Interaction strength combinations and the overfishing of a marine food web

Abstract

The stability of ecological communities largely depends on the strength of interactions between predators and their prey. Here we show that these interaction strengths are structured nonrandomly in a large Caribbean marine food web. Specifically, the cooccurrence of strong interactions on two consecutive levels of food chains occurs less frequently than expected by chance. Even when they occur, these strongly interacting chains are accompanied by strong omnivory more often than expected by chance. By using a food web model, we show that these interaction strength combinations reduce the likelihood of trophic cascades after the overfishing of top predators. However, fishing selectively removes predators that are overrepresented in strongly interacting chains. Hence, the potential for strong community-wide effects remains a threat.community stability ͉ omnivory ͉ trophic chain ͉ trophic cascade ͉ quantitative networks Q uantification of the strength of interactions between species is essential for understanding how ecological communities are organized and how they respond to human exploitation. Food webs are characterized by many weak interactions and a few strong interactions (1-6), which appears to promote community persistence and stability (7-9). However, little is known about how interaction strengths are combined to form the simplest components of complex food webs (10, 11). An example of such a component is a tritrophic food chain (TFC) in which a top predator P eats a consumer C, which in turn eats a resource R (Fig. 1).The cooccurrence of strong interactions on two consecutive levels of a trophic chain has the potential to modify the structure and dynamics of entire food webs through trophic cascades (12-16). Trophic cascades are predator-prey effects that alter biomass or abundance of a species across more than one trophic link (12,16). Reductions in the abundance of a predator through fishing would propagate through the food chain resulting in increased consumer abundance and fewer resources (13).The role of omnivory (the top predator also feeds on the resource) (Fig. 1b) in food web stability has been debated for decades. Although previous results concluded that omnivory destabilizes food webs (17), recent studies have shown the opposite trend (18,19). It is unclear whether omnivory can mitigate the effect of trophic cascades when top predators and consumers are strong interactors (20). Thus, although the overfishing of top predators has the potential to cause trophic cascades, we still do not know how general the community-wide effects of overfishing are.Here we analyze a real, large food web to describe the patterns of interaction strength combinations and explore their implications for food web dynamics. Specifically, we first quantified per-capita interaction strengths (interaction strengths hereafter) between all predator-prey links. Then, we determined how strong interactions are combined within TFCs with and without omnivory. Finally, we linked structure and dynamics by using a biologically para...

Dedication. Acknowledgments

“…Here we expand the quantitative approach to describe links probabilistically, such that link-strengths for a given pairwise interaction have associated probabilities. We define link-strength in terms of the proportional biomass flow from prey to a consumer [14]. Thus, links connecting a single consumer to its prey denote the proportional contribution of biomass of prey to a predator and sum to one ( Fig.…”

“…In a food-web network, link-strengths can be defined as the proportional contribution of resources to the diet of a consumer (cf. [14]). Such a measure necessarily represents a consumer-centric component of trophic interactions, although prey-centric measures can sometimes be subsequently derived.…”

“…Quantitative food-webs have been shown to have long-tailed link-strengths distributions, with many weak interactions and few strong interactions [203,14,188].…”

“…Impacts of perturbations on food-web structure may be immediate or lagged [186]; they can a↵ect communities by reorganizing interactions [137], changing the magnitudes of interactions [89,14], or eliminating species [186,172]. However, observations of community organization across a perturbation event are typically confined to short timescales and to communities with fast turn-over rates.…”

“…Specific patterns of interaction are often observed in ecological networks [14,184,90]. Some of these structures have important dynamical implications and/or may contain information regarding the role of biological factors, such as body size, in shaping the organization of ecological communities [202].…”