Multiple predator effects in an intertidal food web

Co-occurring predators increase biotic resistance against an invasive prey

The presence of multiple predators can lead to variation in predator behavior and ultimately altered risk for shared prey. This concept has seldom been accounted for in studies that consider predator-driven biotic resistance from native marine predators against invasive prey. This study compared the prey selection of whelks and rock lobsters when co-occurring and when foraging in isolation. When in isolation, both predators preferred the native mussel Choromytilus meridionalis, regardless of the abundance of alternative prey. However, when co-occurring, predation risk for all prey species, including the invasive mussel Semimytilus algosus, increased. This was largely driven by greater variation in prey selection by rock lobsters in the presence of whelks. This indicates that predatory efforts from co-occurring predators can result in stronger predation pressure on invasive prey than would be recognized if predators were assessed in isolation.

Combined effects of predator cues and competition define habitat choice and food consumption of amphipod mesograzers

Predation has direct impact on prey populations by reducing prey abundance. In addition, predator presence alone can also have non-consumptive effects on prey species, potentially influencing their interspecific interactions and thus the structure of entire assemblages. The performance of potential prey species may, therefore, depend on both the presence of predators and competitors. We studied habitat use and food consumption of a marine mesograzer, the amphipod Echinogammarus marinus, in the presence/absence of a fish mesopredator and/or an amphipod competitor. The presence of the predator affected both habitat choice and food consumption of the grazer, indicating a trade-off between the use of predator-free space and food acquisition. Without the predator, E. marinus were distributed equally over different microhabitats, whereas in the presence of the predator, most individuals chose a sheltered microhabitat and reduced their food consumption. Furthermore, habitat choice of the amphipods changed in the presence of interspecific competitors, also resulting in reduced feeding rates. The performance of E. marinus is apparently driven by trait-mediated direct and indirect effects caused by the interplay of predator avoidance and competition. This highlights the importance of potential non-consumptive impacts of predators on their prey organisms. The flexible responses of small invertebrate consumers to the combined effects of predation and competition potentially lead to changes in the structure of coastal ecosystems and the multiple species interactions therein.

Functional response and multiple predator effects of two generalist predators preying on Tuta absoluta eggs

BACKROUND

Interactions among invertebrate predators could affect a pest suppression. The hemipteran species Macrolophus pygmaeus (Rambur) and Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae) are natural enemies of several pests in agroecosystems and coexist in tomato crops in Mediterranean countries. By using the multiplicative risk model (MRM) and the substitutive model, the multiple predator effects (MPEs) on prey suppression were calculated when two individuals of the predators foraged at the same densities on South American tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), eggs.

RESULTS

Egg consumption increased with increasing egg density and the two predators exhibited a type III functional response. Predation rates were strongly affected by prey density. Using the MRM, we found risk reduction at intraspecific treatments at high prey density. Applying the substitutive model, we detect risk enhancement at interspecific treatments at high egg density.

CONCLUSIONS

At low prey densities, most of the interactions were independent, whereas at high densities most interactions were not independent and resulted in prey risk reduction, indicating antagonism between the individuals involved. We also showed that N. tenuis is a more competitive predator species for T. absoluta eggs than M. pygmaeus; however, combination of the two predator species will lead to better pest suppression at high T. absoluta population densities. © 2017 Society of Chemical Industry.

Uncovering emergent interactions in three-way combinations of stressors

Understanding how multiple stressors interact is needed to predict the dynamical outcomes of diverse biological systems, ranging from drug-resistant pathogens that are combated and treated with combination drug therapies to ecosystems impacted by environmental toxicants or disturbances. Nevertheless, extensive studies of higher-order (more than two component) interactions have been lacking. Here, we conduct experiments using 20 three-drug combinations and their effects on the bacterial growth of Escherichia coli We report our measurements of growth rates in single, pairwise and triple-drug combinations. To uncover emergent interactions, we derive a simple framework to calculate expectations for three-way interactions based on the measured impact of each individual stressor and of each pairwise interaction. Using our framework, we find that (i) emergent antagonisms are more common than emergent synergies and (ii) emergent antagonisms are more common and emergent synergies are more rare than would be inferred from measures of net effects that do not disentangle pairwise interactions from three-way interactions.

