F. P. Biagioli1,a, K.E. Coblentz2,b, J.P. DeLong1,c1.School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE, USA2.Department of Biology, Colby College, Waterville, ME, USA a. fbiagioli2@huskers.unl.edu b. kcoblent@colby.edu c.jpdelong@unl.eduKeywords: Beddington-DeAngelis functional response, predator-dependent functional response, consumer-resource interactions, interaction strength, time budget, predator-prey interactions, stability, species interactions, meta-analysis, body sizeAbstract:Interference, or the foraging cost incurred from interacting with other predators, plays a key role in shaping predator–prey dynamics, yet its magnitude across taxa and importance relative to other foraging constraints remains unclear. We fit Beddington–DeAngelis functional response models to 41 studies manipulating predator and prey densities to test three hypotheses: that interference scales with space clearance rates but not handling time, increases with predator body mass, and becomes increasingly important as predator abundance rises relative to prey abundance. Interference scaled positively with space clearance rates, showed no relationship with handling time, and increased with predator and prey body mass. Interference costs were comparable to or greater than handling costs and increased with relative predator abundance. These results indicate that interference is a pervasive regulator of predator foraging that is often as important as handling time, highlighting the need to explicitly incorporate interference into predator–prey theory.Introduction :Mutual interference (hereafter interference), or the reduction in per capita foraging rate caused by predators interacting with other predators, plays an important role in shaping predator impacts on prey populations. Interference is common across taxa (DeLong & Vasseur 2011; Novak et al. 2017; Skalski & Gilliam 2001) and has been invoked to explain diverse ecological outcomes including community stability (Upadhyay et al. 2013; Vera et al. 2024), community resilience (Liu & Beretta 2006), predator and prey coexistence (Cantrell et al. 2004; Coblentz & DeLong 2020), and the persistence of complex trophic interactions (Geraldi 2015). Interference also has been linked to broader biological and evolutionary processes including predator diet switching (Hughes & Grabowski 2006), the evolution of prey refuge use (Geritz & Gyllenberg 2014), the long term persistence of multitrophic systems (Mougi 2022), and the paradox of enrichment (Rall et al. 2008).Although interference has been incorporated extensively into theoretical predator-prey models (Arditi & Akçakaya 1990; Liu & Zhang 2008) and syntheses of interference magnitudes have been previously published (DeLong & Vasseur 2011; Novak & Stouffer 2021; Skalski & Gilliam 2001), we lack a general understanding of the factors driving variation in the magnitude of interference across taxa. Moreover, the extent to which interference shapes predator foraging rates remains unclear, especially relative to other factors that limit foraging rates such as prey handling time. Even slight variations in interference can drastically alter the dynamics of both simple and complex ecological communities (Naji et al. 2010). Thus, developing a general understanding of the magnitude of interference across predator populations is essential for evaluating its ecological impact.To explore this, we tested three hypotheses related to interference magnitude. Our first hypothesis is that interference scales positively with predator space clearance rates but is independent of handling time. This predicted relationship emerges from the fact that predator movement that facilitates encounters with prey also may lead to encounters with conspecific predators. Such a connection was observed in theDidinium-Parmecium system across multiple observations and studies (Delong & Vasseur 2013). Because variation in space clearance rate across species also is linked to movement (Pawar et al. 2012), there may be a relationship between space clearance rate and interference across species as well. In contrast, handling time reflects a consumption- and digestion-driven process rather than movement, so it should be less directly associated with interference (DeLong 2021).Second, we hypothesized that interference increases with predator body mass but is unrelated to prey body mass. Because many foraging related processes scale with predator body size (Brose 2010; Coblentz et al. 2025; Uiterwaal et al. 2017), we predicted that interference would show a similar pattern as well. Furthermore, larger predators tend to move more (Hillaert et al. 2018), maintain larger home ranges (Mech & Zollner 2002; Stevens et al. 2014), require more resources (Brown et al. 2004), and forage at higher rates (Peters 1983). Like our rationale behind space clearance rates, increasing the size of these space use features increases the likelihood of encounters among predators, suggesting that interference should scale positively with body mass. However, a previous meta-analysis found that interference may be independent of predator body size (DeLong 2014), highlighting the need to re-examine this hypothesis. Although we did not expect interference to be related to prey mass, predator body mass is broadly correlated with prey body mass (Brose et al. 2006), setting up the possibility that interference may be linked to both predator and prey body mass.Third, we hypothesized that interference imposes relatively greater foraging costs than handling when predators are more abundant than prey. Many studies recognize that the time cost of handling prey is a key constraint on foraging, but often without simultaneously considering the effects of interference (Forbes 1989; Giller 1980; Johnson & Amarasekare 2015; McCann 2012; Rosenzweig & MacArthur 1963; Schreiber & Vejdani 2006). Yet when both interference and handling are considered, interference emerges as a major constraint on foraging rates (Johnson et al. 2001), even outweighing handling time constraints at high predator:prey ratios (Papanikolaou et al. 2016). Additionally, interference has been shown to promote system stability, highlighting its relevance as a driver of population dynamics beyond its direct effects on predator foraging (Arditi et al. 2004; Vance 1984). Thus, quantifying the relative magnitude of interference and handling time across predator–prey abundance ratios could clarify the conditions under which interference most strongly limits predator foraging and has the most stabilizing effects.To test these hypotheses, we fit Beddington–DeAngelis functional response models (Beddington 1975; DeAngelis et al. 1975) to data from functional response studies with multiple predator levels and examined the relationships among parameter estimates, species body masses, and expected abundances derived from mass–abundance scaling relationships. We found evidence for interference in all systems, with its strength strongly linked to space clearance rates and dependent on both predator and prey body mass. Furthermore, the costs of interference increased with predator abundance and was greater than or equal to that of handling at plausible predator and prey densities. Together, these results suggest that interference generates strong constraints on predator foraging and plays a key role in regulating energy transfer through food webs.
