Domestic cats Felis catus, as companion animals provided with supplemental food, are not limited by the availability of wild prey and locally occur at extraordinary high densities. There is growing concern about the potential impact of large cat numbers on native prey populations. In the present study, we quantified the minimum number of animals killed in a rural village in Switzerland by asking owners (1) to estimate the predation rate in advance and (2) to record prey animals returned home by their pets. The frequency distribution of the numbers of prey items was markedly skewed: 16% of the cats accounted for 75% of prey, irrespective of sex, age or breed. A large fraction of owners considerably overestimated their cat's predation, indicating that surveying predation rates by means of a questionnaire alone is not sufficient. The observed average rate of predation within 48 days in spring was 2.29 prey items/cat/month (N = 32 cats); major prey types were rodents (76.1%) and birds (11.1%). The absolute number of prey items taken per area is striking and indicates that cat predation represents an important factor in ecosystems. Its role may be momentous in intensively fragmented urban habitats, where cat densities are especially high. We thus highlight the need to identify the factors determining predation rates of individual cats. Further extended studies, especially in urbanised areas, are needed to quantify the actual impact of cat predation upon the population dynamics of their prey.

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Summary 1.In heterogeneous environments, the risk of predation and parasitism for phytophagous insects varies among different microhabitats. Adult females can escape natural enemies by depositing their eggs on sites unfavourable for prey-searching predators or parasitoids. Host plant characteristics can determine the availability of such spatial refuges. 2.We assessed the importance of host plant exposure for the predation probability of shield beetle ( Cassida rubiginosa Müller) larvae by paper wasps ( Polistes dominulus Christ), hypothesizing that prey on exposed (i.e. free-standing) plants are more likely to be found and eaten by predators than prey on hidden (i.e. surrounded by other vegetation) plants. Because larval predation is a major mortality factor in shield beetles, we further hypothesized that the prey would adapt and avoid high-risk plants for oviposition. Finally, we investigated the influence of egg parasitism on the prey's host plant choice. 3. Host plant exposure significantly affected predation probability of Shield Beetles. Larvae on hidden host plants were less likely to be killed by the wasps than larvae on exposed shoots. However, females did not prefer the low predation risk environment of hidden plants for oviposition. The rate of parasitism was equal on exposed and hidden plants, and thus probably does not contribute to the oviposition habitat selection of prey. 4. Our study shows experimentally that characteristics of the first trophic level (i.e. host plant exposure) can affect the foraging success of insect predators. Different reasons, potentially responsible for the apparent non-selective host plant choice in beetle females, are discussed.

Abstract. Throughout the study of ecology, there has been a growing realization that indirect effects among species cause complexity in food webs. Understanding and predicting the behavior of ecosystems consequently depends on our ability to identify indirect effects and their mechanisms. The present study experimentally investigates indirect interactions arising between two prey species that share a common predator.In a natural field experiment, we introduced different densities of mealworms (Tenebrio molitor), an alternative prey, to a previously studied predator-prey system in which paper wasps (Polistes dominulus) preyed on shield beetle larvae (Cassida rubiginosa). We tested if alternative prey affects predation on the first prey (i.e., the predator-dependent functional response of paper wasps) by modifying either interference among predators or the effective number of predators foraging on shield beetles. Presence of mealworms significantly reduced the effective number of predators, whereas predator interference was not affected. In this way, the experimentally introduced alternative prey altered the wasps' functional response and thereby indirectly influenced C. rubiginosa density. In all prey-density combinations offered, paper wasps constantly preferred T. molitor. This led to an asymmetrical, indirect interaction between both prey species: an increase in mealworm density significantly relaxed predation on C. rubiginosa, whereas an increase in C. rubiginosa density intensified predation on mealworms. Such asymmetrical outcomes of a fixed food preference can significantly affect the population dynamics of the species involved. In spite of the repeated finding of a Type III functional response in this system, our experiment did not reveal switching behavior in paper wasps. The variety of mechanisms underlying direct and indirect interactions within our study system exemplifies the importance of incorporating alternative prey when investigating the impact of a generalist predator on a focal prey population under realistic field conditions.

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