Quantifying the effects of intraspecific variation on predator feeding rates through nonlinear averaging

Abstract

Theory suggests that intraspecific trait variation will alter species interaction strengths through nonlinear averaging when interaction strengths are nonlinear functions of individuals' traits. This effect is expected to be widespread, yet what factors mediate its magnitude in nature and hence its potential effects on ecosystems and communities are unclear. We sought to quantify how nonlinear predator functional responses, variation in prey densities and counteracting variation in attack rates and handling times among predator individuals of similar body size alter their population‐level feeding rates through nonlinear averaging in a natural system, and to determine the processes influencing the net magnitude of this effect. We used a field caging experiment in the rocky intertidal of Oregon, USA to quantify attack‐rate variation and feeding rates of the whelk Nucella ostrina on its barnacle and mussel prey. We also used empirically parameterized simulations to examine the effects of handling‐time variation among individuals on population‐level feeding rates. Within cages, individual attack‐rate variation reduced population‐level whelk feeding rates. However, the magnitude of this reduction differed among prey species and cages depending on cage‐specific magnitudes of attack‐rate variation and functional‐response nonlinearity. The inferred effects of handling‐time variation among individuals were of smaller magnitude than those of attack‐rate variation, yet counteracted them to cause a net weakening of the effect of individual attack‐rate variation on population‐level feeding rates. Across cages, attack‐rate and prey‐density variation had non‐additive effects that produced greater feeding‐rate reductions at the experiment scale relative to the cage scale. Our results indicate that the effects of trait variation via nonlinear averaging depend critically on the features of systems that determine the magnitudes of nonlinearities and trait variation. Because of counteracting trait variation, nonlinear‐averaging effects may be quite complex, involving both the variances and covariances of all traits and environmental variables influencing the ecological process of interest. A free Plain Language Summary can be found within the Supporting Information of this article.

Quantifying predator functional responses under field conditions reveals interactive effects of temperature and interference with sex and stage

“…Second, although using natural variation in conditions ensures that variables are within the range organisms experience, our results show that some conditions may be uncommon, leading to low sample sizes that may hamper inference. One solution to this would be to combine observational approaches and manipulative experiments (Coblentz et al, 2021; Novak et al, 2017). Third, as with any observational study, there is the potential for confounding effects that can influence the results.…”

Prey diversity constrains the adaptive potential of predator foraging traits

“…Data are available from the Dryad Digital Repository: <https://doi.org/10.5061/dryad.j6q573nfn > (DeLong and Coblentz 2021).…”

“…Recent work indicates that there is considerable scope for natural selection to act on a and h given individual variation in functional responses and associated traits (Schröder et al 2016, DeLong et al 2021). This individual variation in functional response parameters is both raw material for selection and likely to have impacts on the ecological dynamics of the predator–prey system through non‐linear averaging effects on the overall interaction strength (Bolnick et al 2011, Coblentz et al 2021).…”

Predator feeding rates may often be unsaturated under typical prey densities

“…Gilg et al, 2006;Messier, 1994;Moleón et al, 2012;Moustahfid et al, 2010;Nielsen, 1999;Nilsen et al, 2009). However, other studies show saturated feeding rates at only one or a few observations at the highest observed prey densities (Angerbjörn, 1989;Coblentz et al, 2022;Korpimaki & Norrdahl, 1991;Quinn et al, 2003;Redpath & Thirgood, 1999;Sundell et al, 2000) or little to no evidence of feeding rate saturation (Beardsell et al, 2021(Beardsell et al, , 2022Coblentz et al, 2021;Novak, 2010;Novak et al, 2017;Preston et al, 2018). Therefore, it is currently unclear how saturated feeding rates are likely to be in general.…”