Deadly competition and life-saving predation: the potential for alternative stable states in a stage-structured predator–prey system

Abstract

Predators often undergo complete ontogenetic diet shifts, engaging in resource competition with species that become their prey during later developmental stages. Theory posits that this mix of stage-specific competition and predation, termed life-history intraguild predation (LHIGP), can lead to alternative stable states. In one state, prey exclude predators through competition (i.e. juvenile competitive bottleneck), while in the alternative, adult predators control prey density to limit competition and foster coexistence. Nevertheless, the interactions leading to these states have not been demonstrated in an empirical LHIGP system. To address this gap, we manipulated densities of cannibalistic adult cyclopoid copepods (Mesocyclops edax) and their cladoceran prey (Daphnia pulex) in a response-surface design and measured the maturation and survival of juvenile copepods (nauplii). We found that Daphnia reduced and even precluded both nauplii maturation and survival through depletion of a shared food resource. As predicted, adult copepods enhanced nauplii maturation and survival through Daphnia consumption, yet this positive effect was dependent on the relative abundance of Daphnia as well as the absolute density of adult copepods. Adult copepods reduced nauplii survival through cannibalism at low Daphnia densities and at the highest copepod density. This work demonstrates that predation can relax a strong juvenile competitive bottleneck in freshwater zooplankton, though cannibalism can reduce predator recruitment. Thus, our results highlight a key role for cannibalism in LHIGP dynamics and provide evidence for the interactions that drive alternative stable states in such systems.

Cannibalism and Intraguild Predation Community Dynamics: Coexistence, Competitive Exclusion, and the Loss of Alternative Stable States

“…This study further shows that cannibalism modifies the strength and qualitative outcomes of these interactions (Toscano et al 2016). For this empirical LHIGP system (Toscano et al 2016) and many others ( fig. 2), we currently lack theory to predict how the alteration of these short-term interactions by cannibalism might scale up to influence longterm LHIGP dynamics.…”

“…The dramatic ecological effects of stage-specific interactions are apparent in a widespread interaction module known as life-history intraguild predation (LHIGP; Pimm and Rice 1987;Walters and Kitchell 2001;Rudolf 2007;Abrams 2011;Hin et al 2011;Toscano et al 2016). LHIGP occurs when juvenile predators compete with their future prey (i.e., an intermediate consumer, hereafter "consumer") for a shared resource ( fig.…”

“…Within LHIGP systems, juvenile and adult predators may feed on separate resources (i.e., a complete diet shift; fig. 1A; Hin et al 2011;Toscano et al 2016), or adults may continue to feed on the juvenile resource, thereby broadening their diet over ontogeny (fig. 1B;van de Wolfshaar et al 2006;Rudolf 2007).…”

“…The ubiquity of cannibalism and the commonness of cannibalistic preference within stage-structured predator-prey systems suggest that cannibalism must be incorporated into current LHIGP theory to better understand and predict natural LHIGP dynamics. For example, a recent study using freshwater zooplankton demonstrates a juvenile competitive bottleneck and the alleviation of this bottleneck by adult predators, the precise interactions predicted to drive alternative stable states within LHIGP systems (Toscano et al 2016). This study further shows that cannibalism modifies the strength and qualitative outcomes of these interactions (Toscano et al 2016).…”

“…For example, a recent study using freshwater zooplankton demonstrates a juvenile competitive bottleneck and the alleviation of this bottleneck by adult predators, the precise interactions predicted to drive alternative stable states within LHIGP systems (Toscano et al 2016). This study further shows that cannibalism modifies the strength and qualitative outcomes of these interactions (Toscano et al 2016). For this empirical LHIGP system (Toscano et al 2016) and many others ( fig.…”