Enhanced identification of synergistic and antagonistic emergent interactions among three or more drugs

Interactions among drugs play a critical role in the killing efficacy of multi-drug treatments. Recent advances in theory and experiment for three-drug interactions enable the search for emergent interactions-ones not predictable from pairwise interactions. Previous work has shown it is easier to detect synergies and antagonisms among pairwise interactions when a rescaling method is applied to the interaction metric. However, no study has carefully examined whether new types of normalization might be needed for emergence. Here, we propose several rescaling methods for enhancing the classification of the higher order drug interactions based on our conceptual framework. To choose the rescaling that best separates synergism, antagonism and additivity, we conducted bacterial growth experiments in the presence of single, pairwise and triple-drug combinations among 14 antibiotics. We found one of our rescaling methods is far better at distinguishing synergistic and antagonistic emergent interactions than any of the other methods. Using our new method, we find around 50% of emergent interactions are additive, much less than previous reports of greater than 90% additivity. We conclude that higher order emergent interactions are much more common than previously believed, and we argue these findings for drugs suggest that appropriate rescaling is crucial to infer higher order interactions.

The enemy of my enemy is my friend: intraguild predation between invaders and natives facilitates coexistence with shared invasive prey

Understanding and predicting the outcomes of biological invasions is challenging where multiple invader and native species interact. We hypothesize that antagonistic interactions between invaders and natives could divert their impact on subsequent invasive species, thus facilitating coexistence. From field data, we found that, when existing together in freshwater sites, the native amphipod Gammarus duebeni celticus and a previous invader G. pulex appear to facilitate the establishment of a second invader, their shared prey Crangonyx pseudogracilis. Indeed, the latter species was rarely found at sites where each Gammarus species was present on its own. Experiments indicated that this may be the result of G. d. celticus and G. pulex engaging in more intraguild predation (IGP) than cannibalism; when the 'enemy' of either Gammarus species was present, that is, the other Gammarus species, C. pseudogracilis significantly more often escaped predation. Thus, the presence of mutual enemies and the stronger inter- than intraspecific interactions they engage in can facilitate other invaders. With some invasive species such as C. pseudogracilis having no known detrimental effects on native species, and indeed having some positive ecological effects, we also conclude that some invasions could promote biodiversity and ecosystem functioning.

Fortune favours the bold: a higher predator reduces the impact of a native but not an invasive intermediate predator

Emergent multiple predator effects (MPEs) might radically alter predictions of predatory impact that are based solely on the impact of individuals. In the context of biological invasions, determining if and how the individual-level impacts of invasive predators relates to their impacts in multiple-individual situations will inform understanding of how such impacts might propagate through recipient communities. Here, we use functional responses (the relationship between prey consumption rate and prey density) to compare the impacts of the invasive freshwater mysid crustacean Hemimysis anomala with a native counterpart Mysis salemaai when feeding on basal cladoceran prey (i) as individuals, (ii) in conspecific groups and (iii) in conspecific groups in the presence of a higher fish predator, Gasterosteus aculeatus. In the absence of the higher predator, the invader consumed significantly more basal prey than the native, and consumption was additive for both mysid species - that is, group consumption was predictable from individual-level consumption. Invaders and natives were themselves equally susceptible to predation when feeding with the higher fish predator, but an MPE occurred only between the natives and higher predator, where consumption of basal prey was significantly reduced. In contrast, consumption by the invaders and higher predator remained additive. The presence of a higher predator serves to exacerbate the existing difference in individual-level consumption between invasive and native mysids. We attribute the mechanism responsible for the MPE associated with the native to a trait-mediated indirect interaction, and further suggest that the relative indifference to predator threat on the part of the invader contributes to its success and impacts within invaded communities.

Foraging decisions and behavioural flexibility in trap-building predators: a review

Foraging theory was first developed to predict the behaviour of widely-foraging animals that actively search for prey. Although the behaviour of sit-and-wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap-building predators (mainly pit-building antlions and web-building spiders), a specific group of sit-and-wait predators that construct traps as a foraging device, with those of widely-foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap-building and widely-foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap-building predators. In each chapter of this review, we summarize the response of trap-building predators to a different factor, while contrasting it with the equivalent response characterizing widely-foraging predators. We provide here evidence that the behaviour of trap-building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.

