An experimental test of the nature of predation: neither prey‐ nor ratio‐dependent

Abstract

Summary1. There is a current debate about the appropriateness of prey-dependent vs. ratiodependent functional responses in predator-prey models. This is an important issue as systems governed by these models exhibit quite different dynamical behaviour. However, the issue is not yet resolved on a theoretical basis, and there is a lack of experimental evidence in natural systems. We used a paper wasp-shield beetle system in a natural setting to assess the validity of either approach. 2. We manipulated the abundance of herbivorous insect prey on thistle plants and of predatory paper wasps in the immediate environment of the prey by opening or closing cages containing wasp nests. 3. The number of wasps foraging at the site increased when cages were opened, but rapidly reached an asymptote, indicating predator interference. The predation rate per predator decreased with the number of wasps in the environment. Thus, the functional response depended on both prey and predator density. 4. Neither a pure prey-nor a pure ratio-dependent model fitted perfectly our observations. However, the functional response of the paper wasps towards shield beetle larvae was closer to ratio-dependence. To our knowledge, this is the first experimental evidence discriminating between ratio-and prey-dependence in a natural setting with unconfined predators and prey. 5. Predator interference was most probably responsible for the specific form of the functional response found. We found indications that both direct (e.g. aggression) and indirect interference mechanisms (e.g. depletion of easy-to-find prey) were at work in our system. We conclude that predator density cannot be ignored in models of predatorprey interactions.

Functional Responses: A Question of Alternative Prey and Predator Density

“…Tschanz, L. F. Bersier, and S. Bacher, unpublished manuscript). Compared to previous studies conducted at the same site (Schenk et al 2005: Fig. 1; B. Tschanz, L. F. Bersier, and S. Bacher, unpublished manuscript), we recorded considerably higher predator numbers in this study.…”

“…The patch was located in the margin of the Institute's courtyard, belonging to a larger, partly uncultivated meadow with a mixture of flowering plants and surrounded by orchard trees and hedges. The same site had been used in earlier studies on paper wasp predation on shield beetle larvae (Schenk and Bacher 2002, Bacher and Luder 2005, Schenk et al 2005, Tschanz et al 2005; B. Tschanz, L. F. Bersier, and S. Bacher, unpublished manuscript). We used last (i.e., fifth) instar larvae of shield beetles and last instar larvae of mealworms in our experiments.…”

“…As similarly described elsewhere (Schenk et al 2005;Tschanz et al unpublished manuscript), we modified the method of Arditi and Saı¨ah (1992) to calculate Type III functional responses, explicitly allowing for prey depletion during the experiment. Without alternative prey, the actual number of C. rubiginosa eaten is given by…”

“…Among other experimental methods, Wootton (2002) proposes to start with a known direct functional relationship between two species and test whether there is any deviation from predicted effects when species composition is changed. Accordingly, we changed species composition in the present study by introducing an alternative prey species to a previously studied predator-prey system (Schenk and Bacher 2002, Schenk et al 2005, Tschanz et al 2005B. Tschanz, L. F. Bersier, and S. Bacher, unpublished manuscript).…”

“…By means of continuous video surveillance, previous investigations reported paper wasps to feed on Cassida rubiginosa larvae (Coleoptera, Chrysomelidae) in the field. In these previous experiments, we manipulated prey density (Schenk and Bacher 2002), prey and predator density (Schenk et al 2005), and prey vulnerability (B. Tschanz, L. F. Bersier, and S. Bacher, unpublished manuscript) in this system and identified the functional response of P. dominulus as predator-dependent Type III, taking an intermediate form between prey-and ratio-dependence (Schenk et al 2005; B. Tschanz, L. F. Bersier, and S. Bacher, unpublished manuscript).…”