PARENT SESSION
Oral Session # 95: Predator - Prey Ecology III: Aquatic Systems.
Presiding: J Vonesh
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 201.

Scale dependent impact of predatory fish on stream benthos: A test of theory.

Englund, Göran^{*,1, 2}, ¹ Department of Ecology and Environmental Science, Umeå University, E-mail:goran.englund@eg.umu.se, Umeå, Sweden, Sweden² Umeå Marine Sciences Centre, Umeå, Sweden, Sweden

ABSTRACT- In open predation experiments the impact of predators on prey densities can be influenced both by consumption and by prey movements. A model predicts that the predator impacts observed in such experiments are scale dependent over the scale range where there is a transition from movement control (of prey densities) to consumption control (Englund 1997). To test this model I examined the impact of predatory sculpins on densities of stream invertebrates in instream channels of different length (0.5 m, 2 m, and 8 m). The predator treatment affected the densities of six taxa. Per capita emigration rates was scale independent for these taxa and comparisons prey emigration and predator consumption rates showed that predator impacts were due to prey movements rather than predation. Thus, the model predicts scale independent predator impacts. In contrast to expectations predator impact on prey density was scale dependent for four taxa. For three large predatory invertebrates the predator impact increased with experimental scale, and for the fourth taxa, a small invertebrate predator, the opposite pattern was observed. These scale effects occurred because prey movement responses to predators were modified by the experimental scale, a mechanism not included in the original theory.

Key words: predation, scale dependence, prey movements