Abstract
The effect of interference competition can be assessed by comparing the capture rate of a predator foraging alone with that of the predator within a group. Since such an effect could be prey density dependent, a constant density of prey must be maintained while assessing this effect, irrespective of the elimination of prey by predation. However, when studying a predator-harvester, such as a planktivorous fish, which collects zooplankton at a rate of up to 1 prey s−1, instantaneous replacement of each consumed prey item is not feasible. This problem was solved in short-lasting mesocosm experiments by minute-by-minute supplementation to replace eliminated Daphnia and maintain a constant average prey density. Such experiments were performed with different numbers of foraging roach (Rutilus rutilus) at three prey densities and in two ranges of ambient temperature. The number of Daphnia required at the start of each experiment to establish the initial prey density and the number that it was necessary to add per minute were determined in experiments conducted without prey supplementation and in preliminary experiments with prey supplementation. The results of this study revealed that fish foraging in a group eat less, due to both exploitation and non-aggressive competition for space. Moreover, the effect of interference competition was stronger at higher temperatures, irrespective of the prey density, indicating that natural populations of roach foraging in shoals may suffer more from competitive interactions in warmer waters.
Highlights
Interference competition occurs when direct interactions between resource-sharing competitors reduce access to shared resources, leading to a decrease in the rate of utilization of those resources (Miller 1967; Hassell and Varley 1969)
The number of Daphnia required at the start of each experiment to establish the initial prey density and the number that it was necessary to add per minute were determined in experiments conducted without prey supplementation and in preliminary experiments with prey supplementation
The aim of this study was to assess the impact of passive interference competition on the prey capture rate of planktivorous fish by excluding the effect of exploitation competition
Summary
Interference competition occurs when direct interactions between resource-sharing competitors (congeners or conspecifics) reduce access to shared resources, leading to a decrease in the rate of utilization of those resources (Miller 1967; Hassell and Varley 1969). This decrease could be due to active (e.g., Mansour and Lipcius 1991; Cresswell 1997) or passive (e.g., Dolman 1995) interactions, the former manifested through food theft and aggressive interactions, and the latter through obstruction of access to resources to other individuals by non-aggressive behavior. In the competition for food, the effect of interference might be obscured by a reduction in the feeding rate due to food depletion (exploitative competition) by competitors. It is clear that exploitative competition may lead to a gross overestimation of the effect of interference, since the latter phenomenon is prey density dependent (Sutherland and Koene 1982; Vahl et al 2005)
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