SPECIAL ISSUE: MOLECULAR DETECTION OF TROPHIC INTERACTIONS Comparing rates of springtail predation by web-building spiders using Bayesian inference KELTON D. WELCH,* 1 MATTHEW R. SCHOFIELD, † 2 ERIC G. CHAPMAN* and JAMES D. HARWOOD* *University of Kentucky, Department of Entomology, Lexington, KY 40546-0091, USA, †University of Kentucky, Department of Statistics, Lexington, KY 40546-0091, USA Abstract A major goal of gut-content analysis is to quantify predation rates by predators in the field, which could provide insights into the mechanisms behind ecosystem structure and function, as well as quantification of ecosystem services provided. However, per- centage-positive results from molecular assays are strongly influenced by factors other than predation rate, and thus can only be reliably used to quantify predation rates under very restrictive conditions. Here, we develop two statistical approaches, one using a parametric bootstrap and the other in terms of Bayesian inference, to build upon previous techniques that use DNA decay rates to rank predators by their rate of prey consumption, by allowing a statistical assessment of confidence in the inferred ranking. To demonstrate the utility of this technique in evaluating ecological data, we test web-building spiders for predation on a primary prey item, springtails. Using these approaches we found that an orb-weaving spider consumes springtail prey at a higher rate than a syntopic sheet-weaving spider, despite occupying microhabitats where springtails are less frequently encountered. We suggest that spider-web architec- ture (orb web vs. sheet web) is a primary determinant of prey-consumption rates within this assemblage of predators, which demonstrates the potential influence of predator foraging behaviour on trophic web structure. We also discuss how additional assumptions can be incorporated into the same analysis to allow broader application of the technique beyond the specific example presented. We believe that such model- ling techniques can greatly advance the field of molecular gut-content analysis. Keywords: alternative prey, detritivore, gut-content analysis, predation rate Received 29 June 2013; revision received 7 March 2014; accepted 12 March 2014 Introduction The rate at which predators capture and consume prey is a primary feature of the structure and function of food webs. It is of interest for both the study of ecologi- cal communities and the management of agricultural systems. In classical ecological theory, predation rate is driven primarily by two factors: the availability of the prey in the habitat, and the efficiency with which the predator can capture and process the prey (e.g. Holling 1959; MacArthur & Pianka 1966). Each of these factors can be influenced by physiological, behavioural and ecological characteristics of the predator and prey and by physical and biotic characteristics of the habitat in which the predator and prey interact (Finke & Denno 2002; Schmitz 2007). While this is relatively straightfor- ward from a conceptual perspective, predation rates have proven difficult to quantify empirically, largely due to methodological constraints (Symondson 2002; Sheppard & Harwood 2005). For example, directly observing adequate numbers of predation events with- out artificial disturbance is often impossible in the field (Symondson 2002). Additionally, many animals (e.g. fluid-feeding arthropods) leave no identifiable frag- ments of meals within their gut, precluding visual iden- tification of meals through gut dissection. Molecular gut-content assays can bypass some of these limitations Correspondence: Kelton D. Welch, Fax: 1 (605) 693-5240; E-mail: keltondouglaswelch@gmail.com 1 Present address: USDA-ARS NCARL, Brookings, SD, USA 2 Present address: University of Otago, Dunedin, New Zealand © 2014 John Wiley & Sons Ltd Molecular Ecology (2014) 23, 3814–3825 doi: 10.1111/mec.12721