteristics and behaviors of cougars within these areas are highly varied; therefore, conflict management is unique to each situation and should target individual behaviors. The ability of individual cougars to learn to exploit these environments with minimal human–cougar interactions suggests that maintaining older age structures, especially females, and providing a matrix of habitats, including large connected open‐space areas, would be beneficial to cougars and effectively reduce the potential for conflict.     

Omnivory and stability of food webs

The role and prevalence of omnivory, defined as feeding on more than one trophic level, are critical to understand food web structure and dynamics. Whether omnivory stabilizes or destabilizes food webs depends on the assumptions of theoretical models. Recently, Tanabe and Namba [Tanabe, K., Namba, T., 2005. Omivory creates chaos in simple food web models. Ecology 86, 3411–3414] found that omnivory can create chaos in a simple food web model with linear functional responses and 12 model parameters. In this paper, first we numerically examined bifurcation diagrams with all the parameters as bifurcation parameters, including self-limitation of the intermediate consumer and predator. Chaos spontaneously appears when the intraguild predator’s consumption rates are low for nutrient-rich intraguild prey and high for nutrient-poor basal resource and the intraguild prey reproduces efficiently feeding on the basal resource. Second, we investigated effects of the addition of a species into the basic model food web which exhibits chaos. The additional species is assumed to consume only one of the basal resource, intermediate consumer, or omnivorous predator. Consequences of the addition greatly depend on the trophic level on which the additional species feeds. While the increased diversity of predators feeding on the intermediate consumer stabilizes the web, the increased diversity of prey feeding on the basal resource induces collapse of the food web through exploitative competition for the basal resource. The food chain with the top predator feeding on the omnivorous predator is highly unstable unless the mortality of the top predator is extremely low. We discuss the possibility of real-world chaos and the reason why stability of food webs strongly depends on the topological structure of the webs. Finally, we consider the implications of our results for food web theory and resource management.     

Multi-predator effects across life-history stages: non-additivity of egg- and larval-stage predation in an African treefrog

The effects of multiple predators on their prey are frequently non‐additive because of interactions among predators. When prey shift habitats through ontogeny, many of their predators cannot interact directly. However, predators that occur in different habitats or feed on different prey stages may still interact through indirect effects mediated by prey traits and density. We conducted an experiment to evaluate the combined effects of arboreal egg‐stage and aquatic larval‐stage predators of the African treefrog, Hyperolius spinigularis. Egg and larval predator effects were non‐additive – more Hyperolius survived both predators than predicted from their independent effects. Egg‐stage predator effects on aquatic larval density and size and age at hatching reduced the effectiveness of larval‐stage predators by 70%. Our results indicate that density‐ and trait‐mediated indirect interactions can act across life‐stages and habitats, resulting in non‐additive multi‐predator effects.     

Forest fragmentation leads to food web contraction

Fragmentation and loss of habitat are critical components of the global change currently threatening biodiversity and ecosystem functioning. We studied the effects of habitat loss through fragmentation on food web structure, by constructing and analyzing plant‐herbivore and host‐parasitoid food webs including more than 400 species and over 120 000 feeding records, in 19 Chaco Serrano remnants of differing areas. Food web structure was altered by habitat fragmentation, with different metrics being affected depending on interaction type, and with all changes being driven by the reduced size of networks in smaller fragments. Only connectance varied in both quantitative and qualitative analyses, being negatively related to area. In addition, the interactions were represented by proper successive subsets, modulated mainly by resource availability (plant–herbivore) or consumer specialization (host–parasitoid), as forest size decreased. The results suggest that habitat loss has led to food web contraction around a central core of highly‐connected species, for plant–herbivore as well as for host–parasitoid systems. The study provides new insights into the effects of human perturbations on complex biological systems.     

Interspecific differences in antipredator strategies determine the strength of non‐consumptive predator effects on stream detritivores

