Food web context modifies predator foraging and weakens trophic interaction strength.

Trophic interaction modifications (TIM) are widespread in natural systems and occur when a third species indirectly alters the strength of a trophic interaction. Past studies have focused on documenting the existence and magnitude of TIMs; however, the underlying processes and long-term consequences remain elusive. To address this gap, we experimentally quantified the density-dependent effect of a third species on a predator's functional response. We conducted short-term experiments with ciliate communities composed of a predator, prey and non-consumable 'modifier' species. In both communities, increasing modifier density weakened the trophic interaction strength, due to a negative effect on the predator's space clearance rate. Simulated long-term dynamics indicate quantitative differences between models that account for TIMs or include only pairwise interactions. Our study demonstrates that TIMs are important to understand and predict community dynamics and highlights the need to move beyond focal species pairs to understand the consequences of species interactions in communities.

Drivers and consequences of structure in plant-lemur ecological networks.

Species interactions shape the diversity and resilience of ecological networks. Plant and animal traits, as well as phylogeny, affect interaction likelihood, driving variation in network structure and tolerance to disturbance. We investigated how traits and phylogenetic effects influenced network-wide interaction probabilities and examined the consequences of extinction on the structure and robustness of ecological networks. We combined both mutualistic and antagonistic interactions of animals (55 species, Infraorder Lemuriformes, Order Primates) and their food plants (590 genera) throughout Madagascar to generate ecological networks. We tested the effects of both lemur and plant traits, biogeographic factors and phylogenetic relatedness on interaction probability in these networks using exponential random graph models. Next, we simulated animal and plant extinction to analyse the effects of extinction on network structure (connectance, nestedness and modularity) and robustness for mutualistic, antagonistic and combined plant-animal networks. Both animal and plant traits affected their interaction probabilities. Large, frugivorous lemurs with a short gestation length, occurring in arid habitats, and with a Least Concern threat level had a high interaction probability in the network, given all other variables. Closely related plants were more likely to interact with the same lemur species than distantly related plants, but closely related lemurs were not more likely to interact with the same plant genus. Simulated lemur extinction tended to increase connectance and modularity, but decrease nestedness and robustness, compared to pre-extinction networks. Networks were more tolerant to plant than lemur extinctions. Lemur-plant interactions were highly trait structured and the loss of both lemurs and plants threatened the tolerance of mutualistic, antagonistic and combined networks to future disturbance.

Les interactions des espèces façonnent la diversité et la résilience des réseaux écologiques. Les caractéristiques des plantes et des animaux, ainsi que la phylogénie, affectent la probabilité d'interaction, entraînant des variations dans la structure du réseau et la tolérance aux perturbations. Nous avons étudié comment les traits et les effets phylogénétiques influençaient les probabilités d'interaction à l'échelle du réseau et examiné les conséquences de l'extinction sur la structure et la robustesse des réseaux écologiques. Nous avons combiné les interactions mutualistes et antagonistes des animaux (55 espèces, Infraorder Lemuriformes, Order Primates) et leurs plantes alimentaires (590 genres) à travers Madagascar pour générer des réseaux écologiques. Nous avons testé les effets des caractéristiques des lémuriens et des plantes, des facteurs biogéographiques et de la parenté phylogénétique sur la probabilité d'interaction dans ces réseaux à l'aide de modèles de graphes aléatoires exponentiels. Ensuite, nous avons simulé l'extinction des animaux et des plantes pour analyser les effets de l'extinction sur la structure du réseau (connectance, imbrication et modularité) et la robustesse des réseaux mutualistes, antagonistes et combinés plante-animal. Les caractéristiques animales et végétales ont affecté leurs probabilités d'interaction. Les grands lémuriens frugivores avec une durée de gestation courte, présents dans des habitats arides et avec un niveau de menace Préoccupation mineure avaient une probabilité d'interaction élevée dans le réseau, compte tenu de toutes les autres variables. Les plantes étroitement apparentées étaient plus susceptibles d'interagir avec la même espèce de lémuriens que les plantes éloignées, mais les lémuriens étroitement apparentés n'étaient pas plus susceptibles d'interagir avec le même genre végétal. L'extinction simulée des lémuriens a eu tendance à augmenter la connectivité et la modularité, mais à diminuer l'imbrication et la robustesse, par rapport aux réseaux pré-extinction. Les réseaux étaient plus tolérants aux plantes qu'aux extinctions de lémuriens. Les interactions lémuriens-plantes étaient fortement structurées par des traits et la perte des lémuriens et des plantes menaçait la tolérance des réseaux mutualistes, antagonistes et combinés aux perturbations futures.

