Omnivory does not preclude strong trophic cascades

Abstract

Omnivory 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.

Grassroots reserves rescue a river food web from cascading impacts of overharvest

“…The reduction in small invertivorous fishes (Figure 1d) and macroinvertebrates (Figure 1e), but not algae (Figure 1, f and g), in reserves suggests that omnivorous fishes preferentially fed on nutritious animal prey rather than algae (Marcarelli et al 2011). Such trophic flexibility is the most plausible explanation for how strong top-down control emerges in reserves despite rampant omnivory and high fish diversity (Fahimipour et al 2019). The result is a fish-driven cascade dynamic akin to that maintained by fewer, more specialized fishes outside of reserves, albeit without additional non-consumptive effects that intensify the cascade pattern.…”

“…Such trophic flexibility is the most plausible explanation for how strong top‐down control emerges in reserves despite rampant omnivory and high fish diversity (Fahimipour et al . 2019). The result is a fish‐driven cascade dynamic akin to that maintained by fewer, more specialized fishes outside of reserves, albeit without additional non‐consumptive effects that intensify the cascade pattern.…”

Inquiline predator increases nutrient-cycling efficiency of Nepenthes rafflesiana pitchers

“…feeds on both E. schuitemakeri detritivores and ant prey carcasses, albeit with a preference for the former (electronic supplementary material, Appendix S1). These equations follow the linear forms of the well-characterized consumer-resource family of models that are used extensively in community ecology [24,25] and that have also been used to model predation [26]. They express resource (R), prey (N 1 ) and predator (N 2 ) densities as functions of the consumption rates (f ij ), conversion efficiencies (a ij ) and mortality rates (d i ) of each species and the resource(s) it consumes (table 1).…”

Food web collapse and regime shift following goldfish introduction in permanent ponds

“…Through consumptive (e.g., predation) or non‐consumptive effects (e.g., fear‐mediated avoidance, kairomones), newly introduced IAS may quickly provoke the exclusion of native predators from invaded systems (Pringle et al., 2019; Winandy et al., 2015, 2017), potentially exacerbating their impact on recipient communities by altering the entire food web structure. Both theoretical considerations (Ward & McCann, 2017) and empirical evidence (Benkendorf & Whiteman, 2021; Gallardo et al., 2016; Pringle et al., 2019) suggest that predators' effects on food web structure depends on traits such as their degree of omnivory (i.e., feeding at multiple trophic levels), multi‐chain omnivory (feeding across multiple food chains) or functional response (i.e., the intake rate of a predator as a function of prey density) because they directly affect the strength and effects of trophic cascades, but their global effects are often difficult to forecast (Fahimipour et al., 2019; Ward & McCann, 2017; Wootton, 2017). By inducing different trophic cascades and/or excluding native predators, the introduction of functionally different IAS, such as omnivores in systems naturally dominated by more selective carnivores, have the potential to reduce food chain length (FCL) and collapse food web structure (Gallardo et al., 2016; Pringle et al., 2019; Sagouis et al., 2015).…”