Vol.:(0123456789) 1 3 Oecologia (2020) 192:791–799 https://doi.org/10.1007/s00442-020-04619-7 COMMUNITY ECOLOGY – ORIGINAL RESEARCH Predator population size structure alters consumption of prey from epigeic and grazing food webs Shannon M. Murphy 1  · Danny Lewis 2  · Gina M. Wimp 2 Received: 13 April 2019 / Accepted: 7 February 2020 / Published online: 22 February 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Numerous studies have found that predators can suppress prey densities and thereby impact important ecosystem processes such as plant productivity and decomposition. However, prey suppression by spiders can be highly variable. Unlike predators that feed on prey within a single energy channel, spiders often consume prey from asynchronous energy channels, such as grazing (live plant) and epigeic (soil surface) channels. Spiders undergo few life cycle changes and thus appear to be ideally suited to link energy channels, but ontogenetic diet shifts in spiders have received little attention. For example, spider use of diferent food channels may be highly specialized in diferent life stages and thus a species may be a multichannel omnivore only when we consider all life stages. Using stable isotopes, we investigated whether wolf spider (Pardosa littoralis, hence- forth Pardosa) prey consumption is driven by changes in spider size. Small spiders obtained > 80% of their prey from the epigeic channel, whereas larger spiders used grazing and epigeic prey almost equally. Changes in prey consumption were not driven by changes in prey density, but by changes in prey use by diferent spider size classes. Thus, because the popula- tion size structure of Pardosa changes dramatically over the growing season, changes in spider size may have important implications for the strength of trophic cascades. Our research demonstrates that life history can be an important component of predator diet, which may in turn afect community- and ecosystem-level processes. Keywords Diet shift · Food web · Multichannel omnivory · Predation · Stable isotopes Introduction Across a wide array of terrestrial and aquatic ecosystems, predators suppress prey densities, which can in turn afect important ecosystem services (Tscharntke et al. 2008). The ability of generalist predators to consume more than one prey resource and switch among prey can stabilize prey populations, and indirectly afect primary production and decomposition. In many food webs, predators link the plant-based, grazing energy channel with the detritus-based, soil-surface epigeic channel by consuming prey from both channels (Polis and Strong 1996; Ward et al. 2015; Wimp et al. 2013; Wolkovich et al. 2014). Here we use the term grazing channel to refer to herbivores that feed on live plants and the term epigeic channel to refer to consumers of plant detritus, its associated microbes, and algae found at the soil surface. Theoretical models show that such channel link- age can stabilize food webs when predators shift between channels in response to changes in relative prey abundance (Rooney et al. 2006; Ward et al. 2015; Wolkovich et al. 2014). However, the stabilizing efect may be considerably weaker if some predator size classes can shift between chan- nels and respond to changes in prey availability, while other size classes cannot, potentially due to negative interactions among size classes of predators (Rudolf 2006). When preda- tors can not only shift between diferent prey species, but also between prey from diferent food webs (multichannel omnivory), these diferent food webs can sustain predator populations at diferent times of the year, which can increase predator population density and stability (Settle et al. 1996). Communicated by Liliane Ruess. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00442-020-04619-7) contains supplementary material, which is available to authorized users. * Shannon M. Murphy Shannon.M.Murphy@du.edu 1 Department of Biological Sciences, University of Denver, Denver, CO 80208, USA 2 Biology Department, Georgetown University, Washington, DC 20057, USA