Interaction modifications lead to greater robustness than pairwise non-trophic effects in food webs.

Considerable emphasis has been placed recently on the importance of incorporating non-trophic effects into our understanding of ecological networks. Interaction modifications are well-established as generating strong non-trophic impacts by modulating the strength of interspecific interactions. For simplicity and comparison with direct interactions within a network context, the consequences of interaction modifications have often been described as direct pairwise interactions. The consequences of this assumption have not been examined in non-equilibrium settings where unexpected consequences of interaction modifications are most likely. To test the distinct dynamic nature of these "higher-order" effects, we directly compare, using dynamic simulations, the robustness to extinctions under perturbation of systems where interaction modifications are either explicitly modelled or represented by corresponding equivalent pairwise non-trophic interactions. Full, multi-species representations of interaction modifications resulted in a greater robustness to extinctions compared to equivalent pairwise effects. Explanations for this increased stability despite apparent greater dynamic complexity can be found in additional routes for dynamic feedbacks. Furthermore, interaction modifications changed the relative vulnerability of species to extinction from those trophically connected close to the perturbed species towards those receiving a large number of modifications. Future empirical and theoretical research into non-trophic effects should distinguish interaction modifications from direct pairwise effects in order to maximize information about the system dynamics. Interaction modifications have the potential to shift expectations of species vulnerability based exclusively on trophic networks.

Predictive power of food web models based on body size decreases with trophic complexity.

Food web models parameterised using body size show promise to predict trophic interaction strengths (IS) and abundance dynamics. However, this remains to be rigorously tested in food webs beyond simple trophic modules, where indirect and intraguild interactions could be important and driven by traits other than body size. We systematically varied predator body size, guild composition and richness in microcosm insect webs and compared experimental outcomes with predictions of IS from models with allometrically scaled parameters. Body size was a strong predictor of IS in simple modules (r2  = 0.92), but with increasing complexity the predictive power decreased, with model IS being consistently overestimated. We quantify the strength of observed trophic interaction modifications, partition this into density-mediated vs. behaviour-mediated indirect effects and show that model shortcomings in predicting IS is related to the size of behaviour-mediated effects. Our findings encourage development of dynamical food web models explicitly including and exploring indirect mechanisms.

Trophic interaction modifications: an empirical and theoretical framework.

Consumer-resource interactions are often influenced by other species in the community. At present these 'trophic interaction modifications' are rarely included in ecological models despite demonstrations that they can drive system dynamics. Here, we advocate and extend an approach that has the potential to unite and represent this key group of non-trophic interactions by emphasising the change to trophic interactions induced by modifying species. We highlight the opportunities this approach brings in comparison to frameworks that coerce trophic interaction modifications into pairwise relationships. To establish common frames of reference and explore the value of the approach, we set out a range of metrics for the 'strength' of an interaction modification which incorporate increasing levels of contextual information about the system. Through demonstrations in three-species model systems, we establish that these metrics capture complimentary aspects of interaction modifications. We show how the approach can be used in a range of empirical contexts; we identify as specific gaps in current understanding experiments with multiple levels of modifier species and the distributions of modifications in networks. The trophic interaction modification approach we propose can motivate and unite empirical and theoretical studies of system dynamics, providing a route to confront ecological complexity.