The consequences of consumer diversity loss: different answers from different experimental designs

Predators are often the most vulnerable group to extinction, yet the consequences of changing predator diversity are poorly understood. One source of confusion has been different experimental designs. The multiple-predator effects literature typically employs an additive design, while the biodiversity ecosystem function literature typically uses a replacement design. Separately, these designs each detect only a subset of the changes in food web interactions caused by predator loss. Here, we measure the impact of consumer diversity on sessile marine invertebrates using a combination additive-replacement design. We couple this with a meta-analysis of previous combination experiments. We use these two approaches to explore how each design can detect different types of interactions among predators. We find that, while high diversity does lead to more negative interspecific interactions, the strength of these interactions is often weaker than negative intraspecific interactions caused by increasing the density of a single species alone. We conclude that a hybrid design is the optimal method to explore the mechanisms behind the effects of changing predator diversity. If researchers merely want to know the consequences of changing predator diversity, at a bare minimum, the experimental design must mimic the actual changes in both predator density and diversity in their system of interest. However, only a hybrid design can distinguish the consequences of shifting the balance of interspecific and intraspecific interactions within a community, an issue of great importance when considering both natural diversity loss and pest biocontrol.

Laboratory observations on the feeding behavior and feeding rate of the nemertean Procephalothrix simulus

The feeding behavior of the nemertean Procephalothrix simulus and the effects of extrinsic factors on the feeding rates of this nemertean were experimentally studied. Procephalothrix simulus showed a typical macrophagous feeding strategy and consumed various food items. When capturing freshwater Tubifex sp., the nemerteans successfully ingested prey in all attacks, and they did not evert the proboscis in 25% of capture events. When capturing marine Saccocirrus gabrillae, the nemerteans failed to consume prey in more than half of the attacks, and they always everted the proboscis. There was a positive relationship between nemertean body weight and the rate of successful attacks, and a negative relationship between nemertean body weight and the duration of feeding events. The feeding rate of P. simulus increased when the temperature was raised from 5 degrees C to 30 degrees C but was significantly inhibited at 32 degrees C. Food intake was significantly reduced in media diluted to a practical salinity of 20 and 10 and in medium with the salinity elevated to 45. Dark conditions induced higher food intake, but prey density had no significant effect on feeding rate. These results suggest that P. simulus is a predator successfully adapted to the variable environmental conditions of the intertidal habitat.

Effects of functional constraints and opportunism on the functional structure of a vertebrate predator assemblage

1. Within mainstream ecological literature, functional structure has been viewed as resulting from the interplay of species interactions, resource levels and environmental variability. Classical models state that interspecific competition generates species segregation and guild formation in stable saturated environments, whereas opportunism causes species aggregation on abundant resources in variable unsaturated situations. 2. Nevertheless, intrinsic functional constraints may result in species-specific differences in resource-use capabilities. This could force some degree of functional structure without assuming other putative causes. However, the influence of such constraints has rarely been tested, and their relative contribution to observed patterns has not been quantified. 3. We used a multiple null-model approach to quantify the magnitude and direction (non-random aggregation or divergence) of the functional structure of a vertebrate predator assemblage exposed to variable prey abundance over an 18-year period. Observed trends were contrasted with predictions from null-models designed in an orthogonal fashion to account independently for the effects of functional constraints and opportunism. Subsequently, the unexplained variation was regressed against environmental variables to search for evidence of interspecific competition. 4. Overall, null-models accounting for functional constraints showed the best fit to the observed data, and suggested an effect of this factor in modulating predator opportunistic responses. However, regression models on residual variation indicated that such an effect was dependent on both total and relative abundance of principal (small mammals) and alternative (arthropods, birds, reptiles) prey categories. 5. In addition, no clear evidence for interspecific competition was found, but differential delays in predator functional responses could explain some of the unaccounted variation. Thus, we call for caution when interpreting empirical data in the context of classical models assuming synchronous responses of consumers to resource levels.

Understanding biodiversity effects on prey in multi-enemy systems

Biodiversity-ecosystem functioning theory would predict that increasing natural enemy richness should enhance prey consumption rate due to functional complementarity of enemy species. However, several studies show that ecological interactions among natural enemies may result in complex effects of enemy diversity on prey consumption. Therefore, the challenge in understanding natural enemy diversity effects is to predict consumption rates of multiple enemies taking into account effects arising from patterns of prey use together with species interactions. Here, we show how complementary and redundant prey use patterns result in additive and saturating effects, respectively, and how ecological interactions such as phenotypic niche shifts, synergy and intraguild predation enlarge the range of outcomes to include null, synergistic and antagonistic effects. This study provides a simple theoretical framework that can be applied to experimental studies to infer the biological mechanisms underlying natural enemy diversity effects on prey.