Consequences of consumer origin and omnivory on stability in experimental food web modules

Monica Granados,Anthony Ricciardi,Katie S Pagnucco

doi:10.1111/fwb.13378

Abstract

Abstract Food web stability, a fundamental characteristic of ecosystems, is influenced by the nature and strength of species interactions. Theory posits that food webs are stabilised by omnivory and disrupted by novel consumers. To test the effects of secondary consumer origin and trophic level on basal resource stability, we constructed crayfish–snail–algae modules using four congeneric species of crayfish (Faxonius spp.), two from native populations (Faxonius propinquus and Faxonius virilis) and two from non‐native populations (Faxonius limosus and Faxonius rusticus). We performed surgical manipulations of crayfish feeding structures to create omnivore food web and predator food chain modules. We compared the temporal stability of these modules using measures of the coefficient of variation of the basal resource (benthic algae). Consistent with theoretical and empirical predictions, food web modules with omnivory had the lowest coefficient of variation. However, contrary to prediction, we did not find consistently higher coefficients of variation in modules with non‐native species. Rather, across species, we found the lowest coefficient of variation in modules with one of the non‐native species (F. rusticus) and one native species (F. virilis), owing to stronger interactions between these crayfish species and their snail and algal food resources. The results suggest that omnivory is indeed stabilising and that very weak interactions or very low attack rates of the consumer on the basal resource can be unstable. Thus, we demonstrate that omnivores may have different impacts than predators when introduced into a novel ecosystem, differences that can supersede the effect of species identity.

Highlights

Food web stability, here defined as temporal constancy, is a fundamental characteristic of ecosystems (Worm & Duffy, 2003) that can be profoundly affected by the presence of omnivores – organisms that feed on more than one trophic level (Pimm & Lawton, 1978; Pimm, 1982)
One of the few direct experimental tests of the effect of omnivory on food web stability was conducted on arthropod assemblages by Fagan (1997), who found that a high degree of omnivory stabilized community dynamics following disturbance; the omnivore and predator species used in the experiment comprised different genera and, aCC-BY 4.0 International license
The coefficient of variation differed between F. rusticus and F. limosus (Fig. 2, analysis of variance (ANOVA), Tukey HSD, P = 0.0023) and between F. virilis and F. limosus (Tukey HSD, P = 0.0213), but not between other species pairs (Table S1 in Supporting Information)

Summary

Introduction

Here defined as temporal constancy, is a fundamental characteristic of ecosystems (Worm & Duffy, 2003) that can be profoundly affected by the presence of omnivores – organisms that feed on more than one trophic level (Pimm & Lawton, 1978; Pimm, 1982). A form of omnivory that has been the focus of theoretical and empirical investigations on stability is intraguild predation, where an omnivore feeds on an intermediate consumer in addition to one of the prey’s resources (Polis, Myers, & Holt, 1989; Holt & Polis, 1997). Experimental studies of the dynamics of simple three- or four-species food webs with and without omnivores have revealed that omnivory is stabilizing (Lawler & Morin, 1993; Morin & Lawler, 1995). One of the few direct experimental tests of the effect of omnivory on food web stability was conducted on arthropod assemblages by Fagan (1997), who found that a high degree of omnivory stabilized community dynamics following disturbance; the omnivore and predator species used in the experiment comprised different genera and, the effects of omnivory on community stability were confounded by potential species effects

Methods

Results

Conclusion