Abstract
AbstractOmnivory has been cited as an explanation for why trophic cascades are weak in many ecosystems, but empirical support for this prediction is equivocal. Compared to predators that feed only on herbivores, top omnivores—species that feed on both herbivores and primary producers—have been observed generating cascades ranging from strong to moderate, null, and negative. To gain intuition about the sensitivity of cascades to omnivory, we analyzed models describing systems with top omnivores that display either fixed or flexible diets, two foraging strategies that are supported by empirical observations. We identified regions of parameter space, wherein omnivores following a fixed foraging strategy, with herbivores and producers comprising a constant proportion of the diet, non‐intuitively generate stronger cascades than predators that are otherwise demographically identical: (1) high productivity relative to herbivore mortality and (2) small discrepancies in producer vs. herbivore reward create conditions in which cascades are stronger with moderate omnivory. In contrast, flexible omnivores that attempt to optimize per capita growth rates during search never induce cascades that are stronger than the case of predators. Although we focus on simple models, the consistency of these general patterns together with prior empirical evidence suggests that omnivores should not be uniformly ruled out as agents of strong trophic cascades.
Highlights
Trophic cascades occur when top predators indirectly effect change in primary producer biomass by directly reducing populations of intermediate herbivores (Paine 1980, Strong 1992, Terborgh and Estes 2013)
A growing number of factors that control the strength of trophic cascades continue to surface from model-based and experimental studies, and their identification has improved our understanding of processes that dampen or enhance indirect effects between species in ecological networks and ecosystem responses to disturbance (Pace et al 1999, Shurin et al 2002, 2010, Borer et al 2005, Estes et al 2011, Heath et al 2014, Fahimipour et al 2017, Piovia-Scott et al 2017)
We first considered the case of fixed omnivores that do not exhibit diet flexibility
Summary
Trophic cascades occur when top predators indirectly effect change in primary producer biomass by directly reducing populations of intermediate herbivores (Paine 1980, Strong 1992, Terborgh and Estes 2013). Omnivores that feed on producers occupy top trophic levels (Arim and Marquet 2004, Thompson et al 2007). This potential for direct consumption of both producer and herbivore species has led to the prediction that omnivory should override. Black curves mark extinction boundaries for either the omnivore or herbivore species (see panel b). Parameter values are a 1⁄4 7:5, b 1⁄4 5:5, f 1⁄4 0:25, d 1⁄4 2, and v 1⁄4 1:05
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