Animal species differ considerably in their response to predation risks. Interspecific variability in prey behaviour and morphology can alter cascading effects of predators on ecosystem structure and functioning. We tested whether species‐specific morphological defenses may affect responses of leaf litter consuming invertebrate prey to sit‐and‐wait predators, the odonate Cordulegaster boltonii larvae, in aquatic food webs. Partly or completely blocking the predator mouthparts (mandibles and/or extensible labium), thus eliminating consumptive (i.e. lethal) predator effects, we created a gradient of predator‐prey interaction intensities (no predator < predator – no attack < predator – non‐lethal attacks < lethal predator). A field experiment was first used to assess both consumptive and non‐consumptive predator effects on leaf litter decomposition and prey abundances. Laboratory microcosms were then used to examine behavioural responses of armored and non‐armored prey to predation risk and their consequences on litter decomposition. Results show that armored and non‐armored prey responded to both acute (predator – non‐lethal attacks) and chronic (predator – no attack) predation risks. Acute predation risk had stronger effects on litter decomposition, prey feeding rate and prey habitat use than predator presence alone (chronic predation risk). Predator presence induced a reduction in feeding activity (i.e. resource consumption) of both prey types but a shift to predator‐free habitat patches in non‐armored detritivores only. Non‐consumptive predator effects on prey subsequently decreased litter decomposition rate. Species‐specific prey morphological defenses and behaviour should thus be considered when studying non‐consumptive predator effects on prey community structure and ecosystem functioning.     

Multitrophic diversity effects depend on consumer specialization and species‐specific growth and grazing rates

Ecosystem functioning is affected by horizontal (within trophic groups) and vertical (across trophic levels) biodiversity. Theory predicts that the effects of vertical biodiversity depend on consumer specialization. In a microcosm experiment, we investigated ciliate consumer diversity and specialization effects on algal prey biovolume, evenness and composition, and on ciliate biovolume production. The experimental data was complemented by a process‐based model further analyzing the ecological mechanisms behind the observed diversity effects. Overall, increasing consumer diversity had no significant effect on prey biovolume or evenness. However, consumer specialization affected the prey community. Specialist consumers showed a stronger negative impact on prey biovolume and evenness than generalists. The model confirmed that this pattern was mainly driven by a single specialist with a high per capita grazing rate, consuming the two most productive prey species. When these were suppressed, the prey assemblage became dominated by a less productive species, consequently decreasing prey biovolume and evenness. Consumer diversity increased consumer biovolume, which was stronger for generalists than for specialists and highest in mixed combinations, indicating that consumer functional diversity, i.e. more diverse feeding strategies, increased resource use efficiency. Overall, our results indicate that consumer diversity effects on prey and consumers strongly depend on species‐specific growth and grazing rates, which may be at least equally important as consumer specialization in driving consumer diversity effects across trophic levels. Synthesis In a microcosm experiment, we investigated multitrophic consumer diversity and specialization effects using ciliate consumers and microalgal prey. Consumer diversity increased consumer biovolume, which was highest in combinations containing both generalists and specialists. Specialist consumers showed a stronger negative effect on prey biovolume and evenness than generalists. These experimental data were supported by a process‐based model, indicating that the large effect of the specialists was based on high per capita grazing rate on the two most productive prey species. Species‐specific traits such as growth and grazing rates were equally important for multitrophic diversity effects than consumer specialization.     

More than a barrier: The complex effects of ecotone vegetation type on terrestrial consumer consumption of an aquatic prey resource

Community structure and dynamics can be influenced by resource transfers between ecosystems, yet little is known about how boundary structure determines both the magnitude of exchanges and their effects on recipient and donor communities. Aquatic and terrestrial ecosystems are often linked by resource fluxes and riparian vegetation is commonly affected by anthropogenic alterations to land use or river hydrological regime. I investigated whether shrubs at the freshwater–terrestrial interface alter the supply, distribution and importance of aquatic prey resources to terrestrial consumers. Shrubs were predicted to alter the larval community composition of aquatic insects and the emergence of winged adults, thus affecting aquatic prey subsidies to terrestrial consumers. In addition, shrubs were hypothesized to alter the microclimatic suitability of the riparian zone for adult aquatic insects, act as a physical barrier to their dispersal and affect terrestrial community composition, particularly the abundance and type of predators that could benefit from the aquatic prey resource. Stable isotope dietary analyses and a survey of shrub‐dominated and open grassland riparian habitats revealed that larval densities of aquatic insects (EPTM: Ephemeroptera, Plecoptera, Trichoptera and Megaloptera) were higher in shrub than grassland habitats; however, reduced emergence and lateral dispersal in shrub areas led to lower densities of adults. The temperature and relative humidity of the riparian zone did not differ between the habitats. Ground‐active terrestrial invertebrate communities had a higher proportion of cursorial spiders in grassland, coinciding with greater abundances of aquatic prey. Aquatic prey contribution to cursorial spider diet matched adult aquatic insect abundances. Overall, riparian shrubs reduced the magnitude, or at least altered the timing, of cross‐ecosystem subsidy supply, distribution and use by consumers through mechanisms operating in both the aquatic and terrestrial ecosystems. Thus, the structure of ecosystem boundaries has complex effects on the strength of biological interactions between adjacent systems.