Strong density dependence in a predatory insect: large‐scale experiments in a stream

Summary1 Empirical information about the intergenerational dynamics of stream insects is scarce, and most field experiments are conducted at small temporal and spatial scales that are inappropriate for assessing effects upon population dynamics. We performed a large‐scale, intergenerational population manipulation of an abundant, stream‐dwelling predator, the alderfly Sialis fuliginosa , by altering its recruitment over 3 consecutive years (it has a 2‐year life cycle). 2 Experimental treatments were assigned to three contiguous 150‐m stretches. Each year at least 92% of S. fuliginosa eggs, that are laid on the streamside vegetation, were removed from an upstream ‘removal’ stretch and transferred below an adjacent ‘control’ stretch to a downstream ‘addition’ stretch, where recruitment was thus effectively doubled. 3 Although manipulations were successful initially, the effects were transient. Strong density‐dependent survival stabilized the population at a similar density in all three stretches within the first 4 months of life for the 1997 cohort and somewhat later for the 1998 cohort. Survey data suggested that intraguild predation (including cannibalism) and/or starvation, particularly early in the life cycle, might be regulating the S. fuliginosa population. 4 It is intriguing that this abundant predator, which is linked very richly within the complex Broadstone Stream food web, has a strongly stabilized population, even in the face of such a dramatic perturbation in recruitment.