Perturbations to trophic interactions and the stability of complex food webs

Abstract

The pattern of predator-prey interactions is thought to be a key determinant of ecosystem processes and stability. Complex ecological networks are characterized by distributions of interaction strengths that are highly skewed, with many weak and few strong interactors present. Theory suggests that this pattern promotes stability as weak interactors dampen the destabilizing potential of strong interactors. Here, we present an experimental test of this hypothesis and provide empirical evidence that the loss of weak interactors can destabilize communities in nature. We ranked 10 marine consumer species by the strength of their trophic interactions. We removed the strongest and weakest of these interactors from experimental food webs containing >100 species. Extinction of strong interactors produced a dramatic trophic cascade and reduced the temporal stability of key ecosystem process rates, community diversity and resistance to changes in community composition. Loss of weak interactors also proved damaging for our experimental ecosystems, leading to reductions in the temporal and spatial stability of ecosystem process rates, community diversity, and resistance. These results highlight the importance of conserving species to maintain the stabilizing pattern of trophic interactions in nature, even if they are perceived to have weak effects in the system. biodiversity and ecosystem functioning ͉ dynamic index ͉ interaction strength ͉ predator-prey interactions ͉ temporal and spatial variability F or decades, scientists have argued over the natural phenomena that allow complex communities to persist in nature (1-3). Randomly assembled communities become less stable with increasing complexity (2, 4), but natural communities are finely structured (5, 6), displaying properties that promote stability despite complexity (7). Experiments (8-10) and theory based on empirical data (11, 12) have shown that real food webs are characterized by few strong interactions embedded in a majority of weak links. It is thought that this nonrandom arrangement of interaction strengths promotes community-level stability by generating negative covariances, which suppress the destabilizing effect of strong consumer-resource interactions (3). Theoretical studies provide overwhelming support for the idea that the pattern of strong and weak predator-prey interaction strengths confers stability to food webs (11-14); however, these predictions have never been tested experimentally in natural systems.One difficulty in testing the importance of interaction strength patterns for the stability of real food webs is the disparity between empirical and theoretical estimates of stability. Theoretical studies often assume that a system is stable only if it is governed by stable equilibrium dynamics (2,7,12,15). Consequently, stability is often measured as the system's ability to defy change, i.e., resilience or resistance (3). In contrast, laboratory and field experiments rarely possess a well-defined equilibrium, so it is difficult to measure resilience or...

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…Byrnes et al, 2011;O'Gorman and Emmerson, 2009;Woodward et al, 2012). This could also lead to an increase in the proportion of top consumers relative to intermediate species, as the latter are effectively promoted to the termini of food Figure 16 Ecological network structure of stream food webs from the Ashdown Forest, UK, shown from local to regional to global networks.…”

L’évaluation des impacts cumulés dans l’estuaire et le golfe du Saint-Laurent : vers une planification systémique de l’exploitation des ressources

“…Des effets non linéaires peuvent également survenir en raison de la complexité des interactions biotiques formant la structure des communautés, qui peut être à la base d'une propagation indirecte des perturbations au sein des écosystèmes (Yodzis, 2000;Montoya et collab., 2009;O'Gorman et Emmerson, 2009). Puisque les impacts des stresseurs peuvent être très contrastés entre espèces, la réponse des communautés aux stresseurs multiples dépend de la co-tolérance des espèces à chaque stresseur (Vinebrooke et collab., 2004).…”

Genetic variation, predator–prey interactions and food web structure

“…Moreover, as the number of predator species that feed on Xysticus would remain maximum and unaltered with genetic variation (table 1), an increase in the number of predator species incorporated in the Xysticus diet as prey and a decrease in the average interaction strengths with increasing genetic variation, suggests that the number of weak and long feeding loops in the web can also increase with genetic variation. All these structural properties have been documented to increase the robustness [20,30,31,36,72] and the stability [28,29,32,38,39,72] of food webs. Interestingly, the effect of higher genetic variation can be more pronounced as predator-prey body size ratios increase.…”

“…Understanding the structure of food webs in terms of connectance, the degree of omnivory, the strength of interactions and its variation across species, as well as the length and density of loops, is crucial to understanding what drives food web dynamics, robustness and stability (e.g. [20,28 -34]; but see [35][36][37][38][39][40]). …”